Strongly charged flexible polyelectrolytes in poor solvents: Molecular dynamics simulations with explicit solvent

The behavior of salt-free solutions of charged flexible polymer molecules in poor solvents is studied using molecular dynamics simulations. The polymer molecules are modeled as chains of charged spheres, the counterions as charged spheres, and the solvent molecules are incorporated explicitly and modeled as uncharged spheres. The equilibrium static and dynamic properties are studied as a function of solvent quality. In many-chain systems, for slightly poor solvents, no peak is observed in the static structure factor at low semidilute concentrations, but a peak appears at higher concentrations. In this regime, chains form bead-necklace structures, and the counterions are strongly correlated with the polyions. When the solvent quality is decreased further, at nonzero but low polymer concentrations, the solution becomes unstable towards phase separation. The dense phase takes on spherical, cylindrical, or lamellar structures depending on the polymer concentration. The mass and charge density profiles of polyions and counterions in the dense phase are an oscillatory function of distance. The phase separation slows down the diffusion normal to the phase interface by two to three orders of magnitude, but has little effect on the diffusion parallel to the interface. These results are qualitatively different from the behavior observed in simulations where the solvent is treated implicitly via a pairwise-induced attraction between polymer beads. (C) 2003 American Institute of Physics.