Computer simulation study on the swelling of a polyelectrolyte gel by a Stockmayer solvent

The swelling of a model polyelectrolyte gel is studied via three-dimensional molecular dynamics simulations, taking into account the counterions and the solvent explicitly. Each network bead carries a charge q(*). The counterion charge is -q(*), and thus the total system is neutral. The solvent is modeled via a Stockmayer fluid, i.e., each solvent particle is a point dipole plus a Lennard-Jones interaction center. A "two-box--particle transfer" simulation method is applied to calculate the swelling ratio of the network as well as the counterion mobility. The swelling of the network shows a broad maximum as a function of q(*) at T(*)(r)=T(*)/T(*)(c)=1.05 and P(*)(r)=P(*)/P(*)(c)=1.0. Here, T(*)(c) and P(*)(c) are the critical temperature and the critical pressure of the pure Stockmayer solvent, respectively, with dipole moments given by mu(*2)=1.0, 2.0, 3.0, and 4.0. The residence time of the counterions is calculated, showing a strong coupling to the charged network beads (condensation) as q(*) increases. Additional simulations at three different charge strengths (i.e., q(*)=0.5, 3.5, and 8.6) illustrate the complicated swelling behavior of the network under supercritical and subcritical conditions.