Makayla R. Branham-Ferrari

arXiv2026

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

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.

cond-mat.soft↗

arXiv2025

Reaction/Diffusion Competition Drives Anomalous Relaxation of Vitrimers

Since their discovery in 2011, vitrimers - covalent associative network polymers - have challenged the traditional understanding of soft matter relaxation dynamics: unlike in typical glass-forming liquids, vitrimers' viscous relaxation can be entirely decoupled from their underlying structural (segmental) dynamics. Beyond this fundamental mystery, the origin of vitrimers' Arrhenius viscosity in the presence of super-Arrhenius structural relaxation behavior has been of high interest due to vitrimers' potential to provide readily reprocessable high-performance plastics. Here, we combine simulations, theory, and experiments to establish a foundational understanding of vitrimer relaxation dynamics. We identify two types of transient networks based on the ratio of atomic displacement scales required for bond exchange to those required to relax a segment. In systems where bond exchange only requires sub-segmental motion, we show that network relaxation is governed by a competition between chemical exchange reactions and segmental diffusion. This competition produces vitrimers' signature network/segment decoupling, while also driving a crossover between Arrhenius and super-Arrhenius behavior that is observed for many vitrimers. This work provides an explanation for longstanding puzzling features of vitrimer dynamics and establishes a foundation for rational vitrimer design.

cond-mat.soft↗