Tunable transparency and superhydrophobicity of chitosan-based fluorine-free coatings

Novel approaches for fabricating safe- and sustainable-by-design coatings are being studied to replace perfluorooctanoic acid or other fluorinated substances that are raising concerns for their potential hazard for human health [1]. In this study, a chitosan-based coating with modified functionality is developed through a solvent-free deposition method. The chitosan backbone is modified with fatty acid side groups to change the polymer wettability from hydrophilic to hydrophobic [2]. The effective chitosan functionality modification with stearoyl groups is assessed and confirmed via combined by 1H and 13C DD-MAS Solid State NMR, FTIR and TGA analysis. The modified chitosan is then deposited by a solvent-free approach consisting of: i) freeze-drying, which allow safe long-term storage, ii) sieving, iii) thermal treatment (Figure 1a). Controlling the amount of deposited coating, transparency and superhydrophobicity can be tuned based on the desired characteristics. A superhydrophobic wetting state (θA = 150°, θR = 144°) is reached for coating amount of 4.8 mg/cm2, obtaining an opaque surface, whereas for lower amount of 0.8 mg/cm2, the coating is hydrophobic (θA = 108°, θR = 67°) with very good transparency (Figure 1b). A morphological characterization by SEM and optical microscope analyses of the coating provides evidence of increased roughness along with increased hydrophobicity. Our innovative and sustainable deposition method for modified chitosan is a simple procedure for creating bio-based transparent hydrophobic coatings, which is fluorine-free and does not require any solvent during deposition. As an outlook, we envision a further development of coatings to better control surface topography to decrease scattering and increase transmittance, and mechanical durability improvement using adhesion promoters or cross-linkers.