Alignment-Assisted Networks of Polyelectrolyte-Grafted Cellulose Nanocrystals

Abstract

This study aims to understand the role of polyelectrolyte grafting on the dispersed cellulose nanocrystal (CNC) rods in water through measuring transport coefficients using depolarized and polarized dynamic light scattering and by measuring the viscoelastic properties using rheometer. Rotational and translational diffusivities are found to slow down with poly(acrylic acid) (PAA)-grafted chains compared to bare CNCs. Translational diffusion is shown to remain constant between pH 3 and 9, indicating the good dispersion and stability of PAA-grafted CNC suspensions. At the overlap solution concentration, chains play a significant role in bridging the CNC and form a network, as measured with the viscoelastic properties of neutral chains. When chains are ionized by altering the pH, the higher viscosity is measured because of the hydrogen bonding between ionized and un-ionized carboxylic groups, as previously demonstrated with PAA-grafted spherical nanoparticles. We further measured the viscoelastic response of PAA-grafted CNC after applying large steady shear. The results show that CNCs with long grafts presented enhanced viscoelastic moduli, and their critical strain value decreased after large shear flow application. Short grafts, in contrast to the long grafts, did not show any changes in the viscoelastic response under shear. These results indicate that the alignment-assisted networks of PAA-grafted CNC enable better entanglements between long grafted chains at the neutral state.