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Ha-Kyung Kwon

HKK

Current Position: Ph.D. Student in Materials Science at Northwestern University

Research Interests: Synthesis and Characterization of Polystyrene-poly(2-ethylhexylmethacrylate) (PS-PEHMA) Copolymers

Diblock copolymers have been extensively studied for their ability to microphase separate into structured nanodomains. The molecular weight at which a block copolymer microphase separates into distinct domains is determined by the Flory interaction parameter, χ. The volume fraction of each block determines the geometry of the domains (i.e. spheres or cylinders of one block dispersed in a matrix of the second). Because of the small feature size (on the order of 10 nm) and the wide range of morphologies that are readily accessible by tuning each of these parameters, block copolymers have been identified as effective etch mask materials for nanofabrication.

 For example, block copolymers of polystyrene-poly(n-hexylmethylacrylate) (PS/PHMA) have been shown to be particularly useful for nanofabrication of polarizers for ultraviolet light. It has also been shown that the height and periodicity of the nano-scale features play an important role in improving the efficacy of the polarizer These properties—height, periodicity, and orientation of the domain structures—are tunable through block copolymer material selection, composition, and molecular weight.

 The objective of my project is to explore materials with even smaller domain sizes than can be achieved with PS/PHMA, thus potentially improving performance of certain applications. My research focuses on diblock copolymers of PS/PEHMA. EHMA has a more massive—and branched—side chain than HMA, and therefore is expected to have a much greater χ. Increasing the Flory interaction parameter reduces the minimum molecular weight required for microphase separation, thus reducing the size of the separated nanodomains. In this case, the expected minimum molecular weight required for microphase separation in PS/PEHMA is half that of PS/PHMA. As the domain size scales to -2/3 power of χ, a 100% increase in χ would reduce the domain size by 37%, also reducing minimum feature size in nanofabrication by 37%. In addition, it is expected that PS/PEHMA will more easily show desirable orientation of in-plane cylinders due to the lower surface energy of EHMA.