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

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.

arXiv commentIntroduction rewritten to focus more strongly on polymers; added text to discussions for clarity; supplementary Information included as appendices. Accepted for publication in Physical Review X at https://journals.aps.org/prx/accepted/4b07cKc0Z331f103561c78546d4988c9c8237e86b

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Cited by in corpus (7)

• Is the Molecular Weight Dependence of the Glass Transition Temperature Caused by a Chain End Effect?
• On the Spectral Shape of the Structural Relaxation in Deeply Supercooled Liquids
• Data-driven identification and analysis of the glass transition in polymer melts
• End-Tethered Chains Increase the Local Glass Transition Temperature of Matrix Chains by 45 K Next to Solid Substrates Independent of Chain Length
• Glass transition temperature of thin polymer films
• A fast transferable method for predicting the glass transition temperature of polymers from chemical structure
• Chain-length-dependent correlated molecular motion in polymers