Orientation aging in polystyrene

As molten polymers are quenched rapidly into the glassy state, several non-equilibrium properties are "frozen-in". Most widely considered of such properties has been the non-equilibrium volume (or ensity), the change of which with time is usually referred to as physical aging. In addition, if the material is flowing or is otherwise being deformed as the material is cooled, the molecules are immobilized in an oriented state. It was the objective of the present work to study the relaxation (or aging) of this oriented state.

Oriented polystyrene was prepared by pulling fiber-like samples in the rubbery state and quenching them to the glassy state. The study was carried out by monitoring the decay of birefringence and the shrinkage of length as a function of time and temperature, below the glass transition temperature. The effect of an externally applied load was also studied. Temperatures of 70°C, 80°C and 90°C were used. In general, it was observed that the decay of birefringence and the shrinkage were closely related, provided the strain measure used to characterize the latter was referenced to the original rubbery length. These relaxations were rapid (taking place in about hours) and essentially reversible at 90°C but with the decay of birefringence preceeding the shrinkage by a small amount. This is probably due to localized disorientation of the phenyl groups, which cause the large birefringence in polystyrene, before the long range disorientation of the extruded molecules. The relaxations were considerably slower at 70°C and 80°C and had decayed only about 35% at 80°C after 45 days.

The data were analyzed with a two-element Voigt model, one element of which accounted for the shrinkage induced by the relaxation of the oriented molecules and the other of which accounted for the extension due to the externally applied load. The compliances associated with the former were rubbery-like, as we would expect from the nature of the orientation mechanism. The compliances associated with the latter ranged between rubbery-like and glassy-like values, depending on the temperature.