Evaluation of Intermolecular Interactions of Hydrogels: Experimental Study and Constitutive Modeling

Abstract

The mechanical properties of hydrogels are significantly influenced by the solvent content. In particular, inelastic deformation occurs in dried hydrogels. In this study, we quantitatively evaluated the inelastic deformation of a hydrogel introduced by intermolecular interactions under uniaxial tensile loading-unloading tests and established a visco-hyperelastic model describing the experimentally observed inelastic behavior based on the transient network theory. The development of the strain field during the test was measured using digital image correlation (DIC) method, and the relationship between the true stress and the true strain was evaluated using the obtained strain field. A significant difference between the loading and the unloading responses was observed for the dried hydrogel specimens. This result indicates that intermolecular interactions induce irreversible deformation when the intermolecular chain distance is smaller. A non-dimensional parameter corresponding to the intermolecular chain distance was introduced to describe the irreversible response of the hydrogel observed in the experimental study. This parameter is a function of the densities of molecular chains, number of segments per chain, and stretching accompanied by swelling and drying. A visco-hyperelastic model was established by introducing the proposed parameter into the transient network theory. The proposed model qualitatively and quantitatively reproduced the experimentally observed features of the mechanical response of the hydrogel.

Keywords: hydrogel, swelling, drying, inelastic, visco-hyperelastic, transient network, intermolecular interaction