Sequence Effects on the Glass Transition - Suppression from Block to Alternating Copolymers

There is evidence that the glass transition temperature (Tg) of copolymers is sensitive to their sequence, potentially providing a new strategy for polymer property control at fixed composition. However, the nature of this sequence-dependence remains poorly understood. Here, we report on results of bead-spring molecular dynamic simulations probing the effect of copolymer sequence on Tg suppression. Results indicate that the Fox equation for mixing fails to predict the qualitative behavior of Tg; however, two mechanisms for Tg suppression emerge. For large positive chi, in the diblock limit, sequence dictates the domain size, modulating interfacial alterations in the glass transition temperature. Notably, as sequences become more like the alternating chain, much larger suppression is observed along with the expected loss of well-defined domains. Results suggest that, in this regime, direct sequence-modulated modification of segmental packing and localization length-scale mediates sequence control of Tg. The findings provide guidance towards design of copolymers with targeted transport and dynamical properties.