Structure Studies of Poly(diallyldimethylammonium chloride-co-acrylamide) Gels/Sodium Dodecyl Sulfate Complex

Abstract

Cationic poly(diallyldimethylammonium chloride) (PDADMACl)-polyacrylamide polyelectrolyte copolymer gels have been synthesized by free radical copolymerization with various degrees of cross-linking over a range of charge densities. The polyelectrolyte gels and an oppositely charged surfactant, sodium dodecyl sulfate (SDS), associate to form complexes even when the SDS concentration in the external solution which is in equilibrium with the polyelectrolyte gel is below the critical micelle concentration (cmc). Small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) have been used to study the structure of the polyelectrolyte gels and the gel/surfactant complex, respectively. SANS results of PDADMACl gels with various degrees of cross-linking suggest that the static concentration fluctuations induced by the cross-links are about 2 orders of magnitude larger than the dynamic solution concentration fluctuations. SAXS results on the spatial arrangements of the PDADMACl−SDS complex suggest a hexagonal supramolecular structure in the gel phase. The intercylinder distance d of the superstructures in the gel phase is estimated from the primary diffraction peak with d = 37 Å. It is surprising that the degree of cross-linking (up to 12 mol %) of PDADMACl gels has no effect on the structure of the complex, due probably to the compensating effects on the mesh size of the collapsed gel with different degrees of cross-linking and to the large spatial inhomogeneity inside the gels. The ordered supramolecular structure is not observed in the PSC when the initial external SDS concentration is above its cmc.