Characterization of stress softening and self-healing in a double network hydrogel

In this paper, a micro-mechanically based constitutive model is presented to describe stress softening and self-healing in alginate-polyacrylamide (PAAm) double network (DN) hydrogels. The stress softening phenomenon in alginate-PAAm DN hydrogels under cyclical deformation is assumed to be the result of the rupture of chain linkages. Therefore, the network evolution method [Dargazany and Itskov, International Journal of Solids and Structures, 2009, 46, 2967] is used to characterize stress softening. The polymer matrix is initially decomposed into reversible and irreversible polymer networks. To model stress softening, the entropic energy of a polymer chain and the chain distribution are taken into account for each network. Unlike conventional DN hydrogels, after deformation alginate-PAAm hydrogels show self-healing. The rate of self-healing is associated with both intermolecular forces and the duration of storage of the samples in a thermal chamber. Broken chain linkages are assumed to rebond due to intermolecular forces and heating. Chemical reaction kinetics and heat transfer equations are utilized to calculate the quantity of the reversible cross-linking rebonding. This model contains few material parameters and demonstrates good agreement with experimental data in stress softening and self-healing.