(505c) Phase Behavior and Mechanics of Triblock Copolymer Elastomers with Interaction-Tuned Additives

Block copolymer (BCP) thermoplastic elastomers are used for a wide range of applications including adhesives, coatings, footwear, automobile parts and medical devices. These materials have rubbery domains that are physically cross-linked by glassy domains, which results in elastic properties along with melt-processability. However, their strength and long-term durability is challenged by displacement of chains within the domains and deformation of the self-assembled structure under stress. In this work, we explore a strategy to tailor the structure and mechanical properties of BCP elastomers by incorporating interaction-tuned additives in the glassy domains.

The BCP elastomer selected for these studies is poly(styrene-b-ethylene butylene-b-styrene) (SEBS), a widely studied material with high oxidative and chemical stability. The additives are linear poly(methyl methacrylate-co-cyclohexyl methacrylate) (PrC) and linear polystyrene (PS) of varying molecular weights. PrC is enthalpically compatible with the glassy polystyrene domain of SEBS, while PS is an athermal additive. Entropic effects that contribute to miscibility are controlled by the molecular weight of the additives. Quantitative analysis of small angle X-ray scattering reveals how additives are distributed in the domains: SEBS assembles into hexagonally perforated lamellae and low amounts of either additive drive assembly into lamellae. PrC additives are evenly distributed in the polystyrene domain of SEBS across a broad range of molecular weights, whereas the PS additives are increasingly segregated towards the center of the styrene domain with increasing molecular weight. The morphology and mechanical properties of SEBS with 10 vol% additive is dependent on the chemistry and molecular weight of the additive. At low molecular weights of PS and PrC additives, the blend shows a well-ordered lamellar structure with increased modulus and yield stress as compared to pure SEBS. SEBS with higher molecular weights of the PS additive adopts a poorly ordered morphology, and both modulus and yield stress were decreased relative to SEBS with low molecular weight PS additive. SEBS with higher molecular weights of the PrC additive shows an ordered lamellar morphology, no reduction in modulus or yield stress, and a delay in onset of strain hardening compared to SEBS with lower molecular weights of the PrC additive. This study provides insight on designing polymeric additives to manipulate nanoscale structure and bulk mechanics of BCP elastomers.