Mechanical behavior and stimuli-response of nanocomposite hydrogels

Abstract

Nanocomposite hydrogels (NC gels) have been intensively studied up to now among all the high performance hydrogels due to the facile preparation, unique composition, and excellent properties of mechanical, optical, and swelling/deswelling. We have achieved some significant results in the mechanical behavior of the NC gels and the synthesis of stimuli-response NC gels during several years. Ultrahigh extensibility was found from the as-prepared and swelling-equilibrated poly(N-isopropylacrylamide) (PNIPAm)-hectorite Laponite NC gels with strain hardening at large strain region. Mooney-Rivlin function was found available to describe the stress-strain behavior under compression, but failed to predict the high strain hardening. Creton model was adopted to interpret the stress-strain curves for the NC gels under large deformation well, especially the strain hardening in these gels. For the NC gels, the effective cross-linking density was estimated from the equilibrium shear modulus under small strain, the hysteresis was observed during a deformation-recovery cycle, and the relaxation exponent was determined based on the relaxation modulus. According to these results, we suggest that the ultrahigh extensibility of the NC gels is originated from low cross-linking density and moderate relaxation. In respect of stimuli-response NC gel preparation, a stability window was found from aqueous Laponite suspensions for NIPAm copolymerization with ionic monomers to avoid flocculation and precipitation of Laponite platelets. In this way, the pH response and pH/temperature dual response homogeneous NC gels with ultrahigh extensibility were successfully synthesized. Finally, the unsolved scientific problems concerning the NC gels and the potential applications in the near future were discussed.