Sequence Effects on the Glass Transition of a Model Copolymer System
William F. Drayer, David Simmons
Abstract
Achieving rational control of the glass transition temperature Tg and associated dynamics is a major fundamental and practical challenge in polymer science, with applications ranging from ion conductivity of polymer electrolytes to thermal stability and processability of engineering plastics. Here we employ molecular dynamics simulations to elucidate a new strategy for rational control of glass transition temperatures by varying copolymer sequences─an approach viable due to advances enabling synthesis of copolymers with increasingly controlled sequences. Our results point to two regimes of sequence control on Tg. For sequences sufficiently blocky to allow microphase separation, Tg alterations emerge from interfacial effects on dynamics, an extensively studied phenomenon, with sequence effects on domain size playing a central role in determining Tg. For sequences that approach an alternating copolymer, we identify a second regime in which Tg is directly tuned by segmental packing. In this regime, Tg is exquisitely sensitive to sequence─far more so than in the interface-dominated regime─so that large fractional changes in Tg can be realized over a range spanning from an alternating sequence to an alternating trimer. This regime represents a largely untapped opportunity for tuning dynamics and improving transport properties in copolymers. Finally, results suggest that sequence-specified copolymers may provide a unique platform in which to probe the physics of the glass transition by leveraging their dynamical variability at fixed composition; results from this study, for example, find that classical free volume theories of glass formation do not account for the sequence dependence of dynamics in these copolymers.