Quantitative evaluation of adsorption behavior of carboxymethylcellulose on carbon nanotubes by time-domain NMR with the aim of obtaining stable dispersions

Abstract

This study applied time domain NMR (TD‐NMR) to investigate the effect of carboxymethylcellulose (CMC) adsorption on the dispersion of carbon nanotubes (CNTs) in water. In many aqueous dispersions, the relaxation time measured by TD‐NMR is dominated by bound water on the particle surface. In contrast, since CMC molecules are adsorbed on the hydrophobic surface of CNTs, we proposed a model where relaxation times of hydration water of CMC adsorbed on CNTs, hydration water of free CMC, and bulk water are dynamically averaged in the TD‐NMR measurement. In this study, conversely, we presented the procedure to calculate the fraction of CMC adsorbed on CNTs from the measured relaxation time. Since the adsorbed layer of CMC can stabilize CNTs, the amount of adsorbed CMC is expected to be closely related to the dispersion state of CNTs. TD‐NMR measurements of CNT slurrys with different CMC concentrations showed that the adsorption ratio reached almost 100% (adsorption saturation) when the CMC concentration became larger than that of CNTs. At the adsorption saturation, it was confirmed that the CNT slurry exhibited the lowest apparent viscosity, and the model electrode showed the highest electro-conductivity. In addition, we demonstrated that the adsorption ratio is helpful to assess the dispersion state of CNTs and to optimize the dispersion process. The current study showed that TD‐NMR is very useful for quantitative evaluation of sterically stabilized dispersions because it can evaluate the adsorption rate of adsorbed polymers without dilution.