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

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.