Polyelectrolyte gels in salt solutions: a computational study*

Polyelectrolyte gels (PGs) demonstrate higher degrees of swelling and lower shear modulus than neutral polymer gels, which enables their applications as water-absorbents, cell cultivation scaffolds, artificial tissues and soft actuators. To understand the factors influencing the PG swelling, we performed coarse-grained molecular dynamics simulations of PGs in salt solutions. The equilibrium swelling ratio Q is shown to depend on the fraction of the charged monomers f and the salt concentration in the reservoir c_ss, which together determine the salt concentration c_s,g and the content of the osmotically active counterions inside PG. In particular, Q monotonically increases with the increasing fraction of charged monomers f on the polymer backbone and decreases with increasing the salt concentration. For studied PG systems, the equilibrium swelling is achieved in the non-linear network deformation regime. Therefore, to describe simulation results we use deformation dependent shear modulus G(Q). Specifically, analysis of simulation data shows that at equilibrium there is universal relationship between G(Q)/Q^1/3 and c_s,g/fc_s,s ratio.