Polyelectrolyte Gels: Swelling and Deswelling upon Nonlinear Deformations

Abstract

Polyelectrolyte gels, made by crosslinking of ion-containing polymers, showcase extraordinary swelling capabilities, with their volumes expanding up to 100 times in salt-free solutions. This swelling behavior of polyelectrolyte gels is attributed to the delicate balance between the gels' elasticity and the osmotic pressure produced by counterions and salt ions localized within the gel volume due to the Donnan equilibrium. We use a combination of the nonlinear version of Flory−Rehner theory and coarsegrained molecular dynamics simulations to explore the swelling and deformation of polyelectrolyte gels undergoing large biaxial deformations in contact with a salt solution. The analysis of the gel deformation points out that the swelling ratio of the polyelectrolyte gels Q eq describing the volume change upon swelling is a nonmonotonic function of the gel deformation ratio α characterizing changes in a gel shape under biaxial deformation with respect to a free-standing swollen state. The swelling ratio first increases with increasing α and then starts to decrease. This intriguing behavior is due to the optimization of osmotic pressure and conformational entropy of charged strands in the nonlinear deformation regime. Furthermore, we observe that the location of the maximum swelling ratio shifts toward large gel deformations with increasing salt concentration and 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.

Ion Partition in Polyelectrolyte Gels and Nanogels

“…However, these models of (poly)electrolyte solutions do not adequately address the solvation of the charged species, and several critiques exist in the literature [53][54][55][56][57]. Despite the reported shortcomings of this type of model, these models remain popular in the scientific community [58][59][60][61][62][63][64][65] and the performance associated with them in comparison with real polyelectrolyte gel materials remains uncertain.…”