Understanding the chain-length-dependent relaxation dynamics in polymeric glass-formers

The glass transition temperature T_g in polymers increases with increasing molecular weight M, but the detailed T_g(M) dependence is not well understood. Here, we present experimental results of the M-dependence of both the structural (α) relaxation process, which controls the glass transition, and faster secondary relaxation processes for polymers of varying chain flexibility. Based on our results, we propose that these relaxations are linked through dynamic facilitation. This leads to the conclusion that the chain-length-dependent α relaxation, and thus Tg(M), is controlled by a relatively ‘local’ fundamental relaxation, for which the relevant metric is linked to local chain flexibility. Thus, following earlier work of many others, we argue that local dihedral barriers play an important role in controlling the dynamics. We identify regimes in M where intra- and inter-molecular relaxation dynamics play different roles in defining the dynamics. We argue that this naturally gives rise to clear differences to the behaviour observed in non-polymeric glass-formers with simpler ‘rigid’ structures, or in barrier-free models of polymers.