Hygroscopic Swelling Determination of Cellulose Nanocrystal (CNC) Films by Polarized Light Microscopy Digital Image Correlation

The coefficient of hygroscopic swelling (CHS) of self-organized and shear-oriented cellulose nanocrystal (CNC) films was determined by capturing hygroscopic strains produced as result of isothermal water vapor intake in equilibrium. Contrast enhanced microscopy digital image correlation enabled the characterization of dimensional changes induced by the hygroscopic swelling of the films. The distinct microstructure and birefringence of CNC films served in exploring the in-plane hygroscopic swelling at relative humidity values ranging from 0% to 97%. Water vapor intake in CNC films was measured using dynamic vapor sorption (DVS) at constant temperature. The obtained experimental moisture sorption and kinetic profiles were analyzed by fitting with Guggenheim, Anderson, and deBoer (GAB) and Parallel Exponential Kinetics (PEK) models, respectively. Self-organized CNC films showed isotropic swelling, CHS similar to 0.040 %strain/%C. By contrast, shear-oriented CNC films exhibited an anisotropic swelling, resulting in CHS,similar to 0.02 and similar to 0.30 %strain/%C, parallel and perpendicular to CNC alignment, respectively. Finite element analysis (FEA) further predicted moisture diffusion as the predominant mechanism for swelling of CNC films.