ИСТИНА

The interaction of star-shaped polyacrylic 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 P4VP∗EtBr (DPw = 570), in aqueous media (pH 7, 0.05 М < [NaCl] < 0.3 M) was examined by means of turbidimetry and analytical ultracentrifugation. This interaction was shown to result in the formation of water-soluble interpolyelectrolyte complexes (IPECs) if the charge ratio Z in the system, Z = [+]/[-] (molar concentrations of the ionic groups of the corresponding polymeric components are given in the brackets), does not exceed a certain critical value of Z*, Z < Z* (Z* ≅ 0.1 ÷ 0.4, depending on N and [NaCl]). In such IPECs, the star-shaped anionic polyelectrolyte whose ionic groups are incorporated in the complex particles in the excess appears to play a lyophilizing part, providing solubility of the complex particles in aqueous media. 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. The homogeneous mixtures of the oppositely charged polymeric components (that is, at Z < Z*) were examined by means of analytical ultracentrifugation and dynamic light scattering. The results strongly suggest the formation of the complex particles of the characteristic composition which appears to be close to Z*: complex particles of the characteristic composition coexist with free (PAA)N stars. For the (PAA)N stars with N = 5 and 8, this finding implies that the particles of IPECs can incorporate several stars, that is, the polycation-induced assembly of the star-shaped (PAA)N takes place. With the rising N, sedimentation coefficient of the complex particles was shown to increase while their hydrodynamic radius was observed to decrease. Due to the screening effect of small ions, the electrostatic interaction between the (PAA)N and P4VP∗EtBr was found to be completely suppressed at rather high concentrations of low molecular weight salts. The salt-induced dissociation of IPECs based on the (PAA)N stars into the individual polymeric components was observed to begin only when the certain threshold salt concentration is reached and to proceed via a gradual release of the cationic macromolecules from the complex particles upon the further increasing ionic strength of the surrounding media. Such a threshold salt concentration corresponding to the dissociation of IPECs was shown to increase in the order NaCl < KCl << (CH3)4NCl, being at the same time virtually independent of N.