Implications of styrenic triblock copolymer gel mechanics on midblock bridging fraction

Abstract

Triblock copolymer gels have garnered considerable scientific interest over the past few decades and are used in a variety of applications including consumer cushioning, pressure-sensitive adhesives, and ballistics gels. While many of their applications rely upon understanding mechanical properties, mechanical studies of block copolymer gels are relatively disjointed with each focusing on specific copolymers, which inherently have a fixed molecular weight and composition. The present study examines the quasistatic mechanical response of styrenic triblock copolymer gels composed of seven unique triblock copolymers at various concentrations. Resultant stress-extension data is fitted with the slip-tube network (STN) theory that describes gel mechanics based upon crosslinked network (Gc) and chain entanglement (Ge) modulus contributions. Collectively, modulus contributions imply that midblock bridging is independent of copolymer identity (i.e., molecular weight and block fraction) and dependent on triblock copolymer concentration via Fb ∝ φABA0.91±0.39 over the concentration range examined (φABA = 0.05–0.39). Gels composed of a complementary diblock-triblock copolymer pair were subsequently analyzed to explore the influence of interlocking loop-loop and loop-bridge pairs within the effective midblock bridging population. These results point to the presence of a physically significant quantity of interlocked loops, but quantitative analysis was inconclusive.