Effect of Polyion Charge Density on the Morphology of Nanostructures in Polyelectrolyte−Surfactant Complexes

The poly(acrylate) (PA)/surfactant and poly(methacrylic acid) (PMA)/surfactant complexes were investigated by synchrotron X-ray scattering at various degrees of neutralization, α, of the polyion and at a fixed surfactant to polyion charge ratio. At low α-values, the surfactant organizes into polymer-induced micelles that are attracted by the polymer through hydrophobic interactions. This is followed by a formation of a Pm 3 n cubic structure at higher α-values as a consequence of very strong electrostatic attractions between the polyion and charged surfactant aggregates. From the position of the maxima in the scattering curves, the characteristic distance d was obtained. In PA/surfactant complexes, d varies linearly with α. This is an indication of a smooth expansion of the PA chain upon neutralization. In PA/dodecylpyridinium chloride (DPC) solutions, d increases from 30.2 Å at α = 0 to 36.6 Å at α = 1, and correspondingly d increases from 44.1 to 51 Å in the PA/CPC case. The d -value at α = 0 almost matches one micellar diameter, whereas the one at α = 1 exceeds this value by 6.4−7 Å. This indicates that micelles induced by the polyion at α = 1 are separated by a layer of the charged polymer chain, whereas the polymer coil at α = 0 does not contribute significantly to the intermicellar distance. The distance d in PMA/surfactant solutions is not a linear function of α. A major increase in d is found in the range of α between 0.2 and 0.5. This agrees with the α-range in which the conformational transition of the PMA chain takes place.