STRUCTURE OF POLYELECTROLYTE STARS AND CONVEX POLYELECTROLYTE BRUSHES

The structure of polyelectrolyte stars and spherical brushes is discussed using a local force balance argument that balances the osmotic pressure and the tension in the chains locally for a given radial position. In general, there are four distinct regions where the segment density profile Phi(r); is dominated by one of the interactions: (i) in the outer region, electrostatic interactions dominate, (ii) in the intermediate region, binary interactions dominate, (iii) in the inner region ternary interactions dominate, and finally (iv) at the center of the star (of the order of a few segment lengths), the density is unity. For poor solvents we predict a collapse induced by the attractive binary interactions that leads to a two-phase star or brush. The inner dense layer is stabilized by ternary interactions whereas the outer dilute layer is dominated by electrostatic repulsions-the segment density changes discontinuously from one to the other (and the intermediate density region disappears). The magnitude of the collapse transition is found to increase with the chain length for stars and spherical brushes, unlike planar brushes. The star is expected to collapse continuously with the reduction in the solvent quality-unlike a first-order transition for planar brushes. Planar polyelectrolyte brushes can also display two-phase regions; however, the full SCF approach is warranged in this case. Collapse induced by n-clusters in uncharged polymers is also shown to lead to two-phase brushes.