(161c) The Effect of Sonication on the Self-Assembly of Cellulose Nanocrystals (CNC) in Suspensions and Dried Films

The extracted cellulose nanocrystals (CNC) in aqueous media can show the chiral nematic self-assembled structure. Using the controlled evaporation-induced self-assembly (EISA) method, the chiral nematic structure can be preserved in dried films. The formation of the chiral nematic structure in dried films, however, depends on the initial CNC concentration and sonication of the suspension. Sonicating the CNC suspensions can expel trapped ions in the bound-water layer around CNC particles into the bulk suspension, and cause the structural changes. Here, we have studied the effect of sonication on the microstructure of both CNC suspensions and dried films at the wide range of CNC concentrations (3, 5, 7, and 9 wt.%). According to the polarized optical microscopy (POM) results, at low CNC concentration (3 wt.%), where there is a large distance between the CNC particles, the sonication treatment results in the formation of an isotropic structure. At medium CNC concentrations (5 and 7 wt.%), however, the internal structure transforms from nematic into chiral nematic. At high CNC concentration (9 wt.%), the sonication treatment reduces the nematic domain size without significant microstructural transformations due to the highly stacked CNC arrangements. The dry films show the iridescence birefringence patterns when the pitch length of the chiral nematic is in the range of the visible light wavelength. The UV-visible transmission spectra show that the pitch length in the dried films increases by increasing the CNC concentration (from 3 to 7 wt.%). Our results show that the POM combined with the UV-visible spectroscopy provides a comprehensive approach to study the self-assembly of CNC particles in suspensions and dried films.