Phase inversion technique

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Several selective interactions by MIP membrane systems have been reported. For example, an L-phenylalanine imprinted membrane prepared by in-situ crosslinking polymerization showed different fluxes for various amino acids [44]. Yoshikawa et al. [51] have prepared molecular imprinted membranes from a membrane material which bears a tetrapeptide residue (DIDE resin (7)), using the dry phase inversion procedure. It was found that a membrane which contains an oligopeptide residue from an L-amino acid and is imprinted with an L-amino acid derivative, recognizes the L-isomer in preference to the corresponding D-isomer, and vice versa. Exceptional difference in sorption selectivity between theophylline and caffeine was observed for poly(acrylonitrile-co-acrylic acid) blend membranes prepared by the wet phase inversion technique [53]. 

Phase inversion technique the external phase is added to the internal phase. For example, if an O/W emulsion is to be prepared, the aqueous phase is added to the oil phase. First a W/O emulsion is formed. At the inversion point, the... 

The methods developed by EBC and others in the late 1990s using hydrocyclones and phase-inversion techniques may be sufficient for separation of the treated oil from the aqueous phase and biocatalyst. However, a cost analysis of such methods may be necessary to determine the economic feasibility. Recent work using hydrophobic membranes, magnetically separable immobilized biocatalysts and other techniques may provide alternate methods for separation of oil and recycling biocatalyst. A comparison of these techniques with each other and the previously investigated hydrocyclone techniques is needed to demonstrate improvements in the separation efficiency. 

Most of the asymmetric mixed-matrix membranes reported to date were prepared from concentrated mixed-matrix dopes via a phase inversion technique [69,... 

Kulprathipanja and coworkers reported the preparation of integrally skinned siUcaUte-1/cellulose acetate flat sheet asymmetric mixed-matrix membranes via phase inversion technique in 1992 [73]. The O2/N2 separation performance of these membranes was investigated. It was demonstrated that the separation factor of... 

The main limitation of this wet phase inversion technique for membrane imprinting is that the use of the resulting imprinted membranes is limited to an aqueous medium, since the membrane generally swells in organic solvents, resulting in a loss of recognition sites. 

Figoli et al. [96,97] reported the preparation of polymeric capsules combining the phase-inversion technique with the membrane process. Polyetheretherketone (PEEKWC) capsules of different size (300-800 micrometer) and morphology (asymmetric with a porous or dense layer) have been prepared. The SEM pictures of the prepared PEEKWC capsules are shown in Figure 21.16. The capsules can find application both in chemical and in food packaging fields [98],... 

Figure 21.16 SEM pictures of the crosssection of the PEEKWC capsules prepared by the phase-inversion technique using a film with a pore size of 550 lm [97],...

Asymmetric membranes are usually produced by phase inversion techniques. In these techniques, an initially homogeneous polymer solution becomes thermodynamically unstable due to different external effects and the phase separates into polymer-lean and polymer-rich phases. The polymer-rich phase forms the matrix of the membrane, while the polymer-lean phase, rich in solvents and nonsolvents, fills the pores. Four main techniques exist to induce phase inversion and thus to prepare asymmetric porous membranes [85] (a) thermally induced phase separation (TIPS), (b) immersion precipitation (wet casting), (c) vapor-induced phase separation (VIPS), and (d) dry (air) casting. 

Effect of Evaporation Condition Previous studies on more traditional applications have investigated the effect of increased air velocity, that is, forced-convection conditions for a combination of dry/wet phase inversion techniques to produce defect-free, ultrahigh flux asymmetric membranes with ultrathin skin layers [115-117]. To investigate the effect of evaporation condition on the release rate of drug, tablets were dip coated with CA solution containing 10% CA, 80% acetone, and 10% water and allowed to dry by blowing air across the surface with a blower (forced convection). As a comparison, tablets coated with the same solution were air dried under natural free-convection conditions. 

