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

Abstract

Abstract13C‐CP/MAS‐NMR (cross‐polarization magic angle spinning), 2D‐WISE (wideline separation experiment) and 1H‐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‐C18 and PSS‐C14. The strong mobility differences between the ionic and alkyl phase in the lamellar complex PSS‐C18 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. 1H‐spin diffusion experiments allow to estimate the distance between mobile and immobile regions of the sample. For the complex PSS‐C14, the length scale determined by NMR is essentially that of the primary lamellar structure. For PSS‐C18, a characteristic length of the density fluctuations within the proposed undulated lamellar structure is estimated.