Polymer International Polym Int 49:334±336 (2000)

Effect of physical ageing on the viscoelasticityof interpenetrating polymer networksYS Lipatov,* VF Rosovitsky, TT Alekseeva and NV BabkinaInstitute of Macromolecular Chemistry, National Academy of Sciences of Ukraine, Kiev, 253660, Ukraine

(Rec* Co

# 2

Abstract: The effect of physical ageing on viscoelastic properties was studied for semi-IPNs based on

crosslinked polyurethane and poly(butyl methacrylate) taken in the ratio 75/25 by mass. The

viscoelastic properties of IPNs were studied after physical ageing and after heat treatment at 60°C.

Signi®cant changes in viscoelastic behaviour after ageing were observed. It was found that

heterogeneous systems such as IPNs have their own speci®c features of physical ageing which are

related to the existence of two glass transition to temperatures. Relaxation processes to establish the

equilibrium state may need a long period of time.

# 2000 Society of Chemical Industry

Keywords: interpenetrating polymer networks; viscoelasticity; physical ageing

INTRODUCTIONBy physical ageing, one means the time dependence of

changes in the behaviour of an amorphous polymer

held at a temperature below the glass transition. Such

changes are normally the result of the continuous slow

relaxation of the glass from its initial non-equilibrium

state at a given temperature and the frequency interval

towards ®nal thermodynamic equilibrium.1 As a rule,

the molecular mobility under such conditions is very

low to reach the true state of equilibrium. In some

works,2±9 physical ageing of amorphous polymers (or

isothermal structural relaxation) has been studied. It

was established that in a broad temperature interval

below the glass transition, there exists a universal

dependence of the relaxation times on chemical

structure, thermal prehistory, and on the temperature

of investigation. Theoretical work in this direction has

been done in the framework of the theory of linear

viscoelasticity, supposing the existence of only one law

of relaxation time distributions.10,11 A fractal descrip-

tion of physical ageing was also proposed.12

It is evident that physical ageing is typical for all

polymeric systems, including polymer blends and

interpenetrating polymer networks (IPNs), but until

now there have been no investigations in this ®eld. The

supposed peculiarity of physical ageing in such systems

is its dependence on the initial degree of microphase

separation typical for polymer blends and IPNs. One

can suppose that ageing in blends and IPNs is related

to incomplete phase separation in the system. Physical

ageing in polymer blends and segmented poly-

urethanes has previously been investigated.13 Such

systems are of great interest from both theoretical and

practical points of view. Physical ageing here is ®rst of

all a time dependent process of changes in the degree

of microphase separation, and only then of changes in

eived 23 June 1999; revised version received 28 September 1999; acrrespondence to: YS Lipatov, Institute of Macromolecular Chemistry,

000 Society of Chemical Industry. Polym Int 0959±8103/2000/$1

the structure of constituent phases. The peculiarity of

such systems consists of the existence of two glass

transition temperatures corresponding to two phases.

Therefore, physical ageing may proceed in the tem-

perature interval below one glass transition and above

the other.

In the present communication, we have tried to

establish some features of the changes in viscoelasticity

of IPNs in the course of their physical ageing.

EXPERIMENTALFor this investigation, we have chosen semi-IPNs based

on crosslinked polyurethane (PUs) and poly(butyl

methacrylate)14 taken in the ratio 75/25 by mass.

Semi-IPNs were prepared by simultaneous formation

of crosslinked PU and radical polymerization of butyl

methacrylate. PU was synthesized from oligo(oxypro-

pylene glycol) with molecular weights 2000 and 500,

and an adduct of trimethylolpropane with 2,4±2,6-

tolyulene diisocyanate in the ratio 3:1 (to give IPN1 and

IPN2, respectively). Momomeric butyl acrylate was

introduced into the reaction system together with an

initiator of polymerization (2,2-azo-bis-isobutyro-

nitrile, 2.96�10ÿ2mol lÿ1). As a catalyst for the

synthesis of PU, 1.4�10ÿ4mol lÿ1 dibutyltin dilaurate

was used. Polymers were investigated as ®lms obtained

by pouring the composition between two glasses

covered with poly(ethylene terephthalate). Curing

was performed at 60°C with subsequent drying and

evacuation up to constant weight. The viscoelastic

properties of IPNs were studied in several steps:

(1) Specimens were taken 2 weeks after synthesis.

