Phase inversion and hollow fibre membrane formation

4 Phase inversion and hollow fibre membrane formation 

The term phase inversion refers to the process by which a polymer solution inverts into a three-dimensional network. Initially, the solvent system is the continuous phase, and after phase inversion the polymer is the continuous phase. Four phase inversion processes exist (1) the dry process, in which a volatile solvent is lost and phase inversion occurs, (2) the wet process, in which solvent is exchanged for non-solvent and precipitation occurs, (3) the thermal process, where a latent solvent (a substance which is only a solvent at elevated temperatures) is used, involving the cooling of the polymer solution which 

5 Finger-type pores in polyetherketone hollow fibres. 

The hollow fibre membranes reviewed in the present work here are formed using the wet phase inversion process. The wet phase inversion process has been very well described by Strathmann. In his work, Strathmann casts various polymer solutions onto glass plates and then immerses the cast film into a bath of precipitation fluid. This kind of precipitation is fast and a skinned membrane structure forms. The skin formation is explained by Strathmann on the basis of the concentration profiles of the polymer, solvent and the precipitant which occurred during the phase inversion process. 

In skin-type membranes, like hollow fibre membranes, two characteristic structures can be formed. One structure is sponge-like below the skin, and one has finger-shaped pores below the skin. These finger-shaped pores can be clearly seen in the polyetherketone hollow fibre (see Fig. 11.5).  

---