Cooperative intramolecular dynamics control the chain-length-dependent glass transition in polymers

TL;DR

This study reveals how the glass transition temperature in polymers depends on molecular weight and chain flexibility, highlighting the role of local molecular relaxations and hierarchical dynamics in controlling glass formation.

Contribution

It introduces a new framework linking local conformational relaxations to the chain-length dependence of Tg in polymers, advancing understanding of polymer glass transition mechanisms.

Findings

Tg(M) correlates with average mass per conformational degree of freedom.

Local molecular relaxations influence larger-scale alpha relaxations.

Hierarchical dynamics explain logarithmic Tg(M) dependencies.

Abstract

The glass transition is a long-standing unsolved problem in materials science. For polymers, our understanding of glass-formation is particularly poor due to the added complexity of chain connectivity and flexibility; structural relaxation of polymers thus involves a complex interplay between intra- and inter-molecular cooperativity. Here we study how the glass transition temperature Tg varies with molecular weight M for different polymer chemistries and chain flexibilities. We find that Tg(M) is controlled by the average mass (or volume) per conformational degree of freedom, and that a `local' molecular relaxation (involving a few conformers) controls the larger-scale cooperative alpha relaxation responsible for Tg. We propose that dynamic facilitation where a `local' relaxation facilitates adjacent relaxations, leading to hierarchical dynamics, can explain our observations including logarithmic Tg(M) dependences. Our study provides a new understanding of molecular relaxations and the glass transition in polymers, which paves the way for predictive design of polymers based on monomer-scale metrics.