Ice-templating hydrogels with high concentrations of cellulose nanofibers to produce architected cellular materials for structural applications

Abstract

Ice-templated cellular materials made of entangled networks of slender cellulose nanocrystals (CNCs) or cellulose nanofibrils (CNFs) exhibit excellent specific physical properties but their mechanical properties are still insufficient: these biosourced systems cannot be used as structural materials, e.g., as cores of composite sandwich structures. To overcome this limitation, we ice-templated hydrogels with high concentrations of CNFs and CNCs by using a specific setup that enabled their unidirectional solidification. The microstructure and the mechanical properties of the freeze-dried foams were investigated using SEM, X-Ray nanotomography and compression tests, respectively. Increasing the content of nanofibers yielded to drastic shifts of (i) the foam microstructures from highly anisotropic (columnar with CNFs, lamellar with CNCs) to more isotropic ones (ii) the mechanical properties towards isotropy together with a significant increase of the foam stiffness, yield stress and absorbed energy. In addition, due to the high aspect ratio of the CNFs, CNF foams with elevated relative densities exhibit noteworthy specific mechanical properties.