Highly Stretchable, Repairable, and Tough Nanocomposite Hydrogel Physically Cross-linked by Hydrophobic Interactions and Reinforced by Surface-Grafted Hydrophobized Cellulose Nanocrystals

Abstract

Highly stretchable, repairable, and tough nanocomposite hydrogels are designed by incorporating hydrophobic carbon chains to create first-layer cross-linking among the polymer matrix and monomer-modified polymerizable yet hydrophobic nanofillers to create second-layer strong polymer-nanofiller clusters involving mostly covalent bonds and electrostatic interactions. The hydrogels are synthesized from three main components: hydrophobic monomer DMAPMA-C18 by reacting N-[3-(dimethylamino)propyl]methacrylamide] (DMAPMA) with 1-bromooctadecane, monomer N,N-dimethylacrylamide (DMAc), and monomer-modified polymerizable hydrophobized cellulose nanocrystal(CNC-G) obtained by reacting CNC with 3-trimethoxysily propyl methacrylate. The polymerization of DMAPMA-C18 and DMAc and physical cross-linking due to the hydrophobic interactions between C18 chains make DMAPMA-C18/DMAc hydrogel. The additional introduction of CNC-G brings more interactions into the final hydrogel (DMAPMA-C18/DMAc/CNC-G): the covalent bonds between CNC-G and DMAPMA-C18/DMAc, hydrophobic interactions, electrostatic interactions between negatively charged CNC-G and positively charged DMAPMA-C18, and hydrogen bonds. The optimum DMAPMA-C18/DMAc/CNC-G hydrogel exhibits excellent mechanical performance with elongation stress of 1085 ± 14 kPa, strain of 4106 ± 311%, toughness of 3.35 × 10⁴ kJ m^-3 , Young's modulus of 844 kPa, and compression stress of 5.18 MPa at 85% strain. Besides, the hydrogel exhibits good repairability and promising adhesive ability (83-260 kN m^-2 toward various surfaces).

Keywords: cellulose nanocrystals; hydrogels; hydrophobic interactions; nanocomposites; repairability.