Polymeric microporous hydrophobic membranes

Polymeric microporous hydrophobic membranes, typically polytetra-fiuoroethylene (PTFE), polypropylene (PP) and polyvinylidenefluoride (PVDF), are major membrane materials. Modified hydrophilic membranes such as cellulose acetate and cellulose nitrate modified with plasma polymerisation, have also been successfully tested in MD operations (Lawson and Lloyd, 1997). Furthermore, modified inorganic membranes, such as ceramic membrane modified with CgFi7(CH2)2Si(OC2H5)3 perfluoroalkylsi-lane molecule (Cg) and carbon nanotube based membranes, have also been developed for MD (Susanto, 2011). 

Evaporative mass transfer of volatile solvents through microporous hydrophobic membranes is employed in order to concentrate feed solutions above their saturation limit, thus obtaining a supersaturated environment where crystals may nucleate and grow. In addition, the presence of a polymeric membrane increases the probability of nucleation with respect to other locations in the system (heterogeneous nudeation)... 

When producing polymeric membranes for crystallization purposes, the selection of the material is mainly driven by the necessity to achieve a good hydrophobicity (low surface energy), high chemical stability, controlled porosity, and thickness. The typology and main characteristics of the polymers frequently used as starting material for microporous hydrophobic membranes are given in Table 10.1. 

The possibility of controlling a crystallization process by a suitable tuning of the physicochemical properties of the polymeric substrate has enhanced interest toward the preparation of specifically modified membranes. In particular, theoretical and experimental investigations that are the subject of this chapter refer to (i) Monbranes prepared from copolymers to modulate the hydrophobicity and (ii) Microporous hydrophobic membranes modified by using additives in the casting solution to modulate the morphology in terms of pore size and porosity. 

The use of liquid membranes in analytical applications has increased in the last 20 years. As is described extensively elsewhere (Chapter 15), a liquid membrane consists of a water-immiscible organic solvent that includes a solvent extraction extractant, often with a diluent and phase modifier, impregnated in a microporous hydrophobic polymeric support and placed between two aqueous phases. One of these aqueous phases (donor phase) contains the analyte to be transported through the membrane to the second (acceptor) phase. The possibility of incorporating different specific reagents in the liquid membranes allows the separation of the analyte from the matrix to be improved and thus to achieve higher selectivity. 

Kong, Y., et al. Plasma polymerization of octafiuorocyclobutane and hydrophobic microporous composite membrane distillation, J. Appl. Polym. Sci., 46, 191, 1992. 

Kong, Y, Lin, X., Wu, Y, Cheng, J. and Xu, J. 1992. Plasma polymerization of octafluorocy-clobutane and hydrophobic microporous composite membranes for membrane distillation. 46 191-199. 

Ciszewski, A., Kunicki, J. and Gancarz, I. 2007. Usefulness of microporous hydrophobic polypropylene membranes after plasma-induced graft polymerization of acrylic acid for high-power nickel-cadmium batteries. Electmchim. Acta 52 5207-5212. 

Papadopoulos and Sirkar (27) employed symmetric microporous hydrophobic polypropylene hollow fibers with a thin nonporous plasma-polymerized skin of silicone on the outside surface. For species like N2, O2, CO2, H2S, SO2 which have high permeability through a thin silicone skin, the extra skin resistance on top of the liquid membrane resistance is limited. Yet, it eliminates liquid membrane breakthrough when Pm exceeds Ppo or P q by 100 psi. Unless the silicone coating is ruptured or the composite fibers break, the membrane liquid remains contained... 

In some cases, the rate-controlling polymeric membrane is not compact but porous. Microporous membranes can be prepared by making hydrophobic polymer membranes in the presence of water-soluble materials such as polyethylene glycol), which can be subsequently removed from the polymer matrix by dissolving in aqueous solution. Cellulose esters, loosely cross-linked hydrogels and other polymers given in Table 4.2 also give rise to porous membranes. 

The acid in the catalyst layer is retained by capiUaiy forces and by the hydrophobicity of the microporous layer while the acid in the membrane is held by the acid-base interaction as well as physical absorption. A recent study showed that a stable interface between the membrane and the catalyst layer can be sustained as long as the proton conducting acid phase is established. Electrodes with no polymeric binder were constructed with improved performance and good stability [48]. 

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