Chapter 1

Introduction

An overview on polymer blends and their compatibilisation is presented in this

chapter. The chapter is constructed in such a way that first we give a brief

description on the important terms, aspects, problems and challenges in the field

of polymer blends. Eventually, we enter the topic of interest, i.e., cornpatibilisation of

polymer blends. Both physical and reactive compatibilisation techniques are

reviewed extensively. Details about the theories, strategies and mechanism of

compatibilisation are highlighted. In physical compatibilisation, we focus on

compatibilisation by the addition of pre-made copolymers and the factors

affecting their efficiency. In reactive compatibilisation, emphasis is put on the

compatibilisation by in-situ formation 3f block or graft copolymers and the

important factors which influence their interfacial activity. Special attention is

given to portray the recent advances in the field of polymer blends. Finally, the

scope of the present area of research and the precise objectives of the present

topic of research are described.

2 Chapter 1

1 .l. Polymer blends

Polymer blends are mixtures of two or more polymers andlor copolymers in

which the minor component contributes at least 2wt%. It is now truism that, in

recent years, polymer blends have exper~enced an important renaissance.

Academic and industrial research in this field is flourishing, and the input of

research papers, reviews and patents is growing exponentially. The exorbitant

use and ubiquitous nature of these materials in modern life can be evidenced by

noting the fact that polymer blends constitute ca. 36wt% of the total polymer

consumption, and their pertinence continues to increase. Polymer blends have

gained significant commercial growth in the last two decades outpacing the

growth rate of existing polymers by at least 2 to 5%. The current worldwide

market volume for polymer blends and alloys is estimated to be more than

700,000 metric tonly, with an average growth rate of 6 to 7% 111. Fig. 1.1

illustrates the pricelperformance profile of commercial polymer blends.

PA. PsS. PET. PC /" FOM, PFO, AES. SMA aP '\

/ HDPE.LDPE.LLLlPE.PP.PS (HIPS). PVC \ Consumption

Figure 1.1: The pricelquality profile of commercial polymer blends (Ref. 1)

Although investigated in lesser detail earlier, the science and technology of

polymer blends had its emergence in the 1970's. Many of the basic principles

existed prior to that time. Commercial blends existed for decades before,

however the concept of miscibility, phase behaviour and the basic nature of

polymer blends were not well understood or appreciated. An initial review of

polymer blends listed only 12 miscible polymer pairs, some of which were minor

variations in copolymer structure 121. It is, perhaps, the commercialisation of

miscible blends of poly(2,6-dimethyl-t,4 phenylene ether) and polystyrene

(PPEIPS) that promoted the interest in polymer blends. This PPEJPS blend

offered a myriad of different pricelperformance characteristics and thus could be

tailored to meet specific application requirements. Since their widespread

commercialisation, the applications of polymer blends have been directed at

replacement of traditional materials, most commonly, metals. Some of the

several applications of polymer blends include automotive, electronics and

electrical, building and constructions, meciical and packaging applications.

One fundamental question which has to be addressed first is: what inspires

polymer scientists and industrialists to focus on polymer blends rather than

synthesizing new materials? Indeed, there are several reasons, among which

the two most important are: first, less development costs. It should be noted that

design of new materials with special properties for demanding applications is

always more expensive than the cost of the constituent existing polymers and

the mixing process. Secondly, maximal diversification and increased use of

existing polymers. Attention should be paid to the fact that combining of

polymeric components in a certain ratio might result in a product with optimal

properties for demanding applications. Additionally, a remarkable broad

spectrum of properties can often be accomplished with polymer blends. Thus,

blending seems rewarding with respect to the price quality ratio as well as

flexibility in realising properties compared to polymer synthesis. Besides these.

other benefits that can be obtained by polymer blending include: (i) Improving

specific properties, viz. impact strength or solvent resistance and (ii) offering the

means for industrial and/or municipal waste recycling. Further more, blending

also benefits the manufacturer by offering: (i) improved processability, product

uniformity and scrap reduction (ii) quick formulation changes (iii) plant flexibility

and high productivily (iv) reduction of the number of grades that need to be

manufactured and stored (v) inherent recyclability etc [I]. Despite all these

positive aspects, there are at least two big problems that should be alleviated to

achieve the desired properties. These are the inherent immiscibility and

4 -- Chapter 1

incompatibility of polymeric materials and a comprehensive and coherent

approach towards these problems is very essential.

1.2. Polymerlpolymer miscibility

The first scientists whc carried out some preliminary experiments on the

miscibility of polymers were Dobry and Boyer-Kawenoki (31. A review published

in 1972 by Krause [4] contained data on 300 mixtures from 135 references. Later

on, in 1978 he published data on 800 polymer mixtures from 281 references [5].

In both these reviews, rniscible polymers comprise 10% of the entries. He called

polymers 'miscible' even when one or more of them is/are crystallisable and the

miscibility is confined to the amorphous state. Using this definition of miscibility,

Krause [6] has summarized data on 324 polymers from 314 references in the

third edition of Polymer Handbook, He also designed a database in which all old

and new data on polymer-polymer miscibility could be placed [7). In this article

he discussed systematically the structure of database, the problems of dealing

with the literature on this subject and the 'goodness' of the data in the literature.

A modified data on polymer-polymer miscibility has been published by Krause

and Goh [8] for fourth edition of Polymer Handbook in 1999 and upgraded and

reproduced by Utracki [I] in Polymer Blends Handbook in 2002.

From a purely thermodynamic viewpoint, miscibility refers to the molecular level

homogeneity that requires the free energy of mixing, AG,, should be negative.

Fig. 1.2 shows the three possible ways in which the AG, of a binary mixture may

vary with composition. In case A, AG, is positive over the whole composition

range implicating that the two components are immiscible with each other to any

extent. In the case of B, AG, is always negative and thus the components are

completely miscible in all proportions. However, a negative AG, does not assure

complete miscibility as case C illustrates. In the case C, AG, shows a negative

curvature in the mid composition range, and thus the mixture can develop an

even lower free energy in this range by splitting into two phases with

compositions given by the two minima resulting in a miscibility gap or partial

miscibility.

Introduction 5

-------------A

Volume fraction

Figure 1.2: Schematic diagram for the variation of free energy change with composition in binary polymer mixture.

In this context, one can conclude that thermodynamic miscibility of polymers

requires two criteria to be satisfied at constant temperature and pressure [9].

These include:

A G , (=AH, -TM;", ) < 0 and (1.1)

in order to ensure stability against phase separation.

where hU, and an, indicate the change in enthalpy and entropy on mixing

respectively, T is the absolute temperature and h,p, andMi are the volume

fraction, density and molecular weight of component i, respectively. Equation (1)

suggests that for spontaneous mixing to occur, A G , must be negative. M,,

will always be positive as mixing increases the disorder of the system, thus

making the second half of equation ( 1 1 ) negative and favourable to mixing. But

in polymer systems, because of the long chain nature of the molecules, i\S, is

often smaller in magnitude than Nf,. It is this fact that causes polymer

miscibility to rely on specific interactions between the species, leading to a

negative AH,$, to achieve miscibility. Many types of interactions may exist

between two polymers. These include London dispersion forces between non-

polar molecules, Columbic ion-ion and ion-dipole interactions, dipole-dipole

interactions between permanent or induced dipoles, charge transfer forces,

hydrogen bonding, etc. The second thermodynamic requirement given in

equation (1.2) is applicable in the case of partially miscible systems and

suggests that the second derivative of AG,,, with respect to the volume fraction

of one of the comporlc?nts at constant temperature and pressure should be

greater than zero to erlsure stability against phase separation since in these

systems the composition fluctuation may lead to phase separation.

The simplest thermodynamic model for describing the AG,,, of two polymers is

based on the extension of the results developed originally for polymer solutions

by Flory and Huggins, according to which

where B is a binary interaction energy density and R is the universal gas

constant. B is related to Flory-Huggin's (FH) interaction parameter,^,?, by the

expression:

where V,+, is a reference volume that usually is taken as the molar volume of

one of the repeat units n the system and H can be estimated from the solubility

parameters (8) of polyrners as:

Scan be calculated from molar attraction constants. Because the terms

contained in the parentheses in equation 3 involve taking the natural logarithm of

fractional numbers, these terms will always be negative. &,is closely

associated with the enthalpic interactions of the blend species. The value of

xI2 may be negative, positive or zero (athermal mixing). However, equations

(1.4) and (1.5) suggest that it cannot have negative values. Therefore, to

achieve miscibiliy, X,: should be made as small as possible

In fact, according to FH model polymerlpolymer miscibility can occur in three

possible cases:

(i) if the polymers are not of very high molecular weight and the combinatorial

entropy of mixing, LLC,~, , IS not negligible (ii) if the polymers have a very small

positive heat of mixing, AH", , arising from a very small exchange energy and

(iii) if the polymers have a negative heat of mixing arising due to specific

interactions. The maln disadvantages of FH theory which is based on simple

lattice model of fixed volume are that it does not take into account the possible

volume changes on rnixing and is not able to predict the phase segregation on

heating (lower critical solution temperature. LCST) which is often observed in

polymer blends. However, these shortcomings were more or less overcome in

more advanced theories such as equation of state theories of mixtures.

1.3. Polymer/polymer compatibility

The most important factor that should be addressed as far as a multi phase

polymer system has been concerned is the 'compatibility' between the

component polymers. It should be noted that the term compatibility has more

technical sense than thermodynamic. It is a relative and is defined in operational

terms. It is relative in the sense that greater or lesser degrees of compatibility

can be displayed by a blend under different conditions or even at different times

(as phase separation progresses) and is operational because the degree of

compatibility measured will depend on the technique used and the scale at

which this method probes the structure of the blends [I]. Thus, compatibility of a

particular blend might related to how 'good' a particular property it is. It is also

important to note that the most important properly of a multi phase polymer

systems is its morphology since most ol the other blend properties, to a lesser or

greater extent, depend on this. In most cases, compatibility of a blend can be

8 - Chapter 1

measured in terms of the type, state and stability of its morphology. Thus

compatibilisation is any modification that improves the compatibility, in terms of

morphology or any other property, between the component polymers and the

species used are usually referred as compatibilising agents (emulsifying or

interfacial agents). However, it should be noted that, before going to the details

of compatibilisation, one must understand the morphology and the most

important factor on which it is evaluated, i.e.. the interface of polymer blends.

1.4. Morphology of polymer blends

In general, the term 'morphology' refers to the form and organization on a size

scale above the atomic arrangement (for example, the arrangement of polymer

molecules into amorphous or crystalline regions) and the manner in which they

are organized into more complex units. On the other hand, morphology of a

multi-component polymer system (polymer blend) indicates the size, shape or

spatial distribution of the component phases with respect to each other (and is

referred as phase morphology). Since it is unequivocally established that most of

the ultimate properties (toughness, strength and crack resistance, optical,

rheological and dielectrical properties) of polymer blends are strongly influenced

by the type and fineness of phase structure, morphology control of polymer

blends has been emerging as an area of continuous interest to polymer material

scientists for the last few decades.

When two immiscible polymers are mixed, the size, shape and relative

distribution of one phase into the other depend on material parameters (blend

composition, viscosity ratio, elasticity ratio and interfacial tension) as well as

processing conditions (temperature, time. intensity and type of mixing and nature

of flow). Therefore the greatest challenge in the field of multiphase polymer

blend research is the manipulation of polymer structure via judicious control of

the melt flow during processing and the interfacial interactions. The mechanism

of development of n~orphology from pellet sized or powder sized particles in

polymer blends is directly derived from the complex interplay of material

parameters and prot:essing conditions. As a result of this, for a given blend,

different types of morphologies are possible. However, from the point of view of

the performance, they may be divided in to two categories: blends with discrete

phase structure (droplet or drop in matrix) and blends with bicontinuous phase

structure (co-continuous). Other important types of morphologies include fibrillar,

composite laminar, core shell, onion ring like, etc. A schematic representation of

different types of morphologies of polymer blends is presented in Fig. 1.3 [lo].

Table 1.1 illustrates a range of properties possible in immiscible blends and

particularly the morphology used to produce these properties [lo]. It is important

to mention that the f~nal morphology is determined by deformation-disintegration

(drop break-up) phenomena and coalescence and therefore before going to the

morphology of blends it is important to mention these aspects briefly.

mn*. h.rmi* exp=ns,on high tlou. W r a d cMduairn, - t a ~ * n c l s . nnmrsr 10nm

Figure 1.3: A schematic representation of different types of morphologies of polymer blends [Ref. 101.

- Product A u f a c t u r e r Components Morpholog~ Property

Dynamar Dyneon PWPTFE dilute drops processability

MB Dow Corning PPIPDMS dilute drops lubricity NorylGTX GE PAG/PPO/SB drops (60%) dimensional control

Zytel ST Dupont PA661EP double emulsion toughness Selar DuPont PE/PA66 lamellar diffusion barrier

Vectra Hoechst PETILCP fibres thermal expansion TSOP Mitsubishil PP/PE/EP co-continuous electrical

Toyota conductivity Stat-Rite Goodrich PPIPU co-continuous electrical

- conductivity --

Table 1.1: Commercial examples of immiscible polymer blends [Ref. 101.

10 Chapter 1

1.4.1. Droplet deformation and break-up

When a neutrally buoyant, initially spherical droplet is suspended in another

liquid and subjected to shear or extensional stress, it deforms and then breaks

up into smaller droplets. Taylor [I 1,121 extended the work of Einstein [13,14] on

dilute suspension of solid spheres in a Newtonian liquid to dispersion of single

Newtonian liquid droplet in another Newtonian liquid, subjected to a well-defined

deforrnational field. Taylor noted that at low deformation rates in both uniform

shear and planar hyperbolic fields, the sphere deforms into a spheroid as shown

in Fig. 1.4.

I" I

Figure 1.4: Deformation of drops in (a) shear and (b) extensional flow field [Ref. 11,121

At low stress in steady uniform shearing flow, the deformation can be expressed

by means of three dimensionless parameters: the viscosity ratio ( A ) , the

capillary number (Cu ), and the reduced time ( t ' ):

where qdand q,,,are the viscosities of the dispersed and matrix phases,

respectively, Y is the stress (either in shear or in extension), R is the droplet

radius, T,, is the interfacial tension coeffic~ent between the two phases and V is

the generated strain. The affine droplet deformation (E) will depend strongly on capillary number (Ca ) and viscosity ratio(1) as follows:

Introduction 11

Ca is a dimensionless number expressing the relative importance of the applied

viscous shearing forces and the counteracting interfacial forces. When Ca

exceeds a critical value (Cacr,, ), the droplet will deform and subsequently break-

up under the influence of the interfacial tension. If Ch is small, the interfacial

forces dominate and a steady drop shape is developed. Figure 1.5 shows a

schematic picture of the modes of affine droplet deformation observed in shear

flow as a function of /I for increasing 1 1 as determined by Rumscheidt and

Mason for a Newtonian droplet [15]. At low Ca the droplets deform from their

initial spherical shape to an ellipsoidal shape, irrespective of A . At higher Ca ,

the deformation and break-up mechanism is clearly different for different

/2values. For IOWA, small droplets are pinched off from the elongated

ellipsoidal droplet during flow. For higherl , but still lower than 1, the droplet

extends into long cylinder threads into the flow direction, which break-up into

larger number of fine droplets. For /2=1, the droplet extended into a cylinder, but

forms a neck in the middle. The neck thins and splits into two identical droplets

and some satellite drops. For very high A(>4), the viscous forces cannot

overcome the interfac~al forces and cannot be broken into simple shear flow. In

essence, two basic rnechanisms exist ior dispersing one liquid into another

according to Rumscheidt and Mason [15] are: (i) stepwise mechanism of

repeated break-up (the steady state break-up or drop splitting) and (ii) transient

mechanism of thread break-up or capillary instability (elongated threads break-

up into a series of droplets) [16]. The second mechanism was theoretically

treated by Rayleigh 1:17], Taylor [I 1,121 and Tomotika [18,19]. Tornotika showed

that the degree of instability can be described by the growth rate parameter of a

sinusoidal distortion:

12 Chapter 1

where A is the distortion wavelength, O(A,A) is a function tabulated by

Tomotika for break up at the dominant wavelength as a function of viscosity ratio

and I$ is the initla1 fibre radius

Figure 1.5: States of droplet formation of Newtonian media in shear fields (Ref. 15)

Later Cox [20] extended Taylor's theory to systems with the full range of 1. Stone et al. [21] have observed another mode of drop break-up, called end

pinching, that is in competition with capillary instability. End pinching is a form of

non-uniform break-up wherein fibre disintegration proceeds initially from the

ends of the threads which was found to be dependent on l a n d initial

lengthldiameter (UD) ratio of the elongated droplet. Break-up through end

pinching can lead to a distribution of particle sizes.

The shear deformation of viscoelastic drops in a Newtonian medium has been

studied by several researchers 122-271. Gauthier et al. [22] found higher values

of Ca',,, than those determined for Newtonian drops. De Bruijn 1231 found that

Ca<,,, for viscoelastic droplets is always higher (sometimes much higher) than

for Newtonian ones and concluded that drop elasticity always hinders drop

break-up. For Newtc~nian drops suspended in viscoelastic fluid. Flumerfelt [24]

reported the existence of a minimum drop size below which break-up cannot be

achieved. The author pointed out that the elasticity of the medium tends to

increase this minimum value for break-up, that is, to stabilize the droplets. In the

case when both the droplets and the suspending medium are viscoelastic liquids

Wu 125-271 reported that drops can break up during extrusion even when /Z > 4.

However, owing to the complex nature of the deformation during flow through an

extruder, it was difficult to even speculate on the origin of this phenomenon.

Van Oene [28] studied the mechanisms of two-phase formation in a mixture of

two viscoelastic fluids;. He pointed out that, besides the viscosity ratio and the

equilibrium interfacial tension of the two liquids, the elasticity of the liquids plays

an important role in deformability of drops. According to the author,

where aefl is the effective interfacial tension under dynamic conditions, a is the

static interfacial tension, d is the droplet diameter, ( N ? ) , and ( N , ) " , are the

second normal stress functions for the dispersed phase and for the matrix,

respectively. Han and Funatsu [29] studied droplet deformation and break-up for

viscoelastic liquid systems in extensional and non-uniform shear flow. The

authors found that ~~iscoelastic droplets are more stable than the Newtonian

ones; in both Newtonian and viscoelastic media they require higher shear stress

for breaking.

1.4.2. Coalescence

For dilute Newtonian systems, the size of the smallest drop that can be broken is

calculable from Taylor's theory [I 1,12]. However for polymer systems, note that

the equilibrium drop size is usually larger than predicted, and the deviation

increases with concentration of the dispersed phase mainly due to coalescence

phenomenon. Coalescence, the dispersed particle recombination in polymer

blends, may be considered to occur by any of the following reasons [30]: (i) van

der Waals' forces between neighbouring particles, (ii) random mechanical forces

exerted on dispersed patticles by irregular motion (Brownian motion), (iii) capillary

forces that cause the further relaxation of a dumbbell-like particle to a sphere,

(iv) Buoyancy resulting from the different gravities of the two components, which

14 - Chapier f

can be neglected for systems with little difference in density and (v) friction

resulting from viscous flow, that is induced by the aforementioned forces.

Different interactions may dominate different coalescence, thus leading to

different coarsening laws. Elmendrop, [31] Fortelny and co-workers 132-351 and

lnoue and co-workers [30,36] divided coalescence process into four stages, viz:

(i) Approach of the particles with radii PI and R2 (R, = R2 = R in the

monodispersed case) and formation of the parallel film between the particles

with thickness h,,. ha can be regarded as the distance at which one particle feels

the existence of another particle; it can be estimated by the normal force

squeezing the film initially being set equal to the Stokes viscous force [37]:

where c = 0.4 is a numerical constant, R," is the smaller particle radius between

the two particles = R for a monodispersed system. Equation (10) shows that h,

increase as 1 increases, approaching asymptotically a value of R/4 as /1

approaches .-, indicating that rigid particles feel the presence of each other at

hSR/4, where his the d~stance between two neighbouring particles.

(ii) Drainage of the matrtx trapped between the particles until the film thickness

reaches its critical value. h,. During this stage the matrix film thins from ha to h,,

where h, can be expressed as [38]:

where A is the Hamaker constant. The Hamaker constant is the representative

of the van der Waals' forces between neighbouring particles (typically for

pure materials). Because of the influence of the matrix on the interaction of the

dispersed particles Hamaker constant can be approximated as [39]:

Introduction 15 -

where i = d or m denotes the dispersed phase and the matrix, respectively, vi is

the surface tension and d is the distance between molecules.