(2) Specimens were taken after 1 year of physical

ageing at room temperature.

cepted 18 November 1999)National Academy of Sciences of Ukraine, Kiev, 253660, Ukraine

7.50 334

Ageing of interpenetrating polymer networks

(3) Specimens were taken after 1 year of physical

ageing and heat treatment at 60°C for 1 month,

and

(4) the same specimens were tested after additional

ageing for 4 months

Viscoelastic properties were studied by dynamic

mechanical spectroscopy (DMS). Temperature de-

pendencies of mechanical loss, tan d were determined

using a mechanical spectrometer previously

described15 with a regime of forced sinusoidal vibra-

tions at 100Hz. Measurements were taken over a

temperature interval of ÿ50 to �120°C. Films with

dimensions 60mm�5mm�0.5mm were used.

Figure 1. Temperature dependence of mechanical losses of IPN1:(a) directly after synthesis; (b) after 1 year of physical ageing; (c) after heattreatment at 60°C for 1 month; (d) 4 months’ ageing at room temperatureafter heat treatment.

Figure 2. Temperature dependence of mechanical losses for IPN2:(a) directly after synthesis; (b) after 1 year of physical ageing; (c) after heattreatment at 60°C for 1 month; (d) 4 months’ ageing at room temperatureafter heat treatment.

RESULTS AND DISCUSSIONThe semi-IPNs chosen for this investigation have a 75/

25 ratio of PU and PBMA constituents, and represent

morphologically a continuous PU-matrix with em-

bedded inclusions of PBMA. However, one has to take

into account that in the systems with incomplete phase

separation each phase consists of both components. In

our case the PU-phase is enriched in PU, whereas the

PBMA phase is enriched in PBMA. In accordance

with published data, this system is characterized by the

existence of two relaxation maxima on the curve of

temperature dependence of the mechanical loss

tangent (tan d) on temperature.14 These maxima

correspond to two phases, a matrix continuous phase

enriched in PU and a disperse phase enriched in

PBMA. The temperature of the tan d maximum for the

two phases corresponds to their glass transition

temperature Tg and differs from the Tgs of the pure

components (ÿ28°C for PU and 68°C for PBMA).

Figures 1(a) and 2(a) show the temperature

dependency of tan d for IPN1 and IPN2, respectively.

The character of the dependency differs sharply from

that of typical phase-separated IPNs where two sharp

maxima are present. For IPN1, the corresponding

maxima are very weak. The maximum for the PU-

phase is shifted to higher temperature compared with

pure PU, whereas the maximum for PBMA is shifted

to lower temperature (from 68 to 45°C). Curves for

IPN2 are characterized by only one broad maximum

in the region between the glass transition temperatures

of the constituent components. According to our

former results,16 the convergence of two loss maxima

or the existence of only one maximum means a very

low degree of component segregation in IPNs com-

pared with the quasi-equilibrium previously

described.17 The lack of maxima may indicate

compatibility between the two components at a topo-

logical level, where the processes of microphase

separation are only in their initial stage. Thus, IPNs

studied directly after synthesis do not show a picture

typical of microphase separation as judged from

dynamic mechanical spectroscopy (DMS). It seems

possible to explain the absence of a two-phase struc-

ture immediately after synthesis by the formation of

Polym Int 49:334±336 (2000)

very small regions of phase separation that cannot be

detected by DMS. Theoretically it was shown18 that

the dimensions of the phase domains at which they

335

YS Lipatov et al

display their own transition temperature should be no

less than 15nm. Reaching this critical dimension

depends on the kinetic conditions of the reaction

which, in turn, determine the phase separation, sizes of

the phase domains and morphology of the system. It is

evident that after completion of the chemical reaction,

the regions of microphase separation are very small and

enlarge because of coalescence over a period of time.

