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Thursday, February 14, 2019 at 4:00pm to 5:00pm
MSE Seminar 2/14: B11 Kimball Hall @ 4pm
University of Minnesota
Thermal Processing of Particle-Forming Diblock Copolymers: Mimicking Metallurgy
Low-symmetry packing structures, usually observed in metals and metal alloys, recently have been identified in various forms of soft materials including dendrimers, surfactants, and block copolymers. In block copolymers these complex phases emerge from the supercooled fluctuating disordered state, comprised of well-formed micelles. Rapid cooling to sufficiently low temperatures below the order-disorder transition extinguishes molecular exchange resulting in non-ergodic “liquid-like packing”. In this talk, I will present findings of new low-symmetry structures obtained as a function of temperature and time from polyisoprene-b-polylactide (PI-b-PLA) diblock copolymers containing 15 to 25 % PLA based on time-dependent synchrotron X-ray scattering and draw connections with the thermal processing of metal alloys. For a given pre-ordered phase, heating above the order-disorder transition temperature (TODT) followed by cooling below TODT returns the system to the same ordered state. Small angle X-ray scattering revealed that the number density of micellar particles, n/V, is retained in the highly structured disordered liquid. We hypothesize that the number of micelles per volume, n/V, imprinted on the liquid during the initial ordering stage, governs the symmetry breaking during subsequent crystallization, and that the metastable structured liquid cannot achieve the equilibrium particle density due to prohibitively large free-energy barriers for micelle fusion and fission. Once the n/V is fixed, facile chain exchange redistributes mass to meet the required particle sizes and packing associated with individual ordered phases. This work reveals the nature of the fluctuating disordered state in asymmetric diblock copolymers in the low molar mass limit.