influence of molecular structure on crystallization behaviour and mechanical properties of...

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Polymer Testing 14 (1995) 173-187 0 1995 Elsevier Science Limited

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Influence of Molecular Structure on Crystallization Behaviour and Mechanical Properties of Polypropylene*

Markus Gahleitner, Klaus Bernreitner, Wolfgang Neil31

PCD Polymere GmbH, St. Peterstr. 25, A-4021 Linz, Austria

Christian Paulik & Ewa Ratajski

Johannes Kepler University Linz, Institute of Chemistry,

Altenbergerstr. 69, A-4040 Linz, Austria

(Received 1 June 1994; accepted 5 July 1994)

ABSTRACT

Two series of polypropylenes with different molar mass-reactor grade (RE) and

peroxide-degraded (CR) types-were investigated with respect to mechanical and

crystallization parameters. A significant influence of the production process on

mechanical parameters was found, which can be attributed to dtrerences in the

crystallization behaviour, mainly to the number of nuclei per unit volume. At a

comparable average molar mass, RE types crystallizefaster and exhibit higher levels

of stiffness, combined with lower levels of impact strength, than CR types.

1 INTRODUCTION

Apart from processing conditions, two factors mainly influence the mechanical properties of thermoplastic materials: flow behaviour and solidification behaviour. Especially in the case of semicrystalline polymers like polyolefins, the importance of the second factor becomes predominant. Both factors are in turn determined by the molecular structure of the material, namely the chain structure (polymer type and chain branching) and the molar mass distribution (MMD). Looking for a feasible way towards ‘tailor-made materials’, one should clearly take a close look at this correlation chain.

*Dedicated to o. Univ. Professor Dr. Hermann Janeschitz-Kriegl on the occasion of his 70th birthday.

173

174 M. Gahleitner et al.

In the literature, one can find various attempts to correlate the molecular structure/MMD of polymers with their rheological properties l-r0 but only few correlations with crystallization behaviour and mechanical properties.“-I4 The scope of the present work was, therefore, to find out such correlations for the case of polypropylene (PP) homopolymers, where MMD/rheology relations have been investigated before.” The main structural differences in PP homopolymers result from the production process: catalyst and polymerization type define the stereochemical chain structure (tacticity) as well as the (MMD), which can be further modified by chemical degradation. PP homopolymers with a branched chain structure, which result from special production pro- cesses,i5*i6 are limited in use to special segments and will not be considered here. Further selective influence on the crystallization can be achieved by the addition of special nucleating agents.r7

2 THEORETICAL BACKGROUND

While the determination of final mechanical properties of the materials is clearly defined by material testing standards, for the investigation of crystallization behaviour a separation of nucleation and spherulitic growth process is necessary. i8 The amount of work which has been carried out in this field cannot be summed up shortly, so only selected examples necessary for understanding the present investigations will be cited.

One of the most widely used techniques for the investigation of polymer crystallization, especially for the spherulitic growth, is the observation of polymer films in a heating stage microscope. In the most simple way, applied, for example, by von Falkai,” Boon et al.,” Martuscelli et al., 21 Ueberreiter22 and Binsbergen and Lange,23 a polymer film is fixed between two glass plates on the heating stage of the microscope, which allows a direct temperature monitoring on the glass surfaces. While the temperature is kept constant after lowering it from the melt region to the crystallization region, photographs of the evolving structures are taken at fixed time intervals. These images are evaluated by plotting the spherulitic radius versus time, thus defining the linear growth speed.

One of the drawbacks of this method, namely the fact that the sample has to be very thin (below 5 pm) to obtain optimum conditions for the observation, which in turn influences the growth process itself, was avoided by Padden and Keith. 24 They used samples between 5 and 20 pm

Influence of molecular structure on properties of polypropylene 175

thickness, which were molten in a nitrogen atmosphere between glass plates and kept at 300°C for about 5 min (to avoid self-nucleation), then transferred to a silicon oil bath at the desired crystallization temperature. After a defined crystallization time, the samples were quenched and the diameters of the spherulites in the samples measured. Assuming initial nucleation and linear growth, this method also allowed a determination of the growth speed.

A completely different approach was applied by Lovinger et ~1.~~ using the ‘zone solidification’ apparatus. The crystallization was performed by passing large rods of the material (1 cm diameter, 10 cm length) through a temperature gradient melting and crystallization apparatus at various speeds. Growth speeds were then calculated from the relation between passing speed and final spherulithe size. This method also allowed the determination of the growth speed for /?-type spherulithes, which can be optically distinguished from the a-type structures. On the other hand, it is restricted to higher measuring temperatures because of the strong increase in nucleation speed in the lower temperature region (i.e. below 120°C).

