Thermoplastic Elastomers via Combined Crystallization and Vitrification from Homogeneous Melts

Block copolymers with crystalline, glassy, and rubbery blocks were synthesized by anionic polymerization of butadiene, styrene, and isoprene followed by chlorosilane coupling and hydrogenation. The performance of two pentablock copolymers, with the block sequence crystalline-glassy-rubbery-glassy-crystalline, as thermoplastic elastomers (TPEs) was evaluated against triblock copolymers having either crystalline or glassy end blocks. Judicious choices of block lengths yielded homogeneous melts for both pentablocks; consequently, the low-shear-rate viscosities of the pentablocks were more than 2 orders of magnitude lower than that of the glassy-rubbery-glassy triblock, which remained microphase-separated in the melt. In the pentablocks, physical cross-linking was achieved by crystallization of the end blocks followed by vitrification of the adjacent glassy blocks, forming composite crystalline-glassy hard domains. All of the polymers studied exhibited desirable mechanical behavior for TPEs, including low Young's modulus, high extensibility, and low permanent set. Increasing the glassy block fraction (at constant hard block content and molecular weight) systematically improved the mechanical performance by reducing the Young's modulus and increasing the ultimate strength; however, the strain recovery was still limited by the crystalline component. Taken in the context of prior work on semicrystalline TPEs, this work highlights the influence of the crystalline morphology on the mechanical properties.