The Interplay Between Liquid-Liquid Phase Equilibria, Sequence, and Tg in Copolymers

TL;DR

This study uses molecular dynamics simulations to explore how copolymer sequence and phase behavior influence glass transition temperature (Tg), revealing that liquid-liquid phase equilibria predict Tg shifts and enabling rational design of polymer properties.

Contribution

It demonstrates that copolymer sequence and phase behavior critically determine Tg variations, providing a new framework for designing polymers with tailored thermal properties.

Findings

Tg shifts are predicted by liquid-liquid phase behavior of comonomers.

Upper Critical Solution Temperature systems show negative Tg deviations.

Lower Critical Solution Temperature systems exhibit positive Tg deviations.

Abstract

Copolymerization is commonly employed to tune polymers' glass formation and improve properties such as ion conductivity and adhesion. Classically, mixing rules such as the Fox equation are employed to explain glass transition temperature (Tg) variations with copolymer composition. However, many copolymers deviate from these mixing rules in a manner that is monomer-sequence sensitive. We perform molecular dynamics simulations to probe the interplay between copolymer sequence, liquid-liquid phase equilibria, and Tg. We find that the direction and sequence-dependence of Tg shift are predicted by the liquid-liquid phase behavior of the comonomers. Systems tending towards Upper Critical Solution Temperature behavior negative Tg deviations, while systems tending towards Lower Critical Solution Temperature behavior exhibit positive Tg deviations. In both cases, this effect is strengthened with increasing alternation - a consequence of bond-induced forced mixing. These results inform strategies for rationally varying copolymer Tg, at fixed composition, via design of polymer chain sequence.