(629h) A Hexagonal Columnar Liquid Crystal Phase Formation in Dilute Solutions of Carbon Nanotubes

Solutions of carbon nanotubes (CNTs) in chlorosulfonic acid (CSA) form liquid crystals at high concentrations. From a fundamental perspective, CNTs are an ideal candidate system for testing theories of liquid crystals for rod-like systems with high degree of polydispersity. From an application point of view, these liquid crystal solutions are of special interest for the fluid-phase self-assembly approach for processing CNT-based macroscopic materials. While CNT-CSA solutions exhibit a phase behavior consistent with the Onsager prediction for non-interacting rigid rods, the ordering nature of the solution and how it depends on the CNT properties and the concentration are not completely understood. Here, we use small angle x-ray scattering to characterize the morphology of the liquid crystal phase and get more insights on the ordering of CNTs in solutions. Our results show that the at high concentration (~5% in volume), our system self assembles into a hexagonally packed columnar phase. Theoretically, for a length polydisperse system with p > 0.18 (polydispersity is defined as p = √(<L^2>/<L>^2 − 1) where L is the length of molecules), the system will exhibit a direct nematic-columnar phase transition at high concentrations (about 50% by volume). Surprisingly, CNTs show the nematic-columnar transition at much lower concentration (~ 5 % by volume). We attribute this difference in behavior to steric forces that could be the result of electrostatic repulsions between CNTs. Such steric forces confine the CNTs to a larger area that increases the effective volume fraction and results in a nematic-columnar transition at lower concentrations. Beside its fundamental importance, this finding should have important implications for processing of multifunctional CNT fibers at high concentrations where the alignment of the CNTs affects the mechanical properties of the final product.