Coupling of Polyelectrolyte Chain Structure with Correlations and Phase Behavior

Using a renormalized Gaussian fluctuation (RGF) field theory that self-consistently accounts for the concentration-dependent coupling between chain structure and electrostatic correlations [Shen and Wang 2017, JCP 146 (084901)], we study the phase behavior of polyelectrolyte solutions. We show that the popular fixed-Gaussian-structure RPA (fg-RPA) greatly over-predicts the driving force for phase separation of charged polymers, even below the critical Manning charge density. This over-prediction is attributable to inaccurate short wavelength chain structure -- while the chain structure is most obviously non-Gaussian at low salt or polymer concentration, the chain's stiffening persists in semi-dilute solution and can qualitatively affect the phase diagram. The RGF's self-consistent renormalization of the chain structure relaxes electrostatic interactions, and better describes much higher charge fractions and a wider concentration window than the regime of validity of fg-RPA. Our work demonstrates how chain structure has important couplings with electrostatic fluctuations and thermodynamics, even before stronger correlation effects like counterion condensation become important.