Synthesis, order-to-disorder transition, microphase morphology and mechanical properties of BAB triblock copolymer elastomers with hard middle block and soft outer blocks

Abstract

A series of BAB-type triblock copolymer elastomers (TBCPEs) with a soft–hard–soft block sequence were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization, in which “A” represents the hard block poly(isobornyl acrylate) (PIBA), whereas “B” represents the soft block poly(n-butyl acrylate)-co-poly(1-vinylimidazole) (P(BA-co-VI)) random copolymer. Two considerably separated glass transition temperatures and two thermal decomposition stages were observed for the as-synthesized BAB-type TBCPEs, indicating the occurrence of microphase separation. The temperature-, frequency-, and time-dependent viscoelastic measurements were conducted to determine the order-to-disorder transition temperature (T_ODT) and to study the microphase separation kinetics for TBCPEs. Increasing the molecular mass of TBCPEs resulted in a linear increase in the T_ODT values, faster phase separation in the first stage and slower one in the second stage. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) measurements disclosed the phase separated morphology, where the inter-domain distance increased upon increasing the molecular mass for each category of TBCPEs. From the mechanical property tests, a greater mass fraction of PIBA in TBCPEs resulted in higher ultimate tensile strength and lower elongation at break. The co-existence of entanglements between the soft outer blocks and the microdomains of the hard middle blocks, with the latter ones being bridged by the former ones, contributed to the observed elastomeric behaviors of TBCPEs. This study provides a picture illustrating the effects of the microstructures on the mechanical properties of BAB-type TBCPEs.