Formation and transition of highly ordered structures of polyelectrolyte-surfactant complexes

Small-angle X-ray scattering studies on the nanostructures of water-equilib rated complexes, formed by slightly cross-linked copolymer gels of poly(sod ium methacrylate/N-isopropylacrylamide) P(MAA/NIPAM) and fully charged sodi um polystyrenesulfonate (PSS), respectively, interacting with oppositely ch arged surfactants of alkyltrimethylammonium bromide (C(n)TAB, with n being the number of carbon atoms in the alkyl chain) at similar to 23 degrees C, are presented. In P(MAA/NIPAM)-C(n)TA complexes, the formation and transiti on of highly ordered structures were investigated in terms of the surfactan t alkyl chain length and the hydrophobicity of the polyelectrolyte chain. T he complexes between fully charged PMAA gel and C(n)TAB showed Pm3n cubic s tructures at 10 less than or equal to n less than or equal to 16 but did no t show highly ordered structures at n = 8 and 18 due to the weak hydrophobi c interaction and the steric hindrance of the long alkyl chains inside the gel, respectively. In complexes formed by moderately charged P(MAA/NIPAM) g el with C(n)TAB, the decrease in the surfactant alkyl chain length could in duce the phase structure transition from Pm3n 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 polyele ctrolyte chains are required to form highly ordered structures inside the r esulting complexes. In PSS-C(n)TA complexes, structures of two-dimensional (2D) hexagonal packing of cylinders were determined. The 2D hexagonal struc tures were different from both the layered structures in the corresponding solid-state complexes and the Pm3n cubic structures in PMAA-C(n)TA complexe s, in which the PMAA chains were more flexible than the PSS chains.