Chain Exchange Kinetics of Bottlebrush Block Copolymer Micelles

Chain exchange kinetics of the bottlebrush core micelles formed by poly(5-perfluorooctyl-2-norbornene-block-N-cyclohexyl-exo-norbornene-5,6-dicarboxyimide) (PF-b-PC) block copolymer in tetrahydrofuran (THF), selective for PC blocks, were investigated by time-resolved small-angle neutron scattering (TR-SANS). The bottlebrush PF core blocks are densely tethered with rigid fluoroalkyl side chains. Despite the unfavorable interaction between the PF cores and the medium, TR-SANS revealed that the chain exchange between the bottlebrush core micelles occurs within a measurable timescale (∼1 h) at 40 °C. The measured exchange kinetics were successfully described by the model based on the thermodynamic barrier for the core block extraction and the solvent plasticizing effect associated with partial swelling of the PF cores. This chain exchange rate is also partly attributed to the bottlebrush architecture of PF blocks, as supported by lower melt viscosities of PF homopolymers than those of the analogous fluorine-containing linear polymers. These results are discussed in terms of current understanding of molecular exchange between block copolymer micelles, and particular attention is paid to the issue of polymer architecture, given the bottlebrush type of the core block.