Porous membranes phase inversion method

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. 

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. 

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. 

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... 

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-... 

The first breakthrough came in 1959 when Sourirajan and Loeb discovered a method to make a very thin cellulose acetate (CA) membrane using the phase inversion method [4]. This technique produces homogenous membranes with an asymmetric (or anisotropic) structure. The membranes were subsequently found to be skinned when examined under an electron microscope by Riley in 1964 [3]. The membranes consisted of a very thin, porous salt-rejecting barrier of CA, integrally supported by a fine CA porous substrate. Pictures of asymmetric membranes are shown in Figures 1.2 and 1.3. These early Loeb-Sourirajan (L-S) membranes exhibited water fluxes that were lOtimes higher than those observed by Reid, and with comparable salt rejection [5]. The membrane flux was 8—18 1/m /h (knh) with 0.05% NaCl product water from a 5.25% NaCl feedwater... 

Wu et al. (1992) treated the surfaces of the hydrophilic porous membranes, such as cellulose acetate, by radiation graft polymerization of styrene to increase their hydrophobicity and to reach the MD membrane characteristics. Kong et al. (1992) employed a cellulose nitrate membrane modified via plasma polymerization of both vinyltrimethylsilicone and carbontetrafluoride and octafluorocyclobutane for the preparation of MD membranes. Fujii et al. (1992) prepared tubular membranes from PVDF polymer dopes by using the dry-jet wet-spinning technique. Ortiz de Zarate et al. (1995) and Tomaszewska (1996) reported on PVDF flat-sheet membranes prepared for MD by the phase inversion method. 

Dense films of PPO were cast from solution in chloroform and prepared in the manner just described. Porous PPO membranes obtained by the phase inversion method were provided by Dr.F.P. Cuperus (The Netherlands) and used as-received. Two samples of different porosity were prepared. Those samples were characterized by surface areas of 70 and 200 mVg. 

The phase inversion method is developed to prepare hollow fiber inorganic membranes. Since the porous substrate and the separation layer are formed in a single step for this method, the preparation process can be... 

During the preparation of a gel copolymer P(VDF-HFP) electrolyte membrane by the phase inversion method, using different solvents such as NMP and NAf-dimethylformamide) (DMF) and nonsolvents such as dibutyl phthalate (DBF) and di-(2-ethylhexyl phthalate) (DEHP) will result in different pore sizes and different porosities. As a result, they are called porous polymer electrolytes, hi fact, they are gel electrolytes. The micromorphology of e membrane is related to the preparation conditions, but the solvents and nonsolvents used do not affect the ionic conductivity much, under certain conditions. After adding a certain amount of plasticizer, the pores will be filled by the plasticizer, and the ionic conductivity is 4.07 x 10 S/cm, with an electrochemical window of 4.5 V. 

The majority of the commereial gas separation membranes are made by wet phase inversion method whieh results in an integrally skinned asymmetrie membrane. This method was first used by Loeb and Sourirajan to produee cellulose acetate membranes for desalination of sea water. An alternative method for making gas separation membranes uses an ultra-porous skinned asymmetric membrane over which a thin polymer film is deposited by either coating or by interfacial polymerization. This method was developed by Cadotte for the creation of in situ dense skin thin film composite membranes for water desalination. These membrane fabrication techniques were made commercially successful for gas separation membranes by a brilliant empirical discovery for in situ sealing of the tiny pinhole defects on the skin of the membrane. 

M.L. Yeow, Y. Liu, and K. Li. (2005). Preparation of porous PVDF hollow fibre membrane via a phase inversion method using lithium perchlorate (LiC104) as an additive, J. Memb. Sci. 268 16-22. 

Yeow, M.L., Y. Liu, and K. Li. 2005. Preparation of Porous PVDF Hollow Fibre Membrane via a Phase Inversion Method Using Lithium Perchlorate (LiC104) as an Additive. Journal of Membrane Science 258(1/2) 16-22. doi 10.1016/j.memsci.2005.01.015. 

Another way of using PVA for UF membranes is by modifying PVA by controlling hydroxyl groups. In this way the pore structure can be easily adjusted by the method phase inversion. Otherwise, once PVA is a water -soluble polymer it is difficult to form porous UF membranes with an ideal morphological structure by the method of wet phase inversion directly when water is used as a coagulation bath. 

Symmetric membranes and asymmetric membranes are two basic types of membrane based on their structure. Symmetric membranes include non-porous (dense) symmetric membranes and porous symmetric membranes, while asymmetric membranes include integrally skinned asymmetric membranes, coated asymmetric membranes, and composite membranes. A number of different methods are used to prepare these membranes. The most important techniques are sintering, stretching, track-etching, template leaching, phase inversion, and coating (13,33). 

Disadvantages of the known porous polymeric membrane preparation processes are that they involve additional process steps after the formation of the fiber to come to a final product. It is therefore desirable to have a more efficient preparation process. A new method to prepare structures of any geometry (Figure 6.13c through f) and large variety of functionality was recently proposed [61]. The authors proposed to incorporate the functionality by dispersion of particles in a polymeric porous structure formed by phase inversion. A slurry of dissolved polymer and particulate material can be cast as a flat film or spun into a fiber and then solidified by a phase inversion process. This concept is nowadays commercialized by Mosaic Systems. The adsorber membranes prepared via this route contain particles tightly held together within a polymeric matrix of different shapes, which can be operated either in stack of microporous flat membranes or as a bundle of solid or hollow-fiber membranes. 

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