ИСТИНА

The interaction of star-shaped poly(acrylic acid) (PAA)N with the number of arms N = 5, 8, and 21 (DPn(arm) ≈ 100) with the strong linear cationic polyelectrolyte, poly(N-ethyl-4-vinylpyridinium bromide) PVP•EtBr (DPw = 600, 3000), in aqueous media at pH 7 (DN(PAA) = 0.9) and 0.05 М < [NaCl] < 0.3 M was examined by means of turbidimetry and dynamic light scattering (DLS). On addition of the PVP•EtBr to the solution of the (PAA)8, no phase separation is observed unless the base-molar ratio Z of the polymeric components, Z = [N+]/[COO- + COOH] (the base-molar concentrations of the corresponding polymeric components are given in the brackets), exceeds a certain threshold value, Z*. At Z < Z*, the mixtures of the (PAA)8 star and PVP•EtBr are transparent, thus strongly suggesting the formation of water-soluble complex species with the star-shaped polyelectrolyte playing a lyophilizing part. The values of Z* were found to gradually decrease with the decreasing number of arms of the (PAA)N stars, this effect becoming more pronounced with the increasing concentration of NaCl. At the same time, the changing length of PVP•EtBr from 600 to 3000 results only to a minor narrowing of the region corresponding to the formation of water-soluble complex species. The homogeneous mixtures of (PAA)N stars with PVP•EtBr were further examined by means of DLS. As a typical example, a distribution of apparent hydrodynamic radius Rh(app) for the mixture of (PAA)8 with PVP•EtBr (DPw = 600) is demonstrated. The distribution is bimodal, indicating the presence of small and large species (for the mixtures of (PAA)5 with PVP•EtBr, only single peak corresponding to large species is detected; for the mixtures of (PAA)N with PVP•EtBr (DPw = 3000), two peaks are poorly separated at [NaCl] < 0.1 and at Z < 0.2). The value of Rh for the small species is close to hydrodynamic radius of the pure(PAA)8 stars, Rh = 8 nm (determined by means of DLS carried out at the angle of 6 ÷ 7°, at 90° light scattering by the pure (PAA)8 stars under experimental conditions is too low to be detected), thus each of the small species is thought to contain only one chain of the polycation and the corresponding minimum number of the (PAA)8 stars needed to provide its solubility. The value of Rh for the large species considerably exceeds the size of the pure (PAA)8 stars, thus each of the large species is assumed to be a complex particle comprising plenty of chains of both polymeric components.