Structure and elastic properties of a block-copolymer bilayer doped with cationic surfactant

Abstract

The triblock-copolymer system EO₃₇-PO₅₈-EO₃₇ (P105) + o-xylene + water is found to organize in a lamellar phase (L_α) upon addition of the surfactant C₈TAB at constant oil-to-water ratio. Small-angle X-ray scattering (SAXS) is used to characterize the internal structure of the stacked bilayers and a detailed model of the structure (interlamellar spacing, bilayer thickness and composition of the amphiphilic film) is proposed on the basis of the SAXS data. Moreover, these data are used to investigate the elastic properties of the bilayers as a function of composition. It is shown that the excess area method and related concepts do not apply to the present system and it is not possible to give absolute values of the bending elastic constant κ. Nevertheless, it can be shown that the elasticity of the lamellar phase strongly decreases with increasing amount of surfactant, but is independent of the block-copolymer concentration. To our knowledge such information about the elastic properties of highly concentrated block-copolymer systems has not been presented in the literature before.