Model development of aqueous diffusion softening transition in thermoplastic polyurethane cellulose nanocrystal composites

The mechanical adaptivity of polymer composites containing cellulose nanocrystals (CNCs) is of broad interest for the development of smart materials. The mechanism leading to diffusion-dependent mechanical softening in response to water uptake in these materials is well understood, however, the mass transport driving this phenomenon has not been completely resolved. Understanding such behavior would further enable the mechanism to be controlled and exploited for various applications. Here, we show that swelling of the bulk polymer composite during moisture uptake results in a decreased apparent diffusivity with increasing CNC concentration, despite the equilibrium water uptake increasing concurrently. Further, we demonstrate that a modification to the widely used percolation model predicts the full time-dependent evolution of storage modulus during the softening process by associating mass transport directly to the softening response. This model will be pertinent to studies of mechanical responses in CNC-polymer composites and is expected to be generally applicable to percolating networks which deactivate in response to a given stimulus.