String-like Collective Motion in the alpha and beta relaxation of a Coarse-Grained Polymer Melt
Jack F. Douglas, Francis W. Starr, Beatriz Betancourt
Relaxation in glass-forming liquids occurs as a multi-stage process involving cooperative molecular motion. First, there is a `fast' relaxation process dominated by the inertial motion of the molecules whose scale grows upon heating, followed by a longer time alpha -relaxation process involving large scale diffusive motion. Our molecular dynamics simulations of a coarse-grained glass-forming polymer melt indicate that the fast collective motion becomes progressively suppressed approaching the glass-transition, necessitating large-scale collective motion by diffusion for the material to relax. In each relaxation regime, relaxation occurs through collective particle exchange motions having a similar geometrical form and quantitative relationships are derived relating the fast `stringlet' collective motion to the string-like collective motion at long times associated with the thermally activated diffusive motion.