Roadmap to Design Mechanically Robust Copolymer Hydrogels Naturally Cross-Linked by Hydrogen Bonds

Although several mechanically strong physical hydrogels bearing H-bond donor and acceptor groups have been reported over the past years, the effect of the complex interplay between competing interactions on the mechanical strength of H-bonded hydrogels remains a challenge. We present here the mechanical properties of six different copolymer hydrogels formed under identical conditions. Methacrylic acid (MAAc), acrylic acid (AAc), N,N-dimethylacrylamide (DMAA), 1-vinylimidazole (NVI), N-vinyl pyrrolidone (NVP), and acrylamide (AAm) monomers were copolymerized to form MAAc/DMAA, AAc/DMAA, AAc/NVI, MAAc/NVI, MAAc/NVP, and AAm/NVP hydrogels, respectively, at various molar ratios in the presence of 60 wt % water. The hydrophobicity of the monomers and the competing interactions between the copolymer chains and copolymer-water were quantitatively elucidated by the all-atom MD simulations in the explicit water, density functional theory calculations, and molecular descriptors by remaining faithful to the experimental compositions. Young's modulus of the hydrogels could be varied between 10-1 and 101 MPa by changing the type and molar ratio of the comonomers. AAc/DMAA and AAm/NVP hydrogels exhibit the lowest moduli, 0.11 ± 0.05 and 0.20 ± 0.04 MPa, respectively, over all comonomer compositions, while for all other comonomer pairs, the resulting hydrogels assume a maximum modulus at a critical composition. MAAc and NVI are the most effective major and minor components, respectively, to generate copolymer hydrogels with a high modulus and strength. The crucial factors determining the mechanical performance of the copolymer hydrogels are the hydrophobicity of the major copolymer component, ionic H-bonds, formation of strong H-bonded nanoaggregates, and stronger and higher inter-chain H-bonding and hence electrostatic interactions.