The effect of corona block length on micelle structure and molecular chain exchange kinetics has been investigated for a series of dilute poly(styrene)-b-poly(ethylene-alt-propylene) (SEP) diblock copolymer micelles with constant PS core block length («Ncore» ≈ 255) but different PEP corona block lengths («Ncorona» = 256-2080), using a combination of dynamic light scattering (DLS), small-angle X-ray scattering (SAXS), and time-resolved small-angle neutron scattering (TR-SANS). Smaller core radii and aggregation numbers, but significantly thicker corona layers (proportional to Ncorona0.7), were observed with increasing corona block length. Furthermore, 2 orders of magnitude more rapid chain exchange was observed in SEP micelles with the longest corona block compared to the shortest. This effect is attributed to the entropic gain arising from the relief of corona chain stretching upon chain expulsion. We further extend a previous theoretical model by explicitly including a corona term associated with the entropy change in the chain exchange process, which successfully explains the influence of the corona blocks on chain exchange. Our results are in excellent agreement with simulation results of Li and Dormidontova but are apparently contradictory with Halperin and Alexander's theory for hairy micelles and with experimental observations in two other systems that the exchange kinetics slow down with increasing corona block length. These discrepancies reveal unanticipated complexity regarding the role of the corona block in chain exchange.
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This work was supported by Infineum USA, L. P. We acknowledge Dr. Dan Zhao, Dr. Andrew Peters, and Dr. Yuanchi Ma for helpful discussions and Dr. Paul Butler (NIST), Dr. Yimin Mao (NIST), and Dr. Lilin He (ORNL) for help with TR-SANS experiments. The TR-SANS experiments were conducted at the National Institute of Standards and Technology, U.S. Department of Commerce, and High Flux Isotope Reactor in Oak Ridge National Laboratory, which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. The SAXS experiments were conducted at DuPont−Northwestern−Dow Collaborative Access Team (DND-CAT) 5-ID at the Advanced Photon Source, Argonne National Laboratory, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357.
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