Volume changes in glassy polymers

Constitutive equations are derived for compressible glassy polymers at non-isothermal loading with finite strains. The model is based on the theory of temporary networks in its version of adaptive links concept. The stress-strain relations are applied to the analysis of uniaxial extension of a viscoelastic bar. Explicit formulas are developed for time-dependent Young's modulus and Poisson's ratio of the bar at small strains. Results of numerical simulation are compared with experimental data for polycarbonate, polyethylene, and poly(methyl methacrylate). It is demonstrated that (i) longitudinal stresses do not affect the specific free volume in the region of linear viscoelasticity at strains up to about 0.2%, and cause substantial changes in the free volume in the region of nonlinear viscoelasticity at strains about 1.0%; (ii) in the latter case, the increment of the free-volume fraction is proportional to the increase in the specific volume.