Coarse-Grained Explicit-Solvent Molecular Dynamics Simulations of Unentangled Polyelectrolyte Chains in Semidilute Solutions

We present results of explicit solvent coarse-grained molecular dynamics simulations of fully charged, salt-free, and unentangled polyelectrolytes in semidilute solutions. The inclusion of a polar solvent in the model allows for a more accurate representation of these systems at higher concentrations, where the assumptions of a continuum dielectric medium and screened hydrodynamics break down. The collective dynamic structure factor of polyelectrolytes, S(q,t), showed that at q > q*, where q*=2π/ξ is the polyelectrolyte peak in S(q) and ξ is the correlation length, the average relaxation time fitted from a stretched exponential was <τ> ~ q^-3, which describes Zimm-like dynamics and un-screened. This is in contrast to implicit solvent simulations using a Langevin thermostat where <τ> ~ q^-2. At q < q*, a cross-over region was observed that eventually transitions to <τ> ~ q^-2 at length scales larger than ξ for both implicit and explicit solvent simulations. The simulation results were also compared to scaling predictions for correlation length, ξ ~ c^-1/2, specific viscosity, η_sp ~ c^1/2, and diffusion coefficient, D ~ c⁰, where c is the polymer concentration. The scaling prediction for ξ holds, however deviations from the predictions for η_sp and D were observed for systems at higher c.