Synthetic polymer membranes preparation

In the early fifties a number of workers demonstrated the synthesis of homogeneous , high capacity, high conductivity synthetic polymer membranes prepared both by condensation and addition polymerization. Both cation and anion exchange membranes based upon cross-linked polystyrene, reinforced by a fabric mesh became commercially available. 

FIGURE 41.2 Basic principle of artificial cells Artificial cells are prepared to have some of the properties of biological cells. Like biological cells, artificial cells contain biologically active materials (I). The enclosed material (I) can be retained and separated from undesirable external materials, such as antibodies, leukocytes, and destructive substances. The large surface area and the ultra-thin membrane allow selected substrates (X) and products (Y) to permeate rapidly. Mass transfer across 100 mL of artificial cells can be 100 times higher than that for a standard hemodialysis machine. The synthetic membranes are usually made of ultrathin synthetic polymer membranes for this type of artificial cell. (From Chang, T.M.S., Artif. Cells Blood Substit. ImmobU. Biotechnol., 22(1), vii, 1994.)... 

Douglas R, Lloyd is Assistant Professor of Chemical Engineering at the University of Texas at Austin which he joined in 1981. Prior to his current position, he was (since 1978) in the faculty in the Chemical Engineering Department at VPI SU. He received his Ph. D. degree in Chemical Engineering from the University of Waterloo, and his research Interests Include the preparation, characterization and application of synthetic polymer membranes. 

HoUow fibers can be prepared from almost any spiunable material. The fiber can be spun directly as a membrane or as a substrate which is post-treated to achieve desired membrane characteristics. Analogous fibers have been spun in the textile industry and are employed for the production of high bulk, low density fabrics. The technology employed in the fabrication of synthetic fibers appUes also to the spinning of hoUow-fiber membranes from natural and synthetic polymers. 

Synthetic ribitol phosphate polymer, unlike teichoic acid in a wall, is readily extracted from the particulate enzyme or membrane preparation by treatment with phenol.18 Similarly, teichoic acid synthesized by intact cells in the presence of penicillin is only loosely attached to the wall,111 and it may be significant that, in each case, synthesis of teichoic acid has occurred without the simultaneous synthesis of glycosaminopeptide. It is now known that, in the normal wall, teichoic acid and glycosaminopeptide are attached to each other, and it has been suggested that the low activity of cell-free synthetase is due to the absence of suitable acceptor molecules of glycosaminopeptide. This possibility could account for the ease of removal of teichoic acid formed when simultaneous synthesis of glycosaminopeptide was not possible. 

Polymers for membrane preparation can be classified into natural and synthetic ones. Polysaccharides and rubbers are important examples of natural membrane materials, but only cellulose derivatives are still used in large scale for technical membranes. By far the majority of current membranes are made from synthetic polymers (which, however, originally had been developed for many other engineering applications). Macromolecular structure is crucial for membrane barrier and other properties main factors include the chemical structure of the chain segments, molar mass (chain length), chain flexibility as well as intra- and intermolecular interactions. 

Complexes of the two polymers, isolated from bacterial plasma membranes or prepared from synthetic polymers, form voltage-dependent, Ca2+-selective channels in planar lipid bilayers that are selective for divalent over monovalent cations, permeant for Ca2+, Sr2+... 

Two main criteria for the membrane selection are pore size and material. As peroxidases usually have sizes in the range of 10-80 kDa, ultrafiltration membranes with a molecular cutoff between 1 and 50 kDa are the most adequate to prevent enzyme leakage [99]. The materials commonly applied to ultrafiltration membranes are synthetic polymers (nylon, polypropylene, polyamide, polysulfone, cellulose and ceramic materials [101]. The adequate material depends on a great number of variables. When enzyme is immobilized into the matrix, this must be prepared at mild conditions to preserve the enzymatic activity. In the case of enzyme immobilization onto the membrane, this should be activated with the reactive groups necessary to interact with the functional groups of the enzyme. If an extractive system is considered, the selection of the hydrophilicity or hydro-phobicity of the membrane should be performed according to the features of reactants, products, and solvents. In any case, the membrane should not interfere with the catalytic integrity of the enzyme. 

Figure 14 shows the plots of the ratio Mw/Mn of the CA fractions prepared by the SSF method 39 42), as a function of their Mw. Mw and Mn values were determined by light scattering and membrane osmometry, respectively. Except for a few fractions of CA(2.46) and CA(2.92), Mw/Mn values of CA fractions lie between 1.2 and 1.5, independently of their Mw values. Most of the fractions of cellulose derivatives reported in the literature were prepared by the SPF method 44) their Mw/Mn values range roughly from 1.2 to 3.7 (most of them 1.5-2.0) and moreover depend markedly on Mw. This indicates clearly that the SSF method is superior to SPF for cellulose derivatives, as it is also the case for synthetic polymers such as polystyrene 38). A computer simulation for a quasi-temary system carried out by K amide and Matsuda also showed the inconditional superiority of the SSF method 34 - 36 ... 