(iii) Rupture of the film due to interfacial instability

(iv) evolution of the dumbbell particle to a coalesced sphere particle (merging Of

particles).

These four stages (schematically showr~ in Fig. 1.6) have different impacts on

coalescence. The first stage is mainly the result of Brownian motion, which

makes more particles collide and increases the touch probability of dispersed

particles. The remaining stages are the coalescence steps of two particles, in

which the time of the rupture of the film IS omitted because it is a relatively quick

stage. Therefore, the total coalescence time, r, of two particles can be written as

the addition of the draining time of the film, r,,,, , and the merging time of the

I I1 Ill I V

Figure 1.6: Schematic diagram of the coarsening process: (I) the approach of the particles and the formation of the matrix film, (11) the drainage of the matrix film, (Ill) the rupture of the matrix film, and (IV) the merge of the dumbbell-like particles (Ref. 31).

Roland and Bohm [40] studied the shear induced coalescence in two-phase

polymeric fluids by small-angle neutron scattering (SANS). The coalescence rate

was high. dependent on the rheological properties of the two phases and the

flow field. Coalescence occurs in shear as well as in quiescent systems. In the

latter case the effect can be caused by molecular diffusion to regions of lower

16 Chapter I

free energy by Brownian motion, dynamics of concentration fluctuation, etc.

Diffusion is the mechanism responsible for coalescence known as "Ostwald

ripening". The process involves diffusion from smaller drops (high interfacial

energy) to the larger one:;. Shear flow enhances the process [41]. Flow-induced

coalescence is accelerated by the same factors that favour drop break-up, e.g.,

higher shear rates, reduced dispersed-phase viscosity, etc. Most theories start

with calculation of probabilities for the drops to collide, for the liquid separating

them to be squeezed out, and for the new enlarged drop to survive the parallel

process of drop break-up. As a result, at dynamic equilibrium, the relations

between drop diameter and independent variables can be derived.

Tokita [42] derived an expression for the particle size of the dispersed phase in

polymer blends that incorporates composition as variable. According to this

theory, at equilibrium, when coalescence and break down are balanced, the

equilibrium part~cle size 11e is given by:

de E 24q0/7% {h, + + ( 4 4 ~ ~ / % ) @ , , ~ ) (1.15)

where T,, is the shear stress. a the interfacial tension, Edk the bulk breaking

energy, Od the volume fraction of the dispersed phase and P, the probability that

a collision to resulting coalescence. In summary, the drop break up and

coalescence processes determine the final morphology of the blends and the

next section gives an idea about how these processes affect the different stages

of morphology developrr~ent.

1.4.3. Morphology development during processing

Morphology development is the evolution of blend morphology from pellet - or

powder - sized particles to sub-micrometer droplets that exist in the final blend.

The morphology development during dissipative melt-mixing in many blends

occurs through a sheeting mechanism as shown in Fig. 1.7 (431.

This mechanism involves the formation of sheets or ribbons of the dispersed

phase in the matrix, which are drawn out of a large mass of the dispersed phase.

When the holes in the sheet or ribbon attain a sufficient size and concentration a

fragile lace structure is formed, which begins to break apart due to flow and

interfacial forces into ~rregularly shaped pieces. These pieces are approximately

the diameter of the particles that are generated in the blend at long mixing times.

These irregular pieces continue to break down until all of the particles become

nearly spherical. Scott and Macosko [43-451 demonstrated this mechanism by

quenching samples from a batch mixer and dissolving the matrix polymer with a

selective solvent. Sundararaj et al. [46-481 verified that the same mechanism

occurs in a twin-screw extruder [46] and in small cup and rotor mixers I47.481.

Sundararaj et al. 147.49-511 also showed that sheets form when large molten

drops are sheared at high enough rates, --

Figure 1.7: Proposed mechanism for initial morphology development in polymer blends (reproduced from Ref. 43).

Several researchers have investigated the morphology development in polymer

blends during extraction and the various facts that determine the final

morphology of the blends. Favis [52] investigated the effect of mixing time (2 to

20 min) on the morphology of polycarbonate/polypropylene (PCIPP) blends and

concluded that the rnorphology was created during the initial short melt mixing.

Laokijcharoen and Ooran [53] also indicated that the major breakdown of the

particles occurs at the very beginning of the mixing process. Thomas and

Groeninckx [54] made a similar observation in PA61ethylene propylene rubber

18 Chapter 1

(EPR) blends. Schreiber and Olguin [55] also found that the bulk of particle size

reduction takes place very early in the mixing process. In an investigation on the

processing of a polystyrenellinear low density poly ethylene (PStLLDPE) blend

in several industrial mixers, Plochocki et al. [56] proposed that the initial

dispersion mechanism might be the abrasion of solid or partially softened pellets

on the walls of the processing equipment. Moldenaers and co-workers [57] have

investigated the morphology development in complex flow fields and found that

flow fields in processing equipment have sign~ficant influence on the morphology

of the blends. Roths et al. [58] have shown that the simulations can contribute to

the understanding of both morphology formation and rheological properties.

Potschke and co-workers [59] have claimed that the melting behaviour and

morphology development that runs parallel to it plays central roles in processing

of polymer blends and have shown that a tine dispersed morphology already

developed at the start of the melting section did not undergo any change as the

blend passed through the extruder. Thomas and co-workers have [54,60,61]

extensively studied the morphology development in various polymer systems.

1.4.4. Influence of material parameters on morphology

The size, shape, uniformity and distribution of the dispersed particles depend on

several factors including the viscosity ratio, composition, elasticity ratio, shear

stress and interfacial modification.

1.4.4.1. Viscosity ratio

Viscosity ratio ( / l j is one of the most critical variables for controlling the

morphology of polymer blends and its effect on blend morphology has been

investigated extensively [62-731. If the minor component has lower viscosity

compared to the major one, it will be finely and uniformly dispersed in the major

continuous phase [62] and otherwise coarsely dispersed. It has been found that

for a given composition of minor component, the relatively less viscous

component forms smaller dispersed droplets in more viscous matrix phase when

all other factors are kept constant [63,64]. It is generally believed that A should

be approximately unity when designing the polymer blends. According to

Rumscheidt and Mason [15] and Grace 1651, the droplets deform into ellipsoids

Introduction 19

under the influence of shear flow but do not break up at d >3.7. In this regard,

Wu (271 modified Taylor's equation as:

The exponent is positive for 1 >I and negative for 2 <I.

Wu's correlation suggests a minimum particle size when the viscosities of the

two phases are closely matched. As the viscosity moves away form unity in

either direct~on, the dispersed particles become larger. The validity of this

equation has been testified by various researchers [66-681. Serpe et al. [66]

have found good agreement with Wu's predictions and modified Wu's equation

by using the blend viscosity rather than the matrix viscosity and by considering a

term of composition. thus coalescence effects as follows:

where rl, is the viscosity of the blends,

On the other hand. Paul and co-workers 1671 and Utracki and co-workers [68]

indicated disagreement with respect to the dependence of particle size on /1 and

the correlation proposed by Wu. It has also been shown that the dispersed particle

size increases with increase in /Z of the components in polymer blends [69] and

there is linear relationship between the number average domain diameters (D,) of

the dispersed particles and d (701. Meanwhile, Favis and Chalioux [71] have

shown that significant particle disintegration occurs even at a toque ratio of 13 in

an internal mixer arld the viscosity ratio has found to have marked effect on the

morphology of the dispersed phase with the phase size increases by a factor of

three to four from a torque ratio of 2 to 13 (Fig. 1.8). Further, Favis and Therrien 1721

have shown that the break-up of the dispersed phase in PClPP blends of high /1

is much more efficient in twin-screw extruder than in an internal mixer. Namhata et

al. 1731 developed a model to predict blend morphology development during rnelt-

flow based on the fact that when two phases of different viscosities are subjected

20 Chapter 1

to a stress field, each phase will have a different velocity resulting in a difference in

volumetric flow rate. These authors stated that /Z should be determined at the

same shear stress instead of using a viscosity ratio criterion based on viscosities

measured at the same shear rate.

5 -

Figure 1.8: The influence of toque (viscosrty) ratio on the number average diameter from a toque ratio of 0.078 to 12.8 for blends of PP and PC (Ref. 71).

Groeninckx and co-workers [74] have studied the influence of h on the phase

morphology development in immiscible PP/(PS/PPE) blends. The results showed

a clear dependence of the blend phase morphology on A ; highly viscous matrices

(A <<I) enhance droplet break-up due to their efficient shear stress transfer

towards the dispersed phase and the higher dispersive forces acting on it; low

viscous matrices (h >I) dten act as a lubricant for the dispersed phase reducing

droplet break-up. The influence of A on droplet break-up is reflected in the particle

diameter in blends with a concentration of the dispersed phase up to 20wt0/.. In the

latter case, blends with a low (h c 1) offer the best approach towards a fine and

stable phase morphology, unlike suggestions in the literature. Blends containing

higher concentrations of the minor phase !>20 wt%) exhibit strong coalescence

during melt mixing; the influence of on the final blend phase morphology

becomes less obvious, and the finest dispersion was observed at h = 1. Only

blends of a lower viscous matrix in which a highly viscous phase has to be

dispersed, do not follow the previous tendency as a result of the strong impact of a

changing overall melt viscosity. Blending a highly viscous component with a low

viscous component seems to counteract quiescent phase coarsening. Recently,

Jana and Sau [75] have studied the effect of 1 on the morphology development

of chaotic mixing of polymers and found that with 2 2 1 the dispersed phase

converted into droplets very rapidly with narrow droplet size distribution 2 - 1. For

higher values of 2 the speed of morphological transitions slowed down, the droplet

size distribution became wider, and much larger droplets were formed.

1.4.4.2. Composition

If the entire composition range is studied for a given A-B blend, three basic

regions may be defined (i) a region for which phase A is dispersed in matrix B (ii)

an intermediate region of phase inversion for which both A and B are continuous

and (iii) a region of phase B is dispersed in mamx A. (I) and (iii) are generally

referred as dropletlmatrix morphology. Figure t.9a demonstrates the dropletlmatrix

morphology of PA121natural rubber (NR) blends in which NR forms the dispersed

phase in PA12 matrix [76]. The studies of Denesi and Porter 1771 showed that for

blends with the same processing history the morphology is determined by /Z

and composition. When the mixed polymers have similar men viscosities, the

resultant morphology will be a uniform distribution of the minor phase in the major

one, no malter which is the minor component. Groeninckx and co-workers 174)

have studied the influence of 1 and composition on the phase morphology of

PP/(PS/PPE) blends and have shown that the concentration of the dispersed

phase is the most important factor determining phase coarsening in blends

having nearly equal melt viscosities. As the concentration of the minor phase in

the blend increases, the dispersed droplet size will increase due to increased

coalescence, until the blend phase inverts and the dispersed phase becomes

matrix and vice versa. Around the phase inversion point a co-continuous phase

structure can be generated, consisting of strongly interpenetrating domains of

both blend phases.

In conventional terms, a co-continuobs phase structure can be defined as the

coexistence of at least two continuous structures with in the same volume in

which each component is a polymer phase with its own internal network like

structure from which its properties result. However in terms of percolation

22 Chapter f

threshold theory, it may be regarded as a structure formed by melt-mixing of

polymers represent a co-existence of different structure types rather than ideal

network [78]. Lyngaae-Jorgensen and Utracki [79] defined a co-continuous

structure as one in which at least a part of each phase forms a coherent

continuous structure that permeates the whole volume. Co-continuous polymer

blends can have a number of advantages, making them ideal for a wide range of

applications due to the fact that in some cases co-continuous phase structure

exhibits synergistic mechan~cal properties, controlled electrical conductivity and

selective permeability 179,801. Potschke and Paul [78] have published an

excellent review on the formation of co-continuous structures in melt mixed

immiscible polymer blends. A typical co-continuous phase structure of a polymer

blend is given in Fig. 1.9b [$I].

(a) (b)

Figure 1.9: SEM micrograph ot (a) dropletlmatrix morphology (Ref. 76) (b) co-continuous morphology (Ref. 81).

The formation of co-continuous structures was long believed to be only possible

in a narrow composition range around the phase inversion concentration. Most

studies have focused on predicting the composition of this phase inversion.

Extensive research has been made to establish the factors affecting the phase

inversion and co-continuous phase structure and developed a number equations

and models for the phase inversion composition [69,77,82-1051. The general

concept of these semi-empirical models is that the less viscous phase tends to

encapsulate the more viscous one. Based on the results of Avgeropoulos et al. [69],

Paul and Barlow [83] reported an empirical model relating A of the pure

materials to the volume fraction ratio of the two phases at the phase inversion

point. Later, Metelkin and Blekht [84] proposed that, at the phase inversion point

the time of break-up of a cylinder A in a matrix B should be equal to the time of

break-up of a cylinder f3 in a matrix A

Utracki 1891 demonstrated the inefficiency of these models regarding high 2 systems. He derived a model from the theory of rigid particles suspended in a

liquid and proposed that at the phase inversion point, the viscosity of a

suspension of A in R should be equal to the viscosity of suspension of B in A.

Further investigations on several systems with different demonstrated that

models based on 2 often failed in predicting the phase inversion point 1961. It

was clearly pointed out that encapsulation phenomena do not depend solely on

the viscosity ratio. Bourry and Favis [98] suggested an elastic contribution in the

encapsulation phenomena and proposed a model based on the elasticity ratio. An

elastic contribution can decrease the dynamic interfacial tension as described by

Van Oene [28]. This predicts that the more elastic phase tends to encapsulate the

less elastic one. Dedecker and Groeninckx [99] have investigated the phase

inversion region and co-continuous phase structure of PAG/polymethyl

methacrylate (PMMA)lpoly(styrene-co-maleic anhydride (SMA) blends and

observed that the region of phase co-continuity is shifted to a lower PA6 content

when the molecular weight (MW) of PA6 is decreased; this shift is in agreement

with the change in viscosity of PA6. In addition, the point of phase inversion is

shifted to a lower PA6 content in the compatibilised blends. Willemse et al.

[100,101] introduced a semi-empirical model, based on geometrical and micro-

rheological considerations. This model presumably predicts both limits of the co-

continuity region rather than a single phase inversion point. It was shown that

increasing interfacial tension increases the onset of co-continuity for several

homopolymer/homopolymer blends. Figure 1.10 summarises different models

describing the phase inversion concentration as a function of 2 .

24 Chapter 1

Figure. 1.10: Diierent models describing the phase inversion volume concentration as a function of viscosity ratio (Ref. 78).

1.4.4.3. Shear stress

It is known from Taylor's equations that shear stress (7) is inversely proportional to

the particle size. Similarly from Tokita's equation (equation 15) also one can note

that as r increases, the particle size decreases. The experimental studies of Wu [27]

were in parallel with Taylol.'~ equation where he noted that changing the T resulted

in an observable difference in the size of the minor phase. White and co-workers

[I061 have observed that shearing at high .r results in much finer morphology in

PEIPS blends. However, Favis [52] and Chalifoux and Favis [71] have reported

that varying the 7 by a factor of two to three has little effect on the particle size.

Thomas and Groeninckx [54] have studied the effect of r on the morphology of

PAGIEPM blends and observed that most significant break down of particles

occurs when particle size increases from 9 to 20 rpm and further increase in shear

rate up to 150 rpm does not have any considerable decrease in disperse particle

size, even though a marginal decrease has been observed. Thus it can be

concluded that large variations in T are required for it to predominate over the /1 in

controlling dispersed particle size. Fortelny et al. (1071 studied the effect of mixing

rate on the droplet size of the dispersed phase and found that for concentrated

systems, the average panicle size increases up to a maximum at 80rpm and then

decreases. Sundararaj and Macosko [I081 reported that there is a critical minimum

drop size as the shear rate is varied, and this can be accounted for through the

Introduction 25 --

polymer elasticity. When the shear rate is increased, the matrix viscosity decreases

and the drop elasticity increases, so that the drop resists the deformation to a

greater extent; consequently, there is an optimum shear rate where the finest

dispersion is obtained. However, it has been found that modifying the applied shear

stress can change the shape of the dispersed phase [ I041 1 I].

1.4.4.4. Elasticity ratio

Van Oene 1281 showed that the elastic contribution to the interfacial tension can

result in a tendency foi the phase of higher elasticity to encapsulate the one with

lower elasticity using the equation 1.9. Macosko and co-workers [I 121 have shown

that in simple shear flow, PP drops stretch perpendicularly to the flow direction in a

more highly elastic matrix. The effect was found to be proportional to the second

normal stress differences between the two phases. Migler [I131 and Hobbie and

Migler [I141 demonstrated that due to the high droplet elasticity, the droplet can

align in the vorticity direction rather than in the flow direction. The influence of

elasticity ratio on the morphology development in polymer blends has been

discussed by several researchers [98,104,115,116]. Bourry and Favis [98] have

found that the interfacial tension under dynamic conditions will be lower for a

system with a more elastic matrix and a less elastic dispersed phase. These

authors have also found that elasticity can influence the phase inversion point and

found that the regton of co-continuity is a function of a combination of both

elasticity and viscosity ratios. Steinmann et al. [I041 studied the influence of

viscosity andlor elasticity ratio on the phase inversion concentration and observed

that most formulae for the calculation of phase inversion concentration depending

on the viscosity ratio fail in the case of blend systems (PMMNPS and

PMMNSAN), but the theoretical approach by Bourry and Favis [98] concerning the

influence of elasticity ratio describe an opposite tendency to their data.

1.4.4.5. Interfacial modification

The fundamental reasons responsible for the unstable morphology are the

unfavourable interfacial interactions between the components which create a high

interfacial energy and low interfacial th~ckness, which would, in turn lead to poor

interfacial adhesion between the phases that may result in premature failure of the

26 Chapter 1

interface upon stress transfer. Another aspect that deserves attention is the

coalescence of the dispersed phase, which makes the dispersed particles larger

and non-uniform, leading to an unstable morphology. Therefore the key to

overcome the problems related to the coarse morphology of multi-component

polymer blends is to (a) reduce the interfacial tension in the melt, (b) diminish the

rate of coalescence under static and quiescent conditions and (c) improve the

interfacial adhesion between the phases in the solid state. These can be achieved

by interfacial modification in the presence of compatibilisers, which are interfacial

agents that can stabilize the immiscible polymer blends, just like surfactants or

detergents so in oil-water emulsions [I 171. Compatibilisation of polymer blends is

extensively reviewed in the following section

1.5. Compatibilisation of polymer blends

Compatibilisation is defined as a process of modification of interfacial properties of

an immiscible polymer blend, leading to creation of polymer alloy which is an

immiscible polymer blend having a modified interface and/or morphology. When

two immiscible polymers are biended without compatibilisation, one generally

obtains a mixture with a coarse, unstable morphology coupled with poor interfacial

adhesion between the phases. As a result the blends exhibit inferior physical

properties to those of either individual polymer coupled with poor structural

integrity and heat stability and their petformance will be irreproducible. As

mentioned earlier, the high interfacial tension derived from the unfavourable

interfacial interactions between the individual phase and coalescence effects are

the elementary reasons for this phenomenon. A poor interfacial adhesion results in

an immature stress transfer which cannot prevent cracks initiation at the interface

from growing until catastrophic failure occurs. Both theories and experiments

support the role of compatibiliser in multiphase polymer systems. The following

section of this chapter is devoted to the compatibilisation of polymer blends and

designed in a such a way that first we discuss the theories of compatibilisation

followed by the different strategies (physical and reactive) employed for it.

Introduction 27

1.5.1. Theories of compatibilisation

1.5.1.1. Helfand and Tagami's theory

For binary A/B blends, Helfand and his co-authors [ I 18-1241 formulated the basic

relations and provided the theoretical guidance to the interfacial properties as well

as interfacial modification in the presence of a compatibiliser. The theory is,

however, based on strong, limiting assumptions (for example, infinitely long

macromolecules) and thus one should not expect a quantitative agreement with

experimental data. For compatibilisation, Helfand's theory furnishes three

important conclusions: (i) the chain-ends of both polymers concentrate at the

interface, (ii) any low MW third component is forced by the thermodynamic forces

to the interface and (~ii) the interfacial tension coefficient increases with MW up to

an asymptotic value. Helfand and Tagami model is based on self consistent field

(SCF) that determines the configurational statistics of macromolecules in the

interfacial region. At the interiace, the interactions between statistic segments of

polymers A and B are determined by the thermodynamic binary interaction

parameter, XAB. Since the polymers are immiscible, there are repulsive enthalpic

effects that must be balanced by the entropic ones that cause chains A and El to

intermingle, The anaiytical solution of the interfacial composition profile was found

to follow an exponeniial decay function:

where b is the lattice parameter and(x/b) the reduced distance across the interphase.

