Coupled chemo-mechanical constitutive equations and residual stress evolution of swelling hydrogels

Hydrogels are cross-linked polymeric materials capable of undergoing large deformation in response to external stimuli, such as chemical gradients and mechanical loading. This article presents a coupled chemo-mechanical model of hydrogel undergoing substantial swelling. A multiplicative decomposition-based framework is adopted to represent simultaneous swelling and mechanical deformation in a consistent thermodynamic way. A nonlinear modified hyperelastic Yeoh–Fleming model is considered for the fully swollen hydrogel to describe the strain energy of the polymer network and is calibrated from the available experiments. After calibrating the model using uniaxial stretching for different volume fractions of the polymer network, the model is then benchmarked with equi-biaxial and pure shear responses. The model calibration at different polymer network volume fractions also allows evolution of the Yeoh–Fleming model parameters with polymer network concentration. Finally, we combine the free energy of mixing of solvent and polymer network and the strain energy of polymer network to solve a coupled boundary value problem (BVP) of free swelling. The solution predicts free swelling of hydrogel and the evolution of residual stresses induced by a slow diffusion phenomenon. The numerical results presented here may provide guidance for significant applications of hydrogels in soft robotics, drug delivery and biomedical systems.