HOMOGENEOUS AND INHOMOGENEOUS SWELLING AND PARAMETRIC STUDY OF HYDROGELS CONSIDERING THE MICROSTRUCTURAL DEFORMATION

An analytical model is proposed to analyze the swelling-induced deformation of the hydrogels considering the microstructural deformation of the polymer chains. It is assumed that the chain is constrained to a tube-like space due to the action of the surrounding chains, and the chains experience the non-affine deformation. Based on the presented model, we studied the free swelling deformation, the swelling along one direction with the pre-stretches in other two directions and the constraint swelling of a spherical hydrogel with a rigid core. For the free swelling case, the deformation is homogeneous and the deformation of the network is equal to that of the chains. The stretches of the network and the chains increase with the increase of the number of polymer segments and decrease with the increase of the density of the chains. The stretches also increase with the increase of the effective tube geometry parameter (ETGP). The effect of the ETGP on the swelling-induced stretch gradually disappears when the external solvent pressure exceeds a certain value. For the swelling along one direction with pre-stretches in other two directions, the equilibrium state with which the stretch of micro-chain equals to that of macro-network can be reached. At this state, the stresses in the hydrogel become zero. For the constraint swelling of a spherical hydrogel with a rigid core, the hydrogel deforms inhomogeneously and the swelling-induced radial stretch is different from the tangential stretch. At the region near the rigid core, the stretch of micro-chain and the stretch of macro-network in the radial direction are larger than that of the free swelling state. At the region far from the rigid core, the stretch of the micro-chain and the radial and tangential stretches of the macro-network approach to that of the free swelling state. The osmotic pressure decreases with the increase of the ETGP, while the volume fraction of the solvent molecules increases with the increase of the ETGP. The presented model can be used to predict the swelling-induced deformation of micro-chain.