A typical dependence is shown in Fig. 1.11. Similarly, for the symmetrical

polymers A and B whose M, 3 *, the interfacial thickness, A,_ and the

interfacial tension coefficient, V_ , were derived as:

A,- = [2b/6XAa""J

v , ~ = bpkT (,yde /6)'12 where k is the Boltzrnann constant.

28 Chapter 1

Helfand and Sapse [I241 removed the restriction of the original theory for the

symmetric character of both polymers.

Figure 1.1 1: Representation of the interface, with the definition of the interphase thickness, Al, and b are respectively the binary interaction and the lattice parameters (Ref. 11 8).

1.5.1.2. Hong and Noolandi's theory

Hong and Noolandi 1125-1271 developed a theory for the interfacial region in three

component polymeric systems complising polymers A and B, and either a co-solvent

or a block copolymer (bcp). The theory was based on the lattice model which

used the mean-field (MF) approximation. It is formulated using reduced

equation of state variables. Finite MWs and conformational entropy effects

were considered, but the excluded volume effects were not (this aspect was

later treated by Broseta et al. [128]). The resulting system of equations can

be solved numerically for the interfacial composition profile, interfacial

tension coefficient, thickness of the interphase, etc. At low values of 2, the

theory well predicts xj2, but for higher %, the prediction was up to 20% too high.

1.5.1.3. Noolandi's theory

According to Noolandi [129,130], the effect of copolymer on surface tension

between the two phases is mainly influenced by the contributions from a series of

factors such as loweririg of interaction energy between the immiscible

homopolyrners, the broadening of the interfac:e between the homopolymers, the

entropy reduction in the system, a decrease in energy of interaction of the two

blocks with each other and a large decrease in the interaction energy of the

oriented blocks with homopolymers. However, it should be noted that the

Introduction 29

localization of copolymer at the interface and the separation of blocks into

corresponding homopolymer phases and the simultaneous reduction in interfacial

tension between the phases depend on various factors such as mixing conditions,

interaction of the compatibiliser with the dispersed phase. MW and composition of

the compatibiliser, the rate of absorption and orientation of the compatibiliser at the

interface. Based on these facts and by neglecting the loss of conformational

entropy, Noolandi derived an equation for the interfacial tension reduction as:

A y = do, [1/2% + l,'Z, -- I/Z< exp(zg L / 2 ) ] (1.22)

where d is the width at half height of the copolymer profile reduced by the Kuhn

statistical segment ength, @,the bulk copolymer volume fraction of the

copolymer in the system, Z, the degree of polymerisation of the copolymer and

x the Flory-Huggins interaction parameter between A and B segments

1.5.1.4. Leibler's theory

Leibler 11311 examined the emulsifying effect of an A-B copolymer in immiscible

blend of polymers A and B and predicted a reduction of interfacial tension caused

by equilibrium adsorption of the copolymer at the interface. He suggested that at

equilibrium, the droplets' size distributior is controlled by rigidity and spontaneous

curnature of radius of the interphase, both dependent on the copolymer's

molecular constitut~on. The theory predicted reduction of the interfacial tension

coefficient, A y , caused by equilibrium adsorption of a copolymer at the interface.

For well-chosen compositions and MWs of the copolymer, low values of Ayare to

be expected. This suggests a possible existence of thermodynamically controlled

stable droplet phase, in which the minor phase homopolymer drops are protected

by an interlacial film of the copolymer, interfacing the matrix polymer.

For long copolymer chains (the "wet brush case" i.e., copolymer overlap strongly

and stretch but still homopolymers can penetrate the brushes), the reduction of the

interfacial tension c~efficient should follow the relation [ I 321:

Ay= -(k~/a')(3/4)"' ( ~ / n ' ) - ~ ' ~ ( 2 , , 2 , Y' + Z,.,Z,-~') (1.23)

where Z , and Z,, are the number of A and B units in the copolymer,

respectively, Z , and Z, the degree of polymerisation of A and B, respectively, a

the monomer's unit length and 2 the interfacial area per copolymer.

In the case of dry brush limit (when copolymers are expected to be stretched but

homopolymers should not penetrate the copolymer layer appreciably, the

interfacial tension reduction ( A y ) obtained by the brush limit which is

independent of the homopolymer MWs.

where yo is the interfacial tension of polymer blend without a compatibiliser and

p is the chemical potential which is given by the equation:

, u = l n @ ' + f , y N (1.25)

where f is the volume fraction of the component in copolymer which is miscible to

homopolymer forming the dispersed phase and

@* = (1.26) [@", + 6, e x p { x ( N . , - N , )I]

where @", @,,! arid @d represent the volume fraction of the copolymer, matrix and

dispersed phase, respectively, N , and N , are the number of segments of the

component in the copolymer miscible to the homopo!ymer forming the dispersed

phase and that miscible to homopolymer forming the matrix phase, respectively.

The surface coverage of one copolymer, i.e., the surface area occupied by one

compatibiliser molecule per unit volume at the interface, is related to p and x as

where b is the Kuhn length.

1.5.1.5. Vilgris and Noolrmdi's' theory

Vilgris and Noolandi [I331 computed interfacial tension ( y ) interfacial thickness

(&I) and the concentration profiles in a system comprising polymer A, polymer B

and an arbitrary bcp X-Y. It was found that addition of X-Y to AIB blends can

reduce y . The reduction increases with MW of the copolymer. Computations were

carried out for different values of X , and 2,. Strong localization effects of X-Y were

observed when the interactions between the blocks and the homopolymers were

increased. Thus, the competitive interactions of the blocks with homopolymers

were shown to promote intertacial activity. The reduction of y in A16 blend upon

addition of copolymer X-Y was expressed as:

for : x, = x,, = x,,, = x,, = x,, and X2 = X,,). = XBA > X, The work suggested possibilities of designing a universal compatib~liser operating

on the princ~ple of competitive interactions between polymers and the copolymet's

blocks.

7.5.1.6. Anastasiadis theory

A generalized gradient theory of the interface was developed by Anastasiads [134].

The approach is based on the assumption that the composition gradient is small

compared to the reciprocity of the intermolecular distances. Under these

circumstances the free energy density, g, can be written as a power series,

truncated after the square term. In essence, the theory determines the difference

in the density fluctuation per unit interfacial area between polymer mixture and a

system in which the properties are homogenous. The theory predicts that,

32 Chapter 1

1.5.1.7. Utracki and Shi's theory

Utracki and Shi [135] derived a semi-emp~rical relation between the interfacial

tension coefficient ( x, ) and compatibiliser concentration by assuming an analogy

between addition of bcp to a polymer blend and titration of an emulsion with

sutfactant. According to the authors:

~l r = {( &cMr + @mtmnrn ~i I/@ + @nicori 1 (1.31)

1.5.1.8. Tang and Huang's theory

Based on the fact that upon the addition of compatibiliser, interfacial tension y

decreases and on assumption that the decrease is directly proportional to the

interfacial tension difference at a particular compatibiliser concentration C and

CMC, then:

where K is the rate constant for the change in interfacial tension with concentration

of the compatibiliser, 7' the interfacial lension at a given compatibiliser

concentration, C and y> the interfacial tension at CMC. From the above

expression, Tang and Huang [l 361 eventually derived the following equation:

R-R, =(R,,-R,)e-KC (1.33)

where R,, R and R, are the average radius of dispersed particles without

compatibiliser, at a given compatibiliser concentration and compatibiliser

concentration at CMC respectively. A plot of In ( R - R, )versus C can be used to

obtain K from the slope.

1.5.1.9. Paul and Newman's theory

According to Paul and Newmarl [I371 the interfacial area per unit volume occupied

by each compatibiliser molecule is given by the expression:

where N is Avogadro number. M the number average MW of the compatibiliser, R

the average radius of the dispersed phase, 4 the volume fraction of the dispersed

phase and W the weight of the compatibiliser required per unit volume of the

blend. When 4 and M are kept constant, C depends on the values of R and W. R

decreases with increase in the weight fraction of the compatibiliser and C may

either decrease or not change or increase.

1.5.1.10. Favis' theory

Favis and co-workers [138,139] have used the geometrical considerations about

the macromolecular size in order to make a critical analysis of the minimum

amount of bcp needed to saturate the intertace in the melt mixing of immiscible

polymer blends. For the case of dispersed spherical domains, an expression has

been developed which contains only molecular parameters such as the number of

bonds, the characteristic ratio and the composition of the bcp used to compatibilise

the blend. According to the authors, the critical saturation concentration (C,, )

value of the bcp is given as:

where MA is the MW of block A, R, is the radius of gyration of A block, N is the

Avogadro number, 17 is the density of the bcp, k is the number of block per cubic

element, W, is the weight fraction of A phase in the copolymer and R is the radius

of the dispersed phase.

1.5.2. Strategies of compatibilisation

As mentioned earher compatibilisers are macromolecular species exhibiting

interfacial activities in heterogeneous polymer blends. Usually, the chains of a

compatibiliser have a blocky structure, with one constitutive block miscible with

one blend compor~ent and a second block miscible with the other blend

component. These blocky structures can be pre-made and added to the

34 Chapter 1

immiscible polymer blend, but they can also be generated in-situ during the

blending process. The former process is usually referred to as physical

compatibilisation where as the latter procedure is called reactive compatibilisation.

It should be noted that compatibilisers retard the formation of Rayleigh

disturbances on the generated threads of polymer, as the result of a decreased

interfacial tension. In addition, the presence of compatibiliser molecules at the

surface of the small generated particles prevents coalescence from occurring

during subsequent processing. Compatibilisers are thus able to generate and to

stabilise a finer morphology. Provided that each block of a poly (A-b-6)

compatibiliser penetrates the parent phase (A and B, respectively) deeply enough

to be entangled with the constitutive chains, the interfacial adhesion is enhanced.

Refinement and stabilisation of the phase morphology and the enhancement of

the interfacial adhesion usually upgrade an inferior and useless immiscible

polymer blend to an interesting material with demanding applications. The

research group of Prol. Thomas has extensively studied the compatibilisation of

immiscible polymer systems. They have peifornied eithei physical or reactive

compatibilisation techr~iques on a series of multiphase blends such as NRIPS

[140-1441, NRIPMMA [145-1501, PSIPMMA [I511 PPl(acrilonitrile-co-butadiene

rubber (NBR) [152-1561, PSlNBR 11571, high density PE (HDPE)/NBR [158-1611,

PSIpolybutadiene (PB) [162.163], PNNBR 1164,1651, PAIPS [166,167],

PEYpolyethylene-co-vinyl acetate (EVA) [168-1711, PNEPR [54,61,172], PElPP

(173,1741, PAIPP [63,175] and polytrimethylene terephthalate/ethyiene propylene

diene terpolymer (PTTIEPDM) [64].

1.5.3. Mechanism of compatibilisation

It is well established that the compatibilisation mechanism (whether physical or

reactive) in a mult~phase polymer blend mainly relies on the interfacial

emulsification efficiency of the compatibiliser. The efficiency of a compatibiliser

depends on its ability to reduce (i) interfacial tension and (ii) coalescence rate. It

has been assumed that bcp suppresses droplet coalescence in immiscible

polymer blends by preventing droplets from approaching each other. For example,

stabilisation against coalescence of co-continuous phase morphology is very

sensitive to the quarlt~tative location of the compatibiliser at the interface [176]. As

mentioned earlier, coalescence of droplets of a blend occurs due to collisions that

are driven either by an externally applied flow, or under quiescent conditions, by

van der Waals' attractions. Several experimental studies have shown that addition

of bcp causes a dramatic decrease in the rate of coalescence [108,177-1881. This

effect is generally termed 'coalescence suppression', and it has been hypothesized

that a bcp promotes morphological refinement more by suppressing coalescence

during processing than by promoting the break-up of droplets. Ramic et al. [I771

reported a drastic reduction in coalescence rate when a very small amount of bcp

was absorbed at the interface. They reported that the coalescence rate decreased

only slightly upon fllrther increasing the bcp concentration. Hu et al. [I781

presented quantitative data for the coalescence efficiency in a linear flow by the

visual observation of two equally sized drops. For very small concentrations of

bcp, which caused negligible changes in interfacial tension, a drastic reduction of

coalescence was observed. Sundararaj and Macosko [I081 proposed that the

steric interaction between bcps balanced the force that drives the droplets to

approach each other and the coalescence of the droplets was prevented. The

minimum bcp coverage was predicted to be independent of the shear rate but

decrease with increasing the MW of the bcps. The original theory was proposed

for static coalescence where the driving force was van der Waals' attraction. In the

case of dynamic coalescence, the shear force drives droplets to approach each

other. On the basis of this theory, the minimum bcp coverage for coalescence

suppression is expected to increase with the shear rate, which disagrees with the

experimental observations. Macosko and co-workers [181] have investigated the

effect of a diblock copolymer P(S-bMMA)) in PMMAIPS blends and reported that

the principle role of bcps in controlling morphology appears to be in preventing

coalescence. Preventing dynamic coalescence leads to droplet size reduction, while

preventing static coalescence results in stability or compatibilisation. They have

estimated that less than 5% of the interface needs to be covered to prevent dynamic

coalescence while 20% is necessary to impart static stability. Karim et al. [I841 have

reported that a copolymer at interface (in reactive blends) leads to a 'frustration' in

coalescence as shown in Fig. 1.12.

36 Chapter 1

Figure 1.12: Schematic ~llustration of droplet coalescence in immiscible polymer blends with and without a copolymer at interface. Note that the presence of copolymer can lead to a frustration in coalescence (Ref. 184).

Simulations of droplets approaching each other under the influence of a specified

force [I 89,1901 suggested that, when the interfacial tension is highly dependent on

the surface concentration of bcp, the droplets behave like rigid particles. The time-

scale of coalescence is then determined solely by the van der Waals' attraction

between the droplets. A similar but more complete numerical analysis [I911

studied the deformation, drainage and rupture of the film between colliding

droplets. When diffusion of the compatibiliser along the surface could be

neglected, the rate of film drainage was found to decrease from that characteristic

of partially mobile interfaces to that of immobile interfaces. Such coalescence

suppression was predicted to be especially significant at low viscosity ratios.

Lyu et al. [192,193] have shown that the minimum bcp coverage (X,,) for

coalescence suppression was independent of, or very weakly correlated with, the

shear rate (y). These authors have examined the role of PS-PE diblock

copolymer on suppression of coalescence in PSIHDPE blends and found that

coalescence was significantly suppressed compared to blends that did not

contain bcp. The minimum bcp required to completely suppress coalescence was

measured to be ahout 0.20chain/nm2 for 20-20kg/mol PS-PE. This value decreased

Introduction 37

with increasing bcp MW and also with shear rate in the range of 0.1-10s". Further,

PS-PE with a larger PS block was more efficient in suppressing coalescence

between HDPE particles. Steric repulsive interaction between the rigid bcp layers

in particle surfaces was thought to contribute to this coalescence suppression.

Milner and Xi [I941 and Milner [I951 assumed that the bcps at droplet surfaces

resisted the droplets to approach each other, and then the coalescence between

droplets was suppressed. Their studies dealt with the modelling of the break-up

and coalescence of polymer droplets in a mixed shear flow. The theory includes

hydrodynamic and repulsive interactions between colliding droplets, and treats the

work done in creating gradients in bcp as droplets approach, as an effective

repulsive potential. The resistant force occurred when the bcps in the gap between

droplets were brought out by the squeezing flow of the matrix and a concentration

gradient formed in the droplet surfaces as the droplets moved close to each other.

This force is called the Marangoni force. The minimum bcp coverage for

suppressing coalescence was calculated to be proportional to the shear rate.

Obviously, several experiments offered apparent conflict with this prediction.

Cristini et al. (1891 proposed a similar theory that gave a similar result. On the

basis of a similar concept, Blawzdziewicz et al. (1901 proposed another

mechanism and did predict the minimum bcp coverage to be independent of shear

rate. In this mechanism, the ratio of Marangoni force to shear force, or the so-

called Marangoni number, was assumed to be much greater than l , which was

different frorn Milner and Xi's mechanism where Marangoni number was smaller

so that the bcp concentration gradient was significant. This large Marangoni

number assumption in Blawzdziewicz et al.'s mechanism led to a situation in which

the bcp layers at droplet surfaces were incompressible and the droplet surfaces

were essentially rmmobile. These two rival mechanisms of coalescence

suppression are illustrated in Fig. 1. 13.

38 - Chapter 1

- -- I::

Figure 1.13: Two mechanisms proposed for block copolymer suppression of coalescence: (a) surface tenston gradient (Marangoni) force and (b) steric repulsion (Ref. 192)

Then the coalescence efficiency was calculated to depend on the mobility and

coverage of the bcps but not on the shear rate. However, Blawzdziewicz et al.

[I901 could not explain the effects of the symmetry of bcps on the droplet

coalescence, e.g.. why the bcps with larger outside blocks suppressed

coalescence more efficiently than that with srnaller otltside blocks even the total

MWs and the mobility were similar. To explain these effects of bcps on

coalescence, Lyu [I961 assumed that the bcps suppress coalescence by

preventing the rupture process of the matrix film between the droplets. Van der

Waals' interaction between the droplets compresses the film. The compression

force increases when there is a fluctuation in the facing surfaces of the droplets.

Thus, the fluctuation tends to grow. However, as the surfaces fluctuate, the

interfacial energy increases due to the increase in surface area, which damps the

fluctuation. When the film is thick, the compression force is negligible; the surface

fluctuation is always damped. However, when the film thickness is smaller than a

critical value (ti,), the van der Waals' compression force exceeds the damping

force, the fluctuation grows, and the film becomes unstable and ruptures (Fig. 14 a

and b) [197,198] and the rupture will be suppressed if there are bcps at the

interface (Fig. 'I .14c) [196]. By using stability analysis, the minimum bcp coverage

was calculated by Lyu [196]. It has been shown that the minimum coverage is

independent of the shear rate that is applied to the polymer blends but decreases

with increasing MW.

Figure 1.14: Schematic of the rupture processes of the film of the matrix between the two droplets. Rupture was suppressed if there were bcp's at the interfaces of droplets (196).

The effect of compatibilisation on the interfacial tension has been studied by

several researchers. Hu et al. (1991 applied a modified pendant drop method to

measure the interfacial tension of immiscible homopolymer blends. They used the

binary homopolymer blend PSIpoly dimethylsiloxane (PDMS) and a nearly

symmetric diblock copolymer P(S-b-DMS). They observed that the interfacial

tension of the blend is initially decreasing with increasing the copolymer

concentration, and then attains a constant value above a certain critical

concentration. It has shown that small amounts (e. g., 1% or less) of bcp can

considerably reduce the interfacial tension by 80- 90% (2001. The dependence of

the interfacial tension on the MW has been studied by Koberstein [201]. He found

that the interfacial tension between immiscible homopolymer melts scales

inversely with the ;i?, . Khattari (2023 has studied the interfacial properties of

diblock copolymers near an interface between two solvents using the exact

Green's function of a Gaussian copolymer chain at an attractive penetrable interface.

The reduction in the interfacial tension was found to decrease with the increase in the

degree of polymensation of the copolymer chain. Lepers et al. 12031 argued that in the

absence of coalescence effects, there is a close 1:l relationship between

morphology and interfacial tension. Further, Liang et al. (2041 have investigated on

the correlation between the interfacial tension and dispersed phase morphology in

interfacially modified blends of LLDPE and PVC and based on the results

40 Chapter 1

demonstrated a direct experimental confirmation of the interfacial tensionfphase

size relationship as predicted by Taylor theory. According to the authors there is a

1 :1 relationship between particle size and interfacial tension that is independent of

the emulsification efficacy of the compatibiliser.

It has been reported that in ternary blends containing a diblock copolymer

(i.e., AIA-b-BIB), the overall interfacial curvature is determined by the balance

between interfacial swelling of the bcp segments on both sides of the interface.

