Membranes

A membrane is defined as a thin sheet, film, or layer, which works as a selective barrier between two phases that can be liquid or gas.113 In other words, a membrane is an interface between two adjacent phases acting as a selective barrier, regulating the transport between the two components.114 Membranes have gained an important place in chemical technology and are used in a broad range of applications.115,116,117 The key property that is exploited is the ability of a membrane to control the permeation rate of a chemical species through the membranes.118 Thus the membrane can also be considered as a permeable medium. A specific amount of energy is required to accomplish the separation of substances or components.113 Membranes can be porous …show more content…
These functional groups are bonded with membrane surface groups by ionic or polar bonding.127 Therefore, the polymer surfaces suitable to this kind of modification must have sites that are able to undergo electrophilic or nucleophilic attack. Such polymer structures include benzene rings, hydroxyl groups, double bonds or halogens.127 The polymer surfaces can be functionalised via nitration,135 amination,135 bromination,136 sulfonation,122,137,138 benzoylation,139 and carboxylation.138 The modification of polyphenylene oxide (PPO) as an example is discussed. The substitution of PPO by either nitro or amino group resulted in increased chain stiffness, increased packing density, and lower thermal stability than unsubstituted PPO.135 Bromination of PPO at the aryl positions had been extensively studied.136,140,141,142 Studies have shown that brominated PPO (PPOBr) has high molecular weight.142 Bromination of PPO increased the permeability of gases without sacrificing the parent PPO polymer permselectivity.136 In order to further improve the performance of the brominated PPO, the brominated PPO was further sulfonated (SPPOBr). The sulfonated brominated PPO membrane had a decreased gas permeability while an increased gas permselectivity when compared to brominated PPO membrane.137 Sulfonation of PPO has also been studied.137 The higher the degree of sulfonation gave denser and more packed membranes. The sulfonated PPO membrane had lower gas permeability and higher gas permselectivity when compared to sulfonated brominated PPO membrane. These were the results caused by manipulation of the polymer backbone stiffness, membranes density and packing.137 The benzoylation of PPO led to an increase in glass transition temperature and a lower thermal stability than unsubstituted PPO.139 Among the