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

Abstract: 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^∗ , where q^∗= 2 π/ ξ is the polyelectrolyte peak in the structure factor S(q) and ξ is the correlation length, the relaxation time obtained from fits to stretched exponential was τ_KWW∼ q^{- 3} , which describes unscreened Zimm-like dynamics. This is in contrast to implicit-solvent simulations using a Langevin thermostat where τ_KWW∼ q^{- 2} . At q< q^∗ , a crossover region was observed that eventually transitions to another inflection point τ_KWW∼ 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∼cp-1/2 , specific viscosity, ηsp∼cp1/2 , and diffusion coefficient, D∼cp0 , where c_p is the polyelectrolyte concentration. The scaling prediction for ξ holds; however, deviations from the predictions for η_sp and D were observed for systems at higher c_p , 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. Graphic abstract: [Figure not available: see fulltext.].