Concentration-Driven Slowdown of Chain-Exchange Kinetics in Block Copolymer Micelle Solutions

Block copolymer micelles find numerous applications including drug delivery, nanoreactors, and nanofabrication. Exchange of polymer chains between micelles is an important equilibration mechanism, primarily studied in dilute polymer solutions, leaving the effects of concentration largely unexplored. Here we investigate the concentration dependence of chain-exchange kinetics for polystyrene-b-poly(ethylene-alt-propylene) (PS-PEP) in PEP-selective squalane. Time-resolved small-angle neutron scattering (TR-SANS) measurements reveal more than a four order-of-magnitude decrease in the rate of chain exchange as polymer concentration increases from 1 to 50 vol%. Small-angle X-ray scattering (SAXS) measurements show a corresponding increase in the micelle aggregation number from about 60 to nearly 200 over the same concentration range. Based on a theory by Halperin, a decrease in interfacial area per chain is expected to induce a steric penalty on chains escaping the micelle core. Activation energies for chain pullout extracted from TR-SANS data using established models are qualitatively in agreement with predicted values. Furthermore, while chain exchange is an important contributor to the ordering of micelles, it appears to be independent of whether micelles adopt a body-centered cubic (BCC) or liquid-like packing (LLP) morphology.