Equilibration and Dynamics in Block Copolymer Micelles

Block copolymers provide a remarkably versatile platform for achieving desired nanostructures by self-assembly, with lengthscales ranging from a few nanometers up to several hundred nanometers. While block copolymers generally adopt the morphologies familiar in small molecule surfactants and lipids (i.e., spherical micelles, worm-like micelles, and vesicles), one key difference is that polymeric micelles are typically not at equilibrium. The primary reason is the large number of repeat units in the insoluble block, Ncore, which makes the thermodynamic penalty for extracting a single chain (“unimer exchange”) substantial. As a consequence, the critical micelle concentration is rarely accessed experimentally; however, in the proximity of a critical micelle temperature, equilibration is possible. We use time-resolved small angle neutron scattering to obtain a detailed picture of the mechanisms and time scales for chain exchange, for systems near equilibrium. The dependence of the rate of exchange on the key variables – concentration, temperature, N_core, N_corona – will be discussed. Interestingly, almost none of the observed features are captured by available theory. Then, when micelles are significantly larger or smaller than the equilibrium size, fragmentation and fusion mechanisms, respectively, can become operative. We will describe measurements using dynamic light scattering, small-angle X-ray scattering, and liquid-phase TEM to follow the fragmentation process in detail.