Toughness improvement and anisotropy in semicrystalline physical hydrogels

Abstract

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 Tm of its crystalline domains followed by cooling below Tm 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 λo. Directional brittle-to-ductile and ductile-to-brittle transitions could be induced by adjusting the prestretch ratio λo. Small- and wide-angle X-ray scattering measurements and mechanical tests highlight a critical prestretch ratio λo at which the hydrogel exhibits the highest microstructural and mechanical anisotropy due to the finite extensibility of the network chains. At λo = 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.