Internal structure and mechanical property of an anisotropic hydrogel with electrostatic repulsion between nanosheets

We recently developed a mechanically anisotropic hydrogel with a large internal electrostatic repulsion. The hydrogel is embedded with negatively charged titanate nanosheets that are cofacially oriented by a magnetic field. Because of the anisotropic electrostatic repulsion between the nanosheets, this hydrogel exhibits a large elastic modulus when compressed orthogonal to the nanosheet plane. In this paper, we systematically investigate how the intensity of the electrostatic repulsion affects the internal structure and mechanical property of this anisotropic hydrogel, where the free-ion concentration in the hydrogel medium was the key parameter for moderating the electrostatic repulsion due to its screening effect. With gradually changing the free-ion concentration in the hydrogel, nanoscopic arrangement and mesoscopic domain structure of the nanosheets were characterized by two-dimensional small-angle X-ray scattering (2D SAXS) and confocal laser scanning microscopy (CLSM), respectively. Furthermore, an anisotropic mechanical property of the hydrogel was evaluated by compression test. These results reveal that extensive deionization of the hydrogel medium and the resultant maximization of the electrostatic repulsion induces the nanosheets to adopt a monodomain lamellar structure that occupies the whole space of the hydrogel, thereby enhancing the mechanical anisotropy of the hydrogel.