For the present investigations, a method developed earlier in the working group of Janeschitz-Krieg12(j was applied. A polymer sample of slablike form is placed in an apparatus between two plates, which can be heated and cooled separately. After melting and relaxation at a sufficiently high temperature, the temperature of one plate is reduced to the desired crystallization temperature. A transcrystalline front is then formed, which progresses into the hot melt. After a predefined time the sample is quenched and the structures developed up to this moment are frozen in. Using a microscope, the thickness of the transcrystalline layer formed can be measured. After a series of measurements, the spherulitic growth speed can be determined from the initial slope of the curve.

A standard method for characterizing the crystallization behaviour of polymers is differential scanning calorimetry (DSC). However, it is not an easy task to determine the parameters of crystallization kinetics from experimental DSC curves. In particular, the heat transfer problem has to be taken into account,27 as will be further described in the data evaluation section.

3 EXPERIMENTAL

3.1 Materials

The materials used for this investigation had been characterized before for a study on correlations between MMD and rheological behaviour.” Two


groups of materials were investigated:

l Reactor grade PP (RE) was produced in a pilot plant (slurry type, stirred tank, commercially available fourth generation catalyst); MFI (IS0 1133, 23O”C/2=16 kg) was varied between 0.7 and 185 g/10 min.

l Degraded PP types (controlled rheology-types, CR) were produced in a single-screw extruder, using appropriate amounts of peroxide to vary the MFI between 2.9 and 36 g/lOmin, starting from a RE type with MFI = 07.

As described in Ref. 10, the MMD and linear viscoelasticity at 230°C of all materials was determined. MMD measurements were carried out on a Waters 150C GPC apparatus, the linear viscoelastic behaviour was characterized by measuring storage and loss moduli as a function of frequency (G’, G”(w)) on a Rheometrics RDS II in cone-plate geometry at 230°C. For the correlations, only the crossover parameters:

Gc = G’(wc) = G”(m,-) and cot (1)

were used.5,‘0

3.2 Mechanical testing

For mechanical testing, all materials were injection moulded into standard specimens according to DIN 16774. The properties used for mechanical characterization in this work are the flexural modulus (determined according to IS0 178; subsequently denominated EF) and Charpy impact strength at 23°C (determined according to IS0 179/leU, subsequently denominated ac).

3.3 Crystallization measurements

The number of nuclei per unit volume were determined through a DSC investigation using a Perkin Elmer DSC 7. The samples, all of the same geometry, were first heated to 240°C and kept at that temperature for 10 min to destroy any traces of crystallinity. The samples were then cooled to 50°C with different cooling rates to obtain different crystallization temperatures. Cooling rates with 10, 15, 20, 25 and 40 K/min were carried out under nitrogen atmosphere, and the observed heat flux was recorded as a function of temperature.

Additionally, the samples were investigated using the front growth apparatus. 26,28 The measurements were carried out as described in the

Influence of molecular structure on properties of polypropylene 177

the distance of the transcrystalline front from the cooled plate of the apparatus (x,) being determined for a sufficient number of quenching times (t). For a comparison between the different materials, a crystallization temperature of 120°C starting from a melt temperature of 200°C was used. As the determination of the initial slope of the x,/t curves turned out to be difficult, a direct comparison of the curve shapes was preferred.

4 RESULTS AND CONCLUSIONS

4.1 Mechanical properties

The results of mechanical testing of the materials, summed up in Table 1, were the primary reason for a further detailed investigation of these samples. The basic fact, a different Mw dependence of mechanical properties in reactor products and degradation products, has been observed before,“*r2 but no explanation had been given there. In particular, the work of 0gawai2 even oversimplifies the observed correlations by stating a simple negative correlation between log(Mw) and the flexural modulus EF. As can be seen from Fig. 1, this is only true for the case of RE types. The correlation is not significantly altered if the crossover frequency IQ or the MFI is substituted for Mw. In the case of CR types, the correlation appears inversed and lower Mw values cause a reduction in the stiffness (&) as well.

The situation appears to be more simple with the impact strength values (see Fig. 2). Here, both series of materials (RE as well as CR) show a

TABLE 1 Molar Mass and Mechanical Parameters of the Investigated Materials (625/06 is starting

material for CR series; n.b. = not broken)

Material Type MFI 230”/2; 16 kg log tMw MwIMN [g/l 0 min] CWmo[l

625106 RE 0.7 625107 RE 2.9 625108 RE 8.8

625101 RE 40 625102 RE 80 625103 RE 185 625109 CR 2.9 625110 CR 8.8 625/l 1 CR 36

6.02 5.6 1241 n.b. 5.85 5.6 1301 147.5 5.71 5.4 1408 113.7

5.45 5.2 1455 74.8 5.34 4.7 1534 57.2 5.18 4.0 1615 28.9 5.76 3.1 1139 n.b.

