Chain Exchange Kinetics of Triblock Copolymer Micelles: Effect of Corona Block Asymmetry

The chain exchange kinetics of block polymer micelles has been observed to be much slower than small surfactants due to the enhanced energy barrier for expulsion of polymer chains. Recently, micelles formed by a symmetric poly(ethylene-alt-propylene)-b-poly(styrene)-b-poly(ethylene-alt-propylene) (PEP-PS-PEP) triblock copolymer were reported to exhibit three orders of magnitude faster exchange kinetics than the equivalent PS-PEP diblock micelles when immersed in squalane, a selective solvent for PEP. Dissipative particle dynamics simulation also revealed faster kinetics in BAB’ triblocks than in the AB diblock, where A is the core block, and B and B’ are distinct corona blocks. This work systematically investigates the structure and chain exchange kinetics of asymmetric PEP-PS-PEP’ triblock micelles by varying the corona block asymmetry at constant core block length and total corona block length, via small-angle X-ray and time-resolved small-angle neutron scattering measurements. Both symmetric and asymmetric triblock micelles display faster exchange kinetics than the diblock. However, the asymmetric micelles exhibit non-monotonic changes in kinetics as corona block asymmetry increases, which we attribute to enhanced compatibility between the core and the shorter corona block.