Nanocrystalline cellulose (NCC) self-assembles in suspension to form iridescent chiral nematic films upon drying that can reflect circularly polarized light at specific wavelengths. Ultrasound treatment has now been found to increase the chiral nematic pitch in suspension and red-shift the reflection wavelength of NCC films as the applied energy increases. Sonication and electrolyte addition combined allow the reflective properties of the film to be predictably tuned. The effects of sonicating an NCC suspension are cumulative and permanent. Suspensions sonicated with different energy inputs may be mixed to give an NCC film having a reflection band intermediate between those obtained from the individual suspensions. The data suggest that the ultrasound-induced red-shift is electrostatic in nature.
Chiral plasmonic films have been prepared by incorporating gold nanorods (NRs) in a macroscopic cholesteric film formed by self-assembled cellulose nanocrystals (CNCs). Composite NR-CNC films revealed strong plasmonic chiroptical activity, dependent on the photonic properties of the CNC host and plasmonic properties of the NRs. The plasmonic chiroptical properties of the composite films were tuned by changing the conditions of film preparation. The strategy presented herein paves the way for the scalable and cost-efficient preparation of plasmonic chiral materials.
Dispersibility is important for nanocrystalline cellulose (NCC) because recovering the unique suspension and particle properties is essential after the product has been dried for storage or transport. It is our goal to produce dried NCC that redisperses in water to yield colloidal suspensions without the use of additives or a large energy input. In contrast with the as-prepared acidic form of NCC (H-NCC), suspensions of neutral sodium-form NCC (Na-NCC) dried by evaporation, lyophilization, or spray-drying are readily dispersible in water. Suspension properties and NCC particle size determined by light scattering were used as indicators of dispersion quality. The neutral counterion content, drying technique, freezing action, drying and redispersion concentrations, and moisture content in the dried NCC were all found to influence dispersibility. When a minimum of 94% of the H(+) counterion is exchanged for Na(+), the neutral salt form is fully dispersible in water even when fully dried. Mild sonication is generally sufficient to recover measured particle sizes identical to those in the never-dried Na-NCC sample. A threshold moisture content of 4 wt % was found, above which dried H-NCC is fully dispersible in water.
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