Morphology of Carbon Nanotube Liquid Crystalline Phases: Insights into Tactoids and Columnar Phase

Solutions of carbon nanotubes (CNTs) in chlorosulfonic acid (CSA) form liquid crystalline phase at high concentrations. CNTs offer a high degree of polydispersity in length which makes them an ideal candidate for testing theories of liquid crystals for polydisperse rod-like systems. Moreover, high concentration CNT-CSA solutions can be processed into macroscopic materials. The properties of the CNT-based macroscopic materials, however, depends on the morphology of the liquid crystalline dope. So far, most of the literature on the liquid crystals of CNTs has been focused on qualitative aspects of the phase behavior. Here, we present our recent results on quantitative understanding of the morphology of the liquid crystal phase and how it is affected by the CNT properties and concentration. Polarized optical micrographs of the semi-dilute solutions of long CNTs shows the formation of the bi-continuous phases, while short CNTs form spindle-shaped nematic droplets, known as tactoids, coexisting with the isotropic phase. Such nematic droplets often partially wet the solid glass surface of the capillary. We developed the first method for characterizing the line tension in lyotropic liquid crystal systems by studying the shape of the partially wetting droplets and comparing that with our theoretical predictions. Additionally, we show that higher concentration solutions of CNTs form a hexagonal columnar phase as revealed by small angle x-ray scattering results. This nematic-columnar phase transition occurs at much lower concentrations than what has been predicted theoretically. We attribute this early transition to steric forces rising from electrostatic repulsions between the CNTs.