Structure and swelling of cross-linked nanocellulose foams

Hypothesis: The water absorption capacity of nanocellulose (NC) foam is tailored by crosslinking with polyethyleneimine (PEI) and hexamethylenediamine (HMDA). The interaction of amine groups in PEI and HMDA with the carboxylic groups (COO⁻) of NC affects the foam structure which reduces its swelling capacity. Experiments: Functionalised NC foams were prepared by TEMPO (2,2,6,6,-tetramethylpiperidine-1-oxyl) oxidation of bleached pulp, followed by fibrillation into a hydrogel, adding a crosslinker and freeze drying the hydrogel into a foam. The structure of the NC foam characterised by rheology, SANS (Small Angle Neutron Scattering), SAXS (Small Angle X-ray Scattering) and cryo-SEM (cryo-Scanning Electron Microscopy) was related to absorption and swelling properties. Findings: The NC foam has the highest water absorption capacity at 132 g water/g foam. PEI-NC foam has a water absorption capacity of 71 g water/g foam, which further decreases to 47 g water/g foam for the HMDA-NC foam. Small angle scattering reveals the elementary fibril of NC is 3–5 nm thick and forms fiber bundles. In water, these bundles swell differently for the different types of foam which affects the water absorption capacity of the network. The structural analysis of the foam was related to the swelling capacity. The structure of NC foam can be engineered for specific applications for biomedical, agriculture or food industries.