Preparation of synthetic membranes (2)

Track-etching

The simplest pore geometry in a membrane is an assembly of parallel cylindrically shaped pores of uniform dimension. Such structures can be obtained by track-etching.

In this method a film or foil(often a polycanbonate) is subjected to high energy particle radiation applied perpendicular to the film. The particles damage the polymer matrix and create tracks. The film is then immersed in an acid or alkaline bath and the polymeric material is etched away along these tracks to form uniform cylindrical pores with a narrow pore size distribution.

Pore sizes can range from 0.02 to 10μm but the surface porosity is low(about 10% at a maximum). ¨The choice of the material depends mainly on the thickness of the film and on the energy of the particles being applied(usually about 1MeV). The maximum penetration thickness of particles with this energy is about 20μm. When the energy of the particles is increases and even inorganic materials(e.g. mica) can be used. The porosity is mainly determined by the radiation time whereas the pore diameter is determined by the etching time.

Template leaching

Another technique for preparing porous membranes is by leaching out one of the components from a film. Porous glass membranes can be prepared by this technique.

A homogenous melt(1000-1500℃)of a three component system (e.g. Na2O-B2O3-SiO2) is cooled and as a consequence the system separate into two phases, one phase consisting mainly of SiO2 which is not soluble whereas the other phase is soluble. This second phase is leached out by an acid or base and a wide range of pore diameters can be obtained with a minimum size of about 0.005μm(5nm).

Phase inversion

Most commercially available membranes are obtained by phase inversion. This is a a very versatile technique allowing all kind of morphologies to be obtained. This technique will be described in detail later in this chapter.

 The membrane in syringe filters can be made by the aboved methods.