Jansen, J. C., Macchione, M., and Drioli, E. (2005), High flux asymmetric gas separation membranes of modified poly(ether ether ketone) prepared by the dry phase inversion technique, /. Membr. Sci., 255,167-180. 

Figures corresponds to the system MD 60-DEA-styrene, where neutralization was achieved by the phase inversion technique (industrial procedure). It can be seen that swelling of e micelles with monomer leads to a linear relationship of ( (]) / (]) ) vs monomer...

One of the oldest methods in preparing NF membranes is the phase inversion technique involving precipitafion of a polymer film from its solution cast film in a nonsolvent bath. The prerequisite for this method is that the solvent of the polymer and the nonsolvent must be thoroughly miscible, while the polymer should not dissolve in the nonsolvent [40-45]. 

Generally, in a phase-inversion technique, a polymer transforms from its liquid phase to solid phase in a well-controlled manner. Usually, the process is carried out by starting the transition from one liquid state to two liquid states. One of them has a high polymer... 

Figure 8. Photomicrograph of the dispersed globules in W/O/W emulsion prepared by the phase inversion technique.

This is the principle for evaluating the durability of the oil layer in W/O/W emulsions. The phase inversion technique in preparing W/O/W type dispersion described was also assessed by the extent of oil layer using this method. 

Figure 12. Stability of the oil layer in W/O/W emulsions prepared by the phase inversion technique. "Reproduced with permission from Ref. 21. Copyright 1983, Academic Press. "...

Phase inversion technique—Preparation of integraiiy skinned asymmetric membranes... 

Phase inversion is a process in which a polymer is transformed from a liquid to a solid state. There are a number of methods to achieve phase inversion. Among others, the dry-wet phase inversion technique and the temperature induced phase separation (TIPS) are most commonly used in the industrial membrane manufacturing. The dry-wet phase inversion technique was applied by Loeb and Sourirajan in their development... 

Cellulose acetate is the material for the first-generation reverse osmosis (RO) membranes. The announcement of cellulose acetate membranes for seawater desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. Cellulose acetate membranes are prepared by the dry-wet phase inversion technique. 

All of the above polymers have TgS higher than 145°C except for cellulose acetate. They are also stable chemically and mechanically. Also, their biodegradability is low. The membranes are made by the dry-wet phase inversion technique. 

Hydrophilic MF membranes can be made by the dry-wet phase inversion technique. The latter technique is also applicable in making PVDF membranes. On the other hand, other hydrophobic MF membranes are made by the TIPS technique. In particular, semicrystalline PE, PP, and PTFE are stretched parallel to the direction of film extrusion so that the crystalline regions are aligned to the direction of stretch, while the noncrystalline region is ruptured, forming long and narrow pores. Hydrophobic membranes do not allow penetration of water into the pore until the transmembrane pressure drop reaches a threshold pressure called liquid entry pressure of water. These membranes can therefore be used for membrane distillation. Tracketching method is applied to make MF membranes from PC. 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

Enzymes or whole cells can be immobilized in ultrafiltration (UF) and reverse osmosis (RO) membranes by several methods. First, cellulose acetate or polysulfone are used to obtain asymmetric membranes by the phase Inversion technique. Albumin and glutaraldehyde are then used for cell Immobilization within the membranes via co-cross-llnklng methods (25,26). 

Virtually the entire membrane manufacture today is based on laminate structures comprising a thin barrier layer deployed upon a much thicker, highly permeable support. Most are formed of compositionaUy homogeneous polysulfone, cellulose acetate, polyamides, and various fluoropolymers by phase inversion techniques in which ultrathin films of suitably permselective material are deposited on prefabricated porous support structures. Hydrophobic polymers as polyethylene, polypropylene, or polysulfone are often used as supports. A fairly comprehensive hst of microporous and ultrafiltration commercial membranes and produced companies are presented in Refs [107-109]. A review on inorganic membranes has been given in Ref. [110]. 

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