Controlling the rate of water-induced switching in mechanically dynamic cellulose nanocrystal composites

Carboxylated cellulose nanocrystals with varying charge densities, ranging from ca. 270 to -1100 mmol/kg, were prepared via partial functionalization of 1100 mmol/kg carboxylated CNCs with different equivalents of propylamine. Incorporation of these differently charged nanofillers into poly(vinyl acetate) produced a series of water-responsive mechanically adaptable nanocomposite materials. Dynamic mechanical analysis and rheological studies of the nanocomposite samples indicated that both the degree of water swelling and the rate of mechanical softening are related to the CNC charge density. Interestingly, however, the mechanical properties of the nanocomposites, either dry or fully swollen, are more-or-less independent of the charge density of the CNC nanofillers. Thus, these results highlight a method to tailor the rate of mechanical switching (or diffusion of water through the nanocomposite material) without significantly impacting its mechanical properties.