Structure and dynamics of polyelectrolyte-surfactant complexes as revealed by solid state NMR

C-13-CP/MAS-NMR (cross-polarization magic angle spinning), 2D-WISE (wideline separation experiment) and H-1-spin diffusion experiments allow to gain new insight into the structure and dynamics of solid polyelectrolyte-surfactant complexes, a material with pronounced mesophase formation. Experiments were performed on two different complexes of polystyrene sulfonate and octadecyltrimethylammonium or tetradecyltrimethylammonium counterions, PSS-C-18 and PSS-C-14. The strong mobility differences between the ionic and alkyl phase in the lamellar complex PSS-C-18 are reflected in the NMR behavior: in the surfactant tails, a mobility gradient towards the terminal methyl group is observed. This fact as well as a high content of gauche conformations suggest a non-interdigitating morphology of the tails at room temperature. The behavior changes during cooling below an endothermic transition centered at 255 K where a high trans content and a homogenization of the side chain dynamics is observed. We attribute this transition which is invisible in the X-ray experiments to the formation of a highly transoid, interdigitated phase of the surfactant tails which is however not crystallized in a classical sense. H-1-spin diffusion experiments allow to estimate the distance between mobile and immobile regions of the sample. For the complex PSS-C-14, the length scale determined by NMR is essentially that of the primary lamellar structure. For PSS-C-18, a characteristic length of the density fluctuations within the proposed undulated lamellar structure is estimated.