Coarse-grained explicit-solvent molecular dynamics simulations of semidilute unentangled polyelectrolyte solutions

We present results from explicit-solvent coarse-grained molecular dynamics (MD) 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 physical representation of these solutions at 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(& lowast;), where q(& lowast;) = 2 pi/xi is the polyelectrolyte peak in the structure factor S(q) and xi is the correlation length, the relaxation time obtained from fits to stretched exponential was tau KWW similar to q(-3), which describes unscreened Zimm-like dynamics. This is in contrast to implicit-solvent simulations using a Langevin thermostat where tau KWW similar to q(-2). At q < q(& lowast;), a crossover region was observed that eventually transitions to another inflection point tau KWW similar to q(-2) at length scales larger than xi for both implicit-and explicit-solvent simulations. The simulation results were also compared to scaling predictions for correlation length, xi similar to c(p)(-1/2), specific viscosity, eta(sp) similar to c(p)(1/2) , and diffusion coefficient, D similar to c(p)(0), where cp is the polyelectrolyte concentration. The scaling prediction for xi holds; however, deviations from the predictions for eta(sp) and D were observed for systems at higher cp, which are in qualitative agreements with recent experimental results. This study highlights the importance of explicit-solvent effects in molecular dynamics simulations, particularly in semidilute solutions, for a better understanding of polyelectrolyte solution behavior.