A known deterrent to a large scale development and use of cellulose nanocrystals (CNCs) is their affinity for moisture, which has a profound effect on dispersion, wetting, interfacial adhesion, matrix crystallization and water uptake. To quantify the hydration and confinement of absorbed water in CNCs and CNC composites we studied sulfated-CNCs neutralized with sodium cations and CNCs functionalized with more hydrophobic methyl(triphenyl)phosphonium cations. Films were cast from water suspensions at 20 C under controlled humidity and drying rate, yielding CNC materials with distinguishably different dielectric properties and cholesteric structures. By controlling the evaporation rate we obtained self-assembled chiral CNC films with extended uniformity, having helical modulation length (nominal pitch) tunable from 1300 nm to 600 nm. SEM imaging, UV-Vis-NIR chiral dichroism and total reflectance spectra revealed tighter and more uniform CNC packing in films cast at slow evaporation rates or having lower surface energy when modified with phosphonium. The dielectric constant was measured by a non- contact microwave cavity perturbation method and fitted to a classical mixing model employing randomly oriented ellipsoidal water inclusions. The dielectric constant of absorbed water was found to be significantly smaller than that for free liquid indicating a limited mobility due to bounding with CNC matrix. In the case of hydrophilic Na-CNCs, a decreasing pitch led to greater anisotropy in the shape of moisture inclusions (spherical to platelet-like) and greater confinement. In contrast, the structure of hydrophobic phosphonium modified CNC films was found to reduce the pitch considerably, yet the shape of confined water remained predominantly spherical. These results provide a useful perspective on the current state of understanding of CNCs materials self-assembly and are beneficial for the realization of CNC functional materials and composites.
Citation: ACS Applied Materials and Interfaces
Pub Type: Journals
dielectric properties, cellulose nanocrystals, water confinement, chirality