## 4 Preparation of Membranes
### Method of Production of Ion Exchange Membranes
Several efforts have been made to avoid the use of chloromethyl ether, with one recent method using bromination instead of chloromethylation [41]. This method can also be applied to the preparation of cation exchange membranes. As mentioned earlier, properties such as low cost, ease of making and good electrochemical properties make heterogeneous ion exchange membranes more suitable to certain applications. Their mechanical strength however is comparatively low. By choosing a suitable binder or reinforcing material, the optimum combination of mechanical strength and electrochemical properties can be produced [21]. Ion exchange membranes have been produced by the interpenetrating polymer network (IPN) method, possessing excellent electrochemical and mechanical properties at a low cost [42, 43]. The process involves the free radical polymerization of two monomers producing a chemical blend of two interpenetrating networks of linear and cross-linked polymers. This blend behaves like a homogeneous type of membrane having less probability of micro-voids compared to heterogeneous type membranes [21]. Semi-interpenetrating polymer network (sIPN) membranes have also been prepared by mixing two polymers followed by cross-linking with gaseous dibromoethane [44]. After the polymers are cross-linked, one of the polymers forms a network in which the chains of the second polymer are immobilised. Asymmetric ion exchange membranes are often produced by phase inversion. In this process, a polymer solution inverts into a swollen, three-dimensional macromolecular network [29]. The production of these membranes involves five stages [45]:  
(i) A homogenous polymer solution with required viscosity is produced. (ii) The homogenous solution is then cast onto a suitable surface. (iii) Part of the solvent is then evaporated off, resulting in the concentration of the polymer at the surface. (iv) The polymer is precipitated in a precipitation bath, causing the exchange of the solvent for a precipitating agent, subsequently forming a gel. (v) The polymer then undergoes tempering, where any imperfections on the thin layer are annealed (closed). During these five stages, changes to certain parameters can influence the membranes final structure and properties. These factors include the annealing temperature, the rate of solvent evaporation, the composition of the polymer solvent mixture and/or the composition of the precipitation bath [45]. The distinction of composite membranes to other asymmetric membranes rests with the mode of formation [46]. With phase inversion membranes, the polymer solution is cast in one step producing the microporous film with the thin dense layer one side. Composite membranes are formed in two steps. The microporous support is cast first and the barrier layer is then deposited on the support layer [46]. Composite membranes are a general improvement on phase inversion membranes as the support and active layers can be produced and optimised separately from a variety of materials for a particular function. Symmetric membranes can also be produced by the phase inversion process. In this case however, the precipitation agent is added to a gas phase, slowly, ensuring that the concentration remains constant over the whole film cross section. Graft polymerization is a process used for the modification of polymers [47]. Graft polymerization yields thin membranes with outstanding mechanical and good electrochemical properties [48]. A graft copolymer is a polymer which consists of one or more types of molecules as blocks connected as side chains to a main chain [49]. In the first step of the process, an active site is created in the polymer either by a free radical or an active group which initiates the polymerization leading to a graft copolymer. There are various ways to activate the polymer; by chemical, photochemical and high energy radiation. High energy radiation is the most versatile method as the grafting process can be more easily controlled.