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

• Published in: Polymer (Guilford) Vol. 177; pp. 43 - 48
• Main Authors: Sano, Koki, Igarashi, Naoki, Arazoe, Yuka Onuma, Ishida, Yasuhiro, Ebina, Yasuo, Sasaki, Takayoshi, Hikima, Takaaki, Aida, Takuzo
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 26.08.2019; Elsevier BV
• Subjects: Anisotropic hydrogel; Anisotropic mechanical property; Anisotropy; Compression; Compression tests; Confocal microscopy; Deionization; Electrostatic repulsion; Hydrogels; Inorganic nanosheets; Ion concentration; Ions; Lamellar structure; Magnetic fields; Mechanical properties; Microscopy; Modulus of elasticity; Nanosheets; Scanning microscopy; Small angle X ray scattering; X-ray scattering; Electrostatic repulsion; Inorganic nanosheets; Anisotropic hydrogel; Anisotropic mechanical property
• ISSN: 0032-3861; 1873-2291
• DOI: 10.1016/j.polymer.2019.05.064

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. [Display omitted] •Effects of the electrostatic repulsion on the anisotropic hydrogel were systematically studied.•It was proved that the nanosheets in the hydrogel under the deionized state adopt a monodomain structure.•The enhancement of the electrostatic repulsion largely increased mechanical anisotropy of the hydrogel.