Comparison of macroscopic and microscopic measurements of segmental dynamics in aging polymer glasses

Optical probe reorientation experiments and mechanical stress relaxation measurements in the linear response regime were performed during aging of glasses of poly(methyl methacrylate) (PMMA) and poly(D,L-lactide) (PLA). For each polymer, across all aging times and temperatures, an excellent correlation is observed between the relaxation times for mechanical and optical experiments with the two observables showing a power law relationship with an exponent near one. For PMMA, relaxation times for the two observables in crosslinked and uncrosslinked materials follow the same correlation. The probe reorientation method has previously been utilized to track changes in segmental dynamics during nonlinear deformations. Our new experiments validate the conclusion that probe reorientation is a good reporter of segmental dynamics in the glass, in the linear response regime. The strong agreement between macroscopic and microscopic measurements of mobility in the aging polymer glasses contrasts to recent work on colloidal and metallic glasses.