Dielectric and mechanic relaxation in polyelectrolyte-supported bilayer stacks: A model for the dynamics of membranes?

The dielectric and mechanical relaxation of two different polyelectrolyte-lipid complexes with a stack-of-bilayer morphology is characterized by broad band dielectric spectroscopy in the frequency range between 10(-2) and 10(7) Hz and by dynamic shear relaxation experiments, With these experiments, we compare a synthetic model system consisting of polyacrylic acid and didodecyldimethylammonium counterions (PAA-C-12(2)) With the complex of a fraction of soybean lecithine and a cationic polyelectrolyte (PDADMAC-Lec). The lamellar morphologies consisting of stacks of bilayers are characterized by quantitative small angle X-ray scattering measurements. PAA-C-12(2) shows in the mechanical experiment two beta-relaxation processes and exhibits at room temperature a storage modulus of G' approximate to 200 MPa which is-considering the observed good deformability-very high. The whole behavior is similar to high-performance loaded rubbers and reflects the extraordinary mechanical response of the lamellar superstructure. For PDADMAC-Lec, a less structured relaxation behavior and lower moduli are found. Above the glass transition, a transition zone is observed which is followed by a rubbery plateau with G' being still on the order of 1 MPa. Considering the very high deformability of this material, the natural polyelectrolyte lipid complex turned out to be an excellent rubbery material. In the dielectric relaxation experiment, both systems show a very similar behavior which is regarded to be typical for a stack-of-bilayer order. At low temperatures, localized beta-relaxations are described which are tentatively assigned to lipid motions. In addition, both systems exhibit a transition from a nonohmic to an ohmic conductivity which occurs close to the softening point of the polyelectrolyte layers.