pH-Responsive Cellulose Nanocrystal Gels and Nanocomposites

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.