Molecular Exchange in Diblock Copolymer Micelles: Bimodal Distribution in Core-Block Molecular Weights

The role of core block size dispersity on the rate of molecular exchange in spherical micelles formed from 1% by volume poly(styrene-b-ethylenepropylene) (PS-PEP) diblock copolymers in squalane (C₃₀H₆₂) was investigated using time-resolved small-angle neutron scattering (TR-SANS). Separate copolymer solutions (total polymer 1% by volume) containing either deuterium labeled (dPS) or normal (hPS) poly(styrene) core blocks were prepared and mixed at room temperature, below the core glass transition temperature. Each preparation (dPS or hPS) contained equal volume fractions of M_n = 26 and 42 kg/mol (h-equivalent) poly(styrene) blocks. Heating to temperatures between 87 and 146 °C resulted in block copolymer exchange as evidenced by a systematic reduction in the SANS intensity; C₃₀H₆₂ and C₃₀D₆₂ were blended so as to contrast match the fully exchanged cores. Following a protocol established in a previous report, the time-dependent intensity data were shifted with respect to time and temperature, leading to a master curve covering nearly 7 orders of magnitude in reduced time. These results are quantitatively accounted for by summing the weighted relaxation functions obtained from the individual components, consistent with a previously published model that accounts for the dramatic sensitivity of the molecular exchange dynamics to core block dispersity.