Evolution of diverse higher-order membrane structures of block copolymer vesicles

Abstract

It is an important challenge to create and reveal subtle higher-order membrane nanostructures of block copolymer vesicles by conventional transmission electron microscopy (TEM) due to the difficulty in trapping them. To solve this problem, a series of hydrolytically cross-linkable poly(ethylene oxide)-block-poly(3-(trimethoxysilyl)propyl methacrylate) (PEO-b-PTMSPMA) block copolymers were self-assembled into vesicles with diverse higher-order membrane structures. Compared to other block copolymers as the building block of vesicles, the advantage of PEO-b-PTMSPMA is that the subtle membrane structures can be readily immobilized by in situ sol–gel reactions in the membranes composed of the reactive PTMSPMA block to afford vesicles with silicified membranes, facilitating the visualization by conventional TEM without staining or freezing samples. Upon manipulating the PTMSPMA length, initial copolymer concentration (C_ini), organic co-solvents and water content (C_w), a wide range of vesicles with diverse higher-order membrane structures have been discovered: genus vesicles, genus-multi-compartment vesicles, multi-compartment vesicles, Janus vesicles, onion-like vesicles, tubule-like vesicles, sunflower-like vesicles, bag-like vesicles, hexagonally packed hollow hoop (HHH)-like vesicles, etc. The specific conditions for creating various membrane structures are summarized in a phase diagram, and their possible formation mechanisms were proposed based on the interaction between copolymer chains and solvents during self-assembly. Finally, we discovered an evolutionary route to polymer vesicles with diverse higher-order membrane structures.