The effect of solvent selectivity on the chain exchange kinetics for two polystyrene-b-poly(ethylene-alt-propylene) block copolymer micelles (SEP 26-70 and SEP 42-64, where the numbers denote the molecular weight of each block in kg/mol) was investigated in pure squalane and in binary mixed solvents of squalane and 1-phenyldodecane, using time-resolved small-angle neutron scattering (TR-SANS). The exchange rate accelerates by 5 orders of magnitude for both micelles when adding only 25 vol % 1-phenyldodecane to squalane, compared to micelles in pure squalane. This acceleration is attributed to two factors: faster relaxation dynamics of the core blocks as more solvent penetrates into the core, serving as a plasticizer, combined with a reduced energy barrier for chain expulsion due to the lower Flory-Huggins interaction parameter χ between the core block and the solvent. By fitting the TR-SANS data to an established theoretical model, the enthalpic penalty of chain expulsion is determined for SEP micelles in mixed solvents. These results are quantified by a χ-dependent function derived from Flory-Huggins theory, where χ values between the polystyrene (PS) core block and binary mixed solvents are estimated by a combination of static light scattering and cloud point measurements with PS homopolymers. This work quantifies the role of χ in chain exchange kinetics of block copolymer micelles.
Bibliographical noteFunding Information:
This work was supported by Infineum USA, L. P. and by the National Science Foundation Polymers Program through award DMR-1707578. The TR-SANS experiments were conducted at the 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. We acknowledge Dr. Lilin He for help with TR-SANS experiments at ORNL. 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. Helpful discussions with Michael Rubinstein and David Morse are appreciated.
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