Waterborne functionalization of cellulose nanofibrils with norbornenes and subsequent thiol-norbornene gelation to create robust hydrogels

Cellulose nanofibrils (CNFs) produced through processes involving oxidation (e.g. TEMPO oxidation) present reactive groups that allow for straightforward modification in aqueous suspension. CNFs fabricated through mechanical refinement alone can be challenging to modify for subsequent reactions due to only having hydroxyl groups present on the surface. To address these issues, CNFs with only hydroxyl groups present were functionalized with norbornene groups in their native aqueous suspension to achieve up to 10% functionalization per anhydroglucose unit. Since quantification of surface functionalization of CNFs is challenging through most methods, a degradation and subsequent nuclear magnetic resonance analysis method was developed to quantify norbornene functionalization. The norbornene functionalized CNFs (nCNFs) were cross-linked through UV and thermally initiated thiol-ene click reactions to create robust CNF hydrogels. By varying the reaction conditions, hydrogels made from nCNFs and a dithiol cross-linker could achieve compression modulus values up to 25 kPa. The materials were stable in aqueous suspensions and the cross-linked hydrogels still exhibited shear thinning behavior with high recovery, which demonstrated that even though effective cross-links were formed, a complete network was not. Through this study, thiol-norbornene crosslinking of CNFs could create robust hydrogels and improve aqueous stability that could have applications in sustainable materials and biomaterials.