Chiral Nematic Solvent-Responsive Actuator Based on a Cellulose Nanocrystal Template

Photonic actuators have attracted much attention because their periodic photonic crystal structures can produce dual responses of color and motion under stimulus responses. However, photonic actuators face some problems in their manufacture, such as their fragility, high cost, and narrow tunable optical range. It is challenging to use low-cost and simple methods to fabricate a flexible, wide-range response and multicolor photonic actuators. Two cellulose nanocrystals (CNCs) are made by sulfuric acid hydrolysis of microcrystalline cellulose or pulp, which emerged blue and red following evaporative assembly, respectively, and their compositions are adjusted to modify the optical color of the final photonic thin films. The introduction of graphene oxide (GO) and the sacrifice of the CNC template result in a mesoporous structure that provides a conduit for polar liquids. When the film absorbs water, the film expands to increase the layer spacing of the film. Due to the asymmetric structure of the film, the film has different expansion rates, which makes a red shift in the color and directional bending. The film changes color from green to red in 25 s when immersed in water and 20 min in dimethyl sulfoxide (DMSO), but no color change occurs in other solutions with less polarity even after 3 months of soaking. Furthermore, the reaction rate of the film in DMSO depends on the temperature. The film changes color from blue to red in DMSO at 30 °C in 10 min but just 3 min at 70 °C, demonstrating that increasing the temperature speeds up the passage of polar solvent molecules and increases the response rate of the film. The application prospects for the prepared nanocomposite films with changing structural colors and solvent-responsive drivability are favorable in optical actuators, solvent viewers, and other applications.