Toughness improvement and anisotropy in semicrystalline physical hydrogels

A major challenge in the gel science is to create mechanically strong hydrogels with anisotropic properties as observed in many biological tissues. Here, we report a simple one-step method of producing high-strength physical hydrogels exhibiting microstructural and mechanical anisotropy. As the precursor material, we use semicrystalline shape-memory hydrogels consisting of poly(N, N-dimethylacrylamide) chains interconnected by n-octadecyl acrylate (C18A) segments forming crystalline domains and hydrophobic associations acting as switching segments and netpoints, respectively. To generate anisotropic microstructure, we impose a prestretching on the isotropic hydrogel sample above the melting temperature T_m of its crystalline domains followed by cooling below T_m under strain to fix the elongated shape of the gel sample. A significant microstructural and mechanical anisotropy was achieved that could be tuned by the magnitude of the prestretch ratio λ_o. Directional brittle-to-ductile and ductile-to-brittle transitions could be induced by adjusting the prestretch ratio λ_o. Small- and wide-angle X-ray scattering measurements and mechanical tests highlight a critical prestretch ratio λ_o at which the hydrogel exhibits the highest microstructural and mechanical anisotropy due to the finite extensibility of the network chains. At λ_o = 1.8, the hydrogel exhibits Young's moduli of 161 ± 14 and 76 ± 7 MPa, and toughness of 16 ± 1 and 1.3 ± 0.1 MJ m⁻³ along and perpendicular to the prestretching direction, respectively.