Screening in solutions of star-branched polyelectrolytes

Equilibrium conformations of star-branched polyelectrolytes in dilute solut ions are studied on the basis of a numerical self-consistent-field (SCF) ap proach and analytical theories. It is shown that, even in a dilute salt-fre e solution, the intramolecular Coulombic repulsion in many-armed stars is s trongly screened by counterions which are localized preferentially in the i ntrastar space. As a result, the dependence of the star size on the number of branches levels off for many-armed stars. Addition of salt results in ad ditional screening and in contraction of the stars. The scaling prediction R similar to c(s)(-1/5) for the star size as a function of the salt concent ration c(s) is well confirmed by SCF calculations. A decrease in the star s ize can also be induced by an increase in the concentration of the polyelec trolyte in the solution. We have observed significant contraction of the st ars with increasing concentration below the overlap threshold, i.e. in dilu te solutions. The latter effect is more pronounced for stars with a small n umber of branches. The screening of the intramolecular Coulombic repulsion due to added salt is compared with that occurring upon increasing the conce ntration of the polyelectrolyte.