From water molecules mobility to water-resistance of swollen oriented and non-oriented cellulose nanofibrils cryogels

Abstract

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 CNF (CNFt) 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), CNFt-ND ou CNFt-UD, with different pore morphologies. Indeed, honeycomb-like or lamellar structures were obtained as evidenced by microscopy and X-ray tomography analysis. Determination of cryogels absorption capacities in water (pH 6) or HCl (pH 2) solution showed different swelling behaviours depending on the charge state of carboxyl groups, but also on pore morphology NFCt cryogels. Measurements of 1H T2 relaxation times using Low-Field (LF) NMR demonstrated the appearance of different population of water molecules characterized by different mobilities due to the structuration of NFCt gel during the freeze-casting procedure. Finally, tests of compression cycles on H2O- or HCl-swollen NFCt-ND and NFCt-UD cryogels demonstrated the higher compressive resistance of swollen-cryogels after protonation and a recovery shape performance of about 87% was obtained after 50 compression cycles.