5.61 2.9 1104 178 5.40 2.4 1066 126.5

h4. Gahleitner et al.

5 5.2 5.4 5.6 5.8 6 6.2

log (Mw Fglmol])

Fig. 1. Correlation between weight average molar mass and flexural modulus for RE (W) and CR types ( + ).

0 I , I I

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

log (Mw (kg/mot])

Fig. 2. Correlation between weight average molar mass and impact strength for RE (W) and CR types (+).

Injluence of molecular structure on properties of polypropylene 179

logarithmic negative dependence on Mw, the CR types always at a higher level than the RE types.

Both facts appear to be common knowledge among PP producers, though no clear explanation had been given so far. The only relevant study in this area, carried out by Dziemianowicz and Coxl’ resulted in a correlation of MMD to the half-time of crystallization from DSC experiments and spherulithic growth speeds, but did not include mechanical data.

4.2 Crystallization behaviour

In principle, different crystallization behaviour can be attributed to two factors:‘* the number of nuclei and the spherulitic growth speed. The investigations were therefore aimed at finding out which of these two factors is relevant in the case of the difference between RE and CR types.

The number of nuclei was determined from DSC experiments, using the following corrections for avoiding misleading results caused by heat transfer problems: if the furnace of the DSC apparatus is cooled with a reasonable cooling rate, the temperature of the sample is different to that of the furnace. Only at unrealistically small cooling rates the heat transfer between the furnace and the sample plays a minor role. On the other hand, crystallization kinetics becomes dominated by surface nucleation in these cases. This means a different crystallization process is observed than originally wanted. With an experimentally determined effective heat transfer coefficient it is possible to calculate the sample temperature and the relative crystallinity as functions of time from the DSC signal without any assumption on a crystallization kinetic model. We use the following energy balance equation for the heat transfer between the sample and the furnace:28,29

d5 -(m,c,+m,c,)~+mshsdt=Y(T,-If,)

where

m = mass; C = specific heat capacity; T = temperature; t = time; h = specific latent heat; 5 = degree of crystallinity; y = heat transfer coefficient.

the subscripts p, s and f refer to pan, sample and furnace, respectively.


The furnace temperature is given through the temperature program of the DSC apparatus, 7’,(t) = T,, + i;(t) with a negative constant cooling rate ?‘r. The heat transfer coefficient y is determined by evaluation of the return of the DSC signal to its baseline.

The DSC signal may be written as

P(t) = y(Ts(t) - Tb(t))

with the baseline temperature given by

G(t)=Ti@)-z and

(3)

with l/a as the characteristic thermal relaxation time of this system. Using these definitions the energy balance (2) reads

After the crystallization we have t = 0, which means that p(t) returns to zero exponentially. In a plot of Frln(p(Tr)) versus Tr one obtains for different cooling rates a family of parallel lines with the slope a. The crystallinity curves t(t) or better LjTr) are given by integration of the energy balance (2). We obtain:27

(5)

( P(Tf) Oc t am,h, m Tf s

p(T) dT

Also, the sample temperature as a function of time is known through

i; P(t) T,(t)= Tf(t)-;+y-

(6)

If p(t) is plotted as a function of T, we often find that crystallization takes place under quasi-isothermal conditions. In these cases (not appli- cable to small cooling rates) one can choose an isothermal Avrami equation to describe the crystallization kinetics:

t(t)= 1 -e- (47d3WcG:V -t~,c)~ (8)

Injhence of molecular structure on properties of polypropylene 181

where

T, is the (mean) crystallization temperature; G, = G( TC) is the spherulite growth rate at temperature T,;

N, = N( T,) is the number of nuclei per volume at T,;

to,c is the time at which T, has been reached.

From this equation one can derive the following relation:

N,=0*147 (’

; $(T,:,,,)

3

c f (9)

with Tf;o,5 being the furnace temperature, at which the degree of crystallinity has reached half of the end value. Since the growth rate as a function of temperature is known from other experiments:29

G,(T)[m/s]=2Q5 x 10-‘2e-0*‘96(r-183) (10)

it is possible to calculate the number of nuclei per unit volume. In Fig. 3 the calculated number of nuclei is plotted as a function of the

corrected crystallization temperature T,. It can be observed that, for a given cooling rate, N, is always significantly higher for RE types than for CR types, at least for the range of T, examined. Additionally, the observed crystalli-

‘O- I

0 0 0

. .

