Effect of charge density on the dynamic behavior of polyelectrolytes in aqueous solution

Dynamic light scattering experiments and measurements of surface tension and viscosity of aqueous solutions of poly(2-vinylpyridine) quaternized with ethyl bromide are carried out as function of the charge density and polyelectrolyte concentration (M(W) = 2.9 x 10(5), M(W)/M(n) = 1.11; alpha(e) degree of quaternization, 0.09 less than or equal to alpha(e) less than or equal to 0.46). The polyelectrolytes are dissolved in an aqueous solution of potassium bromide (c(s) = 1 x 10(-2)c(+), c(+) = 1 mol dm(-3)). Their concentrations <(c)over bar (p)> are in the range <0.1(c)over bar (+)> less than or similar to <(c)over bar (p)> less than or equal to <27(c)over bar (+)> (<(c)over bar (+)> = 1 g dm(-3)). The reduced variable Lambda (=alpha(e)c(m)/c(s)) is used to characterize the composition of the solutions (c(m), molar volume concentration of the repeat unit of the polymer backbone). Monomodal and bimodal dynamic modes characterized by the apparent diffusion coefficients D-s and D-f (index s, slow; index f, fast) are found under the following conditions: monomodal, 0.03 less than or equal to Lambda less than or equal to 0.1; bimodal, 0.1 less than or equal to Lambda less than or equal to 8. A third dynamic mode characterized by the apparent diffusion coefficient D-int (index int, intermediate) appears at Lambda greater than or similar to 1 with small values of alpha(e) in the range 0.09 less than or equal to alpha(e) less than or equal to 0.15 (D-s < D-int < D-f). It is assumed that the third mode reflects dynamic properties of hydrophobic domains formed by uncharged segments of the polymer backbone at low values of alpha(e). Hydrophobic interactions show up also in surface tension and viscosity measurements. A model proposed by Lee and Schurr (J. Polym. Sci. 1975, 13, 873) describes the experimental D-f data of the fast diffusive mode with two fit parameters (Z(eff), effective number of charges of a polyion; D-p, diffusion coefficient of a polyion). Z(eff) is smaller by a factor of 5-7 than the value calculated on the basis of alpha(e).