Again the degree of swelling may be manipulated by varying the MWs of the

homopolymer A and B relative to those of the block segments A and B or by

choosing chemistries of the blend constituents in a fashion that they will produce

exothermic interfacial mixing (2051. Different regimes are distinguished in

dependence on copolymer concentration $, composition f and xN, where x is the

segment interact~on parameter and N the mean degree of polymerisation of the

homopolymers. At low concentration, copolymers segregate to the interface

without interaction between neighbouring chains. Monte Carlo simulations based

on the bond fluctuation model and SCF calculations indicate that no significant

stretching of the chains occurs for this regime [206]. The authors also find no

serious broadening of the interface due to the diblock copolymers, which,

however, is expected for higher concentrations [207].

At higher copolymer concentrations at the interface, the copolymers start to

stretch. Depending on the ratio of the degree of polymerisation of the

homopolymer (NH) to that of the respective block of the copolymer (Nc) the so

called wet (N~>N"',,) and dry (Nc < N~,,) brush regime is defined [132]. The wet

copolymer brushes are swollen with homopolymers, while in the dry brush regime

no penetration of homopolymers into the copolymer brush takes place

[132,208,209]. The cornparison of theoretical work with experimental studies

(207.210-2141 has provided insight into the microscopic structure of modified

interfaces. If the concentration of the bulk copolymer in an incompatible

homopolymer blend exceeds the CMC, the segregation of copolymers competes

with the formation of micelles. Because the formation of micelles is usually not a

spontaneous process, but a highly activated one, the apparent CMC might be

appreciably larger than the thermodynamic equilibrium one [208]. At even higher

copolymer concentrations micro phase separated structures occur where

homopolymers may be confined in the corresponding copolymer domains or form

a second phase [215,216]. The dependence of the interfacial excess (zi*) on

copolymer concentration was investigated by Dai et al. (2141 for different MWs. At

low concentrations a linear increase of zi' with increasing copolymer concentration

was observed with forward recoil spectrometry (FRES). At low 2' the system is in

the wet brush regime and the mixing energy competes only with the loss of

entropy. At higher copolymer concentrations the chains are stretched to reach

higher 2'. Now the rn~xing energy is in competition with the loss of entropy and

with elastic free energy. This explains the observed approach towards a plateau at

higher copolymer concentrations. In contrast to the strongly incompatible polymer

blends, a different behaviour is expected in nearly compatible ternary mixtures

(2<xN< 4(2'")). At low copolymer concentrations the z i increases, but is expected

to decrease again at higher concentrations. In nearly compatible systems the

tendency of the copolymer to segregate to the interface is in competition with the

changing solubility of the polymer phases. For nearly miscible homopolymers and

high copolymer concentrations this might lead to total adjustment of the phase

compositions and the ternary system becomes miscible. Thus in summary, the

prominent mechanism of compatibilisatin of an immiscible polymer blend involves

(a) the copolymer efficiency to diminish the interfacial tension behveen the

components at interfaces by minimising the unfavourable cross correlations andlor

enhancing the favourable interactions and (b) coalescence suppression, mainly by

steric stabilisation.

1.5.3.1. Physical cornpatibilisation

Physical compatibilisation has been achieved by the addition of pre-synthesised

copolymer into the blends. The copolymers may be block, graft or random. The

bcps include di (pure or tapered), tri or multi block (penta, hepta, etc.). Very

recently, Kim et al. [217] demonstrated that compatibilisation of an immiscible

blend can be accomplished by the addition of a gradient copolymer during melt

processing. In physical compatibilisation, it is important that the copolymer stays at

the interface, without dissolving in either of the two polymers andlor forming

mesophase of micelle structure. Even though, a copolymer might be able to locate

at the interface during the first minutes of the compounding process, the stable

location of the copolymer at the interface mainly depends on the composition, MW,

miscibility of the compatibiliser with the individual phase and its molecular

architecture. The 'interfacial situation' of an immiscible polymer blends in the

presence and absence of a pre-made copolymer is presented in Fig. 1.15.

(a) (b)

Figure 1.15: Interfacial situation of an immiscible polymer blend in the (a) absence and (b) presence of a copolymer. Figure 1.15a illustrates the lack of entanglement between the two polymers at the interface results in a material with poor macroscopic properties where as Figure 1.15b demonstrates the way by which the polymer interface is strengthened by a surfactant.

Physical compatibilisation has been successfully used by many researchers since

mid sixties. Some of the important earlier studies of physical compatibilisation

using pre-made graft or block copolymers include those by Molau [218,219],

Molau and Wittbrodt [220]. Riess and co-workers [221-22511 and lnoue et al. [226]

who have used graft and or block copolymers as interfacial agents in solution and

Paul [227,228], Heikens [229,230] and Fay? [231-2371 and co-workers who have

studied the efficiency of graft and or block copolymers as compatibilising agents in

melt state.

Theoretically, each block of a diblock (or multiblock copolymer) is usually either

miscible or has strong affinities, with one of the two homopolymer phases. Thus,

bcps can act by migrating to the interface between the homopolymers. It is believed

that each block then localizes itself in its respective phase, thus reducing interfacial

tension and promoting adhesion between phases. In this case the bcp is an 'entropic

acting copolymer'. Typically in bcp studies A-B bcps are used to strengthen the

interface between homopolymers A and 6. For strengthening to occur, the blocks of

the copolymer must be long enough to allow chain entanglement with the respective

bulk homopolymer. However, long symmetric diblock copolymers tend to form

lamellae at the interface [238]. Since the interfaces between the sub-lamellae within

the copolymer layer are weak, the degree to which diblock copolymers can reinforce

the interface is limited. In contrast, long tri-block copolymers (ABA) are vely effective

as interfacial reinforces despite their tendency to form cylindrical micellas [239]. The

interfacial activity of A-B random copolymers (rcps) which do not form phase

domains has also been studied at A/B inteitaces. In the PS/poly-(2-vinylpyridine).

(PVP), system rcps with nearly a 50:50 composition were found to be surprisingly

effective at strengthening the interface at very low areal chain densities [239-2411.

Both theoretical and experimental studies have shown that the diblock copolymer

arranges itself across an interface in a cylindrical or dumbbell shape, crossing the

interface once [242,243]. As the copolymer becomes less blocky, it will begin to

become more Isotropic at the interface, crossing the interface many times

[242-2451. As the copolymer becomes more random and alternating, it will attain a

pancake-type structure, covering a substantial area of the interface. However, the

number of times these copolymers cross the interface and their ability to entangle

with the homopolymer are less clear than for blocky structures such as the diblock

and multi-block copolymers. If the interface is sharp, diblock copolymer chain will

cross the interface or~ly once, however, as the number of b h k s increases, the

number of interface crossings also increases. For example, a triblock will cross the

interface twice, a penta-block has four interfacial crossings, and a hepta-block

stitches the interface six times. The rcp !s also believed to cross the interface

multiple times although the exact number is not known. It should be emphasized that

this picture is a simplistic version of a real interface but emphasizes the point that the

microstructure of the copolymer can dramatically affect the configuration of a

copolymer at the interface. Additionally, theory predicts that the more times a

copolymer molecule crosses the interface, the more effective interfacial modifier that

copolymer would be 1242-2461. The logic is that there exist more joints or "stitches"

sewing the two phases together, which must be broken or pulled out to allow

fracture. This creates a stronger interface over which stress can be transferred

whether the failure mechanism is by chain pullout or chain break [247.248].

Noolandi (2491 predkted that diblock and triblock copolymers would akgn perpendicular

to the interface plane when placed at a biphasic interface, resulting in dumbbell shaped

confofmat'wffi. Multiiblock copolymers such as pentablock and heptablock ~ ~ p ~ l y m e f f i

lie in the plane of the intertxe and thus form pancake-shaped c o n f o m t ' i . Pancake-

shaped confonnatiins require less matehl to cover a g ~ e n interfacial area than

dumbbell shaped conformatons. Furthemhore, multi-block copolymers are less likely

than diblock copolymers to fom micdles or rnesophases. Supporting this theory,

Kramer and co-workers [250] found that the CMC was higher for tri-block copolymer;

than that for diblock copolymers. PVP-bS-bVP tri-block copolymer had a larger CMC

than that of a similar molecular weight PS-PVP dblock copolymer. Nwlandi 12491

speculated that "multi-block copolymers shoukl be effectiie as polymeric surfactants,

provided that the blocks are large enough to fom loops which extend well beyond the

original homopolymer interface and to enable physical entanglements or chemical

linkages to form with homopolymers". Figure 1.16 illustrates the conformations of

different types ot pre made compatibilisers at the interface of a multiphase polymer blend

(2511. In the following section, we briefly discuss the comwtibiliiing efficiency of dfierent

types of pre-made copolymers in multiphase blend systems.

Figure 1.16: Schematic connecting chain at an interface. (a) diblock copolymers (b) end grafted chains (c) triblock copolymers (d) multiply grafted chains (e) random copolymer (Ref. 251)

Introduction - 45

1.5.3.1.1. Compatibilisation by the addition of block copolymers

(a) Diblock copolymers

Several studies have confirmed that A-El bcps act as interfacial agents between

incompatible homopolymers A and 6 [225,226,252-3221. They concentrate at the

interface with each block dissolved in the relevant homopolymers. It has been

assumed that a thin film of bcp would corffiiderably enhance the adhesion between

incompatible polymers [252,253] though there have been no direct experimental

measurements of this effect. Brown 12541 has shown that the presence of the bcp

can increase the toughness of the PS-PMMA interface by up to a factor of 50. Bcp

car? organize in a simple way at the interface and saturate it. Many experimental

investigations have appeared to study the interfacial and surfactant properties of

bps . In addition to documenting the cornpatibiliing activty in polymer blend

dispersions, these investigations have also provided evidence of the interfacial activity

in polymeric systems, surface activity and dispersant efficiency of bcps [263,264].

Several methods have been used to characterise the morphology of polymer

mixtures containing a bcp. The important recent studies on the compatibilisation

immiscible polymer blends using diblock copolymers are depicted in Table 1.2.

The table has been designed in such a manner that an overview of the study

(system under investigation, compatibiiiser, characterisation, important results and

conclusions) will be obtained.

System Compatibiliser Characterisation StudiesIResults Ref.

PSIPB PS-b-PB Pendent drop method Linear reduction in interfacial tension followed by levelling off. 200

PSIPMMA P(S-b-MMA) Fracture test, SAXS Toughness of interface increased up to a factor of 50. 254

PSIPVP PS-b-PVP FRES Segregation of bcps to interface is a function of the copolymer 265 chemical potential. Equilibration between the interface and a micellar phase is directly affected by the solubility of individual copolymer chains. I

PSIPMMA P(S-b-MMA) Theoretical calculations The excess no. of copolymer chains per unit area at the 266 interface of the homopolymers varied linearly with the volume fraction of copolymer chains in the bulk.

PS/PMMA P(S-b-MMA) Neutron reflectivity Interface could be described by a hyperbolic tangent with an 267 effective width of 7.5nm (50% broader).

MPPOISAN P(S-b-MMA) DSC, SEM Palticle size reduced, location of emulsifier verified 268

PSIPMMA PS-b-PMMA Neutron reflectivity, Pariicie size reduced, location of compatibiliser is deternlined 269 SANS

PSIPVP PS-b-PVP Fracture test, SEM, 4 deformation regimes occurring in the fracture of interfaces 270 FRES, RBS, OIM, XPS were discussed, copolymers with the highest degrees of

polymerisation were the most effective mechanically.

PSlPMMA P(S-b-MMA) SEM, DSC, DMTA Reduction in domain size followed by levelling off, conformation 271 of the copolymer at the interface discussed

PPOlPMMA P(S-b-MMA) Fracture test, SlMS Interfacial toughness is a function of aerial density of copolymer 272

PSIPMMA P(S-b-MMA) Fracturetests, SlMS With symmetric diblocks interface toughness obtainable is 238 independent of the MW, but there is an optimum MW

PSIPVP PS-b-PVP Fracture test, TEM, There are limits to the interlace toughening by bcps, especially 273 FAES for symmetric bcps. Excess copolymer produces secondary

(lamellar) interfaces which are weaker.

8

8 5 e -

PSIPVP PS-b-PVP

PSISAN PMMA-b-PS

PSIPIP PS-b-PIP

PSIPP PS-b-PP

PEBIPMB PEB-b-PMB

PSIPMMA Pis-b-MMA)

PSlPMMA PS-b-PMMA

PSIPI P(S-b-I)

A!B diblock, star. comb copolymers

FRES, Neutron reflectivity

DSC, SEM, SAXS

DMTA, mechanical

TEM

ESR spectra

DSi , DMk, SEiv, mechanical

SANS

TEM, light scattering

Rheoopticai, TEM, light scattering

modeling

SEM, Ellipsometry

SCF calculations

Interfacial roughening (IR) is due to capillary wave, The mean- 214 square IR increases with lfi, where i is the interfacial tension

interfacial thickness improved by 33% (10 wt.% copolymer) 274

Compatibliising efficiency depended on the degree of 275 epoxidation of the butadiene unit in the copolymer, and on the CPIPS ratio in the ternary blend.

By decreasing the AN content of SAN, microstructurai 276 transformations from isolated vesicles to fused vesicles and finally to the phase-inverted network was obse~ved

The rate of migration and final packing density of copolymer at 277 the interface depends on the initial concentration.

Dispersed particle size reduced, mechanical properties 278 improved.

x values are independent of composition, molecular weight and 279 molecular architecture.

Low MW diblocks reduced interfacial tension, and prevent 181 dynamic coalescence but couldn't provide static stability. High MW diblocks were not effective as their CMC was very low.

Mechanisms of coalescence were discussed. Symmetric block 280 copolymer is more effective.

The number of compatibiliser chainshnit cubical element at 138 interface depends on its architecture and independent of MW

interfacial tension and therefore particle size reduced. 281

Diblocks are more efficient. When comparing combs and 282 diblcoks with different MWs, long combs with multiple teeth were more efficient than short diblocks

PSIPP SB SAXS, SEM, TEM, Diblock copolymer has negligible compatibilising efficiency mechanical

AIB AB diblock Theoretical studies Homopolymers longer than the diblocks are expelled from the ordered phases, shorter than the diblocks enter the brush formed by it, comparable to diblocks can swell and very short tend to disorder the microphase

PDMSIPEO PIDMS-b-ECji ir~terlacial tension The total number of DMS segments, turns out to be more (Sessile drop) decisive for their efficiency than their architecture.

PSlEPR SEB SEM, TEM Interfacial area occupied per diblock copolymer - 5.6nm2.

PS/PE PS-b-hPB Neutron eflectivity, TEM, Compatibiliser increased interfacial strength, micelle formation DSC beyond CMC does not decrease the interfacial strength

PSIEPR PS-b-PB NMR, SEM Particle size reduced, NMR technique was used to confirm large scale phase separation in domain sizes.

PBIPMMA PB-b-PMMA TEM Morphology is refined. Best compatibilisation resulted when the MW of the homopolymer is close to that of the block of the bcp.

LDPUPS SEP, S-E SEM, rheology, Diblocks were more efficient in terms of morphology, but mechanical triblocks (SES, SEES) in terms of mechanics.

SANIPCHMA P(S-b-MMA) TEM Coalescence rate decreased.

PPOlEPE P(S-b-4VPy) FTIR, UV, SEM, DSC, Location, miscibility and interaction of the copolymer were mechanical studied. Mechanical properties improved.

PEIPEP PE-b-PEP SANS, rheology, cloud There is common region in phase space over which the PEWPDMS p ~ ~ - b . p ~ ~ s point measurement bicontinuous microemulsion is stable. General phase behaviour PEIPEO PE-b-PEO is independent of molecular weight.

PSIPMMA P(S-b-MMA) Long focal length lens and Addition of bcp significantly increased the amount of area the PS/PP P(S-b-EE) CCD camera deforming drop generates when subjected to simple shear flow. PSIPE P(S-b-E)

PEiPVPh

PPIEPR

PMMNPI

PSISAN

PCIHIPS

PMMAI PCHMA

P(S-b-pMS) Nuclear reaction analysis, Neutron reflectivity

PS-b-PB TEM, rheoiogy, mechanical

P(E-b-MMA) OM, SEM

PP-b-EPR DSC, WAXD, TEM, DMA. mechanical

SEP IR, DSC, TG, TEM

PMS-b-PI Cloud point, TEM, Shear rheometry

P(MMA-b-I) TEM, NEXAFS, SPEX, Impact

PS-b-PMMA Viscosity. TEM

PIP-b-PVP SEM, IGC, mechanical

A-b-B Simulations

PS-b-PAr SEM, TEM, mechanical

PS-b-PMMA TEM

The segregation of the copolymer to the interface is largely controlled by MW and concentration.

Bcp must be designed, such that its order-disorder transition temperature is below the targeted melt blending temperature

Particle size reduced, domain distribution narrowed.

Copolymer was more miscible in PP phase than in EPR phase and therefore better homogenisation is failed.

Compatibility improved

Bcp will act as an effective compatibiliser only when the MWs. of block chains and the corresponding homopolymer lie within a certain range, mobility of bcp plays major role.

Cryogenic mechanics! alloying promotes chemical cross links. toughness improved

Balance of swelling power between the outside and inside segments of bcp at the interface is important and a severe imbalance produces emulsification failure.

Mechanics1 properties improved and depend on mixing procedure

A square gradient theory (SGT) for interfacial tension was proposed

Copolymer successfully compatibilise the blends

Location of compatibiliser, morphology as a function of MW Increasing the Mn of PMMA above block Mn caused all the rnicelles to move to PCHMA matrix

PSIPLLA PS-b-PLLA

LDPEIPB PS-b-PB

PSIPE SB diblock S-B-S-6-S pentablock

AJB A16 diblocks

Pendent drop method, S AXS

P(S-b.1) synthesis, IR, NMR, SEM, IR, mechanical

DMA, SEM, Mechanical

LS230 particle analyser. SEM. TEM

Rheology, interfacial tension, SEM, DMTA, solvent extraction, BET

SEM

SEM, TEM, SAXS. rheology, mechanical

Theoretical studies

SEC, SEM, DSC, rheoloav

Interfacial tension reduction is a monotonic function of the 306 copolymer MW at constant copolymer concentration and passed through a maximum.

Concentration, composition and MW of the bcp, composition of 307 the blend, mode of addition of the compatibiliser, homopolymer MW and processing conditions were found to be significant parameters which determine the blend morphology.

Particle size reduces, mechanical properties improved. 3C8

Coalescence suppression is a function of shear rate, 192 concentration, MW and symmetry of copolymer. The minimum concentration (C,) to prevent coalescence is 0.2chains/nm2 at a shear rate 0.1". Mn 20-20kgmol". C, decreased with shear

rate and increasing gn. Compatibiliser has an asymmetric effect on the blend (effective 309 in 50150 blend but only a marginal emulsifier in blends vuhere PLLA is the matrix.

The design of the copolymer must be based on the optimal 310 interactions balance between its own sequences and the respective individual phases.

Palticle size reduced, mechanical properties improved diblock 31 1 was more efficient.

Bcps suppress coalescence by preventing the rupture process 196 of the matrix film between the droplets. The minimum coverage is independent of shear rate but decreases with increasing MW

The width of the co-continuous region is narrowed. 312

N B IA=19%i A/B dibiock Simulation Bco with ionoer A blocks was more efficient due to their ability to 313 ockupy moreinterfacial area.

PVCIPA12, P(LA-b-CL) SEM. Mechanaai. Mechanical and thermal propefiies were improved. PVC/PP DMTA

PSlPi PS-b-Pi Pendent drop rr~eihod, Effectiveness of ihe interfacia! modifiers is controlled by the 315 SAXS unfavourabie interactions, which drive the additive toward !he

interface and the formation of micellas, which reduce the emulsifying activity.

PPIPS SEP OM, SEM, TEM, Particle size reduced, bimodal distribution of particles merged 316 SAXS, WAXD together, crystallisation process is affected.

SEP SEM, rheoiogy, Lower MW copolymer was more effective inechanica!

PWPS P(E-b-S) Dynamic rheology, When pre made compatibiliser is added inside :he drop phase 318 transparent Couet;e an elastic thin cylindrical tip develops in the vortlcity direction flow cell and then ruptures from the elongated mother drop, copolymer

does not affect initial polymer drop break-up greatly.

sPB90lsPB63 sPB90-b- SIMS, SANS Good agreement between SCFT predictions and experimental 319 sPB63 measurements was obtained.

PMMAILDPE PB-b-PMMA Copolymer synthesis. The best emulsifier was a copolymer with 33% PMMA, efficiency 176 SEM, OM decreases with lncrease in PMMA content.

