Dual Physically Cross‐Linked Hydrogels Incorporating Hydrophobic Interactions with Promising Repairability and Ultrahigh Elongation

Novel dual physically cross‐linked (DPC) hydrogels with great tensile strength, ultrahigh elongation, and promising repairability are designed by introducing cellulose nanocrystal (CNC) or hydrophobized CNC (CNC‐C8) into polymers physically cross‐linked by hydrophobic forces. C18 alkyl chain is grafted to N‐[3‐(dimethylamino)propyl]methacrylamide (DMAPMA) for hydrophobic monomer (DMAPMA‐C18), and C8 to CNC surface for hydrophobic CNC (CNC‐C8). CNC‐C8 (or CNC) DPC hydrogels are synthesized, with monomers N,N‐dimethylacrylamide (DMAc) and DMAPMA‐C18 polymerized to form the first network physically cross‐linked by hydrophobic interactions, on which the secondary cross‐linking points are formed by hydrophobic interactions between CNC‐C8 and DMAPMA‐C18, electrostatic interactions between CNC‐C8 (or CNC) and DMAPMA, as well as hydrogen bonding between CNC‐C8 (or CNC) and DMAc. Compared with optimum CNC DPC hydrogels of the highest tensile strength (238 ± 8 kPa), the optimum CNC‐C8 DPC hydrogel with 0.0675 w/v% DMAPMA‐C18 and 0.4 w/v% CNC‐C8 possesses stronger tensile strength of 331 ± 32 kPa and excellent elongation of 4268% ± 1446% as well, demonstrating the enhanced mechanical property of the hydrogel by introduced hydrophobic interactions. In addition, such DPC hydrogel can be facilely repaired with tetrahydrofuran (THF) on the cut surfaces while retaining good tensile stress and elongation behaviors. A novel dual physically cross‐linked hydrogel with promising repairability, excellent elongation, and great tensile strength is developed, consisting of a first layer of a physically cross‐linked polymer network based on hydrophobic monomers C18 grafted N‐[3‐(dimethylamino)propyl]methacrylamide and N,N‐dimethylacrylamide, and a second layer network based on addition of cellulose nanocrystals or C8‐modified cellulose nanocrystals for physical‐force reinforcement.