Chiral Mesogen-Free Liquid Crystalline Polyethers with Sulfonylated Side Chains and Patchy Brush Nanoparticles
Date of Award
Doctor of Philosophy
Mark Dadmun, Ampofo Darko, Manolis Doxastakis
Ferroelectric liquid crystalline polymers (LCPs) hold promise for various applications driven by low electric fields, e.g., electrocaloric materials, because of the higher molecular motion in the liquid crystalline (LC) state. However, traditional chiral smectic C (SmC*) LCPs exhibit small spontaneous polarizations due to the bulky aromatic mesogens and weak polar groups. This dissertation research is focused on the design of mesogen-free sulfonylated LCPs with a goal of seeking the ferroelectric SmC* phase. Such LCPs are expected to exhibit high polarizations owing to the sulfonyl’s large dipole moment. A series of poly(oxypropylene)s (POPs), with chirality being introduced into either the backbone or sulfonylated side chains or both, were synthesized, and their self-assembly behavior was investigated.
Chirality was first introduced into the backbone of POPs with linear mono- and di-sulfonylated side chains. Although smectic E and A (SmA) phases were observed, no SmC* was found in these polymers. The backbone chirality had an insignificant influence on the LC self-assembly behavior. Two additional strategies were then explored: decreasing the grafting density of sulfonylated side chains and introducing a methyl branch into the short side chains of POPs. SmA and hexagonal columnar structures were observed for isotactic POPs with various degrees of n-dodecylsulfonyl substitution. When a chiral methyl branch was introduced into the short side chains of an isotactic POP, a crystalline phase with titled side chains was discovered. The alkyl tail was then increased and a -CH2CH2CH2O- spacer was incorporated, producing a SmC*-like phase for the single chirality POPs. However, no ferroelectric switching was achieved, likely due to the strong dipole-dipole interactions. These studies provide valuable information for future design of mesogen-free ferroelectric SmC* LCPs.
The last part of this dissertation concerned the morphology of binary mixed homopolymer brush-grafted, 20.4 nm silica nanoparticles (NPs). Transmission electron microscopy showed that the brush NPs phase separated into patchy NPs with 6 to 10 nanodomains (valency) upon solution casting. A linear relationship between valency and core size was observed; the NPs can form physical “bonds” with each other and exhibit an ability to change the valency. This work provides a new approach toward multivalent patchy NPs.
Bohannon, Caleb A., "Chiral Mesogen-Free Liquid Crystalline Polyethers with Sulfonylated Side Chains and Patchy Brush Nanoparticles. " PhD diss., University of Tennessee, 2022.
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