Spatial dimensions of colloidal polyelectrolyte brushes as determined by dynamic light scattering

We present a comprehensive investigation of the spatial dimensions of colloidal spherical polyelectrolyte brushes in solutions. The particles studied here consist of a solid polystyrene core of 50-100 nm radius onto which linear poly(acrylic acid) (PAA) chains are chemically grafted. The grafting is achieved through photo-emulsionpolymerization in which radicals are generated on the surface of the core particles. This procedure leads to a dense blush of PAA chains chemically fixed on the surface of the core particles. The hydrodynamic radius R-H Of these particles is studied by dynamic light scattering as function of salt concentration. The chemical grafting leads to an extreme colloidal stability, and R-H can be studied in KCl solutions which concentrations varying between 10(-3) and 3 M. The resulting R-H decreased monotonically with ionic strength. The data of R-H as function of ionic strength were compared with the theory of Hariharan, Biver and Russel (Macromolecules, 1998, 31, 7514). This theory, which is based on the Daoud-Cotton model of uncharged curved brushes, describes the spherical polyelectrolyte brushes, in terms of an excluded volume parameter nu and the Kuhn length l(K). The local ionic strength of the brush c(s) is predicted to differ widely from the bulk value c(a). Both nu and l(K) depend on the local ionic strength c(s) as expressed through the Debye length kappa (-1) within the brush. Quantitative agreement of theory and experiment is achieved when the following assumptions are made: nu proportional to l(K)(2)kappa (-1), and l(K) proportional to l(B)(-1)kappa (-2), where l(B) denotes the Bjerrum-length.