Enhanced ordering in length-polydisperse carbon nanotube solutions at high concentrations as revealed by small angle X-ray scattering

TL;DR

This study combines small-angle X-ray scattering and microscopy to analyze how carbon nanotubes self-assemble into liquid crystalline phases at high concentrations, revealing the role of thermal undulations in phase behavior.

Contribution

It provides a quantitative characterization of liquid crystalline phases in CNT solutions and explains the influence of thermal undulations on their electrostatic and steric interactions.

Findings

CNTs form liquid crystalline phases with pleated textures at high concentrations

Thermal undulations increase effective diameter and influence phase transitions

Critical concentration for undulation effects is close to the isotropic cloud point

Abstract

Carbon nanotubes (CNTs) are stiff, all-carbon macromolecules with diameters as small as one nanometer and few microns long. Solutions of CNTs in chlorosulfonic acid (CSA) follow the phase behavior of rigid rod polymers interacting via a repulsive potential and display a liquid crystalline phase at sufficiently high concentration. Here, we show that small-angle X-ray scattering and polarized light microscopy data can be combined to characterize quantitatively the morphology of liquid crystalline phases formed in CNT solutions at concentrations from 3 to 6.5 % by volume. We find that upon increasing their concentration, CNTs self-assemble into a liquid crystalline phase with a pleated texture and with a large inter-particle spacing that could be indicative of a transition to higher-order liquid crystalline phases. We explain how thermal undulations of CNTs can enhance their electrostatic repulsion and increase their effective diameter by an order of magnitude. By calculating the critical concentration, where the mean amplitude of undulation of an unconstrained rod becomes comparable to the rod spacing, we find that thermal undulations start to affect steric forces at concentrations as low as the isotropic cloud point in CNT solutions.