Photonic films from surfactant-coated cellulose nanocrystals in toluene for pressure and humidity sensing

Abstract

Most inks use pigments that absorb light and consequently fade with excess sun exposure. Inspiration for a more sustainable, non-absorbing, alternative may be found in nature. Some organisms have developed structural coloration, through millions of years of evolution. This color arises via the interference of light with submicron features. To mimic this, we may use cellulose nanocrystals (CNCs), which can spontaneously self assemble into a cholesteric liquid crystalline phase in aqueous suspensions. Drying these CNC suspensions can result in structurally colored films. While much research has focused on aqueous suspensions of CNCs, less is known about transferring CNCs to apolar solvents, like toluene. Furthermore, drying these apolar suspensions into structurally colored films is virtually unexplored to our knowledge. In this thesis, we present a surfactant: STEPFAC 8170-U, a nonylphenol polyoxyethylene phosphate ester, that is capable of stably suspending CNCs in toluene over extended periods of time. Using these suspensions, we were able to set up phase diagrams for this system with concentrations between ∼18-50 wt.% Drying of the apolar CNC suspensions on a glass substrate resulted in structurally colored films, with a peak wavelength between 805-920 nm. The single-surfactant addition improved the flexibility of the films greatly. This allowed for blue-shifting of the peak wavelength, as measured by UV-vis, by applying pressure to the film. Additionally, introduction of the surfactant induced sensitivity to water and humidity to the films. An increased humidity red-shifted the peak wavelength of dried films.