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

Research output: Contribution to journalArticle

19 Citations (Scopus)

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

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Transition Temperature
Heating
Temperature
Micelles
Crystallization
Particle Size
Polymerization
Hot Temperature
X-Rays
poly(lactide)
isoprene

Keywords

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

Cite this

Origins of low-symmetry phases in asymmetric diblock copolymer melts. / Kim, Kyungtae; Arora, Akash; Lewis, Ronald M.; Liu, Meijiao; Li, Weihua; Shi, An Chang; Dorfman, Kevin D.; Bates, Frank S.

In: Proceedings of the National Academy of Sciences of the United States of America, Vol. 115, No. 5, 30.01.2018, p. 847-854.

Research output: Contribution to journalArticle

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