Liquid-phase processing of carbon nanotube (CNT) solutions is a scalable way of producing conductive, strong macroscopic materials like fibers and tapes. The best solvents for CNTs are corrosive or unstable in air, requiring environmental isolation. Although these conditions can be handled in manufacturing, they complicate rheological characterization due to exposed surfaces that lead to degradation. Capillary rheology offers a promising approach to study CNT solutions because samples are fully enclosed. However, commercially available capillary rheometers are typically designed for melt extrusion with single-pass operation and extrudate ejection into air. We present a simple, low-cost multipass capillary rheometer suitable for CNT solutions, studying the impact of CNT concentration and molecular structure (length and diameter) in chlorosulfonic acid solutions (0.1 to 10 wt. % CNT). In this range, the solutions are liquid crystalline, with a polydomain morphology. The solutions range in viscosity from ∼0.001 (0.1 wt. % CNT sheared at 1000 s−1) to ∼100 Pa s (10 wt. % CNT sheared at 1 s−1). Viscosity and power-law index increase with increasing CNT aspect ratio and concentration. At lower concentrations (1 wt. %), longer CNT solutions have about 10 times higher viscosity than shorter CNTs. At the highest concentrations (10 wt. %), the viscosity is nearly independent of CNT length at low shear rates, suggesting behavior dominated by CNT liquid crystalline domain dynamics. This method and results are a pathway toward a deeper understanding of CNT liquid crystalline solutions that can be used to process CNTs into fibers, tapes, and other macroscopic structures.
Dewey et al. (Mon,) studied this question.