Constitutive equations are derived for the isothermal viscoelastic response of glassy polymers at small strains. An amorphous polymer is modeled as a composite material consisting of a temporary network of long chains, immobile voids, and free volume gas diffusing through the network. The model is applied to analyze stress-induced densification of polymers in tensile and compressive relaxation tests. It is demonstrated that the stress-strain relations adequately predict a decrease in the specific volume observed in uniaxial experiments on polycarbonate.

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