Generality of a Nonlinear Single Relaxation Model Describing Yield Process of Glassy Polymers

The single relaxation time of glassy polycarbonate (PC) and poly (methyl methacrylate) (PMMA) was evaluated during the compressive yield process by using a nonlinear single relaxation model. The model consisted of a spring with a constant modulus and a dashpot with variable viscosity as a single parameter representing strain-induced structural change. A strain-dependent relaxation time estimated by fitting the model to experimental stress-strain relations agreed well with a relaxation time directly determined by short-time stress relaxation experiments performed at various amounts of strain. Since the model had been proved to quantitatively describe the strain-dependent relaxation time during the tensile yield process of PMMA, the model was likely capable of describing the variation of relaxation time during the yield process regardless of differences in material and deformation mode, i. e., tension and compression. Differences between strain-dependent relaxation times for PC and PMMA and between those under tension and compression were also discussed.