Effects of Branching on Conformation, Crystallization, and Self-Assembly of Polymers

Citation

Kim, Joey Dongjin (2019) Effects of Branching on Conformation, Crystallization, and Self-Assembly of Polymers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8AFE-YC54. https://resolver.caltech.edu/CaltechTHESIS:05302019-112215583

Abstract

The central feature of bottlebrush polymers is the stiffening of the main-chain (MC) due to side-chain side-chain (SC-SC) repulsion, amplified by densely grafting long SCs, particularly in good solvent conditions. The expectation of stiffening has led most prior studies to refer to bottlebrush polymers as "worm-like," "cylindrical" or a "self-avoiding walk (SAW) of superblobs". However, there is no direct evidence of stiffening of the main-chain and measurements of the overall segment distribution of the whole molecule have failed to discriminate between competing models. Here, we provide a set of measurements of the main-chain conformation (neutron scattering in a solvent that is contrast matched to the side chains) together with the overall conformation of the bottlebrush as a whole (light, X-ray, and neutron scattering) under conditions that highlight SC-SC repulsion: the side-chains are relatively long compared to prior literature, the concentration of bottlebrushes is low, and the solvent quality is good. Surprisingly, the main-chain has a conformation that does not conform to any prior models: all three main-chain lengths examined showed a window of length scales in which the scattering power increased less than linearly with length scale. In particular, the MC conformation is not worm-like. Direct observation of the main-chain conformation and the overall conformation discriminates among models more powerfully than the overall conformation alone. Inspired by the Paturej-Rubinstein tension blob model, we examined a conceptual model in which tension of the MC accumulates with distance from the ends of the MC and found that it can capture the salient features of both the MC- and whole bottlebrush scattering more gradually than previous theoretical models predicted. The conceptual model also explains our observation of a substantial increase in anisometry with increasing MC length, opposite to a worm-like chain. The results indicate that synthetically accessible bottlebrushes are not fractals; they cannot have self-similar (fractal) conformation because each increase in main-chain length accesses greater side-chain crowding than any of its shorter siblings. We expanded the work to understand the behavior in θ conditions and shorter side-chains expected to have reduced tension as well as the behavior at different concentrations.

In addition, we characterized the interplay of self-assembly and polymer crystallization through analysis of three representative bottlebrush copolymer systems. Our results revealed a surprising number of unexpected behaviors ranging from unexpected morphologies, control of thermal properties even to complete suppression of phase transitions, and control of the orientation of crystal stem with respect to the morphological interface, which highlights the potential of the bottlebrush architecture.

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