Formation and Transition of Highly Ordered Structures of Polyelectrolyte−Surfactant Complexes

Small-angle X-ray scattering studies on the nanostructures of water-equilibrated complexes, formed by slightly cross-linked copolymer gels of poly(sodium methacrylate/ N -isopropylacrylamide) P(MAA/NIPAM) and fully charged sodium polystyrenesulfonate (PSS), respectively, interacting with oppositely charged surfactants of alkyltrimethylammonium bromide (C n TAB, with n being the number of carbon atoms in the alkyl chain) at ∼23 °C, are presented. In P(MAA/NIPAM)−C n TA complexes, the formation and transition of highly ordered structures were investigated in terms of the surfactant alkyl chain length and the hydrophobicity of the polyelectrolyte chain. The complexes between fully charged PMAA gel and C n TAB showed Pm 3 n cubic structures at 10 ≤ n ≤ 16 but did not show highly ordered structures at n = 8 and 18 due to the weak hydrophobic interaction and the steric hindrance of the long alkyl chains inside the gel, respectively. In complexes formed by moderately charged P(MAA/NIPAM) gel with C n TAB, the decrease in the surfactant alkyl chain length could induce the phase structure transition from Pm 3 n cubic to face-centered cubic, and then to hexagonal close packing of spheres. The longer the surfactant alkyl chain, the lower the charge density and the hydrophobicity of polyelectrolyte chains are required to form highly ordered structures inside the resulting complexes. In PSS−C n TA complexes, structures of two-dimensional (2D) hexagonal packing of cylinders were determined. The 2D hexagonal structures were different from both the layered structures in the corresponding solid-state complexes and the Pm 3 n cubic structures in PMAA−C n TA complexes, in which the PMAA chains were more flexible than the PSS chains.