Addition of Corona Block Homopolymer Retards Chain Exchange in Solutions of Block Copolymer Micelles

The exchange of copolymer chains between 1 vol % PS-PEP (poly(styrene-b-ethylene-alt-propylene)) di-block copolymer micelles in squalane (selective for PEP) is investigated using time-resolved small-angle neutron scattering (TR-SANS) as a function of added PEP homopolymer. The solvent squalane, C30H62, is substituted in part or completely with PEP homopolymers that are the same molecular weight as the corona blocks. Polymer solutions/mixtures (1 vol % PS-PEP, plus 2, 7, or 15 vol % PEP in squalane, and 1 vol % PS-PEP in PEP) were separately prepared using normal (h-PS) or deuterated equivalent (d-PS) PS-PEP diblock copolymers. The solvent was contrast matched to a 50/50 mixed h-/d-PS micelle core, so that the scattering intensity decays with the mixing of h- and d-PS-PEP chains undergoing exchange between micelles. The chain exchange rate can therefore be assessed quantitatively. As the concentration of added homopolymer in solution increases above the overlap concentration of PEP chains, the chain exchange rate drops significantly. The results are compared to an earlier study of chain exchange between PS-PEP micelles in a 15% solution in squalane, which was also found to be significantly slower than when the solution is dilute. The primary factor in this slowing down of chain exchange is an increased screening of excluded volume interactions among the corona blocks. The role of increasing micelle aggregation number with PEP concentration is found not to be the dominant effect up to 15% added PEP but may play an increasingly important role in the PEP melt matrix, where no chain exchange could be detected in these experiments.