\textbf{Dynamics of Chain Exchange in Block Copolymer Micelles}

Block copolymer micelles are rarely at equilibrium. The primary reason is the large number of repeat units in the insoluble block, $N_{core}$, which makes the thermodynamic penalty for extracting a single chain (``unimer exchange'') substantial. As a consequence, the critical micelle concentration (CMC) is rarely accessed experimentally; however, in the proximity of a critical micelle temperature (CMT), equilibration is possible. We have been using time-resolved small angle neutron scattering (TR-SANS) to obtain a detailed picture of the mechanisms and time scales for chain exchange, at or near equilibrium. Our model system is poly(styrene)-\textit{block}-poly(ethylene-\textit{alt}-propylene)) (PS-PEP), in the PEP-selective solvent squalane (C$_{30}$H$_{62})$. Equivalent micelles with either normal (hPS) or perdeuterated (dPS) cores are initially mixed in a blend of isotopically substituted squalane, designed to contrast-match a 50:50 hPS:dPS core. Samples are then annealed at a target temperature, and chain exchange is revealed quantitatively by the temporal decay in scattered intensity. The rate of exchange as function of concentration, temperature, $N_{core}$, $N_{corona,}$ and chain architecture (diblock versus triblock) will be discussed.