Influence of PEG on toughness, humidity sensitivity and structural color of cellulose nanocrystal films

Polyethylene glycol (PEG) / Cellulose nanocrystal (CNC) structural colored films with good flexibility and humidity sensitive were prepared by using the evaporation-induced self-assembly method. The influence of PEG’s molecular weight and content on the change of structural color, toughening and humidity-sensitive of CNC films were systematically investigated through optical properties, morphologies, mechanical properties, and humidity responsiveness. Optical measurement results showed that the structural color and the maximum reflected light wavelength (λ_max) of CNC films were red-shifted by adding PEG. The higher the PEG molecular weight and content, the more obvious the red-shift was. Morphology characterization demonstrated the red-shift was attributed to the increasing pitch of the chiral nematic phase structure. Mechanical results showed that adding PEG significantly improved the toughness of CNC films, the higher the PEG molecular weight and content, the higher toughening effect. The toughness enhancement can be attributed to PEG being soft and having good interaction with CNCs, can be regarded as an energy dissipating binding phase which increased the energy dissipation during CNCs movement. Humidity responsiveness showed that the humidity sensitivity of CNC film was improved with PEG concentration increasing, but was independence of PEG molecular weight. Furthermore, the rheological behavior and thixotropy recovery were studied to explain the influence of PEG on the self-assembly behavior of CNCs in suspension. This study provides a foundation for regulation of CNC structural colored films with good toughness and humidity-sensitive.