Synthesis of Polystyrene-Polylactide Bottlebrush Block Copolymers and Their Melt Self-Assembly into Large Domain Nanostructures

High molecular weight polystyrene-polylactide (PS-PLA) bottlebrush block copolymers have been shown to self-assemble into highly ordered lamellae structures with domain spacings as large as 163 nm, as identified by ultrasmall-angle X-ray scattering. Bottlebrush block copolymers were synthesized by a combination of living radical and ring-opening polymerizations. The backbone was prepared by RAFT block copolymerization of solketal methacrylate (SM) and 2-(bromoisobutyryl)ethyl methacrylate (BIEM). Polystyrene branches were grafted by ATRP from poly(BIEM) block, and PLA branches were grafted from the poly(SM) block after the removal of ketal groups. The investigation into the self-assembly of PS-PLA bottlebrush block copolymers with varying lengths of branches and backbones revealed a number of unusual trends, which were attributed to their dynamic, three-dimensional structure. The results suggest that in phase-separated melts the bottlebrush block copolymer backbone, while extended, still possesses a certain degree of flexibility to accommodate for different interfacial areas necessary to pack into lamellae microstructures.