Chain Exchange Kinetics of Spherical Block Copolymer Micelles: Effect of Core Size

Numerous evidence suggests that chain exchange among discrete micelles plays a vital role in establishing the nano-scale structures resulting from self-assembly of block polymers. In the past decade, the time-resolved small angle neutron scattering (TR-SANS) technique has been utilized to reveal the critical molecular characteristics, including core block chain length, dispersity and core block/solvent interaction parameter, that can affect the kinetics of molecular exchange processes. In this work, we examine the effect of micellar structure on the chain exchange kinetics in spherical micelles, decoupled from chain and solvent characteristics. This was realized by preparing two sets of micelles using different protocols, but from the same block copolymer in the same solvent. SAXS reveals a 30% difference in the core size of these two micelles. More importantly, TR-SANS shows that the chain exchange in the micelles with a larger core size is retarded by a factor of 3.5 compared to that in the smaller ones. Possible causes for this finding will be discussed. These results clearly indicate that, in addition to the molecular characteristics of the block polymer/solvent system, the geometrical structure of the micelle plays an important role in unimer exchange.