Mechanical Rejuvenation in Poly(methyl methacrylate) Glasses? Molecular Mobility after Deformation

Optical photobleaching experiments were used to directly measure the molecular mobility of PMMA during creep and recovery to study the interaction between mechanical deformation and physical aging. Experiments were performed both on PMMA glasses with different aging histories at T g − 14 K and on PMMA melts aged to equilibrium as much as 6 K below the conventional DSC T g . Our results show that plastic deformation increases the mobility of a polymer glass, makes the system more dynamically homogeneous, and erases the predeformation aging history. After removing the stress, the highly mobile and dynamically homogeneous system quickly relaxes to a dynamically heterogeneous system. After this initial transient, glasses produced by plastic deformation follow the same aging trajectory of the thermally quenched glasses unless the strain is very large. In contrast, deformation in the preflow regime transiently increases the molecular mobility of polymer glasses but this enhanced mobility quickly returns to the original aging trajectory after unloading the stress, indicating that preflow deformation does not change the “age” of the polymer.