Wet phase inversion process

In an alternative approach, MIP membranes can be obtained by generating molec-ularly imprinted sites in a non-specific matrix of a synthetic or natural polymer material during polymer solidification. The recognition cavities are formed by the fixation of a polymer conformation adopted upon interaction with the template molecule. Phase inversion methods have used either the evaporation of polymer solvent (dry phase separation) or the precipitation of the pre-synthesised polymer (wet phase inversion process). The major difficulties of this method lay both in the appropriate process conditions allowing the formation of porous materials and recognition sites and in the stability of these sites after template removal due to the lack of chemical cross-linking. 

Fig. 16 SEM image of the cross section of a theophylline-imprinted membrane asymmetric structure of imprinted membrane prepared by the wet phase inversion process. Reproduced with permission from [217]...

Blanco, J.F., Sublet, J., Nguyen, Q.T. and Schaetzel, P. (2006) Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process. Journal of Membrane Science, 283, 27-37. 

For many years polymeric membranes have been utilized widely for material separation without detailed characterization of the pore size and the pore size distribution. Most of the commercially available membranes are prepared by either a dry or a wet phase-inversion process. These membranes are formed by the phase separation of multicomponent polymer-solvent systems, the underlying principle being phase separation of the polymer solution. 

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]. 

The majority of polymer membranes used for microfiltration and ultrafiltration of liquids are prepared by the wet phase inversion process. Such membranes exhibit a typical asymmetric structure characterized by a thin dense surface layer and a thick microporous bulk. Poly(phthalazinone ether sulfone ketone) (PPESK) copolymers, c.f. Figure 7.10, show glass transition temperatures in the range of 263-305°C. The polymers show an outstanding chemical stability. They are soluble only in 98% H2SO4. Concentrated aqueous solutions of sodium chlorate, hydrogen peroxide, acetic acid, and nitric acid show no effect. ... 

Ren, J., Li, Z. and Wong, F.-S. 2004. Membrane structure control of BTDA-TDl/MDI (P84) co-polyimide asymmetric membranes by wet-phase inversion process. 

When solvents are removed solely by evaporation, the membrane formation is known as a dry phase inversion process (Resting 1985). When the phase separation and structure formation are achieved by immersion of a cast membrane in a quench medium, the process is known as a wet phase inversion process (Heffelfinger 1978). The latter process is used to prepare asymmetric membranes for either microfiltration (Roesink 1989), ultrafiltration (Michaels 1971), reverse... 

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. 

The hollow fibre membranes reviewed in the present work here are formed using the wet phase inversion process. The wet phase inversion process has been very well described by Strathmann." " In his work, Strathmann casts various polymer solutions onto glass plates and then immerses the cast film into a bath of precipitation fluid. This kind of precipitation is fast and a skinned membrane structure forms. The skin formation is explained by Strathmann on the basis of the concentration profiles of the polymer, solvent and the precipitant which occurred during the phase inversion process. 

Polyaciylonitrile hollow fibers were spun by the diy/wet phase inversion process by David and Ismail [31], Thermostabilization was performed by oxidative stabilization process at 250°C for 30 min under pure oxygen or compressed air. The carbonization was done at 500°C under nitrogen for a certain amount of time, which is called soak time . 

Recently, an in-depth review on molecular imprinted membranes has been published by Piletsky et al. [4]. Four preparation strategies for MIP membranes can be distinguished (i) in-situ polymerization by bulk crosslinking (ii) preparation by dry phase inversion with a casting/solvent evaporation process [45-51] (iii) preparation by wet phase inversion with a casting/immersion precipitation [52-54] and (iv) surface imprinting. 

Today the majority of polymeric porous flat membranes used in microfiltration, ultrafiltration, and dialysis are prepared from a homogenous polymer solution by the wet-phase inversion method [59-66]. This method involves casting of a polymer solution onto an inert support followed by immersion of the support with the cast film into a bath filled with a non-solvent for the polymer. The contact between the solvent and the non-solvent causes the solution to be phase separated. This process involves the use of organic solvents that must be expensively removed from the membrane with posttreatments, since residual solvents can cause potential problems for use in biomedical apphcations (i.e., dialysis). Moreover, long formation times and a limited versatihty (reduced possibUity to modulate cell size and membrane stmcture) characterize this process. 

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. 

Most UF membranes are asymmetric, having a thin separating layer or skin layer with small pores on one side of the membrane, and a much thicker layer with larger pores below the membrane which provides structural support with minimum flow resistance. Asymmetric membranes are manufactured by wet phase inversion casting. In this process, a casting solution of a polymer in a water-miscible solvent is spread in a thin layer onto a flat surface and then immersed in water. The water causes extraction of solvent and precipitation of the polymer as a porous flat sheet. The skin layer is formed on the upper surface that was in direct contact with water, and the underlying... 

Sol 2 is present either when one phase separates into two phases or when two phases are prevented from recombining into a single phase. It is expedient to entitle this factor inoompatibilityt and to discuss the various phase inversion processes in terms of the reasons for incompatibility. In the sections to follow four phase inversion processes are discussed a dry process, a wet process, a thermal process and a polymer assisted phase inversion process. 

The wet, or combined evaporation-diffusion, technique Is that variation of the phase inversion process in which a viscous polymer solution Is either allowed to partially evaporate prior to Immersion In a nonsolvent gelation bath, or immersed directly Into a nonsolvent gelation bath. Upon Immersion, any residual solvent or poreformer Is exchanged for nonsolvent. 

Membranes are prepared from polymer(s) dissolved in a solvent using either a dry process or a wet process. In the dry process, a volatile solvent is used for dissolving the polymer(s) and the extruded polymer solution is transferred into an evaporation chamber to yield a porous, isotropic or anisotropic membrane. In the wet process, on the other hand, the extruded mixture is coagulated by exposing the mixture to a nonsolvent in the form of vapor or liquid. The latter process is often referred to as the phase inversion process. 

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. ... 

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]. 

The phase-inversion process consists of the induction of phase separation in a previously homogeneous jx)lymer solution either by temperature change, by immersing the solution in a nonsolvent bath (wet process) or exposing it to a nonsolvent atmosphere (dry process). 

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]. 

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. 

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