Relaxation dynamics of salt-free polyelectrolyte solutions using flow birefringence and rheometry

Relaxation dynamics of salt-free, aqueous solutions of sodium poly(styrene sulfonate) (NaPSS) were investigated by mechanical rheometry and flow biref ringence measurements. Two semidilute concentration regimes were studied in detail for a range of polymer molecular weights. At solution concentration s c < 10 mg mt, limiting shear viscosity eta(0) was found to scale with mol ecular weight and concentration as eta(0) similar to c(0.5)M(w) over nearly two decades in concentration. At higher solution concentrations, c > 10 mg mt, a change in viscosity scaling was observed eta(0) similar to c(1.5)M(w )(2.9), consistent with a change from simple Rouse dynamics for unentangled polyions to near-perfect reptation dynamics for entangled chains. Characte ristic relaxation times tau deduced from shear stress and birefringence rel axation measurements following start-up of steady shearing at high rates re veal very different physics. For c < 10 mg mi,, both methods yield tau simi lar to c(-0.42)M(w)(-0.9). and tau similar to c(0)M(w)(0) for c > 10 mg mi, . Curiously, the concentration scalings seen in both regimes are consistent with theoretical expectations for salt-free polyelectrolyte solutions unde rgoing Rouse and reptation dynamics, respectively, but the molecular weight scalings are not. Based on earlier light scattering studies using salt-fre e NaPSS solutions, we contend that the unusual relaxation behavior is likel y 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 collec tive of aggregated chains rather than the dense polymer aggregates previous ly supposed. Nonetheless, polyion aggregates of either variety cannot accou nt for the inverse dependence of relaxation time on polymer molecular weigh t for c < 10 mg mt. (C) 1999 John Wiley & Sons, Inc.