Self-assembly of Carbon Nanotubes into a Columnar Phase at Low Concentrations Revealed by Small Angle X-ray Scattering

Solutions of carbon nanotubes (CNTs) in chlorosulfonic acid (CSA) exhibit phase behavior consistent with the Onsager prediction for rigid rods interacting via a repulsive potential, including the appearance of an ordered liquid-crystalline phase upon increasing the concentration. However, the nature of the ordered phase, and how it depends on the properties and the average concentration of the CNTs in the solution, is only partially understood at high concentrations. We apply small-angle X-ray scattering, paired with polarized optical microscopy, to characterize the morphology of the liquid-crystalline phases formed in CNT solutions at concentrations from 3 to 6.5 % by volume. Theoretically, for high length polydispersity of rods, such as CNTs, a direct transition is expected to occur from a nematic to a columnar phase only at high concentrations (~50 % by volume). Surprisingly, we find a hexagonal columnar phase at a relatively low concentration of about 4.3 % by volume in CNT-CSA solutions, implying a large inter-particle spacing. We attribute this early transition to undulation-enhanced electrostatic forces, which increase the effective diameter of the CNTs in solution.