Self-assembly of lyotropic chromonic liquid crystals: Effects of additives and applications

Heung-Shik Park

Kent State Universisy

Lyotropic chromonic liquid crystals (LCLCs) are composed of plank-like molecules with rigid polyaromatic cores and two or more ionic groups at the periphery. These molecules typically stack on top of each other leaving the ionic solubilizing groups at the aggregate-water interface. As the concentration of LCLC increases, the aggregates multiply, elongate, and align parallel to each other and then form mesophases. The two most commonly met phases in LCLCs are the uniaxial nematic (N) phase and the hexagonal columnar phase with aggregates forming a hexagonal lattice in the plane perpendicular to the average orientation of aggregates. This thesis explores how the aggregate structure and the phase diagrams of LCLCs in water depend on their concentration, temperature, pH of the solution, and the presence of various additives, such as salts and neutral polymers. The two main mechanisms associated with the role of additives are (1) electrostatic interactions within and between the aggregates and (2) excluded volume effects induced by the neutral additives. Mono- or divalent salts enhance the stability of the N phase when the concentration of LCLCs and salts is small, while they suppress the mesophases when the concentration of LCLCs and salts is large. The addition of non-ionic additives such as PEG to the SSY solution leads to phase-separation into a condensed liquid crystalline (LC) region with a high concentration of SSY and a PEG-rich isotropic region. In the condensed LC region, the distance between the SSY aggregates decreases and the average length of the aggregates increases when the concentration of PEG increases. This dissertation also describes a potential application of LCLCs as a functional material for nanofabrication, namely, a controlled and reversible assembly of gold nanorods. The anisotropic electrostatic interaction between the metallic NRs and chromonic stacks allows one to achieve either side-by-side or end-to-end assembly, depending on the surface charge of the NRs. The assembly of NRs can be controlled by a number of factors influencing the self-assembly of chromonic materials, such as the concentration and pH of the solution. We hope that these studies provide a basic understanding of phase behavior and the physical properties of the reversible self-assembled chromonic materials and expand the opportunities for practical applications of LCLCs.

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