Formation of ordered micro-porous membranes

Regular micro-porous polymeric membranes have recently been discovered by r apidly evaporating a solution of CS2 containing poly(p-phenylene)-block-pol ystyrene [1]. 1,2-dichloroethane (a chlorated solvent in which polystyrene gel phase has never been observed) is also found to produce ordered structu res, which definitively excludes eventual effect of the gelation process du ring the membrane formation. The observation of the solution surface during the solvent evaporation reveals the growing of micron-sized water droplets trapped at the surface and forming compact aggregates. The study of the so lution/water interface shows that the water droplets profile is in agreemen t with the pore shape observed in the membranes. Moreover, the copolymer wa s found to precipitate at the interface, forming a layer encapsulating the droplets and preventing their coalescence. In that way, the final structure results from the droplets stacking under the action of large surface curre nts. Finally, we argue that the decisive element in the formation of ordere d structures is the ability of the polymer to precipitate at the solution/w ater interface, which seems to be related the star-polymer microstructure.