## 4 Preparation of Membranes
### Method of Production of Ion Exchange Membranes  
The fabrication of membranes is a multistage process, beginning with the production of the polymer followed by post-treatments to impart the selectivity and durability to the membrane [35]. Most polymer membranes are produced by conventional melt, dry or wet extrusion processes. Melt and dry extrusion is often the more feasible option due to the higher production rates than the wet process [35]. In melt extrusion, a polymer melt is extruded into a cooler atmosphere, which induces the phase transition, producing a membrane which is dense and isotropic [35]. A latent solvent (a compound miscible with the polymer at the extrusion temperature) may be added to produce a secondary phase separation. The removal of the solvent adds porosity. In dry extrusion, the polymer mixture is dissolved in a volatile solvent. Using an evaporation chamber, a porous isotropic or anisotropic membrane is created [35]. The wet extrusion method can be used to produce a greater variety of membrane morphologies. Here, the extruded mixture is coagulated by exposure to a nonsolvent in the form of a liquid or vapour (the basis of phase inversion). It is important to note that commercial polymers can display inconsistencies resulting from batch type production, even when obtained from the same manufacturer [35]. Other methods of membrane manufacture include the \"paste method\", and membrane casting. The paste method [36, 37, 38] is used to produce the commercially available NEOSEPTA(r) range of membranes (Tokoyama Soda Co. Ltd., Yamaguchi, Japan). The process begins with a paste consisting of a monomer with a functional group appropriate to introduce an ion exchange group, divinylbenzene (DVB), a radical polymerization initiator, and a finely powdered poly (vinyl chloride) (PVC). This paste is then coated onto a PVC cloth as reinforcing material, which is wound, together with separating film, onto a roll. The monomers are then copolymerized by heating to prepare the base membrane, onto which the ion exchange group is introduced. The ion exchange membranes produced by this method have been found to contain two continuous, finely and closely intertwined phases, the PVC and the ion exchange resin component [38]. Membrane casting is a method which produces ion exchange membranes by directly imparting the ion exchange functionality (such as sulphonic acid groups) to a base polymer material (such as polystyrene). According to this method [39]:  
(i) Membranes are produced with relatively inexpensive materials. (ii) The reaction is carried out in a homogenous liquid phase and thus the product is homogeneously sulfonated. (iii) Application to large-scale processing is achievable. Ion exchange membranes are often symmetrical, homogenous membranes. Three general methods are used to prepare homogeneous ion exchange membranes [21]:  
(i) By the polymerization or polycondensation of monomers containing a moiety that can be (or can be made) cationic or anionic. (ii) By the introduction of cationic or anionic moieties onto a polymer film. (iii) By the introduction of cationic or anionic moieties into a polymer, followed by the dissolution of the polymer and casting it into a film. Styrene and DVB are the most commonly used monomer starting materials for membrane polymerizations [19]. Cation exchange membranes are given their character by the attachment of the weakly acidic carboxylic acid groups, or the strongly acidic sulphonic acid groups [19]. Anion exchange membranes are produced by a two step process, chloromethylation followed by quaternary animation [19]. Strongly basic tertiary ammonium can then be added, whilst weakly basic groups include primary, secondary or tertiary amine groups. The preparation of anion exchange membranes is often more complicated and costly [19]. In addition, the chloromethyl ether used in the chloromethylation step is harmful to humans as it is classified as a carcinogen [40].