Facile surface modification of cellulose nanofiber with "cross-linkable" internal alkene by syringaldehyde in aqueous medium towards mechanically robust nanocomposite hydrogels.

Incorporating "cross-linkable" double bonds into cellulose nanofibers (CNFs) is crucial for fabricating high-performance composite hydrogels. Herein, TEMPO-oxidized CNFs syringaldehyde propiolate ether (TS), functionalized with "cross-linkable" internal alkenes (-C=C-), was synthesized via hydroxyl-yne click chemistry under mild aqueous conditions. This rapid modification yielded 0.24 ± 0.04 mmol/g alkenes in 5 min and 0.50 ± 0.03 mmol/g in 4 h. The internal -C=C- groups enabled efficient UV-induced gelation of TS dispersions (0.15-0.6 wt%) within 10 min. The TS-polyacrylamide (PAM) nanocomposite hydrogels exhibited a dual-network structure, with a crosslinked TS network providing mechanical support and a PAM network ensuring flexibility. At TS-PAM mass ratios of 0.45-0.9 wt%, stress and toughness increased by 108 ± 11.1 % and 673 ± 58.2 %, respectively, due to enhanced crosslinking and stress transfer. These hydrogels exhibited strain sensitivity (10-200 %) and stable dynamic response across strain rates (50-250 mm/min), with a maximum gauge factor (GF) of 4.3. Real-time motion tracking highlights their promise for wearable sensors and sustainable flexible electronics. Notably, the use of green hydroxyl-yne click chemistry and natural lignin-derived syringaldehyde ensures environmentally friendly and efficient modification, avoiding harmful reagents and minimizing waste. This study presents a sustainable and efficient approach to high-performance cellulose nanofiber hydrogels with promising applications in flexible electronics, motion sensing, and soft robotics.