We show that functionalization of the surface of cellulose nanocrystals (CNCs) with either carboxylic acid (CNC-CO2H) or amine (CNC-NH 2) moieties renders the CNCs pHresponsive. 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.
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