Chain-length-dependent correlated molecular motion in polymers

TL;DR

This study investigates how dynamic heterogeneities in polymers depend on chain length and flexibility, revealing complex behaviors and potential links to relaxation processes relevant for glass transition understanding.

Contribution

It provides new insights into the chain-length and flexibility dependence of correlated molecular motions in polymers, connecting DH length-scales with relaxation mechanisms.

Findings

Flexible polymers show chain-length independent correlated motion at Tg.

Less flexible polymers exhibit three regimes of N_c(M) behavior.

N_c(M) correlates with the ratio of activation barriers of relaxation processes.

Abstract

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 ($N_c\sim500$) 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 ($\lesssim$ 2 Kuhn steps), a transition from mainly $\it{inter}$molecular correlations and $N_c\sim 200$, to strongly $\it{intra}$molecular 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 ($\alpha$) and secondary ($\beta$) relaxations. Our results suggest a link between the DH length-scale and the number of $\beta$ relaxation events jointly-activated to facilitate the $\alpha$ relaxation.