(713e) In Situ Rheodielectric Investigation of Shear-Induced Alignment of Lyotropic Liquid Crystal Mesophases

Lyotropic liquid crystals (LLCs) can be formed through the self-assembly of amphiphilic block copolymers in the presence of selective solvent(s). By altering the temperature and concentration, a variety of nanostructures, such as lamellar, hexagonal, cubic, and gyroid structures, can be obtained. Nanostructured cross-linked networks known as polyLLCs can be obtained by polymerizing templates of LLCs containing monomers. However, through the self-assembly process, a polygrain structure is obtained with locally anisotropic ordered domains (grains). The randomly oriented grains throughout polyLLCs result in a macroscopically isotropic material. Since most advanced applications require the nanostructured materials to be macroscopically aligned, it is essential to investigate the mechanism and underlying processes for alignment especially for shear-induced alignment. It has been reported that both simple shear flow and large amplitude oscillatory shear (LAOS) flow can result in the alignment of mesophases systems. However, LAOS has been found more effective for macroscopic orientation of symmetric block copolymers as it allows for different orientations of the unit normal of the lamellae (reverse, perpendicular, and parallel orientation). Poloxamers are amphiphilic triblock copolymers with basic chain structure of polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO). Due to the stability and safety, they are used in many biological and pharmaceutical applications. In this work, we investigate the orientation of LLCs prepared with two different types of poloxamers P123 (PEO₂₀-PPO₆₉-PEO20) and F127 (PEO₁₀₀-PPO₆₅-PEO₁₀₀) under LAOS through in situ rheodielectric measurement. Our results show that due to differences in viscoelastic properties, the LLCs prepared with F127 and P123 reach to parallel and perpendicular orientations relative to LAOS flow direction, respectively. In addition, comparing the conductivity of polyLLCs with and without LAOS before and after curing indicates that the ordered nanostructure can be retained after thermal polymerization. The results from this work can be used in templating fabrication of ordered nanostructured materials for microelectronic, biomaterial, and separation applications.