Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation

Abstract

Carbon nanotubes were dispersed in a sodium dodecylsulfate/decanol/water nematic fluid. The long-term stability of the dispersions is ensured by the small density gradients existing between nanotubes and the nematic fluid, and by its viscosity, as well. Presumably, surfactant or nematic micelles adsorb onto nanotubes and concur to stabilize them. A Rheo 2 H NMR characterization was performed. It was supported by classical 2 H quadrupole splitting and pulsed field gradient spin−echo NMR, allowing to ascertain the diffusive trends therein. The nematic fluid shows uniaxial spectral profiles and marked diffusion anisotropy. No such effects were observed in nanotube-containing nematic dispersions. In addition, the measured water self-diffusion values are substantially lower than the pure nematic fluid. In the absence of shear, dispersed nanotubes do not modify the quadrupole splitting amplitude, but affect the spectral profiles. The reasons for the observed behavior are briefly outlined. In the presence of shear, the spectral modifications are substantial and lead to the onset of isotropic dispersions, after long-time shearing.

Anisotropic micro-cloths fabricated from DNA-stabilized carbon nanotubes: one-stop manufacturing with electrode needles

“…Therefore, an isotropic-to-nematic phase transition has been found to occur in concentrated DNA-wrapped CNT suspensions particularly at the optimal ratio of α =1 [11,[33][34][35][36]. This is similar to the transition observed in lyotropic rigidrod polymers [38].…”

“…Accordingly, we investigated the effect of density ratio, α =C CNT/C DNA, in the range of 0.6≤α ≤2 while maintaining the CNT density at C CNT=0.1 wt.%. It is to be noted that the formation of a nematic phase has been reported at α =1 in more concentrated suspensions (C CNT>1 wt.%) [11,[33][34][35][36].…”

“…Denatured DNA has been proven to be efficient to stabilize CNT densities lower than 1 wt.% in water [ 11,23,[25][26][27][33][34][35][36]. We first prepared aqueous solutions of singlewalled CNTs (purity, >95%; Cheap Tubes, Grafton, VT, USA) using salmon DNA (Wako, Richmond, VA, USA) as follows: 10 mg of raw CNT soot was suspended in 5 ml of a 0.12 wt.% density solution of salmon DNA (Wako) denatured by 10-min heating at 90° C. The DNA-CNT mixture was sonicated for 60 min at a 20-kHz frequency at 50 W in a water-ice bath using a homogenizer (Branson Sonifier 250, Branson Ultrasonics, Danbury, CT, USA).…”

“…The DNA-stabilized CNTs [11,23,[25][26][27][33][34][35][36] disperse and stabilize at a considerable range of concentrations. Therefore, an isotropic-to-nematic phase transition has been found to occur in concentrated DNA-wrapped CNT suspensions particularly at the optimal ratio of α =1 [11,[33][34][35][36].…”

“…To this end, we used the dielectrophoretic assembly method [12][13][14][15][16][17][18][19] using AC electric fields in DNA-stabilized CNT suspensions [11,23,[25][26][27][33][34][35][36] to produce aligned arrays of CNTs. The dielectrophoretic method provides a notable advantage over other techniques because this electronic method is able to selectively collect metallic CNTs and separate them from semiconducting CNTs [15,16].…”