Polymeric microporous hydrophobic

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). 

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. 

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)... 

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. 

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. 

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. 

This chapter focuses on the fixation of lyotropic liquid crystalline phases by the polymerization of one (or more) component(s) following equilibration of the phase. The primary emphasis will be on the polymerization of bicontinuous cubic phases, a particular class of liquid crystals which exhibit simultaneous continuity of hydrophilic — usually aqueous — and hydrophobic — typically hydrocarbon — components, a property known as bicontinuity (1), together with cubic crystallographic symmetry (2). The potential technological impact of such a process lies in the fact that after polymerization of one component to form a continuous polymeric matrix, removal of the other component creates a microporous material with a highly-branched, monodisperse, triply-periodic porespace (3). 

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

Mesoporous silicas with various pore sizes are hydrophobic by silylation with silanes. Changes in the pore structure as a result of the silylation reactions are monitored in order to assess the distribution of the hydrophobic groups. Extensive polymerization of dimethyldi-chlorosilane causes blocking of the micropore fraction. For silica with pore sizes in the supermicroporous range (2nm), this leads to hydrophobization of almost exclusively the outer surface. While for trimethylchlorosilane a smaller number of molecules react with the surface, modification is more homogeneous and an open structure is optimally preserved. Both silanes lead to lower surface polarity and increased hydrothermal stability, i.e., preservation of the porous structure during exposure to water.12231... 

The microporous alumino-silicate zeolites (Types A, X, and mordenite are frequently used) provide a variety of pore openings (3-10 A), cavity and channel sizes, and framework Si/Al ratios. They are also available in various cationic exchanged forms (Na, K, Li, Ag, Ca, Ba, Mg), which govern their pore openings and cationic adsorption site polarities. They are highly hydrophilic materials and must be dehydrated before use. The amorphous adsorbents contain an intricate network of micropores and mesopores of various shapes and sizes. The pore size distribution may vary over a wide range. The activated carbons and the polymeric sorbents are relatively hydrophobic in nature. The silica and alumina gels are more hydrophilic (less than zeolites) and they must also be dehydrated before use. 

Another mechanism proposed by some authors for the enhanced hydrophobicity evokes a closure of large micropores in cell walls [43]. Indeed, change in crystallinity of cellulose or conformational reorganization of polymeric components of wood could lead to more perfectly aligned bonds which may no longer be separable by intrusion of water. This would explain why water drops are not absorbed in wood for certain experimental conditions as seen in wettability with water measurements. Blantocas et al. [44] and Ramos et al. [45] observed partial closure of surface pores in wood following ion irradiation. For low ion energies, ion bombard-... 

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. 

The polymeric resins and the carbonaceous polymers are significantly more hydrophobic than activated carbon. A comparison of water vapor isotherms is shown in Figure 9.25. With such highly hydrophobic surfaces, it is not clear whether the micropores are indeed wetted upon the pretreatment described above. 

The polymeric resins are substantially more hydrophobic than activated carbon. The non-wetting types are subjected to the pre-wetting procedure described above (i.e., wetting wi a solvent, such as methanol, followed by aqueous solution). The pores are presumably wetted (completely) upon this treatment. Undoubtedly, the mesopores (i.e., voids between the microspheres within each bead) are wetted. It is not clear, however, whether the micropores within each microsphere are completely wetted by this procedure. 

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