Versatile Assembly of Metal-Phenolic Network Foams Enabled by Tannin-Cellulose Nanofibers

Abstract

Metal–phenolic network (MPN) foams are prepared using colloidal suspen-sions of tannin-containing cellulose nanofibers (CNFs) that are ice-templatedand thawed in ethanolic media in the presence of metal nitrates. The MPNfacilitates the formation of solid foams by air drying, given the strength andself-supporting nature of the obtained tannin–cellulose nanohybrid struc-tures. The porous characteristics and (dry and wet) compression strength ofthe foams are rationalized by the development of secondary, cohesive metal-phenolic layers combined with a hydrogen bonding network involving theCNF. The shrinkage of the MPN foams is as low as 6% for samples preparedwith 2.5–10% tannic acid (or condensed tannin at 2.5%) with respect to CNFcontent. The strength of the MPN foams reaches a maximum at 10% tannicacid (using Fe(III) ions), equivalent to a compressive strength 70% higher thanthat produced with tannin-free CNF foams. Overall, a straightforward frame-work is introduced to synthesize MPN foams whose physical and mechanicalproperties are tailored by the presence of tannins as well as the metal ion spe-cies that enable the metal–phenolic networking. Depending on the metal ion,the foams are amenable to modification according to the desired application.