Rheological characterization of BCC and FCC structures in aqueous diblock copolymer liquid crystals
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Low molecular weight, amphiphilic diblock copolymers in selective solvent exhibit complex phase behavior and macroscopic properties that affect the processing and application of these materials. The mechanical properties of the crystalline phases seen in concentrated solutions are dependent on nanoscale structure and sample history. The goal of this study is to characterize macroscopic properties and thermal history effects of lyotropic liquid crystals in aqueous diblock copolymer solutions. Rheological temperature ramps are used to characterize three aqueous concentrations of diblock copolymer [Brij-58®, C16H33(CH2CH2O)20OH]. Between these three samples the order-disorder transitions (ODTs) for BCC and FCC are accessible in addition to the order-order transition (OOT) between BCC and FCC. These transitions are distinguished using rheology. Frequency sweeps are performed across a range of temperatures and parameterized with a loglinear fit to the phase angle data to extract the crossover frequency. We find that a single frequency sweep does not distinguish BCC and FCC structures. By normalizing the temperature with respect to the ODT, we are able to use a series of frequency sweeps to distinguish characteristic trends in the response of BCC and FCC structures to thermal history.
Keywordsliquid crystals block copolymers viscoelasticity rheology hysteresis
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The authors would like to thank Mahesh Mahanthappa and Ashish Jayaraman at the University of Minnesota for providing access to the pre-print phase diagram of Brij- 58® and water in addition to complementary SAXS data.