Attempt to correlate the formation of free volume and the plastic deformation process in glassy polymers

Abstract

Both the formation of free volume and the elementary motion characterizing the plastic deformation in the glassy state are described here by the same molecular process, consisting of the transition of a given number of chain segments from fundamental to activated states. Using this basic assumption, a model is proposed which makes use of classical laws of thermodynamics. Five parameters need to be adjusted to take into account the thermodynamic properties related to the free volume: one more is necessary to characterize the molecular process of plastic deformation. The pressure dependence of T g and the difference between the specific volumes found in the glassy state from isothermal and isobaric measurements can be calculated from the model. Fairly good agreement is found between the theoretical and experimental values of these quantities. The physical meaning of constants belonging to a previously-proposed yield criterion is clarified, and these constants are given as a function of the molecular process. Moreover, the treatment confirms that the glass transition temperature is related to a constant fraction of activated segments and not to a constant value of the free volume.