2024/05/04 by Reynolds, Matthew, Baker, Daniel L., Olmsted, Peter D. +1
#FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci) #Soft Condensed Matter (cond-mat.soft)
paper · doi:10.48550/arxiv.2405.02733
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 (Nc∼500) undergo correlated motion at the glass transition temperature Tg, independent of molecular weight (M). In contrast, less flexible polymers show a complex Nc(M) behaviour divided into three regimes, consistent with observation in both Tg(M) and chain conformational structure. For short oligomers (\lesssim 2 Kuhn steps), a transition from mainly \itintermolecular correlations and Nc∼ 200, to strongly \itintramolecular correlations and Nc< 50 (roughly the molecular size) is observed; for longer chains, Nc increases weakly, before saturating. For poly(methyl methacrylate), a remarkable similarity is found between Nc(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.