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Small-angle X-ray scattering analyses reveal that the hydrated diblock oligomer n-C 16H 23(OCH 2CH 2) 20-OH (C 16E 20 or Brij 58) forms lyotropic liquid crystals (LLCs) exhibiting face-centered cubic (FCC), body-centered cubic (BCC), Frank-Kasper (FK) A15, and cylindrical (H I) morphologies over the concentration range 30-65 wt % amphiphile. Heating LLCs comprising 54-59 wt % C 16E 20 drives the temperature-dependent phase transition sequence: A15 BCCH I. However, rapidly quenching the resulting H I phase from 70 to 25 °C initially forms a BCC phase that isothermally transforms into a complex, tetragonal FK σ phase comprising 30 quasispherical micelles. The metastability of this micellar σ phase is shown to depend on the sample cooling rate, thermal quench depth, and isothermal annealing temperature. We rationalize the preference for the A15 structure at 25 °C in terms of minimizing unfavorable water/hydrophobic contacts, while maximizing local particle sphericity. The symmetry breaking transition kinetics in these micellar LLCs apparently stem from the temperature-dependent activation barriers for phase nucleation and growth, which are intimately coupled to the time scales for micelle reconfiguration by amphiphile chain exchange and their spatial rearrangement. These findings highlight how thermal processing influences nucleation and growth of the self-assembled morphologies of intrinsically reconfigurable, soft spherical particles.
Bibliographical noteFunding Information:
This work has been supported by National Science Foundation Grants CHE-1608115 and CHE-1807330 (A.J. and M.K.M.) and funding through the MRSEC program under DMR-1559833 and DMR-1420013 (D.Y.Z. and B.L.D.). Preliminary SAXS analyses were acquired in the Characterization Facility at the University of Minnesota, which receives partial support from NSF through the MRSEC program (DMR-1420013). Synchrotron SAXS experiments were conducted at the 12-ID-B beamline of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota (http://www.msi.umn.edu) for providing resources for reconstructing electron density maps using the SUPERFLIP program. We thank Grayson L. Jackson, Ronald M. Lewis, III, Frank S. Bates, Timothy P. Lodge, and Kevin D. Dorfman for helpful discussions, and we are grateful to Akash Arora for providing assistance with various calculations.
Copyright © 2019 American Chemical Society.
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- Journal Article