Membrane characterization

It is important to realize the wide range of pore sizes which must be covered. In general, it may be stated that membrane characterization becomes progressively more difficult as the pore size decreases. Various pore sizes have their own methods of characterization methods. Again , the membranes will be classified in two main groups, which have been depicted schematically porous and nonporous membranes.

In order to avoid confusion in defining porous membranes, we will use the term ‘porous’ for both the microfiltration and ultrafiltration membranes instead of the frequently used definition of microporous. The definition for porous is more in agreement with the definitions adopted by the IUPAC:

- macropores >50nm

- mesopores 2 nm < pore size <50 nm

- micropores < 2 nm

The pore size classification given here is referred to pore diameter or more arbitrarily pore width.

This implies that microfiltration membranes are porous media containing macropores and ultrafiltration membranes are also porous with mesopores in the top layer. Hence, the definition porous covers both the macropores and mesopores.

With membranes of these type it is not the membrane (material) which is characterized but the pores in the membrane. Here the pore size( and pore size distribution) mainly determines which particles or molecules are retained and which will pass through the membrane. Hence, the material is of little importance in determing the separation performance.

On the other hand, with dense pervaporation/gas separation membranes, no fixed pores are present and now the material itself mainly determines the performance. The morphology of the polymer material (crystalline, amorphous, glassy, rubbery) used for membrane preparation directly affects its permeability. Factors such as temperature and the interaction of the solvent and solute with the polymeric material, have a large influence on the segmental motions. Consequently, the material properties may change if the temperature, feed composition,etc. are changed. The syringe filter we provide are usually made by porous membranes.