Temperature, Frequency, and Small Static Stress Dependence of the Molecular Mobility in Deformed Amorphous Polymers near Their Glass Transition

Abstract

For the first time, we report on using dynamic mechanical analysis (DMA) to directly follow molecular mobility in PMMA and PC deformed by small constant tensile stress (σ) over the temperature interval from Tg – 60 K to Tg + 60 K. At Tα′ > Tg, a pronounced secondary α′-relaxation was observed exhibiting larger frequency shift and stronger σ-dependence compared to the main α-relaxation. After reaching its maximum at approximately the Tg, the strain rate was progressively reduced at the onset of the α′-relaxation, attaining its minimum value at approximately the Tα′. Above the Tα′, the strain rate increased gradually again, suggesting that mobility has again been enhanced. We ascribe the observed variation in mobility to the strain-induced transition of segments from less mobile into more mobile domains, resulting in the alteration of conformational and orientational distribution. The macroscopic deformation (ε) dependence of the transient modulus, defined to characterize the system stiffness between the α and α′ relaxation (Γ*(ε)) and above the α′-relaxation (Ω*(ε)), collapsed to a single linear master curve for Γ*(ε) and the nonlinear Ω*(ε) master curve. Our results seem to indicate that the observed α′-relaxation may be caused by the process of intrabasin jump diffusion of small portions of the backbone chain over the characteristic length scale smaller than or equal to the dynamic Kuhn segment length (lK). The existence of the α′-relaxation may be a manifestation of the large strain-altered chain packing associated with the polyamorphic phase transition as the process accompanying yielding in polymer glasses.