Transition Behavior of Cellulose Nanocrystal Networks Induced by Nanoconfined Water

Abstract

Hydrogen bonding (HB) is essential for the mechanical properties of cellulose-based materials. However, the plastification of cellulose nanocrystals (CNC) caused by the transition of HB in the presence of water is still insufficiently understood. In this work, the rigid–soft transition of nanoconfined chains in non-ordered regions of CNC surfaces is quantitively described by comparing their strain behaviors with amorphous cellulose. Moreover, this softening (referred to as the “hydro-glass transition”) with increasing relative humidity (RH) is explored, and a threshold RH value (RH_t) is identified to characterize the transition. The phenomenon is attributed to the monolayer to multilayer adsorption and eventually capillary condensation of water molecules in wedged mesopores of the CNC films. This triggers a rapid transition of HB from cellulose–cellulose to cellulose–water type in the vicinity of RH_t. The hydro-glass transition is promoted by higher temperatures, for example, RH_t at 65 °C decreases to 50%. In addition, the presence of surface groups with lower acid dissociation constant (comparing SO₃⁻ and OH/COO⁻ moieties) also accelerates this hydro-glass transition process. Thus, a detailed understanding of the thermodynamic changes in hydrogen-bonded nanoconfined polymer chains in the presence of humidity, with implications for developing nanomaterials with RH-controlled properties, is provided.