Relaxation processes in polymer glasses: a hierarchical approach

The glass transition temperature Tg in polymers increases with increasing molecular weight M, but the detailed Tg(M) dependence in polymers is not well understood. Here, we present experimental results of the M-dependence of both the structural (alpha) relaxation process, which controls the glass transition, and faster (beta and gamma) secondary relaxation processes for a range of polymers of varying chain flexibility. Based on our results, we propose a hierarchical relaxation scenario that links these relaxations and controls Tg in polymers, where the fundamental metric is linked to ‘local’ flexibility. We identify regimes in M where intra and inter-molecular relaxation dynamics play different roles in defining the dynamics (and thus Tg). We argue that dihedral barriers play a crucial role in controlling the dynamics, which gives rise to distinct differences in behaviour from what is observed in glass-formers with simpler ‘rigid’ structures, or in simplified bead-spring models of polymers.