Elastic properties of polymer-doped dilute lamellar phases: A small-angle neutron scattering study

We investigate experimentally using small-angle neutron scattering the elas tic properties of polymer-doped dilute lamellar phases. In our system the p olymer is water-soluble but nevertheless partially adsorbs onto the negativ ely charged surfactant bilayers. The effective polymer-mediated interaction between bilayers is less repulsive than the weakly screened electrostatic interaction that prevails at zero polymer content. It el en becomes attract ive in some regions of the phase diagram. Small-angle neutron scattering al lows us to measure directly the Caille exponent eta characterizing the bila yer fluctuations in lamellar (smectic A) phases, and thus indirectly estima te the compression modulus (B) over bar as a measure of the strength of the bilayer-bilayer interactions. The compression modulus appears to be vanish ing at a point located on the lamellar-lamellar phase separation boundary, a candidate critical point.