Soo-Hyung Choi Thesis defense of Ph.D, August 18, 2010

Block Copolymer Self-Assembly in Solution: Structure and Dynamics

Thesis defense of Ph.D. candidate Soo-Hyung Choi
Department of Chemical Engineering and Materials Science.

Advisors: Frank Bates and Tim Lodge of IPrime’s Microstructured Polymers program.

Held Wednesday, August 18, 2010, 9:00 a.m., 240 Amundson Hall, University of Minnesota.

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Abstract
Block copolymers can self-assemble into micelles or vesicles when dispersed in a selective solvent. In this study, spherical micelles were formed by poly(styrene-b-ethylene-alt-propylene) (PS-PEP) in squalane, highly selective to PEP blocks, leading to PS cores and swollen PEP coronas. The micelle structure was characterized by dynamic light scattering (DLS) and small-angle x-ray scattering (SAXS). The experimental results provide a detailed picture of micelle structure and intermicelle interaction as a function of block copolymer molecular weight and composition, concentration, and temperature. Based on this structural information, the single molecular exchange kinetics between the spherical micelles in dilute solution was examined by time-resolved small-angle neutron scattering (TR-SANS). Two pairs of structurally matched partially protonated and deuterated micelles were prepared and each pair was blended to provide an initially isotopically segregated state in solution. The SANS intensity is directly related to the concentration of protonated chains in the micelle cores. Therefore, a reduction in the measured scattering intensity can be quantitatively correlated with the exchange of chains. This measurement was aimed at probing the dependence of molecular exchange kinetics on temperature, molecular weight, and concentration. The temperature dependence of the chain exchange rate R(t) can be explained based on the core block dynamics, while the documented quasi-logarithmic decay of R(t) is shown to be consistent with single chain exchange that is hypersensitive to the core degree of polymerization and therefore polydispersity.