Origins of low-symmetry phases in asymmetric diblock copolymer melts

Kyungtae Kim, Akash Arora, Ronald M. Lewis, Meijiao Liu, Weihua Li, An Chang Shi, Kevin D. Dorfman, Frank S. Bates

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102 Scopus citations

Abstract

Cooling disordered compositionally asymmetric diblock copolymers leads to the formation of nearly spherical particles, each containing hundreds of molecules, which crystallize upon cooling below the order–disorder transition temperature (TODT). Self-consistent field theory (SCFT) reveals that dispersity in the block degrees of polymerization stabilizes various Frank–Kasper phases, including the C14 and C15 Laves phases, which have been accessed experimentally in low-molar-mass poly(isoprene)-b-poly(lactide) (PI-PLA) diblock copolymers using thermal processing strategies. Heating and cooling a specimen containing 15% PLA above and below the TODT from the body-centered cubic (BCC) or C14 states regenerates the same crystalline order established at lower temperatures. This memory effect is also demonstrated with a specimen containing 20% PLA, which recrystallizes to either C15 or hexagonally ordered cylinders (HEXC) upon heating and cooling. The process-path–dependent formation of crystalline order shapes the number of particles per unit volume, n/V, which is retained in the highly structured disordered liquid as revealed by small-angle X-ray scattering (SAXS) experiments. We hypothesize that symmetry breaking during crystallization is governed by the particle number density imprinted in the liquid during ordering at lower temperature, and this metastable liquid is kinetically constrained from equilibrating due to prohibitively large free energy barriers for micelle fusion and fission. Ordering at fixed n/V is enabled by facile chain exchange, which redistributes mass as required to meet the multiple particle sizes and packing associated with specific low-symmetry Frank–Kasper phases. This discovery exposes universal concepts related to order and disorder in self-assembled soft materials.

Original languageEnglish (US)
Pages (from-to)847-854
Number of pages8
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number5
DOIs
StatePublished - Jan 30 2018

Bibliographical note

Funding Information:
Byeongdu Lee is acknowledged for his help with experiments conducted at Beamline 12-ID-B at the Advanced Photon Source (APS). Marc Hillmyer provided a valuable review of the manuscript. This research was supported by the National Science Foundation under Grants DMR-1104368 and DMR-1333669, the National Science Foundation of China under Grants 21774025 and 21320102005, and the Natural Science and Engineering Research Council of Canada. The work was enabled by resources at the APS, a US 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. Part of this work was performed at the DuPont–Northwestern–Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the APS. DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company, and Northwestern University. Parts of the computations were made possible by the Minnesota Supercomputing Institute at University of Minnesota, as well as the facilities of the Shared Hierarchical Academic Research Computing Network (www.sharcnet.ca) and Compute/ Calcul Canada.

Funding Information:
APS, a US 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. Part of this work was performed at the DuPont–North-western–Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the APS. DND-CAT is supported by E.I. DuPont de Nemours & Co., The Dow Chemical Company, and Northwestern University. Parts of the computations were made possible by the Minnesota Supercomputing Institute at University of Minnesota, as well as the facilities of the Shared Hierarchical Academic Research Computing Network (www.sharcnet.ca) and Compute/ Calcul Canada.

Funding Information:
ACKNOWLEDGMENTS. Byeongdu Lee is acknowledged for his help with experiments conducted at Beamline 12-ID-B at the Advanced Photon Source (APS). Marc Hillmyer provided a valuable review of the manuscript. This research was supported by the National Science Foundation under Grants DMR-1104368 and DMR-1333669, the National Science Foundation of China under Grants 21774025 and 21320102005, and the Natural Science and Engineering Research Council of Canada. The work was enabled by resources at the

Keywords

  • Frank–Kasper phases
  • block copolymers
  • fluctuating disorder
  • sphere-forming diblocks
  • structured liquids

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