Flat sheet membrane with controlled variation of pore density and pore size in a direction parallel to the surface

We describe the manufacture of track-etch membranes with controlled gradients of pore size and density along the length of the membrane. Gradient of track density is obtained by controlled motion of a screen between the swift ion beam and the polymer material. Linear gradient of pore size is obtained by a dipping process at constant velocity in alkali solution. The two steps are linked by the desired characteristics of the membrane. Membranes were characterized by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Examples are given with pore radius gradient of 3 nm/cm and a theoretical model of flow in a channel with such kind of porous wall is presented. This two-step process is rather simple and presents numerous variants at will. The method could be extended easily to produce any desired parameter variation, or to maintain some characteristics constant along the membrane, as for instance porosity, pore internal area or hydraulic resistance.