MSE Seminar: Kyungtae Kim, 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.
Kyungtae Kim earned his B.S. in Chemistry (Magna cum Laude) from Pohang University of Science and Technology (POSTECH), Pohang, South Korea in 2008 and Ph.D. in Polymer Chemistry from POSTECH as well in 2014. During the graduate study, he has revealed structure-property relationships of thin film electronic data storage polymers utilizing electronic property analysis and synchrotron X-ray scattering. After graduation, Kyungtae has been pursuing his postdoctoral research with Prof. Frank S. Bates in the Department of Chemical Engineering & Materials Science at University of Minnesota since February 2015. His postdoc research involves two very different projects. One aims to reveal the underlying principles of microphase separation in sphere-forming block polymers at equilibrium and non-equilibrium conditions. The other focuses on interfacial properties of commercial polyolefin blends and multilayer films, in collaboration with Prof. Christopher W. Macosko. These projects rely on the integration of polymer synthesis and molecular characterization, synchrotron X-ray scattering, rheological characterization, and imaging techniques such as scanning electron microscopy (SEM) or transmission electron microscopy (TEM).