Cross-linked cellulose nano-sponges: a small angle neutron scattering (SANS) study

Cellulose nano-sponges (CNS), obtained by cross-linking TEMPO oxidized and ultra-sonicated cellulose nano-fibers (TOUS-CNFs) with branched polyethyleneimine (bPEI), underwent here a systematic small angle neutron scattering investigation, by varying the amount of cross-linker and the water content. The aim was to provide experimental evidence of nano-porosity in the TOUS-CNF network of these nano-sponges (CNSs) by investigating the water nano-confinement geometries in the adsorbent material. Moreover, we also verified how the breaking/reformation of specific intermolecular hydrogen bond interactions between water and the chemical groups present in the architecture of the CNSs could contribute to regulate the water adsorption process observed at macroscopic level. The analysis of the experimental data, performed in terms of the correlation length model, allowed us to extract the short-range correlation length xi, interpreted as a very first indirect estimation of the effective nano-dimension of the cavities produced by the cross-linking of the reticulated cellulose nano-fibers. From the model, power-law (n) and Lorentzian (m) exponents have been also obtained, associated with the density of TOUS-CNFs at high (larger than hundreds of angstrom) and low (similar to 10-100 angstrom) spatial scales, respectively. These parameters were all sensitive to the structural variations induced by the progressive uptake of water on the bPEI/TOUS-CNF sponges with different bPEI:TOUS-CNF (w/w) ratios. Finally, we investigated the effect of the addition of citric acid in the CNS formulation, confirming its role in increasing cross-linking density and sponge rigidity. The obtained results appear crucial in order to rationalize the design of these sponges and to track the changes in the ability of the final products as efficient nano-confinement systems for water.