On the origin of the slow mode and low q upturn in salt-free polyelectrolyte solutions

Polyelectrolytes in salt-free solvents are theoretically expected to scatter weakly at low q due to their high osmotic pressure and low osmotic compressibility. However, experiments have consistently revealed an unexpected upturn in this regime, puzzling researchers for decades. Similarly, dynamic light scattering (DLS) often shows a slow relaxation mode with a diffusion coefficient orders of magnitude lower than that of single-chain diffusion, even though other techniques detect no unusually slow dynamics.

Here we present small-angle neutron and light scattering data on polyelectrolytes in deionized water and show that both the low-q upturn and the slow mode can be eliminated by careful sample filtration. When the slow mode is present, analysis of the DLS autocorrelation function allows the total structure factor to be decomposed according to the relative amplitudes of the fast and slow relaxations. Extrapolation of the fast component to zero angle yields osmotic pressures consistent with independent measurements such as membrane osmometry and freezing-point depression, in agreement with theoretical predictions. Filtration does not alter the correlation length, density, or viscosity of the solutions, indicating that only a negligible fraction of chains, if any, are responsible for the low-q excess scattering.