PUPMMA PB-b-PMMA SEM, tensile The best copolymer had a PMMA sequence that was 320 approximately half that of the HPB block.

PSIHDPE PS-b-PE SEM Copolymers with different mi. Wts. (6 to 200kgmol.') were 321 used. 40komol" was most effective. . -

PSllonomer P(S-b-VP) SEM, DMTA, mechanical Particle size reduced, mechanical properties improved. 322

Table 1.2: Representative examples for the cornpatibilisation by the addition of diblock copolymers

52 Chapter 1

(b) Triblock and multiblock copolymers

Triblock copolymers have been found to be effective compatibilising agents by

several researchers [323-3471. However, note that the strengthening effect and

failure mechanisms at the interface strongly depend on the MW of each block,

the areal chain density and the chain conformation of the triMock copolymers at

the interface [239]. Dai et al. [239] suggested that the triblock copolymer (PVP-b-

S-b-VP) forms a hairpin or a "staple" structure at the interface with the dPS block

forming a loop on the PS side of the interface and the PVP ends anchoring the

"staple" in the PVP side as schematically shown in Fig. 1.17. Thus, as discussed

earlier, a triblock copolymer can cross the interface twice as illustrated in the

figure. On the other hand, multi-block copolymers such as penta or hepta block

copolymers can cross the interface more than a triblock copolymer can. The

important recent investigations on the emulsifying effects of tri or multi-block

copolymers on immiscible multiphase systems are summarised in Table 1.3.

Staple tail

Figure 1.17: Schematic representation of a triblock copolymer at the interface between PS and PVP showing the staple or the tail structure contributing respectively two and one connector per chain (Ref. 251).

1.5.3.1.2. Compatibilisation by the addition of grafi copolymers

Graft copolymers (gcps) have also been used extensively as compatibilising

agents in multiphase polymer systems and in most cases were found as efficient

as bcps [348-3701. Cho et al. [356] have cla~med that if one adds a gcp with an

incompatible main chain back bone and compatible grafts to the immiscible

polymer blend, gcp will have a better chance than a bcp to be situated at the

interface due to the repulsion between the back bone polymer and the

immiscible polymers. Table 1.4 presents the representative examples of

compatibilisation studies by gcps.

System PUPS

LDPUPVC PSILDPE

PSIPE

PSIEPR

PSIPVP

PAGIPES

PPIPS

AJB

PSIEPR

PPIPS

PPOIPEO

Compatibiliser Characterisstion Studies/results/conclusion Ref. SEBS SEM, DTA, Stress Internal stress level in the specimen increased and the creep rate reduced due to 323

SEBS

PVP-b-PS-b- PVP

SBS

ABA. BAB

SBS, SEES

relaxation, creep SEM, DSC, mechanicai DMS, DSC, SEM. rheo- optical measurements, I n Rheology, SEM, solvent extraction, interfacial tenslon (breaking thread) SEM, mechanical

FRES, fracture test, TEM

Copolymer synthesis, IR NMR, DSC, TEM. TGA SEM. TEM. WAXD, mechanicai

Simulation

SEM, impact

SEM, TEM, Mechanical

Shearing cell

interfacial adhesion. 1% bcp was sufficient for large improvement of the mechanical properties. 324 Rheo-optical technique could be used as a valuable tool to study the effect of 325 compatibiliser on orientation of phases Morphology is stabiiised, the interfacial tension is found to be reduced from 5.6 to 326 l.lrnN/m by the addition of 20 parts of the copolymer to the blend

For the lower MW. interfacial agent, a transition in fracture mechanisms, lrom 327 fragile to ductile, m u r s which is not obsewed with the high MW, interfacial agent. Most of the triblcck copolymers lm a "staple" structure at the interface and 239 therefore the saturation areal chain density of the tribiwk copolymer at the intetface is half of that lor the diblcck copolymer of similar MW.

Particle size reduced, thermal properties improved 328

Dispersed particles were complex "honey comb like" aggregates enveloped and 329 ioined tooether with the comoatibiliser. cwstallinilv increased sliahtlv. mechanical - " . properties improved. When A lorms a dispersed phase, 8AB and ABA triblcck copolymer chains tend 330 to Stretch on the interface, so that they are more efficient than A-B diblcck copolymers. Compared with diblccks. Dibicck and triblccks improved mechanical properties. 331 The styrene block in the triblcck copolymer is below its entanglement molecular weight. Particle size decreased, mechanical propenies improved. Low MW SEBS was 332 superior to high MW SEES Coalescence was suppressed by 0.1% copolymer by reducing droplet collision 177 and film drainage and/or rupture.

PSIPB PSIPB

PSIPMMA

PSIHDPE PSIPP

HDPEIPS

PUlPE

PSILDPE PPIHiFS

LDPElPS

PSIPP SANIPPE HIPSIPE

PSIPB

PPIPS PSIPMMA

SBS SBSItriblock, penta block Triblock, multiblocks

SBS SEES

SEES

SEES

SBS SEES P(S-b-B-b-S), SEES SBS P(Sb&bMMA) SEES

SBS

SEES Triblock, multiblocks

TEM, DSC, DMTA Morphology and thenomechanical properties have been studied. 333 Rheology, mechanical, SEM. Triblock was the best emulsifier 334 TEM. SAXS Fracture toughness

MFI. SEM, mechanical, DSC Mechanical. SEM

Rheology, SEM, BET. TFR, solvent extraction

SEM, TEM

SEM, tensile SEM, rnecharricai SEM, OM. annealing

Rheology, SEM, TEM. SAXS TEM, rheology Rheology, SEM. DSC. NMR, mechanical SEM, NMR

Copoiymer architecture and block iength played cnrcid role in determining 335 compatibilising efficiency. Mechanical properties improved. Crystallisation behaviour does not change. 336 Toughness improved: particle size decreased. 337.

338 Microstructural features are controlled by reduced droplet-droplet coalescence. 339 the generation of fresh interface during droplet deformation resuns in a system that is only partialiy emulsified. Micellelike structures only occurred when the compatibiliser was partially or 340 completely compatible with one of the phases. Triblock was inferior to tapered dibicck copolymer 31 0 Mechanicai properties improved 34 i Compatibiiisation efficiency is compared with a tapered block copolymer which 342 was found to be more efficient. Location of the copolymer depends on the length of the blocks. 343 Raspberv morphology is observed, 344 Compatibility improved, mechanical properties improved. 345

Compatibilty improved, ttiblcck copolymer outperformed random copolymer in 163 com~atibilisina efficiencv. NMR was found to be an efficient tool for comoatibilitv - , . stdes.

Dynamic rheology, SEM The values d the stresses increased. 346 NMR. Fract~re toLgnness Compahbdfstng eff~c~ency in terms of strengtn of nterlace. pentaolocotnolocb 347 nedtron ref*ect v ty GPC d b ocrvhepaDlocbrandom

Studleal System Compatibilistr ResultslConclusions Ref.

Characterisation

- 3 I

PCIPMMA PC-g-PMMA NMR, TEM, DMS Copolymer acts as effective surfactant, producing fine 348,349 dispersions and creating significan: amounts of in!erfaclal area. I I The relative MW of the PC homopolymer to that of the PC block controls the position of the CMC.

NRIPS NR-g-PS DSC, DMA, SEM Morphology is reftned, properties are improved. 140-144 rhaology, TGA, mechanical

NRiPMMA NR-g-PMWP. nSC. DMA, SEM Morphology is refined, properties are improved. 145-150 rheology, TGA, mechanical.

PS/PVC S-b-8-g-CHMA SEM Compatibiliser refined morphology. 350 S-b-B.g-MMA

PPOIPMMA P(S-g-€0) SEM, DSC. DMA. The ternary blends showed an additional transition at temp. 351 NMR between 60 and 100°C due to the existence of an interphase.

POILCP PO-g.LCP Copolymer Reduced the interfacial tension, reduce the coalescence rate, 352-354, synthesis, improve the minor phase dispersion. 356.362. morphology, OM, lnterfacial tension

364

PSlEVA EVA-g-PS SEM, rheology. interfacial adhesion improved, mechanical properties improved. 355 mechanical

PCIABS P(AC-g-CL) NMR, mechanical. Mechanical properties improved, interface smoothened due to the 356 SEM interfacial enrichment ol the graft copolymer

%

PBAIPMMA P(BA-g-MMA) TEM, DMA Graft copolymers decrease the interfacial tension between core and shell layers in the composite particles.

LLDPEISBS LLDPE-a-PS NMR. DSC LLDPE-a-PS chains connect the crystalline region of LLDPE by ~somor~h~sm, with sertous effects on the supe;mo~ecular structure of LLDPE which depends on composition.

PSIPEQ, PSI P!S-pEO) SEM, DMTA, DSC Even very short parts of the backbone of a graft copolymer can PMMA, contribute to compatibii~sation, especially when the backbone has PPOIPMMA a negative heat of mixing with one of the blend components.

PVCIPS CPE-g-PS DMA, mechanical Interfacial adhesion improved, mechanical properties enhanced

PPfPS PP-g-PS DSC, SAXS, OM Morphology is stabilised, crystallisation behaviour is affected.

PPSIPET PPS-g-PET Synthesis, PPS-g- Particle size decreased, interfacial activity depends on MW ratio, PET, SEM, DSC Crystailisation temperature increased.

PPIPS AVM-g-PP SEM, mechanical Morphology reflned, propenies improved.

PPIPMMA PP-g-PMMA DSC, OM, SAXS Morphology is refined, crystailisation behaviour is affected.

PSIEVA EVA-g-PS SEM, Mechanical Mechanical properties improved.

PPIPS PP-g-PS Synthesis, NMR, PP-pPS with short PS side chains showed betier DSC, GPC, SEM compatibilisation efficiency than those with long PS chains at

comparable composition.

PPIPS PP-g-PS Synthesis, NMR, Compatibilisation ability improved with increasing either the GPC, SEM length of the PP or PS sequences or the no, of the side chains.

Table 1.4: Representative example for the cornpatibilisation by the addition of graft copolymers

1.5.3.1.3. Compatibilisation by the addition of random copolymers

There are several theoretical and experimental results that support the

compatibilising action of a rcp in an immiscible polymer system (240,272.371-3801

although the mechanism whereby a rcp strengthens the interface between two

immiscible polymers is still controversial It has been suggested that each

copolymer molecule can form loops that penetrate into the homopolymers on

both sides 1239,3731 of the interface, and !so, each copolymer molecule fomls a

number of stitches. Note that diblock copolymers, however, represent only one

possible architecture for copolymers (Fig. 1.18a). As discussed earlier, triblock

copolymers form "hairpins" at interfaces [240,381] as shown schematically in

Fig. 1.18b. In the case of a random copolymer, a single polymer chain may

make multiple crossings at the interface 1240,371,382-3841. Hence, the number of

times one chain crosses the interface can be large, as shown in Fig. 1.18c, and,

thus, the copolymer may effectively "stitch" the two immiscible homopolymers

together. However, the effectiveness of each stitch is unknown since the loop may

not be long enough to entangle well with the homopolymers. There were

suggestions that the loops may be longer than expected from the average Flory-

Huggins parameter 2 between the rcp and the homopolymer due to selection of

particular random sequences, but it is now clear that such an effect is unlikely [385].

Hence, the mechanism of reinforcement is unclear in-situations where x is large

and the interfaces between the copolymer and the homopolymers are expected

to be narrow.

58 Chapter 1

ie4 / ~

Figure 1.18: Schematic diagram of the organization of copolymers at the interface between hornopolymers comprised of the same basic units: (a) a diblock copolymer; (b) a triblock copolymer; (c) a random copolymer. Note the increase in the number of crossings of the copolymer chain as the sequence distribution of the monomers in the chain is varied.

Dai et al. [240] have done an elegant set of experiments on the reinforcement of

polymer interfaces with rcps. They have shown that long rcps with symmetric

monomer lractior~, f - 0.5, are more effective than those with an asymmetric

monomer fraction f - 0.8, in strengthening the interfaces between immiscible

homopolymers, and also more effective than diblock copolymers. The

exceptional effectiveness of the symmetric rcp was attributed to the rcp crossing

the homopolymel. interface multiple times, thereby maximising the number of

entanglements with each of the immiscible hornopolymers. Note that a long rcp

with f - 0.5 is likely to have the same (Gaussian) conformation in either

homopolymer and therefore also at the interface, since on the average there is

no thermodynamic driving force to break the spherical symmetry, unlike the case

of an A 6 diblock copolymer which has the A block mostly in homopolyrner A and

the B block mostly in the homopolymer B at A-B interface (3861. This process will

be particularly effective il the homopolyrner MWs are much less than those the

rcps, as in the experiments; carried out by Dai et al. [240]. A beneficial side effect

of this process is the formation of numerous entanglements of both

homopolymers with the rcp, with the maximum number occurring for f - 0.5. For

f < 0.5 or f > 0.5, the homopolymer corresponding to the majoriiy monomer

fraction will form a larger number of entanglements with the rcp than for f - 0.5,

but the other homopolymer will form a smaller number, and since the strength of

the interface is determined by the weakest link, the strength decreases rapidly

for asymmetric copolymers, as observed. Noolandi and Shi [387] have reported

that for asymmetric rcps the orientational ordering at the interface is important

and conformational entropy is minimum. In addition, a smaller number of

entanglements are expected for one of the homopolymers with the copolymer

than for f - 0.5. Note that for the symmetric case (f - 0.5) the conformational

entropy of the copolymer is maximised, as is the number of entanglements of

both homopolymers with the copolymer.

On the other hand, Balazs and co-workers [388] examined the effect of

molecular architecture of potential compatibilisers (random, alternating and

diblock copolymers) on their compatibilising performance using the SCMF

approach. These authors calculated the extent of the interfacial tension

reduction as a measure of the efficiency of the compatibilisers, and predicted

that long rcps are more effective than short diblocks when they compare rcps

with diblocks of different MW. However, at fixed MW, diblock copolymers are

more effective in reducing interfacial tension. Note that the spreading of rcps

between immiscible phases is thermodynamically favoured. Winey et al.

demonstrated 1389) that encapsulation by rcps can occur during drying of a

solvated blend.

Macosko and co-workers [390] have discussed whether rcps can act as effective

compatibilisers for blends prepared by melt mixing. They have observed that rcp

(SMMA) moves to the interfaces between PS and PMMA domains during melt

mixing and forms encapsulating layers. As a result, the size of the dispersed

phase is significantly reduced. When SMMA is used in excess, separate

domains of pure SMMA are formed. However, when the blends are annealed at

high temperature, the size of the encapsulated particles increases with

annealing time. The authors claimed that the encapsulating layer of rcps does

60 Chapter 1

not provide stability against static coalescence. Figure 1.19 illustrates how

particles with encapsulating layers grow during annealing. Further, these authors

have demonstrated that rcps with the same monomeric units as blended

homopolymers A and B have a strong tendency to encapsulate the minor phase

in AIBIA-ran-B ternary systems [391]. Transmission electron microscopy (TEM)

demonstrated that for all cases SMMA moves to the interface between the

matrix and dispersed phases during melt mixing, and forms an encapsulating

layer. However, the resulting average size of a dispersed phase droplet is not

significantly decreased by the addition of SMMA. Moreover, this size increased

significantly upon further annealing, except for the blend with a PPO matrix

which has a very high melt viscosity, demonstrating that encapsulation by SMMA

does not provide stability against static coalescence. Table 1.5 summarises

recent studies on the effect of rcps on the morphology and properties of

immiscible polymer blends.

Figure 1.19: Coalescence of encapsulated particles: (a) approach and overlap of encapsulating layer; (b) deformation of dispersed particles and squeezing out the encapsulating layer; (c) rupture of the encapsulating layer; (d) mass flow and coalescence. Note that these pictures do not represent a time series of two particular drops (Ref.. 391).

System Compatibiliser CharacterisationIStudies Resultdconclusion Ref. PSIPBD SBR Cloud point, theoretical The temperature coefficient of the interaction energy density 392

PSIPMMA P(S-r-MMA) Fracture test PSIPVP PSt-r-PVP,., FRES, NMR

HDPElPET EVA.EPR,NBR lnteriacial tension

between styrene and butadiene polymers is determined. Fracture toughness improved by a factor of 20. 271 Long random copolymers (f = 0.48) was more effective than a 240 iona svmmetric block coooivmer. As f increased from 0.48 to 0.77, the effectiveness decreased markediy. Com~arison of different co~olvmers. Blocbarafi>random 393

PAJPMMA P(S-r-MMA) Fracture test, neutron The maximal fracture toughness was found when f = 0.68 where 373 reflectivity the interfacial broadening on the PS and PMMA sides of the

interface was symmetric. PSIPMMA SMMA TEM The encapsulating layer of SMMA does not provide stability 390

against static coalescence. PSIPMMA P(S-r-MMA) Fracture test, TEM, FTiR PS/copolymer interface width increases as the styrene copolymer 378

composition increases from 48 to 73mol%. PMPSiPDMS P(MPS-r-EESS) !nteriacial !ensian, phase Efficiency of copolymer for reduction of y is bound 10 the condition 394

diagram that it is sparingly soluble in blend components. PSIPB SBR TEM. Mechanical Random cooolvmers with a com~osition near 50150 were 395

PS/PC.PPO/ SMMA TEM PMMA, PPOIPC PSIPMMA P(S-r-MMA) Fracture test, OM, Ion

scattering technique LLDPWPMMA-co- E M U IR. SEM, XPS, NRET, 4VP mechanical PSIPVP PS-r-PVP Fracture toughness (G,)

PSINBR S AN OM, SEM, mechanical

PSISAN PS-r-PSPh ADCB test, AFM

successful as kterfaciai agents. ' SMMA moves to the interface, and fotms an encaosulahna laver 391 - . Panicle sze s not s~gntcantly reducea The enectiveness of the copolymer as a tou nening agent 39E ~ncreasea as ns MW ircreaseafrom 16xtdPto 4.510' Mechanism of compatibilisation is discussed. 397

G, is governed by Af,, the largest difference in adjacent 398 compositions and by width of the narrowest interface (wmin). G, increased as w,, increased from 3 to 5nm. Conformation of the compatibiliser at the interface was 157 discussed, mechanical properties improved. Random copolymer was less effective than block. 399

Table 1.5: Representative examples for the compatibilisation by the addition of random copolymers

1.5.3.2. Factors affecting physical compatibilisation by pre-made copolymer

1.5.3.2.1. Molecular weight of the blocks

Several researchers have used high MW bcp compatibilisers because of their

relatively large stabilisation effect [237,400,401]. Eastwood and Dadmun 13351

have shown that MW's of the blocks must be large enough to obtain significant

anchoring in the homopolymers. Lyu et al. [I921 have studied the role of bcps

(PS-b-PE) on the suppression of droplet coalescence in PS/PE blends and

found that high MW bcp was more efficient in suppressing coalescence. They

have also observed that bcps which have MW greater than a critical value was

capable to prevent coalescence. However, Radonjic [332] has reported that low

MW SEBS appeared to be a more efficient compatibiliser in PP/PS blend than

high MW SEBS. Recently, Choi et al. [317] have used two diblock copolymers of

SEP with different MW's to compatibilise PSIEPR blends and observed that the

lower MW diblock copolymer was more efficient. Very recently, Macosko and

co-workers [321] have investigated the effect of bcps on the co-continuous

morphology of 50150 (wlw) PSI HDPE blends using symmetric PS-PE with

MW's varying from 6 to 200kg/mol. An intermediate MW PS-PE, 40kg/mol,

showed remarkable results in reducing the phase size and stabilising the blend

morphology during annealing. Overall, the bcps were ranked in order of

stabilisation effectiveness: 6k < 2M)k c lOOk < 40k. The existence of an optimal

MW bcp is due to a balance between the ability of the block copolymer to reach

the inteltace and its relative stabilisation effect at the interface. Maric and

Macosko 14021 have examined the effect of adding the bcp on the blend

morphology as a function of the bcp MW, concentration and h. When blended

together with the PS and PDMS homopolymers, most of the bcp appeared as

micelles in the PS matrix. Adding 16kglmol PS-PDMS block copolymer

dramatically reduced the PDMS particle size, but the morphology, as examined

by SEM microscopy, was unstable upor1 thermal annealing. Adding 156kglmol

block copolymer yielded particle sizes similar to those of blends with 40 or

83kglmol bcps, but only blends with 83kglmol bcp were stable after annealing.

Introduction - 63

The effect of the MW and concentration of compositionally symmetric diblock

copolymer additives on the interfacial tension between two immiscible

homopolymers has been investigated hy Anastasiadis and co-workers 13061.

