Exploring the inverse poroelastic behavior of hydrogels: the roles of finite extensibility of polymer chains and unfolding of polymer domains

Abstract

Hydrogels under stretching can exhibit pronounced deswelling—a phenomenon termed as “inverse poroelasticity”—yet its underlying mechanisms remain indistinct. In this paper, two mechanisms of the inverse poroelasticity of hydrogels are proposed: (1) the finite extensibility of polymer chains, and (2) the unfolding of polymer domains which induces a hydrophilic/hydrophobic transition of polymer chains. A novel constitutive model incorporating the two mechanisms is developed to reproduce the inverse poroelastic behavior of hydrogels. The finite extensibility of polymer chains is captured through the Langevin chain, while the unfolding of polymer domains leads to a varying chain length. Additionally, the hydrophilic/hydrophobic transition is modeled via a newly proposed mixing energy density function. The constitutive model is validated by comparing the results from the model with the experimental data of double network (DN) hydrogels and fibrin hydrogels, both of which exhibit an inverse poroelastic behavior. Furthermore, the proposed constitutive model is applied to investigate the inverse poroelastic fracture of hydrogels through the finite element method. The rate-dependent fracture and delayed fracture of hydrogels with a permeable crack are investigated. It is found that both the rate-dependent fracture and the delayed fracture differ between the hydrogels exhibiting the inverse poroelastic behavior and those displaying the conventional poroelastic behavior. This work deepens the fundamental understanding on the inverse poroelastic behavior of hydrogels and provides insights in designing mechanically robust hydrogels.