Kinetics of\nChain Exchange in Asymmetric ABA′\nTriblock Polymer Micelles by Time-Resolved Small-Angle Neutron Scattering

The effect of corona block asymmetry\non chain exchange\nkinetics\nhas been investigated using time-resolved small-angle neutron scattering\n(TR-SANS) with a series of AB di- and ABA′ triblock micelles\ncomposed of poly(ethylene-<i>alt</i>-propylene) (A) and\npolystyrene (B) block copolymers in squalane. The four copolymers\nmaintain constant molecular weight and total block composition, but\nhave different asymmetry ratios of the end blocks, denoted as SEP,\nEPSEP′-1, EPSEP′-2, and EPSEP. The TR-SANS results reveal\na 10-fold acceleration in the chain exchange rate for asymmetric triblock\ncopolymer micelles (EPSEP′-1 and EPSEP′-2) and a hundred-fold\nacceleration for symmetric triblock copolymers (EPSEP), compared to\nthe SEP diblock copolymer. This enhanced exchange rate is primarily\nattributed to differences in the micelle structure associated with\ncorona chain entropy during extraction, supported by calculations\nof grafting chain density and stretching of corona chains. Thus, this\nstudy demonstrates an interesting and significant role of block copolymer\nasymmetry in molecular exchange dynamics.