These authors observed dependence d the interfacial tension was a non-

monotonic function of the copolymer additive MW at constant copolymer

concentration in the plateau region. The emulsifying effect increased by

increasing the additive MW for low MW's, whereas it decreased by further

increasing the copolyiner MW, thus going through a maximum. The presence of

micelles for high MW additives and their absence for low MW's was supported

by SAXS data. Schnell et al. [294] have investigated the segregation of diblock

copolymer to the interface between two weakly incompatible homopolymers by 15 N nuclear reaction analysis ( ' 5 ~ - ~ ~ ~ ) and neutron reflectivity, and reported

that the segregation of the copolymer to the interface is largely controlled by MW

and concentration, resulting finally in a regime, where copolymer segregation to

the interface decreases with increasing concentration and where the system

becomes completely compatible.

Compatibil'sation of blends of PB and PMMA with butadiene-methyl methacrylate

diblock copolymers has been investigated by Zhao and Huang 12891 and found that

when the MW of PMMA is close to that of the corresponding block of the

copolymer, the best compatibilisation result would be achieved. Favis and

co-workers [331] have compared the efftciency of two SEE diblock copolymers

and a SEP diblock copolymer as compatibilisers. All the interfacial modifiers

demonstrated an excellent emulsificatior~ efficacy for the EPWPS system, and

no effect was observed when changing the block structure from EB to EP.

Despite the excellent capacity of all three diblock modifiers to emulsify the blend,

the impact testing demonstrated significantly different behaviours for the various

copolymers. The higher MW SEE and SEP interfacial modifiers displayed a

brittle fracture behaviour over a wide range of interfacial modifier concentrations.

The lower MW SER diblock displays a brittle to ductile transition in impact

closely related to the concentration of modifier required for saturation of the

interface. The PSlEPR system was used to compare the efficiency of diblock

and triblock copolymers [327,403]. For the diblock copolymers a small MW effect

64 -- Chapter 1

was observed, and it was shown that the high MW modifier was the best

emulsifier with a lower critical concentration (Cm,) value for interfacial saturation

than the other two. Since higher MW copolymers are considered to be more

likely to form rnicelles, this was taken as an indication that micelle formation for

the diblocks, below C,:.,, was likely not a serious concern in this system [404].

Similarly, Horak et al. [334] found in PSIPB system that bcps containing styrene

blocks of MW - 40,000 showed a finer and more regular phase structure and

higher impact and tensile strengths than the blends compatibilised with bcps

containing styrene blocks of MW - 10,000. Thus, in summary, MW of the

copolymer is one of the most critical factors that determines its compatibilisation

efficacy and for efficient emulsification to be occurring the copolymer must have

some critical MW (below which not much compatibilisation has been attained)

which in turn depends on the MW (viscosity) of the homopolymers.

1.5.3.2.2. Type of cornpatibiliser

It is believed that a bcp is more efficient than a gcp which in turn is more efficient

than a rcp. Among bcps, a diblock copolymer is more efficient tlran a

triblocklmulti-block copolymer and a tapered diblock copolymer will be more

efficient than a pure diblock copolymer. However, this trend need not be true in

all cases and some researchers have found that rcps were surprisingly effective

in reinforcing polymer/polymer interfaces [371-3801 and long rcps with monomer

fraction nearly equals to 0.5 can be even more effectwe than a long diblock

copolymer in strengthening interfaces [240]. It was reported that when triblock, pure

diblock, and tapered diblock copolymers were compared, a tapered was found to

exhibit the most efficient compatibilisation [342,405]. Jer6me and co-workers [405]

examined the efficiency of a tapered diblock P(S-bhB) and triblock P(S-hB-S)

copolymer in the stability of a co-continuous morphology of LDPEIPS blends and

o b s e ~ e d that the tapered di block copolymer was more efficient particularly at

low concentrations of copolymer. The superiority of the tapered diblock over the

triblock might be due to its ability to quantitatively locate at the interface and

consequently form a more efficient barrier against the subsequent break-up of

the elongated structures of the co-continuous phase morphology. Dadmun and

co-workers 1335.3471 have shown that multi-block or blocky copolymers are the

Introduction 65

most effective interfacial strengtheners among linear copolymers for polymer-

polymer interfaces. Asymmetric double cantilever beam (ADCB) and neutron

reflectivity experiments provide direct evidence that multi-block copolymers that

have blocks that are long enough to entangle with a homopolymer are most

effective at strengthening the interface. According to the authors the efficiency of

copolymers in terms of their ability to strengthen the copolymer interface is:

pentablock > triblock > diblock > heptablock > random copolymer. As we go from

the penta-block to the triblock to the diblock copolymer, the number of times the

copolymer crosses the interface decreases from 4 to 2 to 1, as illustrated in Fig.

1.20 and this logically corresponds t,o a decrease in the strength of the interface.

However, as the number of blocks, increases to seven, the hepta-block

copolymer will cross the interlace six times, and thus should be even stronger

than the penta-block copolymer, however, this was not observed. This is due to

the fact that as the copolymer becomes more blocky ti.@. contains more blocks),

the length of each block must decrease. Thus, the block length of the hepta-

block was insufficient to efficiently entangle with the homopolymers, and

therefore does not strengthen the interface as effectively as the penta-block,

tribfock or diblock copolymers.

Figure 1.20: Illustration of the ideal number of crossing of a given multi-block copolymer at a sharp polymer-polymer interface (Ref. 347).

Lyatskaya et al. [282] have used analytical arguments and numerical SCF

calculations to calculate the intelfacial tension (y) for a system of AB copolymers

66 - Chapler 1

localised at the interface between two immiscible homopolymers, A and B. They

fixed the MW's of the copolymers but vary their architecture from diblock, star,

and various comb copolymers. Diblock copolymers were the most efficient at

reducing y and at fixed MW, as the number of teeth (N) is increased, the comb

becomes significantly less efficient and the disparity between combs and

diblocks developed larger. However, when c:ombs and diblocks with different

MW's were compared, they observed a trade-off between MW and N. In

particular, long combs with multiple teeth are more efficient than short diblocks.

Kramer and co-workers [379] have used a series of styrene-d814-hydroxystyrene

graft and bcps for interfacial strengthening studies at the PSIPVP interface.

These included the following copolymers in which A and B segments represent

poly(styrene-08) and poly(4-hydroxystyrene), respectively: poly(A-graft-B),

poly(B-graft-A), poly(B-block-A-block-B), poly(A-block-B-block-A-block-B-block-

A). In general, the amphiphilic copolymers acted as interfacial reinforcers but

were susceptible to the formation of microphases (lamellae or micelles). The

penta-block copolymer showed better strengthening behaviour than the triblock

copolymer especially at very low areal chain density. The strengthening ability of

the gcps was found to depend on the lengths of the PS and poly(4-

hydroxystyrene), PS(OH.1, segments. In both graft and block copolymers the

PS(0H) segments were found to resist pull-out from the bulk PVP even at low

degrees of polymerisatiori (NPS,, = 29).

On the other hand, Horak and co-workers [283,334] have studied the effects of

the block length and block number of linear styrene-butadiene (S-B) bcps on

their compatibilisation efficiency in PS/PB blends and observed that

independently of the styrene block length, the triblock was the most efficient

compatibiliser. This was observed for both symmetrical and asymmetrical

polymer systems. They have also used di-, tri-, and penta-block types of S-B

bcps as compatibilisers for HIPSIPP blends and reported that the PS-terminated

multi-blocks influenced development of the interfacial layer around dispersed PP

particles and, consequently, improved mechanical properties compared with

diblock copolymer, whic:h showed negligible compatibilisation efficiency.

Cavanaugh et al. [395] have studied the effectiveness of various S-B copolymers

as compatibilisers in PSlPB blends and found that the most effective

compatibiliser proved to be a long, asymmetric diblock, which could entangle in

both homopolymer phases. Diblocks with short PS segments were inadequate

as interfacial agents, presumably because their short PS blocks were easily

pulled from the matrix Rcps with a composition near 50/50 were successful as

interfacial agents, but the impact strength declined with a decrease in PS

composition of the copolymer. Koklas and Kaifoglou 12751 have examined the

compatibilising efficiency of epoxidised S-B of varying architecture for the immiscible

PSIchlotinated polymer (CP) blends [PVC and CPE]. The copolymers were a

diblock (ESB), a three-arm star (ESB), and a random ESBR. Compatibilising

efficiency was found to depend on the degree of epoxidation of the butadiene unit in

the copolymer, which in turn relates to the degree of chlorination of the CP, and

also on the CPIPS ratio in the ternary blend and the molecular architecture of the

E(S-b-B) compatibiliser. Other things being equal, this efficiency varies as:

ESB>(ESB)+ESBR. Guo et al. [290] have studied the phase behaviour and

mechanical properlles of a LDPEIPS blends after compatibilisation with various

S-E, SEB and SEP type bcps. These authors have observed that although the

diblock copolymers were more efficient in reducing the phase size, the triblock

copolymers were more effective in improving the mechanical properties.

7.5.3.2.3. Block length and sequence of distribution (symmetry)

Eastwood and Dadmun [335] have demonstrated that block length in multi-block

copolymers played a pivotal role during interfacial modification. Increasing

lengths of blocks in multi-block copolymers increased interfacial adhesion.

Dadmun [242] has reported that the copolymer structure at the biphasic

interface, however, does vary significantly with copolymer sequence distribution.

This variation in configuration was interpreted in terms of the ability of the

copolymer to form loops and entangle homopolymer so that a macroscopic

reinforcement of the interface will be found. With this interpretation, it is found

that both alternating and bcps should efficiently strengthen the interface due to

their extension along and/or across the interface. The purely rcp, however, is

less efficient at strengthening the interface, as this copolymer tends to crumple

in on itself and does not offer the opportunity for entanglements to occur.

Anastasiadffi and co-workers 13151 have investigated the effects of macromolecular

architecture, copolymer composition, and concentration of the addiives utilising

PSlpolyisoprene (PI) Mends in the presence of a series of gcps of almost constant

MW and varying composition. The interfacial tension at interfacial saturation is a

nonmonotonic function of the copolymer composition, which can be understood

to be due to the competition between the decreased affinity of the copolymer

within the homopolymer phase when the size of the "incompatible" block

increases, which increases the driving force of the copolymer toward the

interface, and the possibility of micelle formation. Moreover, the interfacial

tension at saturation depends on the side of the interface where the copolymer is

introduced. This is due to the asymmetric architecture of the copolymer and

points to the fact that a local equilibrium can only be attained in such systems:

the copolymer reaching the interface from one homopolymer phase probably

does not diffuse to the other phase.

Bcp symmetry is another critical parameter that might affect coalescence

efficiency. To check the effects of block symmetry on prevention of coalescence,

Lyu et al. [I921 compared the increases in particle size at 0.1s.' in blends that

were compatibilised with three PS-PE diblock copolymers with number average

MW values of 20-20, 28.5-10.5, and 33-5kglmol. In this series the total MWs are

approximately the same, but the ratios of radius of gyration (PSIPE) were 0.58,

0.96 and 1.52, respectively. The bcp with radius gyration ratio of 1.52 had the

most significant effect. This bcp has the greatest PS block MW, a result that

agrees with the steric interaction mechanism. Balazs and DeMeuse [406]

studied the effect of the sequence distribution of a copolymer on the miscibility of

ternary mixtures which contain a copolymer and two homopolymers. They

characterised the copolymer architecture by the parameter 0, which defines the

proportion of unlike nearest-neighbour monomers in the copolymer chain. The

limiting values of 0 for a symmetric copolymer (i.e. f - 0.5) are 0 = 0 for a bcp,

0 =1 for an alternating copolymer and B = 0.5 for a rcp. The results showed that

the sequence distribution plays an important role in the phase behaviour of these

ternary mixtures and that a bcp may not be the best thermodynamic compatibiliser in

a temary mixture. Jo et at. [281] have shown that the reduct'mn in interfacial tension

of asymmetric diblock copolymer having longer isoprene block is more significant

than the symmetric diblock copolymer or the styrene-rih diblock copolymer. The

reduction in particle size by the addition of the isoprene-rich asymmetric diblock

copolymer is more significant than the cases of the symmetric or styrene-rich

asymmetric diblock copolymer.

Fortelny et al. [311] examined the effect of mixing conditions on the morphology,

molten state viscoelastic properties and tensile impact strength of PSIPE blends

compatibilised with S-B bcps containing various numbers and lengths of blocks.

The effects were stronger for S-B diblock with a short styrene block than for

S-5s-8-S penta-block with long styrene blocks (where S represents styrene

and B represents butadiene). Harrats et al. [320] have studied the stress-strain

diagrams and ultimate tensile properties of uncompatibilised and compatibilised

hydrogenated hPH-b-PMMA blends with 20wl% PMMA droplets dispersed in a

LDPE matrix. Four copolymers, in terms of the MWs of the hydrogenated PB

and PMMA sequences (22,000-12,000, 63,300-31,700, 49,500-53.500 and

27,700-67,800), were used. The best copolymer, resulting in the maximum

improvement of the tensile propert~es had a PMMA sequence that was

approximately half that of the hPB block. Dai et al. I3131 used Monte Carlo

simulatior)~ to investigate the compatibilising effects of diblock copolymers in

A/B/A-B diblock copolymer ternary blends and triblock copolymers in

A/B/triblock copolymer temary blends, respectively. The simulation results showed

that diblock copolymers with longer A-blocks are more efficient as compatibilisers,

and symmetric triblock copolymers with a shorter middle block length are easily able

to bridge each other through the association of the end blocks.

1.5.3.2.4. Concentration of compatibiliser

It should be noted that the amount of added copolymer is one of the key factors

controlling the phase dispersion and phase dimensions in all type of

compatibilisation processes. In general, the dispersed particle size drastically

70 Chapter I

decreases up on addition of a few percentage copolymer, reaches a critical

concentration beyond that the copolymer has no interfacial compatibilising

efficiency since it (copolymer) usually dissolves in one of the phases and forms its

own micelles. This point where the emulsification curve shows a levelling off (plot of

dispersed particle size vs. compatibiliser concentration) is usually referred as CMC

which indicates 'interfacial saturation'. The shape of the emulsiiication curve is highly

dependent on surfactant type and on processing technique and is characterised by

an initial rapid drop in phase size followed by the attainment of an equilibrium value

at higher concentrations of modifier. The emulsification curve for polymer blend

systems can be potentially useful in evaluating the overall efficacy of an interfacial

modifier for a given blend system. It can indicate the point of interfacial saturation,

which is important for estimating the amount of modifier necessary for a given

system [407]. Another point that should be considered is that since both block and

graft copolymers are generally expensive, it is important to optimise their efficiency

so that only small amount is required. Figure 1.21 illustrates the characteriitii shape

of a typical emulsification curve [408].

.I

% * ~ . --

2 du l r i rn ) . e, (urn) 3 3 .,,

B , ' i, - e - i 4. d

\ critical rommtnliom - J%

i e 7 . ' \,

H , ',,. --..A a

, : . = -. . - - . I a ,

Figure 1.21: Emulsif~cation curve for PA6lABS 70130 blends prepared on the twin-screw extruder (Ref. 408).

The quantity of surfactant (compatibiliser) required to fully cover an interface is

related to variables such as the degree and type of mixing, the affinity of the

surfactant for the dispersed phase, the size of the dispersed phase, the rate of

adsorption and orientation of the surfactant at the interface and the extent to

which the interface is stabilised against flocculation and subsequent

coalescence. In classical oiltwater emulsions stabilised by surfactants, the

efficacy of the interfacial modifier for the interface is often characterised by

emulsification curves which essentially follow the evolution of dispersed phase

size with modifier concentration (4091.

1.5.3.2.5. Ratio of molar mass of blocks

Macosko and co-workers (3051 have investigated the morphology and the

aggregation of the PS-bPMMA as a function of MW and MW fraction. At low

MW PS-bPMMA, a PMMNPS-b-PMMA macrophase is formed. At higher ;i?, , small minor phase (PMMA) drops coated with copolymer are formed, but these

drops also containea micelles even at low PS-bPMMA concentration. Increasing

the MW of the PMMA first caused the drop size to increase, and then the

copolymer micelles to relocate from the PMMA to the PCHMA phase. Harrats

and Jer6me [I761 have studied the emulsifying ability of HPB-bPMMA pure

diblock copolymer in an LDPEIPMMA immiscible blend. The copolymer

efficiency was evaluated based on its ability to reduce the particle size of the

PMMA minor phase in the droplet-in-matrix phase morphology and to stabilise

against evolution ot the co-continuous one. It was demonstrated that the best

emulsifier, among the ones investigated in the study, was a copolymer

containing about 33W% of PMMA. This efficiency decreased when the PMMA

content increased (5(!-70%) while keeping the same total MW (100,000). In fact, a

copolymer having a MW of 34,000 and a good composition balance (33wP/0 PMMA)

was more efficient in terms of particle size reduction than a copolymer of the same

composition but of higher MW, particularly at low concentration (2wt%). This is

believed to be mainly due to a viscosity effect.

Eastmond and co-workers [348,349,410] have reported on the general effect of

the relative MW of the minor component (homopolymer) to that of the copolymer

block and copolymer concentration on the interfacial activity of gcps on

PCIPMMNPC-g-PMMA system. They have shown that the maximum interfacial

72 - Chapter 1

activity of the copolymer is achieved when the MW of the PC copolymer block is

significantly greater than that of the PC homopolymer. They have also studied

the effect of varying the homopolymer MW, corresponding to the minor phase,

on the interfacial activity of gcps in blends of homopolymers. The MW of the PC

homopolymer was increased from 3.6 to 52.4kg/mol, always maintained lower than

that of the PC block (143.8kglmol). MWs of all other components were kept

constant. Copolymer interfacial activity, in terms of its efficacy deteriorated

considerably with increasing PC homopolymer MW. They have concluded that the

relative MW of the PC homopolymer to that of the PC block controls the position of

the critical copolymer-homopolymer concentration relative to that of the overall

system. Besides the aforementioned factors the other important factors which

influence the compatibilising efficiency of the copolymer comprise order of addition

of the compatibiliser, blending conditions, interaction parameter balance and heat of

mixing, viscostty of the compatibiliser and the location of the copolymer [411-4131.

1.5.3.3. Disadvantages of physical compatibilisation

Even though physical compatibilisation technique is beneficial in terms of

scientific, technical and commercial point of view, there are some drawbacks

that should be considered seriously before employing this strategy to an

immiscible multiphase system. First, it should be noted that this technique often

requires a new product synthesis with expensive and time consuming process

development, and, hence, a significant number of years before profitability, since

scale of copolymer manufacture will be initially low. Secondly, adding the

copolymer as a separate species to the blend requires that the copolymer

diffuses to the phase interface of the immiscible polymers to be effective as a

compatibiliser which (diffusion) may not be efficient within the residence time of a

typical extrusion blendiny process (usually 2-5min). Further, high concentrations of

added copolymer may form micelles as a third, distinct phase that does not

contribute to compatibilisation. A third potential disadvantage is that for optimum

interfacial interaction a copolymer synthesised in a separate reaction step must

have carefully controlled segment lengths to best match the MW of the bulk

phase in which the segment must dissolve. It 1s often desired to offer a series of

commercial blends containing the same two polymers but with different MWs for

Introduction - 73

the polymers in each blend. Therefore, in recent years, more attention has been

focused on reactive compatibilisation technique, as it is very fast, easy and cost

effective alternative. However, it should be noted that from a fundamental point

of view, physical cornpatibilisation is considered to be more convenient. The

molecular characteristics of the pre-made copolymers are well defined and

therefore it is possible to make a judicious correlation between the chain

structure of the copolymer and its compatibilising efficiency. Note that the

copolymers formed during reactive blending are difficult to separate and

characterise.

1.5.3.4. Reactive compatibilisation

It should be emphasised that reactive compatibilisation is similar to physical

compatibilisation in some respects: (i) the compatibilising agent is expected to

be located at the interface between the phases (ii) in both cases,

cornpatibilisation results in morphology refinement and interfacial adhesion

enhancement and (iii) both methods lead to the design of polymer alloys with

attractive properties. However, as pointed out earlier, an obvious advantage of

reactive cornpatibilisation strategy is that copolymer is made only as required

and a separate copolymer commercialisation process need not be developed.

A second and most important advantage is that the copolymer is formed directly

at the interface where it can serve as a compatibiliser. A third advantage is that

the copolymer automatically has segment MWs similar to the MWs of the bulk

phases in which the segments must dissolve, which should promote optimum

interaction between copolymer and bulk phases.

