Deposition of hierarchically porous cellulose microfibril films via emulsion templating and drying at ambient temperature

Porous films and coatings based on biocompatible natural renewable materials such as cellulose are of interest to diverse fields such as biomedical devices, insulation, separations, ion exchange, sensing, and packaging. We report the fabrication of hierarchically macro- and nanoporous films of microfibrillated cellulose (MFC) using a novel single-step method based on emulsion templating with recyclable volatile oils. The hierarchical (dual-level) porous cellulosic films are deposited at room temperature by drying of volatile oil-in-water emulsions stabilized by MFC and surfactants. A three-step formation mechanism, based on the evolution of the emulsion droplet size, is proposed. One key factor controlling the resultant structure of the porous films is the vapor pressure difference between the oil and water. Templating with droplets of oil with vapor pressure of the same order as, or lower than, that of water results in formation of macroscopic porous structures. Addition of surfactant can significantly increase the porosity and the thickness of the films by suppressing the coalescence/coarsening of the oil droplets and reducing the flattening effect caused by capillary forces. We demonstrate that these films can be functionalized by silylation to obtain hydrophobic hierarchical dual-level porous films with water contact angles of 70∼116°.