The interplay between liquid-liquid phase equilibria, sequence, and Tg in copolymers

Recent work shows that the copolymer glass transition temperature (Tg) can deviate significantly from standard mixing models such as the Fox equation. For example, some copolymers exhibit Tgs outside of their component homopolymers and in others Tg depends on chain sequence at fixed composition. Previous work shows that an alternating sequence maximizes both Tg suppression and enhancement, with the former observed in Kremer-Grest copolymers with high-chi monomers, and the latter observed in poly(MA-VDC). To understand this, we perform molecular dynamics simulations of copolymers with positive or negative chi monomers. We find that the sequence-dependence of copolymer Tg involves a novel interplay between liquid-liquid phase behavior, glass formation, and molecular packing. In particular, our results suggest that UCST vs LCST copolymers may exhibit quite distinct behavior. This work points to a pathway towards rational modulation of Tg in copolymers at fixed composition.