Chain-length-dependent correlated molecular motion in polymers

We show how dynamic heterogeneities (DH), a hallmark of glass-forming materials, depend on chain flexibility and chain length in polymers. For highly flexible polymers, a relatively large number of monomers (N_c∼500) undergo correlated motion at the glass transition temperature T_g, independent of molecular weight (M). In contrast, less flexible polymers show a complex N_c(M) behaviour divided into three regimes, consistent with observation in both T_g(M) and chain conformational structure. For short oligomers (≲ 2 Kuhn steps), a transition from mainly intermolecular correlations and N_c∼ 200, to strongly intramolecular correlations and N_c< 50 (roughly the molecular size) is observed; for longer chains, N_c increases weakly, before saturating. For poly(methyl methacrylate), a remarkable similarity is found between N_c(M) and the M-dependent ratio of the activation barriers of the structural (α) and secondary (β) relaxations. Our results suggest a link between the DH length-scale and the number of β relaxation events jointly-activated to facilitate the α relaxation.