Linear (IS)nI Multiblock Copolymers: Tailoring the Softness of Thermoplastic Elastomers by Flexible Polyisoprene End Blocks

Nano-segregated morphologies and the resulting thermoplastic elastomer (TPE) properties, e.g., in multiblock copolymers based on isoprene (I) and styrene (S), depend on chain architecture (overall molar mass, block number, composition profile). In this work nonconventional (IS)nI type multiblock copolymers possessing two flexible polyisoprene end blocks are introduced. Molar masses between 120 and 400 kg·mol–1 and block numbers ranging from 3 to 13 with either precisely defined blocks or tapered segment structures were obtained by (i) sequential monomer addition or (ii) statistical S/I copolymerization steps, respectively. All polymers are based on a 30/70 molar ratio styrene/isoprene (36 vol % PS). The materials were investigated with respect to their self-assembly and mechanical response, respectively, by small-angle X-ray scattering (SAXS) and tensile tests. While the sequential multiblock copolymers generally exhibited strong segregation into well-ordered lamellar morphologies and more extended chain conformations, their tapered counterparts showed only weak order and a smaller scaling exponent δ with d ∼ Nδ. In addition, a higher bridge-to-loop ratio was found for the sequential multiblock copolymers. On the other hand, the tapered structures exhibited accessible order–disorder transition temperatures for low molar masses and high block numbers, which is relevant for processing. Generally, tensile tests revealed high toughness (20–70 MJ·m–3) for the multiblock copolymers bearing at least two polystyrene blocks, with the tapered structures being softer and more flexible. The stiffness also increased with increasing block number, reflecting shrinking of the domain sizes. Moreover, the maximum deformation was found to increase with the overall molar mass. With their soft, albeit resilient properties, these materials with low Tg polyisoprene end blocks are promising as a substitute for plasticized polymers, e.g., poly(vinyl chloride), avoiding migration and release of a plasticizer.