Structure and dynamics of polymer-surfactant complexes: Dielectric relaxation studies

The isothermal phase diagram of the system composed of water, sodium dodecyl sulfate, and hexadecyltrimethylammonium polyacrylate, PACTA, at 25 degreesC, was investigated by dielectric relaxation, ionic conductivity, and viscosity. The polymer is not water-soluble and dissolves only in the presence of surfactant in micellar form. Dielectric relaxation spectra indicate the occurrence of one relaxation in the surfactant solutions, above the critical micellar concentration, and of two relaxation processes in the presence of PACTA and sodium dodecyl sulfate, C12SO4Na, in micellar form. They are ascribed to free and polymer-bound micelles, respectively. Data have been interpreted in terms of a chemical equilibrium of micelles partitioned between polymer-bound and free states. The dielectric results indicate that the distance between micelles adsorbed on the polymer backbone is close to 40 nm and slightly changes, depending on the surfactant content. These results support the hypothesis of a necklace structure for the polymer-micelle complex. According to viscosity findings, in fact, the stiffness of the PACTA-C12SO4Na complex decreases on increasing the amount of bound micelles.