Relaxation Dynamics of Salt-Free Polyelectrolyte Solutions Using Flow Birefringence and Rheometry

摘要

Relaxation dynamics of salt-free, aqueous solutions of sodium poly(styrene sulfonate)(NaPSS) were investigated by mechanical rheometry and flow briefringence measurfements. Two semidilute concentration regimes were studied in detail for a range of polymer molecular weights. At solution concentrations c < 10 mg mL, limiting shear viscosity #eta#_0 was found to scale with molecualr weight and concentration as #eta#_0 ～ c~0.5M_w over nearly two decades in concentration. At higher solution concentrations, c > 10 mg mL, a change in viscosity scaling was observed #eta#_0 ～ c~1.5M_w~2.9, consistent with a change from simple Rouse dynamics for unentangled polyions to near-perfect reptation dynamics for entangled chains. Characteristic relaxation times #tau# deduced from shear stress and birefringence relaxation measurements following start-up of steady shearing at high rates reveal very differnet physics. For c < 10 mg mL, both methods yield #tau# ～ c~-0.42M_w~(-0.9) and #tau# ～ c~0M_w~0 for c > 10 mg mL. Curiously, the concentration scalings seen in both regimes are consistent with theoretical expectations for salt-free polyelectrolyte solutions undergoing Rouse and reptation dynamics, respectively, but the molecular weight scalings are not. Based on earlier light scattering studies using salt-free NaPSS solutions, we contend that the unusual relaxation behavior is likely due to aggregation and/or coupled polyion diffusion. Simultaneous stress and birefringence measurements suggest that in concentrated solution, NaPSS aggregates are likely well permeated by solvent, supporting a loose collective of aggre-gated chains rather than the dense polymer aggregates previously supposed. Nonetheless, polyion aggregates of either variety cannot account for the inverse dependence of relaxation time on polymer molecular weight for c < 10 mg mL.