SMECTIC COMPRESSIBILITY OF POLYMER-CONTAINING LYOTROPIC LAMELLAR PHASES - AN EXPERIMENTAL TOOL TO STUDY THE THERMODYNAMICS OF POLYMER CONFINEMENT

We investigate a mixed amphiphilic system formed from a brine swollen surfactant lamellar phase in between the bilayers of which is incorpor ated a neutral water-soluble non-adsorbing polymer: polyvynilpyrilidon e (PVP) with a very large molecular weight. This system allows an expe rimental realization of the various regimes of polymer confinement wit hin infinite slits. The presence of polymer will induce a new effectiv e interaction between the bilayers which depends on the confinement re gime of the macromolecular solution. We calculate the contribution of this polymer mediated interaction to the smectic compressibility in th e four expected regimes of polymer confinement and show that it is a n on monotonic function of the polymer concentration. This smectic compr essibility is of particular interest because it can be readily derived from small angle scattering measurements. Our model allows a quantita tive interpretation of the critical L(alpha)/L(alpha) phase separation which is observed upon progressive addition of salt at a particular c omposition of the system.