C.D. Emiroglu, B. Natarajan, J.W. Gillman, J.A. Liddle, J. Obrzut
National Institute of Standards and Technology,
Keywords: cellulose nanocrystals, water confinement, dielectric properties, chirality
Summary:Nanocellulose has been a continuously emerging area of interest over the recent years due to inexpensive, abundant, and renewable nature of the material source. Cellulose nanocrystals (CNCs) are regarded to be highly useful candidates as a sustainable way of polymer reinforcing. In this regard, investigating the confinement of absorbed water in CNCs is essential to understand the effects on dispersion, wetting, interfacial adhesion, matrix crystallization, and water uptake, all which have critical importance in the utilization and high-scale production of nanocomposites. In this work, we study cellulose nanocrystal films of known mesostructure and functionality, cast from water dispersions under controlled drying rate. Continuous monitoring and control of the drying rate, together with the relative humidity and temperature of the ambient, yields films that are self-organized into well-defined and distinguishably different chiral structures with a characteristic helical modulation length (pitch). Films are studied via reflectance spectroscopy for measurement of their structural periodicity. In addition, scanning electron microscopy (SEM) is used for imaging, and numerical evaluation of the pitch distributions. Reflectance spectroscopy and the SEM analysis results are found to be in strong agreement and directly correlated with the drying rate of the films. For dielectric characterization, a non-contact microwave cavity perturbation method is used. The method enables permittivity measurements with adequate accuracy to investigate water confinement in the CNC films. Through an approach using classical mixing models with randomly oriented ellipsoidal water inclusions, it is found that in the case of hydrophilic Na-CNCs, a decreasing pitch led to greater anisotropy in the shape of moisture inclusions (ellipsoidal or platelet-like). In contrast, the structure of hydrophobic phosphonium-cation modified CNC films are found to have little influence on absorbed water inclusions, which remain free and predominantly spherical. These results provide a useful perspective on the current state of understanding of CNCs materials and are beneficial for the realization of CNC functional materials and composites.