The Glass-Rubber Transition and Thermal Shrinkage of Stretched Polymethyl Methacrylate Films

Abstract

Hereunder is presented a report of an experiment made of polymethyl methacrylate films that were stretched to a predetermined length at 140°C in a silicone oil bath, and kept at the temperature, until the stress relaxed adequately. The temperature was then lowered stepwise, leaving the films at certain fixed length. At each temperature the sample was kept for about 30 minutes. Observations were enacted of various specimens with different extensions with respect to their reversible stress vs. temperature relations; the glass-rubber transition temperature decreased linearly with increasing extension (102°C and 98°C for 16% and 120% extension, respectively). The volume of the specimens increases slightly with stretching, and this could be interpreted reasonably by considering that the configurational entropy should be frozen at a constant value in glassy state; it relates to the number of 'holes' in the system.The film held at constant length was cooled to room temperature below Tg, then the stress was removed; its length almost remained unchanged. The specimen thus obtained was heated in the bath, and the length vs. temperature relation was measured. It began to shrink abruptly at the transition temperature and the length changed irreversibly to an equilibrium value. The transition point varied more rapidly with extension than that in the reversible stress vs. temperature relation (106°C and 97°C for 16% and 120% extension, respectively, at a heating rate of 1°C per minute). It depends on the heating rate; the slower the heating, the lower the temperature is. The time effects on the recovery were measured at various temperatures in the transition region for 5% and 40% extended samples. The isotherms were closely superposable by horizontal shifts. Temperature dependence of the logarithmic shift factors (logaT) was in fair accord with the WLF equation for the temperature range below 115°C, but diverged from the equation at higher temperatures.In case the specimen with previous history was fixed at constant length and heated, no stress was observed in the temperature range below Tg. The stress due to thermal shrinkage was observed, however, at several degrees above the transition temperature; the value increased and the temperature where it appeared decreased with increase in the extension. The time effects were also measured of various extensions in this case, but they behaved in very complicated manner. The mechanism of the thermal stress could be explained qualitatively based on a thermodynamic consideration.