Study of humidity-responsive behavior in chiral nematic cellulose nanocrystal films for colorimetric response

This study focuses on the preparation and characterization of an iridescent chiral nematic cellulose nanocrystal (CNC) film in relation to their use as a humidity-responsive material. A free-standing film with Bragg reflection was successfully prepared. Thermogravimetric analysis demonstrated gradual thermal transitions with two major weight loss regions, while the crystallinity index obtained from X-ray diffraction method was about 78%, implying the presence of disordered regions. A scanning electron microscope confirmed the orientation of CNCs in a chiral nematic pattern with a pitch in the wavelengths of visible light. A dynamic vapor sorption study showed that water vapor can penetrate into the CNC film, leading to an increase of overall mass of up to 20% at 95% RH. The moisture sorption and desorption rates of the CNC film were faster at higher humidity (80% RH). The exposure of CNC film to elevated humidity was simulated to investigate the effect of humidity on their optical semaphore. As the humidity increased, the complementary color was red-shifted upward by about 60–100 nm, which was observable to the naked eye. This is potentially explained by the swelling of the nanocrystals and the expansion of the interspace between the quasi-nematic layers of the CNCs.