Microphase Separation in Block-Random Copolymers of Styrene and Hydrogenated Isoprene

The capacity to synthesize block-random copolymers, which are block copolymers with one or more random copolymer blocks, allows for continuous tuning of the interblock segregation strength, $\chi $, through the composition of the random copolymer. The ability to tune $\chi $ effectively decouples the block copolymer molecular weight from its order-disorder transition temperature. By lithium-initiated anionic polymerization with added triethylamine, we synthesize near-monodisperse and near-symmetric block-random copolymers of styrene and isoprene: PI-PSrI (50{\%} wt. styrene). In comparison to PS-PI diblock copolymers, the number of unfavorable segmental contacts in the disordered state is decreased and hence the effective interblock $\chi $ is reduced. Isoprene-hydrogenated derivatives of these block-random copolymers exhibit microphase separation into well-ordered lamellae and display sharp thermally-induced order-disorder transitions via small-angle x-ray scattering. The observed reduction in $\chi $, as gauged by the molecular weight required to achieve a desired T$_{ODT}$, matches well with the mean field prediction.