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Recently, attention has been directed toward understanding the dynamics and relaxation kinetics of block copolymer micelles, including mechanisms such as micelle fragmentation and fusion. The few prior studies on block copolymer micelle fragmentation relied on ensemble averaging techniques such as small-angle X-ray scattering and dynamic light scattering; some individual particles were imaged by ex situ transmission electron microscopy. Here we report the direct observation of fragmentation for three molecular weights of 1,2-polybutadiene-block-poly(ethylene oxide) (PB-PEO) micelles in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide using high-temperature liquid-phase transmission electron microscopy (LP-TEM). The use of in situ LP-TEM provides unique insights into the evolution of block copolymer micelles during fragmentation. Specifically, upon heating to 170 °C, a sequence of morphological transitions from a spherical micelle to a prolate ellipsoid, then a "peanut" shape, followed by a two-spherical-compartment micelle was observed, where the last is presumed to be the transition state.
Bibliographical noteFunding Information:
This work was supported primarily by the National Science Foundation (DMR-1707578) and by the U.S. Department of Energy (DOE), Office of Science, Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program. The SCGSR program is administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE. ORISE is managed by ORAU under Contract No. DE-SC0014664. Portions of this work were performed at the Center for Functional Nanomaterials, and the National Synchrotron Light Source II, Brookhaven National Laboratory, which are supported by the U.S. DOE Office of Science under Contract No. DE-SC0012704. Data were collected using an instrument funded by the National Science Foundation under Award No. 0960140. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program (DMR-1420013). We thank Dr. Ruipeng Li and Dr. Esther Tsai for assistance with TR-SAXS measurements at the 11-BM CMS beamline at the NSLS-II and Dr. Lihua Zhang for insightful discussions on high temperature LP-TEM.
© 2020 American Chemical Society.
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