Comparing Segmental Dynamics in Polymer Glasses during Deformation: PLA and PMMA

Polymer glasses are ubiquitous engineering materials, but their deformation is not well understood at a fundamental level. Here, the segmental dynamics of glassy poly(lactic acid) (PLA) were measured with a probe reorientation technique during constant strain rate tensile deformation at temperatures between T_g – 25 K and T_g – 15 K and local strain rates between 10^-5 s^-1 and 10^-4 s^-1. The relaxation time of the undeformed PLA is about 10⁴ s and decreases by up to a factor of 30 during deformation. The relaxation time in the plastic flow regime shows a temperature dependence of about 30 K/decade and is related to the local strain rate through a power-law relationship with an exponent near -1 for all investigated temperatures. We also find that the KWW β parameter for PLA increases during deformation from 0.42 to 0.60, indicating a narrowing of the distribution of relaxation times. These results are very similar to those previously reported on lightly crosslinked poly(methyl methacrylate) (PMMA) glasses, suggesting that these are universal effects of constant strain rate deformation on the segmental dynamics of polymer glasses.