Dry-wet phase inversion

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. 

Pinnau, I., and Koros, W. (1991), Structures and gas separation property asymmetric polysulfone membranes made by dry, wet, and dry/wet phase-inversion, J. Appl. Polym. Sci., 43,1491-1502. 

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... 

An integrally skinned asymmetric membrane with a porous skin layer (hereafter called substrate membrane) is prepared from a polymer solution by applying the dry-wet phase inversion method and dried according to the method described later, before being dipped into a bath containing a dilute solution of another polymer. When the membrane is taken out of the bath, a thin layer of coating solution is deposited on top of the substrate membrane. The solvent is then removed by evaporation, leaving a thin layer of the latter polymer on top of the substrate membrane. 

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. 

Membranes for vapor removal from air have a structure similar to the prism membrane, but they are prepared on a different principle.Aromatic PEI is used to produce a porous substrate membrane by the dry-wet phase inversion method. This polymer was chosen over PS/PES because of the higher durability of PEI to organic vapors. Unlike an asymmetric PS substrate for the prism membrane, the top layer of asymmetric PEI membrane has a large number of pores, the size of which is equivalent to those of UF membranes. When a layer of silicone rubber is coated on the top layer of the porous substrate membrane, the silicone rubber layer will govern the selectivity and the porous support will provide only mechanical strength to the composite membrane. Because the permeabilities of water and organic vapors through the silicone... 

Membrane structures can also be formed by a microphase separation process in which the outermost region of the cast membrane undergoes phase separation induced by solvent evaporation, while the bulk of the structure is formed by sol vent/non-sol vent exchange during a quench step. This type of structure formation is defined as a dry/wet phase inversion process [Pinneau et al., 1990]. 

PEEK-WC is a modified PEEK, as shown in Figure 6.2. Ultrathin asymmetric gas separation membranes of modified PEEK can be prepared by a dry/wet phase inversion technique. Under optimized conditions, membranes with an open cellular morphology and an ultrathin dense skin... 

In addition, PEEK-WC membranes have been prepared by using a phase inversion process with supercritical fluids. The supercritical fluid acts as a non-solvent. In comparison to the dry/wet phase inversion method, the supercritical fluid allows the cell size and the membrane morphology to modulate by changing the experimental conditions, such as polymer concentration, temperature, and pressure. A dry membrane can be obtained rapidly and without additional post-treatments. ... 

J. C. Jansen, M. G. Buonomenna, A. Figoli, and E. Drioli. Ultra-thin asymmetric gas separation membranes of modified PEEK prepared by the dry-wet phase inversion techrrique. Desalination, 193(l-3) 58-65, May 2006. 

Hollow fiber UF membranes have been prepared from PPESK with a dry/ wet phase inversion technique. Ethylene glycol mono methyl ether, diethylene glycol, and methyl ethyl ketone were used as non-solvent additives and NMP was used as a solvent in membrane preparation. ... 

The fastest growing desalination process is a membrane separation process called reverse osmosis (RO). The most remarkable advantage of RO is that it consumes little energy since no phase change is involved in the process. RO employs hydraulic pressure to overcome the osmotic pressure of the salt solution, causing water-selective permeation from the saline side of a membrane to the freshwater side as the membrane barrier rejects salts [1-4], Polymeric membranes are usually fabricated from materials such as cellulose acetate (CA), cellulose triacetate (CTA), and polyamide (PA) by the dry-wet phase inversion technique or by coating aromatic PA via interfacial polymerization (IFP) [5]. 

I. PiNNAu, J. Koros, a. qualitative skin layer formation mechanism for membranes made by dry/wet phase inversion. Journal of Polymer Sdence B Polymer Physics 31 (1993) 419. 

An RO membrane acts as a barrier to flow, allowing selective passage of a particular species (solvent) while other species (solutes) are retained partially or completely. Solute separation and permeate solvent (water in most cases) flux depend on the material selection, the preparation procedures, and the structure of the membrane barrier layer [5,15]. Cellulose acetate (CA) is the material for the first generation reverse osmosis membrane. The announcement of CA membranes for sea water desalination by Loeb and Sourirajan in 1960 triggered the applications of membrane separation processes in many industrial sectors. CA membranes are prepared by the dry-wet phase inversion technique. Another polymeric material for RO is aromatic polyamide [16]. 

Membrane permeation properties are largely governed by the pore sizes and the pore size distributions of UF membranes. Rather, thermal, chemical, mechanical, and biological stability are considered of greater importance. Typical UF membrane materials are polysulfone (PS), poly(ether sulfone), poly(ether ether ketone) (PEEK), cellulose acetate and other cellulose esters, polyacrylonitrile (PAN), poly(vinyKdene fluoride) (PVDF), polyimide (PI), poly(etherimide) (PEI), and aliphatic polyamide (PA). All these polymers have a Tg higher than 145 °C except for celliflose esters. They are also stable chemically and mechanically, and their biodegradabflity is low. The membranes are made by the dry-wet phase inversion technique. 

Membranes for vapor removal from air have a structure similar to the Prism membrane, but they are prepared on a different principle [22]. Aromatic poly(etherimide) is used to produce a porous substrate membrane by the dry-wet phase inversion method. This polymer was chosen over polysulfone/poly(ether sul-... 

Feng et al. [32] studied the morphology of the inner and outer surfaces of hollow fibers fabricated from poly(etherimide) by TM-AFM. The hollow fibers were fabricated by the dry-wet phase inversion method at two different bore fluid flow rates, 0.1 and 0.4 mLmin and their effect on the surface morphology was investigated. The average pore sizes on the inner surface were 39.8 and 81.9 nm, respectively, for 0.1 and 0.4 mLmin while those on the outer surface were 218.4 and 93.4, respectively, for 0.1 and 0.4 mLmin h It is interesting to note that the pore size increased with an increase in the bore fluid flow rate at the inner surface, while the opposite was the case at the outer surface. 

Kawakami et al. prepared dense and asymmetric membranes from 2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and bis[4-(4-aminophen-oxy)phenyl]sulfone (APPS) by solvent evaporation (dense) and by the dry-wet phase inversion technique [47]. The surface morphology was studied by AFM. They reported that the solvent evaporation method adopted for the preparation of the dense membrane influenced the formation of nodules, while the dry-wet process in which solvent/nonsolvent exchange was involved determined the roughness of the skin layer. 

Membranes prepared by the dry-wet phase inversion method from glassy polymers are composed of polymeric spheres. 

Hollow fiber UF (ultrafiltration) membranes have been prepared from PPESK with a dry/wet phase inversion technique. 

L., Tang, B. and Wu, P. 2009. An experimental investigation of evaporation time and the relative humidity on a novel positively charged ultrafiltration membrane via dry-wet phase inversion. 326(1) 168-177. 

Scheme of dry-wet phase inversion to (a) tailor flat and (b) hollow fibre membranes. 

Wang Z.Y, Li J.L., Kong X.S., Yang Z.S., Li C.L. (2011), Effect of octanol on wetta-bihty and permeability of PVDF porous membrane via dry-wet phase inversion, Tianjin Daxue Xuebao (Ziran Kexueyu Gongcheng Jishu Ban)/Joumal of Tianjin University Science and Technology, 44(7), 628-632. 

Hollow fiber spinning is usually based on the dry-wet phase inversion process that involves the following four steps ... 

Hou et al. (2010) applied the DCMD process to remove F from brackish groundwater. They used PVDF hollow fibers that were self-prepared by a dry/wet phase inversion process and assembled into a polyester tube. The main membrane properties and the module characteristics are reported in the Tables 13.15 and 13.16. 

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