We study the morphology and optical properties of a variety of cellulose nanocrystals (CNC) obtained from various natural sources with different dimensions and composition of individual nanocrystals and their selective light reflectance in the solid state. A library of CNCs with different dimensions, sulfate contents, and crystallite sizes was prepared from five distinct sources representing traditional choice ranging from soft and hard wood pulps to microcrystalline cellulose under identical hydrolysis conditions. High-resolution atomic force microscopy, scanning electron microscopy, scanning transmission electron microscopy, and X-ray diffraction confirmed that all CNCs have a well-defined rod-like morphology with different aspect ratios and axi-asymmetric diamond-shape cross-sections. Varying the cellulose sources while utilizing constant processing conditions resulted in wide variability of the CNC dimensions with lengths from 120 to 210 nm, aspect ratios from 30 to 70, heights from 2.9 to 3.6 nm and widths from 6 to 11 nm. Specifically, the CNCs from microcrystalline sources have large cross-sectional dimensions and produce straight CNC bundles, but CNCs from various wood pulps have small cross-sections and facilitate twisted local bundles of a few individual nanocrystals. Chemical composition and surface potentials were found to be less critical to the resulting chiral characteristics and structural colors while the CNCs with high aspect ratios form chiral films with large pitch values and thus longer wavelengths of selective reflection. Such flexible chiral CNC materials with controlled optical signature can be further considered for development of advanced materials for colorimetric sensors, tunable and active photonic materials, optical coatings, chiral inks and 3D printed photonic structures.

Flexible photonic materials based on cellulose nanocrystals with different morphologies

256th National Meeting (2018)

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