(393c) Effect of Shear Induced Alignment on the Optical Properties of Cellulose Nanocrystal Coatings

The effects of original dispersion microstructure and imposed shear stresses on the microstructure and optical properties of cellulose nanocrystals (CNC) coatings has been investigated. CNC are predominantly isolated through the sulfuric acid hydrolysis of natural cellulosic fibers, where the amorphous regions are preferentially hydrolyzed. During hydrolysis, negatively charged sulfate groups are introduced to the surface of the CNC, enabling the CNC to be easily dispersed in water through ionic stabilization. As rod-like particles the CNC follow lyotropic liquid crystalline phase behavior, and form a cholesteric polydomain structure at high enough concentration. It has been shown that upon drying, the cholesteric microstructure is preserved in the dried coating. Additionally, shear can be used to alter the microstructure of a CNC dispersion. At high enough shear rates, all of the CNC can be aligned with a single director to form a monodomain nematic structure, with the order further increasing with increasing shear rate. In this study, we investigate the preservation of shear alignment in dried CNC coatings. Rheology and cross polarized rheo-optics were first used to measure the response of CNC dispersions at different concentration to shear. CNC coatings were then prepared on quartz slides using doctor blade coating over a range of shear rates. CNC alignment in the coatings was investigated using atomic force microscopy and cross polarized optical microscopy, while optical properties were explored using a CRAIC Technologies UV/Vis microspectrometer with both cross and circularly polarized light. In the cross polarized rheo-optical study, at certain concentrations, interference colors were seen to change with applied shear rate. Based on  the Michel-Lévy interference color chart, the changes in color are the result of results from increasing birefringence with increasing shear rate, and therefore increasing CNC alignment. Microscopy of the dried coatings showed that when the original dispersion concentration was high enough, the shear induced alignment could be preserved in the coating. Therefore, the birefringence of these coatings can be tuned by simply controlling the applied shear rate during application. These coatings have the potential to be applied as easily fabricated, tunable optical filters. Their feasibility as both bandpass and notch filters in were tested using UV/Vis spectrometry.