Kinetics of Chain Exchange in Asymmetric ABA′ Triblock Polymer Micelles by Time-Resolved Small-Angle Neutron Scattering

The effect of corona block asymmetry on chain exchange kinetics has been investigated using time-resolved small-angle neutron scattering (TR-SANS) with a series of AB di- and ABA′ triblock micelles composed of poly(ethylene-alt-propylene) (A) and polystyrene (B) block copolymers in squalane. The four copolymers maintain constant molecular weight and total block composition, but have different asymmetry ratios of the end blocks, denoted as SEP, EPSEP′-1, EPSEP′-2, and EPSEP. The TR-SANS results reveal a 10-fold acceleration in the chain exchange rate for asymmetric triblock copolymer micelles (EPSEP′-1 and EPSEP′-2) and a hundred-fold acceleration for symmetric triblock copolymers (EPSEP), compared to the SEP diblock copolymer. This enhanced exchange rate is primarily attributed to differences in the micelle structure associated with corona chain entropy during extraction, supported by calculations of grafting chain density and stretching of corona chains. Thus, this study demonstrates an interesting and significant role of block copolymer asymmetry in molecular exchange dynamics.