Bending moduli of a nonadsorbing-polymer-containing lyotropic lamellar phase: An experimental study

We investigate the effect of the incorporation of a neutral water-soluble nonadsorbing polymer polyvinylpyrrolidone (PVP) on the bilayer bending moduli {κ} and κ of the lamellar phase L_α composed of cetylpiridiniumchloride (CPCL)/hexanol/H₂O. The mean bending modulus {κ} is obtained from the measurement of quadrupolar splittings of perdeuterated hexanol by deuterium solid state NMR. It is found that {κ} is insensitive to the amount of polymer in the lamellar phase, in agreement with the theoretical predictions. Observations with the polarizing microscope of topological defects in the lamellar phase allow a qualitative study of the Gaussian bending modulus κ. Addition of a small amount of polymer in the lamellar phase induces a large proliferation of focal conic domains of second species (positive Gaussian curvature). Cryo-TEM confirms that the polymer drastically modify the texture of the lamellar phase. These results indicate clearly that the presence of the nonadsorbing polymer strongly decreases κ, in contradiction with theoretical predictions.