SAXS study on complexes formed by anionic poly(sodium methacrylate-co-N-isopropylacrylamide) gels with cationic surfactants

Small-angle X-ray scattering (SAXS) has been used to study the nanostructur es of complexes formed by slightly crosslinked anionic copolymer gels of po ly(sodium methacrylate-co-N-isopropylacrylamide) [P(MAA/NI-PAM)] interactin g with cetylpyridinium bromide (CPB), and alkyltrimethylammonium bromide (C (n)TAB, 10 less than or equal to n less than or equal to 18), respectively. Both the charge density of polyelectrolyte gels and the surfactant alkyl t ail length could induce the phase structure transition from Pm3n space grou p cubic to hexagonal close packing of spheres (HCP), while the different po lar groups of pyridinium and trimethylammonium with the same hydrophobic ce tyl chain in surfactants had no significant effects on the structures of co mplexes formed with the same gels. The highly ordered structures were shown to be formed by the self-assembly of ionic surfactants inside the anionic gel network, driven by both electrostatic and hydrophobic interactions. Fre eze drying the water-equilibrated complexes could collapse the formed order ed structures. However, the highly ordered structures could be restored aft er the dried complexes were reswollen by water under the same conditions, i ndicating that the highly ordered water-equilibrated complexes were thermod ynamically stable. Copyright (C) 2000 John Wiley & Sons, Ltd.