pH-Responsive Cellulose Nanocrystal Gels and Nanocomposites

Abstract

We show that functionalization of the surface of cellulose nanocrystals (CNCs) with either carboxylic acid (CNC-CO₂H) or amine (CNC-NH₂) moieties renders the CNCs pH-responsive. At low pH, where the amine groups are protonated, CNC-NH₂ forms aqueous dispersions in water on account of electrostatic repulsions of the ammonium moieties inhibiting aggregation. However, a transition to hydrogels is observed at higher pH where the CNC-NH₂ are neutral and the attractive forces based on hydrogen bonding dominate. The opposite behavior is observed for CNC-CO₂H, which are dispersible at high pH and form gels in an acidic environment. We further show that these pH-responsive CNCs can be incorporated into a poly(vinyl acetate) matrix to yield mechanically adaptive pH-responsive nanocomposite films.