The basic principle underlying reactive compatibilisation is that one can generate

graft or block copolymers in-situ by making use of the functionalities present in

one or more polymets, during meii processing as presented schematically in Fig.

1.22. These in-situ formed copolymers act as compatibilisers by reducing

interfacial tension, c!oalescence rate and improving interfacial adhesion. As a

result, the particle size is reduced, the ultimate mechanical properties are

improved and the blend retains its phase morphology upon thermal annealing or

re-extrusion and re.melting operations [414]. Reactive polymers undergo the

74 - Chapter 1

usual chemical reactions seen in low MW materials. However, there are a

number of factors that limit the extent of reaction [415]. Further, the reaction rate

can also be reduced by the restricted diffusional mobility of the functional groups

in the melt stage. In addition, the low concentration of the functional groups and

the short reaction time are two other limiting factors. Therefore in order to

achieve successful cornpatibilisation of polymer blends, the polymers must have

sufficiently reactive functional groups, the reaction should be fast, selective and

preferably irreversible and mixing conditior~s should be designed in such a way

to minimise mass transfer limitations to reaction. The different types of chemical

reactions that have been used in compatibilising polymer blends are listed in

Table 1.6. Among these reactions, four possible reaction schemes are generally

applied: chain cleavage and recombination reactions (transesterification), graft

copolymer formation, block copolymer formation and covalent cross-linking

reactions. The following section will provide an ove~ iew of different types of

reactive compatibilisation strategies.

Figure 1.22: Schematic of the interfacial reaction of reactive chains. Polymer chains on either side of the interface contain functional groups that can mutually react together, effectively forming a copolymer in-situ (Ref. 251).

Reactive Reaction type Co-reactive group Remarks

-- group

Amidation Carboxylic acid Amine Addition1 Substitution

lmidation Aiiydride Amine

Carboxylic Esterification Hydroxyl acidanhydride

Concerted MaleateIMA Double bond addition

Urea formation Carbodiimide Carboxylic acid

Urethane formation isocyanate Hydroxyl

Substitution Anline Hydroxyl, halide

Ester interchange Ester Ester Interchange reaction

Transesterification Ester HydroxyVphenol

Amide-ester Amide Ester exchange

Aminolysis Ester Amine

Amide Amide interchange Amide

Acidolysis t3er Carboxylic acid

Ring-opening Epoxide Carboxylic acid, MA. Ring.opening reaction reaction hydro@, amine

Ring-opening Carboxyiic acid. MA. Oxazoline reaction amine, halide

Ring-opening Lactam Amine reaction

lonic bonding Acid Pyridine, amine, imidazole ionic bonding

Ionic bonding lonorner lonomer -- - Table 1.6: Types of compatibilising reactions used in blending (Ref. 414)

1.5.3.4.1. Compatibilisation by redistribution to produce block and random copolymer

Redistribution reactions (sometimes referred to as 'Yransreactions") occur by

chemical interchange of block segments of one polymer chain for corresponding

segments of a second polymer chain. Such reactions may be homogeneous

(self-reaction) or heterogeneous. In the homogeneous case the MW distribution

of a polymer may reach equilibrium. In the heterogeneous case redistribution

reactions can forrn a copolymer between two different polymers. A common

feature of all redistribution processes is that the MW of at least one segment of

76 Chapter 1

the initially formed bcp is less than that of the bulk polymer phase from which it

is derived and therefore, the copolymer may not be as efficient a cornpatibiliser

as a similar type of bcp formed by an end-grouplend-group reaction. Table 1.7

shows typical possible interchange reactions during the melt blending of two

polyesters, two PAS and polyester and a PA. Several researchers have employed

this cornpatibilisation strategy in rnultiphase polymer systems [417-4431 and some

of the representative examples are given in Table 1.8.

Type of reaction -- Representation

Alcoholysis PI-O-CO-PZ+OH-P~ - PI-OH + PJ-O-CO-Pz

Transesterification P,-O-C:O-P2+P3-CO-O-Pn - PI-O-CO-P3+P2-CO-O-Pa

Aminolysis PI-NH-C:O-P2 + H2N - P3 PI P1NH2+P3NH-CO-Pz

Transimidation P,-NH-C:O-P2 + P3 40-NH-Pa - Pj-NH-CO-P3 +PrCO-NHPd

Acidolysis PI-O-CO-Pz+ HOOC-P3 - P,-O-CO-Pj+ HOOC-P2

Ester-amide PI-NH-CO-Pz+ P340-O-P4 - PI-NH-CO-P3+ P&O-O-P4 interchange

Table 1.7: interchange reactions between polycondensates (Ref. 416).

-- System Characterisation Resultrlconclusion Ref. PAW Mechanical. DMTA. EHects of annealing and trans amidation levels 417. PMXDPE DSC. NMR on ~rowlties have been studied. Blends were 41 8

hom~eneous due to interchange reactions. PA46lPA61 Injection moulding, Transamidation process occurred altered the 426,

DSC melting and crystallisation behaviour. 427 PClPET Phase contrast The transesterification product, rcp. 438

microscope.DSC decreased the dispersed phase size. PPS LCP Tensile, impact Crystallinity marginally decreased, tensile 439

SEM and impact properties improved. PBTlPU NMR FTIR. DSC. PBT-PU copolymer is formed by amlde-ester 440

DMA. SEM, tens le mterchange reaction, mechan~cal properties improved.

PA61 FTIR,NMR.DSC,SE Evidences of the occurrence of interchange Vectra M, mechanical reactions are given. PAGIPC. TGA Thermal stability improved. PAGIPCI PPO PAWPET DSC. XRD. DMA, Mechanical properties improved.

WR. SEM, impact --

Table 1.8: Representative examples of compatibilisation of blends through redistribution reactions

Introduction 77

1.5.3.4.2. Compatibilisation by grafi copolymer formation

Gcp formation (Fig. 1.23), which involves reaction between one polymer

containing reactive sites along its main chain and a second polymer with reactive

sites only at end-groups, is the most widely employed method of forming a

c~mpatibi~sing copolymer during reactive cornpatibilisation. Generally, four types of

basic chemical reactions are employed for the formation of in-situ graft copolymer.

These comprise: (i) amine-cabxylic acid (ii) amine-anhydride (iii) amine-epoxide

and (iv) carboxylic acid-epoxide reactions.

(i) Amine-carboxylic acid reaction

A representative example of schematic of interfacial chemical reaction of SAN-

amine with bisphenol -A- polycabnate to form SAN-g-PC is given in Fig. 1.23.

Interfacial chemical reaction between amine end group and cabxylic acid group

leading to graft copolymer fonation has been employed for cornpatibilisation by

several researchers 1444-4681 and a few recent representative examples are

presented in Table 1 9 .

PC 2 +- t - 4 - 8 ----r * ,,. , $ dn,cn, n - c - o L

\ .~t CHIC"> l(T'NH

i i PC

SA".ilrni"~ P r n n a t g .aw%won

Figure 1.23: Reaction of SAN-amine with PC to form SAN-g-PC [Ref. 4591

PAG/PVOF/PVDF-MA SEM, hwiogy ~&hanki ope& improved 458 PCISANJSAN-amine TEM Relarive mpnkrse of re&e 459, (32.5% AN) mmpatlbiIkWm and matrix phase vismsv 460

on me morphdqlcal Wlity was st& PA10101LLDPUPE-q-AA SEM. DSC Three types of morphologies for 46 1 -

compatibilised blends. PAGILOPULOPE-pA.4 TSC, DMA. SAXS Compatibiliser shined the Tg. interfacial 462

thickness was calculated PAGIABYIAA TEM.DSC,impact Impact strength improved. 463

465 ~. PAGIHDPEIEAA Permeation. SEM Pain1 sotvent permeation improved, 466 PAGILDPUEAA DSC, SEM. intetlaciai adheston improved. rate of 467

rheology, FTIR coalescence reduced PA6lPVDFPiMMAco- DSC, WAXS, DMA. Heat treatment improved compatibility. 468 MMA) SEM. tensile resuted in a significant increase in the

-- energy 01 rupture in tensile testing.

Table 1.9: Representative examples tor compatibilisation through amine- carboxylic acid reaction

78 Chapter 1

(ii) Amine-anhydride reaction

This reaction (involving maleic anhydride, MA) has been the most widely used

one mainly due to the fact that it is relatively easy to graft MA on to many polymers

at normal processing temperature without homo polymerisation (Fig. 1.24). Large

number of researchers have used this reaction route for the compatibilisation of

multiphase systems since 1980's [469-5371. A brief review of the important studies

appeared in last few years on reactive compatibilisation of polymer blends involving

amine-anhydride reaction has been presented in Table 1 .lo.

Figure 1.24: Scheme of interface grafting by reaction between a carboxyl group of maleic anhydride and a polyamide amino end group (Ref. 526).

System Characterisation Resultslconclusions Ref.

Aromoatic PA copolymer/PP/PP-g-MA

fracture energy (G,), TEM, DSC, ESCA

DSC, SEM, mechanical, DM A

MFI, rheology, SEM

Mechanical, SEM, rheology, FTlR

NMR, SEM. DSC, mechanical, DMA

DSC, DMA, permeation, contact angle

TEM, DMA, mechanical

FTIR, NMR, SEM, rheology. mechanical

TEM, DMA. impact

Mechanical properties

When PP forms conlinuous phase, PA domains alters crack propagation path leading to increase in G,, when PA forms the matrix, crack propagates by jumbing between PP domains.

Impact strength improved significantly

Uniformity of phase structure ot the blends improved

Reduced particle size. improved impact strength

Impact and fractirre properties improved.

Compatibilised blends were transparent, slerilisable. dimensionally stable, which have very high barrier properties.

Impact strength improved, HDT increased

Mechanical properties improved

Particle in particle morphology of dispersed phase has been studied

Mechanical properties improved, Compared the efficiency of different compatibilisers

.- - XRD, FTRarnan, DSC Compatibilised Mends are well

otiented during melting than uncmpatibilised blends

Palierne emulsihn type model was used to correlate between the meology and morphology.

Morphology is refined, interfacial chemical reaction is characterised.

Power law relation was used to measure interfacial tension. SEES-g-MA was most efficient

Rheoiogy. SEM, TEM

FTIR. DSC, SEM. TEM,

PAWPP/PPMA,EPR-g MA. SEBS-9-MA

PAGIPSIPS-g-MA

Steady state torque, SEM

SEM. FTIR, DSC, impast Dispersed partrcle size decreased, crystallisation behaviour atfected

Toughness increased nith the annealing time, passed a peak, and reached a plateau.

Toughness. XRD

DSC, DMTh TEM Dtsperseo pamcle s ze oecreased cwslaillsanon behaviour is affected

Broadband dielectric spectroscopy

tlnmodified blend absorbed more water than the compa~D~11sW b end lne segmental rlynam ss are unanected.

Tensile and impact properties improved.

PP dispersion became finer and good interfacial bonding between PA6 and PP.

The strength and stiffness, storage (G') and loss moduli (G) and apparent shear viscosity of the PAWPP increased.

The dispersion of PA 1212 in the PS matrix and mechanical properties improved.

The crystallisation behaviour of nylon 11 and PE, improved cmpatibility.

Impact and tensile strengths increased, coalescence decreased.

SEM, mechanical

AFM. TEM

SEM. TEM. XRD. DMTA. rheology

SEM, DSC, mechanical

DSC, SEM, hOl09\

ATR-IR. Impact, tensile, SEM

Table 1.10: Representative examples for compatibilisation through amine- carboxylic acid reaction

(iiv Amine-epoxide reaction

Several researchers have employed this reaction route for the compatibilisation

of polymer blends [538-5451. The Schematic representation is given in Fig. 1.25.

Some representative examples are presented in Table 1.1 1

Figure 1.25: Scheme ot reaction between an amine and an epoxide groups (Ref. 5431

Svstem Characterisation Ref. --

PAIO~O/PP/PP-g- Viscometer, SEM, GMA Tensile, Flexural

PSIPPOISGMA Rheology, SEM, DMA, mechanical

PA6,10,11,(6,10), FTIR,SEM,DSC,DMA, (6,12)/ PE-g-GMA tensile

PA101O/PPl ESCA, FTIR, SEM, PP-g-GMA TEM. SAXS,

mechanical

PA6JABSI TEM, Impact, rheology SAN-GMA

PA6lPPl DSC, SEM, mechanical PP-g-GMA

PAGILDPEIEGMA SEM, DSC, solvent fractionation

No significant change in phase inversion point, dispersed particle size reduced, mechanical properties improved.

D~spersed particle size reduced, viscosity increased, mechanical propelties improved

Most efficient grafting occurred i~ PA1 IIPE-g-GMA, copolymer formation was confirmed by solvent extraction.

C:opolymer formation is confirmed. Fracture energy ir~creased, mechanical properties improved, dispersed particle size decreased.

Reactive nature of the compatibiliser on morphology and toughening was studied.

Compatibilising efficiency was compared with PP-g-MA and found to be lower.

PA-g-EGMA compatibiliser was separated by solvent extraction method and the composition has been determined.

Table 1.11: Representative examples for compatibilisation through amine- epoxide reaction

(iv) Carboxylic acid-epoxide reaction

Even though rlot widely used, researchers have used carboxylic acid-epoxide

reaction for the compatibilisation of immiscible blend systems [546-5491. The

Schematic representation is given in Fig. 1.26. Some representative examples

are given in Table I.>>!

Figure 1.26: Scheme of reaction between a carboxylic and an epoxide groups (Ref. 5471

-- System Characterisstion Resultslconclusion Ref.

PE-r-ANPS-g- SEM, TEM, Dispersed particle size reduced 546 GMNPS, PP-g- DMTA, significantly in PE-r-ANPS-g- AAIPS-g- rheology GMNPS but not in PP-g-ANPS-g- GMNPS GMNPS due to existence of an

inhomogeneous distribution of AA functional units in PP-g-AA and the PAA homopolymer.

PE-AA/PS/PS- SEM, Dispersed particle size reduced 547 GMA. PBT/PS- rt\eology significantly in both systems, reduced PS-GAM domain size with PS-GMA contents

was collapsed into one master curve regardless of blend compositions in each blend system.

PAGIABSISAN- TEM, Impact, The importance of the reactive 543 GMA rheology nature of the compatibilising species

on blend morphology and toughening capacity was studied.

PP-g-AA/PS/P SEM, rheology The existence of PAA significantly 548 SGMAwith and affects the rheology and morphology.

without PAA

PBTIABSfMMA- TEM, DSC Evidence for carboxyl/epoxy reaction 549 GMA-EA) Impact is presented, morphology is

stabilised, toughness improved. - Table 1.12: Representative examples for compatibilisation through carboxylic

acid-epoxide reaction

82 Chapter 1

1.5.3.4.3. Compatibilisation by block copolymer formation

Generally, bcps are formed by four different routes:

(i) By the direct interfacial chemical reaction between the functionalised end-

groups on some fraction of chains in each of the polymers across a melt

phase boundary. It is believed that the average MW of the copolymer

corresponds to the sum of the average MWs of the reacting polymers.

(ii) By reaction of the end-group on one polymer with a condensing agent

(phosphate esters) that activates the end-group for reaction with a

nucleophilic end-group on a second immiscible polymer. In this case, a by-

product from the condensing agent is always formed in the copolymer

reaction and is often removed by devolatisation of the blend melt.

(iii) By the reaction of end-groups on each of the immiscible polymers with a

coupling agent (multifunctional epoxy resins, oxazolines, carbodiimides, and

isocyanates).

(iv) By a degradative process in which end-groups on one polymer undergo

transreaction with linkages in the main chain of a second, immiscible

polymer. The by-product formed is generally, a low MW fragment of the

second polymer. The bcp has lower average MW than the sum of the

average MWs of the reactants.

Several researchers have reported reactive compatibilisation technique through

the bcp formation. Bcp formation may be due to amine-carboxylic acid reaction

(550-5531, carboxylic acid-epoxide reaction [554-5581, amine-anhydride reaction

[559-5851 or alcohol-carboxylic acid reaction [586-5901.

1.5.3.4.4. Compatibilisation by covalent cross-linking reaction

The cross-linking reactions have been performed by at least four methods:

1. Direct cross-linking by covalent bond formation between functionalities on

each of the two immiscible polymers, without degradation of either polymer.

Common examples include reactions of pendent acid or amine or alcohol

nucleophiles on one functionalised polymer with pendent electrophilic groups

such as epoxide 1591-6001, oxazoline [494,601-6061, or anhydride ortho

ester (607-61 I ] or1 a second functionalised polymer.

Introduction 83

2. Direct cross-linking by covalent bond formation arising from radical

generation and recombination (mechano-chemical radical generation, during

extrusion) (612-6161. When two immiscible polymers form radicals on their

main chains in the absence of added radical initiator, a copolymer may form

by recombination at the phase interface of radical sites on the two polymers.

3. Cross-linking mediated by a third, added reagent that serves as a coupling,

condenstng, or activating agent.

4. Ionic cross-linking: The ionisable groups are at least partially neutralised by

a mono-, di-, or tri-valent metal cation, such as multivalent cations (2n2+, or

A?') may form a bridging linkage between the ionisable groups of the two

immiscible polymers resulting in interchain copolymer formation by ion-ion

association. Mono-valent cations (Na' or Kt) may also be used to promote

association through ion-dipole association. With either type of cations,

morphology is formed in which there are concentrated domains of

associated ionic species (ion clusters) in a matrix of the immiscible

homopolymers Generally, ionic cross-linking comprises three mechanisms

(a) ion-ion association mediated by metal cations 1617-6221, (b) ion-neutral

donor group association mediated by metal cations (623-6261 and (c) ionic

association mediated by interchain orotonation 1627-6291.

It should be noted that besides, the aforementioned routes there are still some

miscellaneous techniques which provide compatibilising agents during

processing through interfacial chemical reactions (630-6421. In addition, there are

some other established methods widely used for compatibilising immiscible polymer

systems. These include: (a) dynamic vulcanisation (b) addition of a third polymer

(partially) miscible with all blend phases (c) addition of reactive fillers as

compatibilisers (d) addition of low molecular weight compounds and (e) solid

state shear pulverisation (SSSP).

(a) Dynamic vulcanisation

Dynamic vulcanisation is a cost effective technique to achieve better properties

which involves vulcanising the elastomer during its intimate melt mixing with a

non-vulcanising thermoplastic polymer. Improvements in properties include

reduced permanent set, improved ultimate mechanical properties, fatigue

resistance and high temperature utility, greater fluid resistance, melt strength

and phase stability and more reliable thermoplastic fabricability. The introduction

of cross-links into one of the phases increases the viscosity of this phase leading

to a change in the morphology of the blend [643,644]. Because dynamic

vulcanisation takes place inside the mixer, the mix cycle should be as short as

possible, for economic reasons. Several researchers have successfully

employed this technique for the preparation of multiphase polymer systems with

improved performance [645-6801.

(b) Addition of a third polymer (partially) miscible with all blend phases

According to this concept a polymer C: (partially) miscible with the two

constitutive polymers (A and 6) of a two phase binary blend can act as a

common solvent and accordingly can promote complete or partial miscibility of

the originally immiscible polymers. The choice of polymers A, B and C is such

that the binary interaction parameter is negative for the A/C and BIC pairs and

positive for the A/B pair. Lee and Chen 16811 have investigated tricomponent

blends of PVCICPEJEPDM where EPDM is the very low temperature impact

modifier and CPE is the compatibiliser, because of its similarity in chemical

structure with PVC and EPDM.

(c) Addition of reactive fillers

This compatibilisation strategy is similar to the one described earlier using a third

polymer. Shifrin et al. [6823 have proposed that a two phase polyblend can be

compatibilised by the addition of a mineral filler. The selective localisation of the

filler at the interface is a pre-requisite, which is fulfilled (or not) depending on the

balance of interaction between the filler and each constitutive polymer 16831. In

the case of a comparable affinity of the filler for each blended polymer, the filler

may be expected to accumulate in the interfacial region and provide the

immiscible polyblend with an enhanced stability.

(d) Low molecular weight substance

This is a completely different strategy for polymer blend compatibilisation which

comprises the addition of a (mixture of) low MW chemicals such as peroxide and

multifunctional chemicals. The actual compatibiliser, a branched, block or graft

copolymer, is formed during a reactive blending process. Various procedures

may be distinguished depending on the added chemicals. Sato and co-workers

[684,685] have found that addition of small amount of maleic anhydride

produced profound dierence in molecular structure, crystallinity and

morphology of nylon 121PE blends.

