Physically cross-linked cellulose nanofiber (LCNF/CNF) hydrogels: impact of the composition on mechanical and swelling properties

Lignocellulose nanofibers (LCNFs) are highly regarded for their ability to significantly enhance the rigidity of formed structures. When integrated into cellulose nanofiber (CNF) hydrogels, they hold substantial promise in augmenting mechanical strength, as well as improving adsorption capacity. Herein, the preparation of hydrogels from an aqueous suspension of CNFs and LCNFs extracted from eucalyptus cellulose pulp through a homogenization process is outlined. Suspensions of different concentrations were prepared to assess the influence of lignin and nanofiber content on the properties of the hydrogels. The hydrogels cellulose nanofibers (HCNF) and lignocellulose nanofibers (HLCNF) were formed through a freeze-thaw process, revealing an enhancement in rigidity with increasing nanofiber concentration. DFT (density functional theory) calculations illustrated the cross-linking mechanism between cellulose chains induced by the crystallization of water molecules, thus, corroborating the postulated hydrogel formation mechanism. Microstructural analysis revealed honeycomb-shaped matrices in longitudinal sections, with HLCNF hydrogels presenting less smooth walls. Studies on water adsorption capacity showed rapid swelling in both hydrogels, correlated with the nanofiber content reaching 8750% and 5500% for HLCNF and HCNF, respectively. HLCNF hydrogels exhibited higher adsorption capacity due to the influence of lignin on cross-linking rates. Mechanical compression tests demonstrated exceptional resilience in all hydrogels. Despite having a lower cross-linking density compared to hydrogels made from 2 wt.% cellulose nanofibers, hydrogels composed of 2 wt.% lignocellulose nanofibers exhibited a Young's modulus of 2.83 kPa. This underscores the superior mechanical properties of lignin-based hydrogels, highlighting the effect of lignin on the hydrogel matrix.