Relaxations of Entropically Stressed Polymer Glasses

Deviations of chain conformations from Gaussian statistics induce entropic stress that affects the dynamics of polymer materials. Understanding its role is further complicated by the presence of enthalpic stresses, arising from the concurring change in bond energy and intermolecular interactions. By fabricating oriented poly(methyl methacrylate) brushes free from backbone deformation, we successfully identified the role of entropic stress on the nonisothermal relaxation of chains at both global and local levels. We show that the entropic stress unjams the local backbones and is responsible for the occurrence of a stress-sensitive glassy state near Tg, where the local backbone motion provokes chain contraction upon heating. Remarkably, the intrachain stress does not affect local secondary relaxations involving side-group rotation nor thermal expansion in the deep glassy state. These results cast light on the dynamics and fundamental molecular mechanics of oriented, stressful polymer glasses, such as textile fibers.