New evidence of the nonequilibrium nature of the "slow modes" of diffusionin polyelectrolyte solutions

The time-dependent behavior of the dissolution of polyelectrolyte powders i n pure water and moderate ionic strength aqueous solvent was monitored by f lowing dissolving material through an online filter, and then through a mul tiangle light scattering unit, a refractometer, and a capillary viscometer. When the polyelectrolytes were dissolved in solutions of moderate ionic st rength, their dissolution behavior was similar to that of neutral polymers. When dissolved in pure water, however, there was consistently a small popu lation of aggregates that appeared at the beginning of the dissolution proc ess, which then rapidly diminished. For large pore filtration, the aggregat es reached a final low level, and slowly disappeared over the span of many days, whereas for small pore filtration the aggregates disappeared complete ly over a scale of minutes. The real-time data, together with size exclusio n chromatography analysis, shed light on previously unanswered questions co ncerning the nonequilibrium nature of this small population of polyelectrol yte aggregates in low ionic strength solutions, and its relation to the "ex traordinary phase" of diffusion (or "slow modes "). Further evidence is als o provided that both angular scattering maxima due to interpolyion correlat ions and the maximum of reduced viscosity vs polyion concentration ("electr oviscous" effect) at low ionic strength are equilibrium properties that are unrelated to these aggregates. (C) 2000 John Wiley & Sons, Inc.