THERMALLY ACTIVATED DEFORMATION OF GLASSY POLYSTYRENE

The deformation of polystyrene in compression at constant strain rate was studied in the temperature range 150 - 330 K. Thermal pretreatments of the samples to prevent possible discrepancies in compression tests are discussed. The experiments were analysed in terms of thermally activated processes by using the activated slip analysis for crystalline solids. The activation parameters and particularly the entropy contribution were measured in order to estimate the free energy associated with the deformation mechanism. The results show that two different deformation modes should be considered, depending on temperature, above and below some critical temperature near 280 K. It was established that at lower temperatures the stress dependence of the strain rate is purely exponential, i.e. there is a true stress activation volume; its value does not vary much and corresponds to a coherent fluctuation of about five or six monomers. At higher temperatures, the stress contributes to the strain rate probably only in the pre-exponential factor in the form of some power law; moreover, the activation energy seems to be lower than in the low temperature mode. © 1978.