Polyelectrolyte Gels: Swelling and Deswelling upon Nonlinear Deformations

Polyelectrolyte gels, made by crosslinking of ion-containingpolymers,showcase extraordinary swelling capabilities, with their volumes expandingup to 100 times in salt-free solutions. This swelling behavior ofpolyelectrolyte gels is attributed to the delicate balance betweenthe gels' elasticity and the osmotic pressure produced by counterionsand salt ions localized within the gel volume due to the Donnan equilibrium.We use a combination of the nonlinear version of Flory-Rehnertheory and coarse-grained molecular dynamics simulations to explorethe swelling and deformation of polyelectrolyte gels undergoing largebiaxial deformations in contact with a salt solution. The analysisof the gel deformation points out that the swelling ratio of the polyelectrolytegels Q (eq) describing the volume changeupon swelling is a nonmonotonic function of the gel deformation ratio & alpha; characterizing changes in a gel shape under biaxial deformationwith respect to a free-standing swollen state. The swelling ratiofirst increases with increasing & alpha; and then starts to decrease.This intriguing behavior is due to the optimization of osmotic pressureand conformational entropy of charged strands in the nonlinear deformationregime. Furthermore, we observe that the location of the maximum swellingratio shifts toward large gel deformations with increasing salt concentrationand decreasing fraction of the charged groups on network strands.This behavior is a direct result of the reduced influence of the ions'osmotic pressure on strand prestretching in the free-standing gels.