8 ’ 0 0

. .

: .

.

0

. .

0.01 I 110 112 114 116 118 120

Tc PC1 Fig. 3. Number of nuclei per unit volume as calculated from DSC-results (W-RE type

625/01; O-CR type 625/11; a-RE type 625/07; O-CR type 625/09).


zation temperatures indicate a faster crystallization rate for the reactor products.

Further information about the crystallization behaviour, mainly the growth speed Gc, resulted from the front growth measurements.26,28 Basically, Gc was assumed to be independent of Mw3’ above the critical molar mass M, = 2M,, M, being the molar mass of an average chain length between two entanglements of the polymer chain. Dziemianowicz and Cox claim in their work” a negative correlation between log(Mw) and the spherulitic growth speed with no difference between RE and CR products. The effects, however, appear to be rather small compared to those induced by the differences in the number of nuclei mentioned above. If the results of the front growth experiments (see Figs 4 and 5) are compared directly, two major facts are obvious. On the one hand, the differences in crystallization behaviour within a group of materials (RE and CR types, respectively) are negligible, the main influence factor being the temperature; on the other hand a comparison between RE and CR types at the same Mw level shows a significant difference. As mentioned in Section 2, Gc can in principle be determined from the initial slope of the curves. Though, as the linearity in this part of the curves is rather poor, this seems difficult and results only in

0.5

T .

. I’

I :

. . . 3’ =

. . . . . . .

.: : . . :

. .

00

00 q :

B 0

0, a” , I

0 500 1000 1500 2000

t Is1

Fig. 4. Front growth results for materials 625/01 (D, RE type 625/01) and 625/11

(CL CR type).


3.5

3

2.5

F 2

-

0

1

. . . . . . . .

. . . . . %‘.’ .’ l . . . . . . . . . .* . . . . . .

. ‘.’ .E 00 0 0

:: ooxxc? 0 0

.: 0 0

0 o”o 0

.: Ei 000 . . 008Oo

.*=‘,oo 0 al

l $00 0

. . m . . : 0

cg

. 0

&” 1 ,. .____,-_-

7

0 500 1000 1500 2000

t Is1

Fig. 5. Front growth results for materials 625/07 (@, RE type) and 625/09 (0, CR type).

TABLE 2 Crystallization Parameters for Two Comparisons RE/CR at Comparable Molar Mass; All Values for T,= 120°C. For Nc Determinations a Cc Value of 8.5 x lo-* ms- ’ was

Generally Assumedz9

Material Type

625101 RE 625/l 1 CR 625107 RE 625109 CR

log (MdWnoO) NJm-31 G C.Fro&- ‘1

5.45 1.50 x 10’3 3.4 x 10-s 540 6.02 x 10” 3.2 x 1O-8 5.85 9.52 x 10” 2.4 x lo-’ 5.76 2.85 x 10” 2.1 x 10-8

estimations. Table 2 sums up the results for the four materials more closely investigated concerning their crystallization (625/01 vs. /ll and 625/07 vs. 09). Considering the dependences outlined in the Avrami equation (eqn (8)), the differences in crystallization behaviour mainly result from the differences in Nc.

4.3 Conclusions

It is known from the literature 31 that there exists a correlation between crystallinity and the elastic modulus. To obtain a final verification whether

Znjbence of molecular structure on properties of polypropylene 185

or not this effect closes the correlation chain mentioned in Section 1, the crystalline structure of the samples used for mechanical testing was investigated using polarizing light microscopy. Figure 6 gives a direct comparison of the structure of two of the products in the core zone of the test specimen, clearly showing a difference in the average spherulite size, which is lower in the case of the RE type. This at least partly explains the observed mechanical effects and can be attributed to the difference in crystallization behaviour.

Additionally, however, differences in the thickness of the highly oriented skin layer of the specimens were observed, which cannot be systematized that simply. The effects of shear induced crystallization’3 also seem to be influenced by the molecular structure of the materials, and the conse- quences of these effects will have to be investigated more closely.

Finally, it can be concluded that there clearly exists a significant influence of the production process on mechanical parameters, which can be attributed to differences in the crystallization behaviour, mainly to the number of nuclei per unit volume. At a comparable average molar mass, reactor grades crystallize faster and exhibit higher levels of stiffness (&) combined with lower levels of impact strength. It seems unclear, however, whether this effect can be attributed to the differences in the broadness of the MMD, which is always reduced in the case of CR types produced by degradation with peroxides, or whether the degradation process induces changes in the molecular structure of the material leading to this behaviour. Further investigations about this point are under way and will be reported in due course.

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