From water molecule mobility to water-resistance of swollen oriented and non-oriented cellulose nanofibril cryogels

The development of porous, water-resistant cellulose-based materials with shape-recovery performance requires control of the swelling behaviour of these materials. In this context, TEMPO-oxidized cellulose nanofiber (TCNF) cryogels, were prepared by non-directional (ND) and unidirectional (UD) freezing step followed by freeze-drying to obtain lightweight porous materials (22.6 kg m⁻³ and 98% air content), TCNF-ND or TCNF-UD, with different pore morphologies. Indeed, honeycomb-like or lamellar structures were obtained as evidenced by microscopy and X-ray tomography analysis. The determination of the absorption capacities of these cryogels in water (pH 6) or HCl solution (pH 2) showed different swelling behaviours depending on the charge state of carboxyl groups, but depending also on the pore morphology of the TCNF cryogels. Measurements of ¹H T₂ relaxation times using Low-Field (LF) NMR demonstrated the appearance of different populations of water molecules characterized by different mobilities due to the structuration of TCNF gel during the freeze-casting procedure. Finally, uniaxial cyclic compression tests were conducted on H₂O- or HCl-swollen TCNF-ND and TCNF-UD cryogels. A higher compressive resistance of swollen-cryogels after protonation and a recovery shape performance of about 87% were obtained after 50 compression cycles.