"Raft" Formation by Two-Dimensional Self-Assembly of Block Copolymer Rod Micelles in Aqueous Solution

Block copolymers can form a broad range of self‐assembled aggregates. In solution, planar assemblies usually form closed structures such as vesicles; thus, free‐standing sheet formation can be challenging. While most polymer single crystals are planar, their growth usually occurs by uptake of individual chains. Here we report a novel lamella formation mechanism: core‐crystalline spherical micelles link up to form rods in solution, which then associate to yield planar arrays. For the system of poly(ethylene oxide)‐block‐polycaprolactone in water, co‐assembly with homopolycaprolactone can induce a series of morphological changes that yield either rods or lamellae. The underlying lamella formation mechanism was elucidated by electron microscopy, while light scattering was used to probe the kinetics. The hierarchical growth of lamellae from one‐dimensional rod subunits, which had been formed from spherical assemblies, is novel and controllable in terms of product size and aspect ratio. A novel mechanism of lamella formation is described, which involves the hierarchical self‐assembly of poly(ethylene oxide)‐block‐polycaprolactone micelles in water. Spheres transform first into rods, which then align spontaneously and associate laterally to form extended lamellar sheets. These self‐assembled lamellar “rafts” are thus quite different from the traditional chain‐folded crystalline lamellae.