As a result of physical ageing for 1 year at room

temperature, the temperature dependence of tan dchanges signi®cantly (Figs 1(b) and 2(b)). Physical

ageing manifests itself ®rst of all in continuation of the

microphase separation. For IPN1, one observes a

typical picture with two maxima being expressed. The

maxima temperature are shifted for the PU phase from

5 to ÿ5°C and for the PBMA phase from 45 to 70°C.

The increasing distance between the two maxima

shows higher degree of microphase separation reached

as result of physical ageing. For IPN2, two maxima of

losses also appear, but they are less pronounced. The

distinction in behaviour of IPN1 and IPN2 is con-

nected with the molecular weight of the oligomer used

for PU synthesis (see Experimental). The diminishing

loss level in both cases is also a sign of phase trans-

formations in the systems. Thus, it can be concluded

that semi-IPNs in which one of the components is

below the glass transition temperature approaches a

higher equilibrium physical state during ageing.

Consider now the data on the physical ageing of

IPNs as a result of heat treatment (Figs 1(c) and 2(c)).

The IPNs were subjected to heat treatment at 60°C(above the Tg of PBMA) for 1 month. The data display

only one broad relaxation maximum showing the

transition of the systems into another non-equilibrium

state, with very low degree of segregation. At the same

time, the existence of two shoulders at 20 and 40°C for

IPN1 is evidence for the preservation of low segrega-

tion in the system. One can suppose that such changes

are the result of increasing molecular mobility of both

components at temperatures above the glass transition

temperature of PBMA.

Of interest is the question about the reversibility of

the heat treatment in the systems under investigation.

Figures 1(d) and 2(d) show the temperature depen-

dency of mechanical losses of heat-treated specimens 4

months after heat treatment at 60°C. It is seen that the

character of the curves tan� = f(T) is practically the

same as after the heat treatment for both IPN1 and

IPN2. For IPN2, curves c and d in Fig 2 are identical

but the loss maximum is slightly narrower, and the

maximum is shifted from 60 to 40°C. However, in no

case was the restoration of the state of the system

before heat treatment observed. It seems improbable

that the structure existing before heat treatment can be

recovered at temperatures below the glass transition of

PBMA for a time as short as 4 months.

CONCLUSIONSExperimental data for semi-IPNs show that their

336

dynamic mechanical properties are a function of the

conditions of physical ageing. It may be supposed that

the main process taking place during ageing is

microphase separation of the system, whose equili-

brium state consists of two phases. Immediately after

synthesis, IPNs are characterized by one diffuse

maximum mechanical loss in the region of the glass

transition of PU (the effect being different for PU

based on oligoglycols of different molecular masses).

After 1 year of physical ageing, IPNs reveal the typical

features of a two-phase structure that was observed in

previous work for IPNs studied after some storage.

Thermal treatment of IPNs lead to the convergence of

loss maxima and to the appearance of only one broad

maximum. The data show a complicated transforma-

tion of the structure as a result of various physical

actions on the systems in which the components are

thermodynamically incompatible and having a two-

phase equilibrium state. During the synthesis, the

process of formation of the phase morphology lags

behind the onset of phase separation, ie it proceeds

with some shift in time. This effect, found for the ®rst

time, needs additional investigation. At the same time,

it is evident that heterogeneous systems such as IPNs

have their own speci®c features of physical ageing

connected with the existence of two glass transition

temperatures. The relaxation processes for the estab-

lishment of the equilibrium state may need a long

period of time.

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Polym Int 49:334±336 (2000)