Polymolecular and Unimolecular Micelles of Triblock Copolymers

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Gao, Yang
mesogen-driven , cylinder , micelle , macrocycle , triblock copolymer
Reported in this thesis are the studies of micellar aggregates of four triblock copolymers and the unimolecular micelles of a triblock copolymer. The micelles were prepared from BCF and ACF copolymers. Here A, B, C, and F denote poly(acrylic acid), poly(tert-butyl acrylate), poly(2-cinnamoyloxylethyl methacrylate), and the liquid crystalline poly(perfluorooctylethyl methacrylate) block, respectively. At room temperature (21 oC) in solvents that were selective for the A or B blocks, three of the four copolymers formed exclusively cylindrical micelles regardless of their block ratios. Cylindrical micelles were formed because their geometries best accommodated the mesogen-ordering requirement of the core-forming F block, as supported by the results from wide angle X-ray scattering and differential scanning calorimetric studies. Mesogen-driven cylinder formation was further supported by the observation of ridges formed by collapsed coronal chains on the surfaces of dried cylinders. We also observed a morphological transformation from other micellar morphologies to cylindrical micelles at 70 oC, which is near the isotropic-to-smectic A phase transition temperature for the F blocks. This inter-conversion between the vesicular and cylindrical micelles of an ACF sample could be reversed repeatly by temperature cycling. These results provided additional evidence for the mesogen-driven micellization hypothesis. Unimolecular micelles were prepared from CDC triblock copolymers, where D and C denote poly(dimethylaminoethyl methacrylate) and poly(2-cinnamoyloxylethyl methacrylate), respectively. In selective solvents for the D block at high dilutions, the D chain formed a loop, and the terminal C blocks of the isolated unimer chain associated together as a globule, thus closing the loop and rendering a cyclic structure. Alternatively, the terminal C blocks formed individual globules, thus yielding a pompom-coil-pompom structure. To lock in these structures, the globules were photo-crosslinked. The D block chain was subsequently enlarged for AFM observation through a quaternization step, which increased the chain’s diameter and introduced cations to the chain. The semi-flexible thickened polymer chains and the globules were observed by AFM, confirming unambiguously the hypothesized architectures of the unimolecular micelles. The AFM images also allowed the quantification of the macrocyclic structures, and a correlation between the direct AFM results and determined from a traditional size exclusion chromatography technique.
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