Liquid Crystalline Behaviors of Single-Walled Carbon Nanotubes in an Aqueous Sodium Cholate Dispersion

Abstract

Controlling the alignment of single-walled carbon nanotubes (SWCNTs) on the macroscopic scale is critical for practical applications because SWCNTs are extremely anisotropic materials. One efficient technique is to create an effective SWCNT dispersion, which shows a liquid crystal (LC) phase. A strong acid treatment can realize SWCNT liquid crystalline dispersions. However, strong acids pose a substantial safety risk, which renders the process unfit for mass production. Herein, an isolated SWCNT dispersion displaying an LC behavior is prepared using sodium cholate without an acid treatment, and its phase transition behaviors are systematically investigated across the isotropic to biphasic to nematic phases. As the SWCNT concentration increases, the dispersion undergoes an isotropic-to-nematic phase transition in which the spindle-shaped LC droplets, or the so-called tactoids, and the Schlieren textures can be observed in the intermediate biphasic state and the nematic phase, respectively. The arrangements of SWCNTs in the tactoids and the Schlieren structures are directly investigated by polarized optical microscopy. The clear LC behaviors of the CNT dispersion suggest that the CNT orientations can be controlled by the normal surfactant-assisted method, which is a crucial advantage for the liquid-phase processing of CNT fibers and films.

Liquid-crystalline behaviors of single-walled carbon nanotube aqueous dispersions with different nanotube aspect ratios and surfactants

“…SWCNTs were dissolved in the dispersion as the isolated SWCNT and/or the isolated bundles and showed the firstorder nematic LC transition as increasing the concentration of SWCNTs. 19) Especially, the Schlieren textures of +1/2 and −1/2 strength disclinations in the nematic phase were directly observed by the sensitive tint technique, which was the first empirical demonstration.…”

“…S3(a)]. From the curve fitting, f iso bip ˜and f bip nem ˜were ∼0.16 and ∼0.27 vol%, which are lower than those for the previous SC-low dispersion, respectively (f iso bip = ∼0.24 vol% and f bip nem ˜= ∼0.32 vol%) 19) (note that the transition concentrations of SC-low were reanalyzed by the sigmoid curve fitting [Fig. S3(b)]).…”

“…We previously presented the LC behaviors of sodium cholate (SC) stabilized SWCNTs by optimizing the dispersion condition with the optical microscope observation and the differential centrifugal sedimentation analysis. 19,20) The SC is one of the bile salts and thus much safer than CSA. SWCNTs were dissolved in the dispersion as the isolated SWCNT and/or the isolated bundles and showed the firstorder nematic LC transition as increasing the concentration of SWCNTs.…”

“…In the previous report, the LC behaviors of the SCstabilized SWCNTs with L/D = 665 were studied in detail. 19) To investigate the effect of SWCNT aspect ratio on the LC behaviors, we prepared well-dispersed SWCNT-SC solutions with higher L/D in this study. Based on the SEM and the AFM observations, the average length (L) and diameter (D) of SWCNTs were estimated to be ∼1.7 µm and ∼1.5 nm, resulting in L/D = 1088 (Table I, Fig.…”

“…The average diameter of single SWCNTs used here is 1.65 nm. 19) From the distributions of the SWCNT diameter in the dispersions, both the isolated SWCNTs and the bundles of several SWCNTs were contained. Figure 1(a) shows the phase diagram of the SC-high dispersion.…”