Applying a stress to a glassy polymer accelerates its dynamics as one goes from low stress up to plastic regime. For decades, the phenomenological Eyring's model has been used to describe plastic flow in polymers. This model, however, raises fundamental issues which makes its use deleterious in glassy polymers. We propose an alternative model in which the elastic energy stored at the length scale of dynamical heterogeneities $\xi \approx 3–5$ nm reduces the free energy barrier for relaxation. Contrary to the Eyring's activation volume, which has no clear interpretation, this length scale is derived from physical arguments, based on a detailed account of relaxation mechanisms at the molecular scale. Recent creep experiments in glassy polymers by Ediger and coworkers allow for discriminating the two pictures. It is shown that the whole evolution of the ${\tau }_{\alpha }$ relaxation time under stress can be reproduced quantitatively, using as the only adjustable parameter the scale $\xi$. The obtained value of $\xi$ is the same as the value previously determined by considering the whole set of properties of glassy polymers. This confirms the coherence and completeness of the theory of relaxation processes in nonpolar glassy polymers that we proposed in previous works.

• Received 2 May 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.2.105601