Interpolyelectrolyte complex and coacervate formation of poly(glutamic acid) with a dendrimer studied by light scattering and SAXS

Interpolyelectrolyte complex (IPEC) formation between sodium poly((L)-glutamate) and poly(amido amine) dendrimer in aqueous solution containing 0.25 M NaCl was studied as a function of pH by light scattering. The formation of soluble IPECs upon protonation of the dendrimer at 11 > 1 pH > 9.1 was followed from the increase of the extrapolated scattering intensity at zero scattering angle, the indirect Fourier transform method being used for the fitting of the scattering curves and for evaluation of the particle size. Coacervation was observed at pH similar to9.1 which increased slightly with the dendrimer excess. Dynamic light scattering revealed three relaxation modes. The relaxation times tau of the two faster modes scale with the scattering vector q as tau proportional to q(-2) and are attributed to the diffusion of dendrimer and of microgel inhomogeneity. An additional slow mode (tau proportional to q(0)) dominates the multiexponential dynamic structure factor of the IPEC solutions until breaking down at the pre-transition to the coacervation, which is also associated with a sharp increase of the radius of gyration R-g of mesoscopic particles from similar to50 to similar to250 nm. The microscopic structure of the coacervate was analyzed by small-angle X-ray scattering. The R-g similar to1.7 nm obtained from Guinier analysis of the smooth intermediate scattering function was close to that of a dendrimer. A low-q upturn of the scattering curves I(q) with I proportional to q(-6) was attributed to the superimposed scattering from compact spheres with a radius of similar to20 nm. A high-q upturn exhibits a power-low decay of I(q) proportional to q(-3), which may be attributed to a smooth surface.