Effects of nanocellulose on the structure and properties of poly(vinyl alcohol)-borax hybrid foams

Nanocellulose-borax-polyvinyl alcohol (PVA) hybrid foams were prepared using a facile approach in an aqueous medium followed by a freeze-casting technique. Nanocellulose was well-dispersed in the PVA-borax (PB) matrix and acted as a cross-linking agent and nanofiller to bridge the 3D network, leading to enhanced mechanical and thermal performance. The effects of particle size, aspect ratio, surface charge and crystallinity on the microstructure and performance were investigated. With the increasing size and aspect ratio, cellulose nanofiber-PB foam with a density of similar to 0.110 g/cm(3) exhibited the most pronounced honeycomb-like structure with a porosity of 92.2%, the smallest cell diameter (similar to 0.93 mu m) and the highest mechanical strength (bearing more than 7560 times its own weight). Chemical cross-linking of nanocellulose-PVA foams with borax led to uniform porous structure, small pores and high mechanical strength. Possible lyophilization-induced assembly mechanisms, relationships between microstructure and mechanical properties, and complexation reactions between building blocks are proposed.