Synthesis of (hard-soft-hard)x multiblock copolymers via RAFT emulsion polymerization and mechanical enhancement via block architectures

A large variety of block copolymers have become accessible due to the advances in controlled/living radical polymerization. However, it remains scarcely explored that how one can improve the properties of block copolymers via the chain architectures. Herein, three sets of well-defined multiblock copolymers (SBAS)x (S for poly(styrene)180 and BA for poly(n-butyl acrylate) blocks) with 25, 33, and 50 wt% PS are synthesized via RAFT emulsion polymerization. All three sets of (SBAS)x are weak/mild phase separated. Compared with their SBAS counterparts, all tensile properties of (SBAS)2 are enhanced. Particularly interestingly, both the tensile strength and toughness are simultaneously much enhanced but the modulus is less influenced. It is suggested that a soft yet strong and tough elastomer could be synthesized via changing widely-applied hard-soft-hard triblock copolymers to (hard-soft-hard)2 pentablock copolymers. The enhancement of mechanical properties increases as the increase of PnBA composition. In the case of 75 wt% PnBA, the tensile strength, toughness, and modulus of (SBAS)2 are enhanced by a factor of 2.73, 2.51 and 1.30. (SBAS)2 also show better mechanical properties than its counterpart of triblock copolymer S2BA2S2 with close molecular weight which is strongly phase separated. It is suggested that the combination of pulling-out end PS blocks and bridging middle PS blocks might contribute to the mechanical enhancement of (SBAS)2, simultaneously in toughness and tensile strength.