Relaxations of Entropically Stressed Polymer Glasses

Deviations of chain conformations from Gaussian statisticsinduceentropic stress that affects the dynamics of polymer materials. Understandingits role is further complicated by the presence of enthalpic stresses,arising from the concurring change in bond energy and intermolecularinteractions. By fabricating oriented poly(methyl methacrylate) brushesfree from backbone deformation, we successfully identified the roleof entropic stress on the nonisothermal relaxation of chains at bothglobal and local levels. We show that the entropic stress unjams thelocal backbones and is responsible for the occurrence of a stress-sensitiveglassy state near T (g), where the localbackbone motion provokes chain contraction upon heating. Remarkably,the intrachain stress does not affect local secondary relaxationsinvolving side-group rotation nor thermal expansion in the deep glassystate. These results cast light on the dynamics and fundamental molecularmechanics of oriented, stressful polymer glasses, such as textilefibers.