(e) Solid state shear pulverisation

This is a recently developed technique [686-693) and is done at temperatures

below the melt transition (semicrystalline polymers) or glass transition

(amorphous polymers). Polymer pellets or flakes undergo compression and at

high shear/extensional conditions, resulting in fragmentationtfracture into a fine

powder without melt~ng (689-6911. Depending on SSSP conditions, chain

scission may occur, y~elding highly reactive free radicals at chain ends. Radical

combination at interfaces and in highly mixed regions may yield bcps (6921 like

those achieved with reactive compatibilisation during melt mixing of

functionalised polymers.

1.5.3.5. Factors affecting the efficiency of reactive compatibilisation

It should be noted that the action of reactive compatibiliser at the interface

between two immiscible polymer blends is similar to that of a pre-made

copolymer and therefore most of the factors which influence the physical

compatibilisation are also valid in this case also. Besides, the other important

factors which influence the efficiency of a reactive compatibiliser include (a) the

reactive group content and end-group configuration (b) effect of miscibility of the

reactive compatibiliser with one of the phases and (c) the stability of the

copolymer at the interface.

1.5.3.5.1, Reactive group content and end-group configuration

The reactive group content will affect the extent of the interfacial chemical

reaction and the molecular architecture of the in-situ formed compatibiliser. By

increasing the amount of the reactive groups on a reactive polymer from one to

multiple groups, the structure of the graft changes from a single to a multi-graft

comp like structure. At the same time, the extent of interfacial reaction which is a

86 Chapter 1

function of the content of the reactive groups available will affect the stability of

the formed copolymer a1 the interface as well as its physical entanglement with

the compatibilised phases. Paul and co-workers [456,464,486,495,498,499,508,

509,543,694-6971 have performed extensive studies on the influence of reactive

group content and end group configuration of homopolymer and compatibilisers

on the compatibilising efficiency of copolymers.

Oshinski et al. [486] have reported that PA6 is monofunctional and PA66 is

difunctional in their reactions with anhydride group and this difference can make

a marked difference in morphology. In another study, Kudua et al. [464,543]

have shown that the grafting reactions which can occur between the

compatibilising species and the blend components have paramount importance

in the morphology and properties of immiscible blends. These authors have also

shown that the difunctional nature of the PA6 matrix with respect to the

compatibiliser leads to a poor dispersion of rubber particles in PA6IABS blends.

Gonzalez-Montiel et al. [498,499,697] studied the effect of MA content on the

morphology of PA6IPP,'PP-g-MA blends and found that the size and size

distribution of PP depended on the content of MA in PP-g-MA and on the extent of

miscibility between PP and PP-g-MA. Majumdar et al. [695] have shown that the

extent of graft reaction between SEES-g-MA and members of the nylon x,y series

increases as the CHdNHCO content increases. These authors have also reported

that a very high level of reactive functionality on the compatibiliser adversely affect

the compatibilising efficiency [696]. In another study, Majumdar et al. [495] have

shown that there should be an optimum level of functionality on the copolymer for

maximum performance. Loyens and Groeninckx [698] used different

compatibilising agents to obtain a morphology refinement of the PETIEPR blend

system and established that the GMA-induced compatibilisation reaction

(E-GMAB, E-GMAl2 or EPR-g-GMA1.5) was much more effective than the

reaction between MA (EPR-g-MA0.6) and the PET hydroxyl end groups.

Groeninckx and co-workers [699] have performed the quantitative analysis of the

interfacial thickness and the interfacial reaction and reported that the interfacial

layer thickness of the bilayer system PA12JSMA increased as a function of the

Introduction 87

MA content in SMA. At higher MA content, the interfacial layer thickness

decreases again. It was also shown that the % reacted MA groups (a) increased

as a function of the MW of PA6 in PAGIPSISMA blends (b) as the % dispersed

phase decreases, the particle size being independent of the % dispersed phase

and the % reacted MA groups decreased as the concentration of SMA2 is

increased.

1.5.3.5.2. Effect of miscibility of the reactive compatibiliser with one of the phases

The level of miscibility of the precursor with the phases will certainly affect the

final location of the in-situ formed copolymer during reactive compatibilisation

and can strongly influence its efficiency [495]. It is expected that reactive

compatibiliser that is fully miscible with the phase where it is initially incorporated

can provide the most efficient particle size reduction through interiacial tension

reduction and enhanced interfacial stabilisation (Fig. 1.27a). If the compatibiliser

would show thermodynamic affinity to one of the phases, but is not miscible with

it, an interlayer can be formed at the interface of the blend phases. Note that the

compatibilisation in this situation is not expected to be as effective in

coalescence suppression as in the formed case (Fig. 1.27b). At the same time, a

very low affinity of the compatibiliser to one of the phases can cause the

migration of it to the other phase (Fig. 1.27~). However, it ca be assumed that in

reality, the three above scenarios co-exist in a real reactively compatibilised

blends as illustrated in Fig. 1.27d.

Groeninckx and co-workers (7001 have studied the effect of reactive

compatibilisation on the morphology development and stabilisation of PA61

PMMAISMA blends and stated that the mechanical stress exerted by the screws

on the PA side chains of the gcp (PA-6-g-SMA) tends to remove the gcp from

the interface and to coarsen the morphology at long extrusion times. This is

counteracted by thermodynamic forces resulting from the miscibility of the SMA

main chain sequences with PMMA. It is the miscibility of reactive SMA

compatibiliser with the PMMA phase that determines the phase stability in the

blend at long extrusion times.

88 - Chapter 1

Figure 1.27: Schematic representation of possible modes of location of graft copolymers in a reactively compatibilised blend depending on the level of its interaction with the dispersed phase (Ref. 495).

1.5.3.5.3. Stability of the copolymer at the interface

It is well known that the location of the compatibiliser at the interface between

the individual phase of a blend is a prime and necessary requirement for an

efficient compatibilisation and phase morphology stabilisation upon further re-

processing or thermal annealing. Thus the ability of the compatibiliser (either

physical or reactive) to stabilise the developed morphology can be used as an

additional criterion for the evaluation of compatibilisation efficiency. For example,

a co-continuous morphology exhibits an intrinsic instability when subjected to

further re-processing or thermal annealing [310,326,701-706). Indeed, a

copolymer might be able to locate at the interface during the first minutes of the

compounding process reducing efficiently the size of the dispersed particles and

preventing their coalescence. The in-situ generated copolymer should not leave

the interface upon further melt processing.

Charoensirjsomboon et al. 17071 found that in-situ formed PSU-PA bcps via the

reaction of PSU-g-MA with PA6 amine end groups were pulled out from the

interface by the external shearing forces during melt mixing. The pull out of the

compatibiliser from the interface depends on the accumulation of the copolymer

chains at the interface, their molecular architecture and the processing

conditions (707,7081. It may be believed that gcps are more stable at the interface

on account of the more branched structure. Groeninckx and co-workers [99,709],

however, observed that this mechanism also occurs for gcps. An increase of PA6

particle size in PA6I(PPE/SMAB) compatibilised blends was reported with

increasing mixing time, explained by the expelling of formed PA6-g-SMA8

copolymer from the interface under influence of the strong forces (high viscous

PPE matrix phase). A similar result was found by Thomas and Groeninckx [710]

for PA6lEPMlEPM-g-MA blends. Recently, Harrats et al. [711] have investigated

the behaviour and structural stability of the blends exhibiting a co-continuous

phase morphology when the compatibiliser is generated and found that

significant differences were found in relation to the MA content of the PP-g- MA

reactive compatibiliser precursor.

1.5.4. Recent developments in polymer blends

Before concluding this chapter, it is important to mention the recent

developments in polymer blends, since as this chapter begins with stating that

polymer blends have gained significant commercial growth in the last two

decades outpacing the growth rate of existing polymers by at least 2 to 5%. The

important renaissance in recent times are mainly because of the developments

of microfibrillar composites (MFC) [712-7151, electrically conducting polymer

blends (716-7181. nanostructured polymer blends [719-7451, bio-degradable

polymer blends [746,747], high temperature polymer blends [746] and polymer

blends as bio-materials. However, we would like to mention only nanostructured

polymer blends not only because it is more related to the present topic of

investigation but also due to the fact that the present investigation (second part)

will be extended in such a way that nanostructured polymer blends could be

derived in future.

Nanostructured blends very often exhibit unique properties, which are directly

attributed to the presence of structural entities having dimensions in the nano

meter range. The idealized morphology of these polymer blend systems is

characterised by the ~nolecular level dispersion of the phases, which leads to a

considerable enhancement in the mechanical, electrical and optical properties.

90 Chapter 1

Nanostructured polymer blends can be prepared by reactive blending (in-situ

polymerisation and in-situ compatibilisation), micro-emulsion polymerisation,

solvent casting, spin casting, controlled evaporation or thermal treatment of

initially miscible system. The important characterisation techniques include AFM,

TEM, XRD and time resolved SAXS. Figures 1.28 and 1.29 illustrate the nano

structure developed during reactive extrusion of PA12 by a reactive triblock

copolymer [729].

Figure 1.28: Core shell morphology of nanophases formed by the in-situ compatib~lisation PA1 2 by a reactive triblock copolymer (Ref. 729).

Figure 1.29: Sketch of the internal structure of the vesicular shell of PS-b-PIP- b-PA in the core shell morphology of nanophases (Ref. 729).

It is well established that bcps self-assemble to form a variety of morphologies

such as spherical, cylindrical, lamellar, and gyroid phase [748,749]. Furthermore,

blending bcps with thermoplastic homopolymers have been widely employed to

produce polymeric materials with different nanoscale structures 17501. However,

it should be noted that the preparation of nanostructured polymer blends for

Introduction 91

immiscible polymers, with a phase size of less than 100nm, is very challenging

using normal processing methods currently available. Very recently,

nanostructured blends have been produced from block copolymers by using

conventional melt processing [727-7291 but the method shows obvious limitation

for the practical application. Therefore the main challenge in front of polymer

scientists as far as nanostructured polymer blends are concerned is to design an

easy, economic and efficient method to develop polymer blends with

nanostructured morphology as these materials find a wide range of applications.

For example, Leibler and co-workers [740] have recently developed

nanostructured transparent PSISBS blends that can replace PS, which is often

too brittle and HIPS, which is opaque in packaging applications. The TEM image

of these toughened PS is given in Fig. 1.30.

Figure 1.30: Toughened nanostructured PS with SBS (Ref. 740).

Finally, it can be concluded that there is great future and scope in the field of

multjphase polymer blends, especially in compatibilised systems as polymer

blending offers an extraordinary rich range of new materials with enhanced

characteristics regarding mechanical, chemical, or optical pedormances.

However, it should be noted that compatibilisation in the most cases is very

essential since most of the polymer pairs are immiscible and incompatible. With

respect to ultimate properties, there are, unfortunately, two extremes as George

Bernard Shaw told Greta Garbo when she expressed her desire to engender

with him a child that would combine beauty and brains: (a) the blends with

jnferior properties compared to the individual component and (b) the blends with

92 Chapter 1

superior properties compared to the individual component. Uncompatibilised

blends come under the former category while compatibilisation is required to

satisfy the other extreme. It is also important to mention that since the ultimate

properties depend on the interfacial situation that in most cases is determined by

the morphology which again is decided by two counteracting processes, drop

break-up and coalescence, all these factors are critical in designing a multiphase

polymer systems with attractive properties. The mechanism of compatibilisation

(whether physical or reactive) mainly comprises interfacial tension reduction and

coalescence suppression. At the same time, interfacial adhesion, which is

essential for efficient stress transfer between the components, enhanced. All

these factors tend to stabilise the morphology which in turn result in superior

blend properties. In physical cornpatibilisation, one can use pre-made block,

graft or random copolymers where as in reactive compatibilisation graft or block

copolymers can be generated in-situ by making use of the functionalities present

in one or more polymers, during melt processing. Both these processes are

affected by several factors and all these aspects are discussed with

representative examples in each case.

1.6. Scope and objectives of the work

The present study is entirely devoted to investigate the effect of compatibilisation

on polymer blends. In order to have a complete understanding of cornpatibilisation,

both physical and reactive compatibilisation techniques have been employed. The

work has been designed in such a way that the thesis is divided into two parts.

The first part of the study is committed to the physical compatibilisation of

polypropylenelhigh der~sity polyethylene (PPMDPE) blends. The second part is

dedicated to the reactive compatibilisation of polyamide 12 (PA12)/PP blends.

Before going into details we would like to explain why we select plastic/plastii

multiphase systems for cornpatibilisation studies and to justify the selection of

component polymers, as out of three components two are commodity plastics and

third is not strictly an engineering thermoplastic. This is important since

researchers are usually more interested in rubberlplastic systems to obtain

thermoplastic elastomer to minimise the gap between elastomer and plastics or

other wise interested in engineering thermoplastics to obtain high performance

polymeric materials. Note that out of 16 Ph. D theses awarded from our research

group in this field, 15 deal with rubberfplastic or rubberlmbber blends. The other

one deals with high performance engineering thermoseVthermoplastic blends. In

order to justtfy our selection of plasticlplastic system we would like to quote a

sentence in the recently published and interesting review article by Leibler in

Progress in Polymer Science 17411: 'there is great future in plastics'. He justified

this statement by presenting the statistics. In 1967, about 14 millions metric tons of

plastics have been produced. Thirty years later some 200 millions metric tons

were produced and the volume of plastics produced exceeded that of steel. Their

(plastics) lightweight opens more and again new applications, the most obvious

being those in airspace or in packaging industry. Few materials can match the

versatility and economy of modern plastics. Note the fact that at present there is

no 'other material' that can replace plastics and it is almost certain that that new

material cannot be generated in near future!

Now why we went for commodity plastics? In 1975, Chemical Company made a

prediction that the majority of commodity plastics, plastics made from cheap easily

available monomers (PE, PS. PP and PVC) which occupied 82% of the market in

1975 will be replaced in 1995 by engineering and speciality polymers and these

three categories of plastics will equally share the market. Yet, in reality in 1997,

engineering plastics (PAS, PET, polyacrylics) accounted for 18% of the market,

exactly the same share as in 1975 and speciality polymers (LCP. PEEK, etc.)

'conquered' less than 1% of the market, as they were 20 years before. Hence, the

commodii polymers proved themselves as workhorse materials since the 1960s

impossible to displace (thanks to their low cost and versatility!). It tells us (i)

superior properties do not always grant or mean market success (ii) cost and

availability are important and (iii) working with simple (old) monomers and

materials can be both challenging and rewarding.

Polyolefins are by far the most important class of commodity plastics 17511. Most

of the commercial applications of PE, PP or their copolymeffi are determined by

their physical and mechanical properties, the ease of manufacturing and

94 Chapter I

processing which mostly depends on the composition and architecture of the

polymer. Hitherto, polyolefins production has been largely limited to homopolymers

and random copolymers mainly due to restrictions imposed by Ziegler-Natta or

even metallocene polymerisation methods. Over the last half decade a marvellous

progress has been made in living synthesis of polyolefins [752-7541.

It is also noteworthy that polyolefins constitute the major polymeric components of

plastic wastes and their separation in to individual polymers is costly and complete

sorting is some times impossible. However, these can be recycled easily by

converting into the form of polymer blends and this will provide greatly increased

motivation for improving the properties of polyolefin blends. At the same time, the

combination of PE and PF' is particularly important because it is very difficult to

separate these polymers from each other in waste recovery operations. The

difficulty arises due to the structural similarity of these polymers. PPlPE blends are

commercially very important because of their high impact strength and low

temperature toughness. Addition of PE into PP increases the impact strength of

PP and addition of PP into PE improves the environmental stress crack resistance

of PE. Hence these blends are technologically very important. The quantum of

research work on this system itself justifies their importance.

A careful and systematic survey revealed that nearly around 200 research articles

have been published to investigate various properties of PEIPP blends 1755-9341.

Note that PE refers all types of polyethylenes such as HDPE, LDPE, LLDPE, VLDPE,

UHMWPE, etc. The important studies include the morphology, mechanical properties,

rheology and crystallisation behaviour. A number of researchers have reported on the

compatibilisation of these blends. However, we could hardly seen any serious and

systematic study to analyse how the morphological changes derived in presence of

compatibiliers affect and other blend properties such as mechanical, dynamic

mechanical, ctystallisation and thermal properties of these blends. This is very

important since it is the nwrphology that determines the ultimate properties and a

change in morphology wo~~ld result in change in properties. At the same time, to the

best of our knowledge, no research has been done to investigate the effect of

composlin on the compatibilising efficiency of copolymers in PUPP system. In this

research work we mainly concentrate on these two 'missing aspects'. Our prime

objective is to investigate the effect of ethylene propylene random copolymers (EPDM)

on the morphology and there by on the other properties of PPPE blends. However,

the main attraction of t h i study is that we have used three different EPDM rubbers

(with different ethylenelpropylene ratio, f) as compatibiliers to analyse how the

symmetry of these copolymers (in terms of the composition) affects the compatibiliing

efficimy of these copolymers. In this respect, there is great scope to put forth a novel

model to explain the compatibilising efficiency of random copolymers. The efficiency of

the cornpatiilisers has been mainly estimated in terms of the refinement of

morphology. One of the motivations behind this work is the interesting finding of Dai et

al. [240] in which they have shown that long random copolymers with symmetric

monomer fraction, f -. 0.5, is more effective than those with an asymmetric monomer

fraction f - 0.8, in strengthening the interlaces between immkible homopolymers,

and also more effective than diblock copolymers. However, Kramer and cc-wotkers

have studied the effect of compositional drift during the synthesis of random

copolymers on their compatibilising efficiency [398]. In this point of view we have

carried out extensive investigations on PPMDPE blends using random copolymers

with dierent symmetry in order to find out whether the symmetry of the random

copolymers affects its compatibiliing efficiency in these blends. We selected two

blends for compatibilisation: one with matriddroplet morphology and the other with co-

continuous morphology. Regarding the matriddroplet morphology, we are more

interested in blends w~th PP matrix, since PP possesses more strength, environmental

stress crack resistance, thermal stability, transparency and light weight. Additinally, it

is one of the components in the second system under consideration.

The motivation behirid the reactive compatibilisation of PA12JPP blends is mainly

the fact that physical compatibilisation alone cannot provide a complete study on a

subject like compatibilisation. Further, we are also interested to know how different

compatibilisation strategies differ in aspects such as properties, performance, cost,

processability, etc. However, note that a complete comparison study was not our

intention for which same system should be selected for different compatibilisation.

Therefore we selecled PAIZIPP blends so that PP is still there and a functionalised

polymer, which does not differ considerably with HDPE. Unlike, PPlHDPE blends,

96 Chapter 1

we could find only a few studies on PA121PP blends [935-9401. None of them didn't

provide a systematic study on morphology/property/pmessing relationships in the

presence and absence of compatibiliser.

PA12 has several advantages compared to other PAS, for example, relative low

melting point which is comparable to PP, low water absorption, etc. and polyesters

since amine functionality avoids the complexity of the interfacial chemical reaction.

In terms of technological point of view also, we can justify the selection of these

blends. PA12 exhibits excellent mechanical (including impact) and thermal

properties, but is expensive, while PP is one of the cheapest and lightest

commodity thermoplastics w~th good strength and solvent resistance but poor

impact properties. Blending these two polymers would lead to a new cost effective

polymeric material with good mechanical and thermal properties coupled with

excellent solvent resistance. However, these blends are highly immiscible and

incompatible owing to the unfavourable intelfacial interactions. PP-g-MA is

selected as compatibiliser precursor as it is miscible with PP phase of the blend

and can facilitate interfacial chemical reaction. In this case, we selected three

blends for compatibilisation: two with dispersed phase morphology (70130 and

30170 compositions) and one with co-continuous morphology.

Precise objectives of the work

P Preparation of PPtHDPE and PA12/PP blends in the presence and

absence of compatibilisers

> Evaluation of phase morphology of the blends

> Study of the mechanical and dynamic mechanical properties of both

compatibilised and uncompatibilised blends

i Analysis of the effects of blend ratio and compatibilisers on the thermal

and crystallisation properties of the blends

> Investigation of the effect of blend ratio and compatibilisers on the

rheology of the blends

> Correlation of the mechanical, viscoelastic, thermal, crystallisation and

rheological properties of the blends with the phase morphology in

presence and absence of compatibilisers

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