Solvent-Stable Polymeric Membrane Materials

Traditional polymers for separations in water have a limited chemical resistance and are not useful for solvent separations. Some may be applicable in nonaggressive solvents such as methanol and ethanol due to crosslinking, additives or additional interlayers, but not in any other solvent modified polyamide membranes and poly (ethersulfone) membranes are typical examples. 

A list of typical commercial pervaporation membranes [23] is given in Table 3.1. Commercial hydrophilic membranes are very often made of polyvinyl alcohol (PVA), with differences in the degree of crosslinking. Commercial hydrophobic membranes often have a top layer in polydimethyl siloxane (PDMS). However, a wide variety of membrane materials for pervaporation can be found in the literature, including polymethylglutamate, polyacrylonitrile, polytetrafluoroethylene, polyvinylpyrrolidone, styrene-butadiene rubber, polyacrylic acid, and many others [24]. A comprehensive overview of membrane materials for pervaporation is given by Semenova et al. [25], 

Solvent resistant nanofiltration membranes are a much more recent evolution. Historically, the membranes developed by Membrane Products Kyriat Weizmann (Israel) - now Koch - (MPF 44, MPF 50, MPF 60) were the first nanofiltration membranes intended for application in organic solvents, although other membranes (e.g., PES and PA membranes) also have a limited solvent stability. The Koch membranes are based on PDMS, similarly to pervaporation membranes, although the level of crosslinking is quite different. 

Other membrane materials include mainly polyimide, polyacrylonitrile and polybenzimidazole. An overview of commercially available membranes is given in Table 3.2. These membranes are manufactured in procedures usually derived from practical experience by using high-throughput screening, it was shown that optimization is possible [26]. Many other membrane materials are described in the scientific literature and in patents an overview is given by Cuperus and Ebert [27]. 

Brand name Manufacturer Material Hydrophilic/ hydrophobic 

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The synthesis of polymeric membranes is mostly achieved through the phase inversion process. Phase inversion is one of the most versatile and economical processes used to develop polymeric membranes. In this process, a membrane is prepared from a thermodynamically stable solution, in its most basic form consisting of a polymer and a solvent. This polymer solution is often called the casting solution, and is sometimes also called dope. The casting solution is cast onto a supporting material, which is typically nonwoven, but sometimes also woven, resulting in a thin liquid-cast-polymer film, further referred to as cast film, which is later transformed into a solid membrane. The phase transformation from liquid to solid is induced by disturbing the stability of polymer solution. 

A typical matrix material for ion selective sensors and sensors utilising the ion selective response, is PVC. Macrocyclic receptor molecules are dissolved in a plasticiser as solvent and retained within the polymeric matrix. The ratio of membrane components is critical (39) and deviation leads to excess leaching of the plasticiser or insufficient receptor molecule to obtain a response. The PVC matrix is only able to retain a limited proportion of plasticiser and the ionophore has limited solubility in the plasticiser. In fact the most stable membranes have the lowest proportion of plasticiser, but these are also the least sensitive and selective, since they contain a low ionophore ratio some stability must be lost at the expense of sensitivity (Duschl C and Hall EAH Hall, unpublished data). 

Due to the upper temperature constrain for polymeric-based membranes (Mulder, 1997), there is a growing market for membranes based on solvent-resistant materials able to withstand high temperatures. Ceramic materials (silicium carbide, zirconium oxide, titanium oxide) endure harsh temperature conditions and show stable performance in solvent medium and so are excellent materials for membrane preparation. On that basis a new generation of OSN membranes have been developed, the inorganic composite membranes. 

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