The influence of temperature on cellulose swelling at constant water density

We have in this paper investigated how water sorbs to cellulose. We found that both cellulose nanofibril (CNF) and cellulose nanocrystal (CNC) films swell similarly, as they are both mainly composed of cellulose. CNF/CNC films subjected to water at 0.018 kg/m³ at 25 °C and 39 °C, showed a decrease in swelling from ~ 8 to 2%. This deswelling increased the tensile index of CNF-films by ~ 13%. By molecular modeling of fibril swelling, we found that water sorbed to cellulose exhibits a decreased diffusion constant compared to bulk water. We quantified this change and showed that diffusion of sorbed water displays less dependency on swelling temperature compared to bulk water diffusion. To our knowledge, this has not previously been demonstrated by molecular modeling. The difference between bulk water diffusion (D_WW) and diffusion of water sorbed to cellulose (D_CC) increased from D_WW - D_CC ~ 3 × 10^-5 cm/s² at 25 °C to D_WW - D_CC ~ 8.3 × 10^-5 cm/s² at 100 °C. Moreover, water molecules spent less successive time sorbed to a fibril at higher temperatures.