This book has been written and computer-drawn to present the wealth of membraneous structures that have been realized by chemists mainly within the last ten years. The models for these artificial molecular assemblies are the biological lipid membranes their ultimate purpose will presumably be the verification of vectorial reaction chains similar to biological processes. Nevertheless, chemical modelling of the non-covalent, ultrathin molecular assemblies developed quite independently of membrane biochemistry. From the very beginning of artifical membrane and domain constructions, it was tried to keep the preparative and analytical procedures as simple and straightforward as possible. This is comparable to the early development of synthetic polymers, where the knowledge about protein structures quickly gave birth to simple and more practical polyamides. 

Several investigators have demonstrated the feasibility of immobilized liquid membrane gas separations in applications where large pressure differences are encountered such as acid gas removal from synthetic natural gas. The immobilized liquid membranes prepared by Kimura et al. (16) using 100 pm cellulose acetate supports withstood CO2 partial pressure differences of up to 6.89 10 N/m. Matson et al. (15) used mlcroporous cellulose acetate and polyethersulfone films of 25-75 pm thickness to successfully immobilize potassium carbonate solutions for H2S transport at pressure differences of up to 2.07 10 N/m. The ILMs were supported by macroporous non-wetting polymer films such as polypropylene and polytetrafluoro-ethylene to increase the resistance to high transmembrane pressures. 

Three different techniques are used for the preparation of state of the art synthetic polymeric membranes by phase inversion 1. thermogelation of, a two or more component mixture, 2. evaporation of a volatile solvent from a two or more component mixture and 3. addition of a nonsolvent to a homogeneous polymer solution. All three procedures may result in symmetric microporous structures or in asymmetric structures with a more or less dense skin at one or both surfaces suitable for reverse osmosis, ultrafiltration or microfiltration. The only thermodynamic presumption for all three preparation procedures is that the free energy of mixing of the polymer system under certain conditions of temperature and composition is negative that is, the system must have a miscibility gap over a defined concentration and temperature range (4). 

Artificial enzymes may be divided into two categories semisynthetic artificial enzymes and synthetic artificial enzymes. Semisynthetic artificial enzymes are partly prepared by biological systems. Catalytic antibodies are typical examples of semisynthetic artificial enzymes. Semisynthetic artificial enzymes are also prepared by modification of a known protein or enzyme at a defined site with a cofactor or new functional group. Synthetic artificial enzymes are prepared totally by synthetic methods. Synthetic artificial enzymes may be either relatively small molecules with well-characterized structures or macromolecules. The term syn-zymes has been coined to designate synthetic polymers with enzyme-like activities. In addition, synthetic artificial enzymes are also obtained with molecular clusters such as micelles and bilayer membranes formed by amphiphiles. 

In membrane-separation processes, a feed consisting of a mixture of two or more components is partially separated by means of a semipermeable membrane through which one species moves faster than the others. That part of the feed that passes through the membrane is called the permeate, while the portion that does not pass is called the retentate. The membranes may be thin layers of a rigid material such as porous glass or sintered metal, but more often they are flexible films of synthetic polymers prepared to have high permeability for certain types of molecules. 

Higuchi S, Ozawa T, Maeda M, Inoue S. pH-induced regulation of the permeability of a polymer membrane with a transmembrane pathway prepared from a synthetic polypeptide. Macromolecules 1986 19 2263-2267. 

Polymersomes, self-assembled polymer shells composed of block copolymer amphiphiles. These synthetic amphiphiles with amphiphilicity similar to lipids constitute a new class of drug carriers. They are spontaneously formed in aqueous media, as unilamellar vesicles up to tens of microns in diameter. Amphiphilic block copolymers form a range of self-assembled aggregates including spherical, rod-like, tubular micelles, lamellae, or vesicles, depending on polymer architectnre and preparation conditions. Polymers having low hydrophobicity (less than 50%) favor the formation of micelles, however, intermediate level of hydrophobicity (50%-80%) favors the formation of vesicles. Polymeric vesicles, which have a liposome-like structure with a hydrophobic polymer membrane and hydrophilic inner cavity, are called polymersomes. 

Chitosan membranes have been proposed as an artificial kidney membrane because of their suitable permeability and high tensile strength. Chitosan and its same derivatives are used to prepare scaffolds for tissue engineering applications. It can also be used for designing artificial skin, treatment of brain-scalp damage, and in plastic skin surgery. Chitosan has replaced the synthetic polymers in ophthalmo-logical applications due to its characteristic properties such... 

As documented in and expressed by these various contributions, the topic Polymers for Fuel Cells is a vast one and concerns numerous synthetic and physico-chemical aspects, derived from the particular application as a solid polymer electrolyte. In this collection of contributions, we have emphasized work which has already led to tests of these polymers in the real fuel cell environment. There exist other synthetic routes for proton-conducting membrane preparation, which are not discussed in this edition. Furthermore, certain polymers are utilized as fuel-cell structure materials, e.g., as gaskets or additives (binder, surface coating) to bipolar plate materials. These aspects are not covered here. 

Zeng, M., Fang, Z., Xu, C. Effect of compatibility on the structure of the microporous membrane prepared by selective dissolution of chitosan/synthetic polymer blend membrane. J. Membr. ScL 230, 175-181 (2004)... 

Most early efforts focused on the development of thin layer matrix precoated membranes, including those made of nylon and other synthetic polymers, nitrocellulose, anion- and cation-modified cellulose and regenerated cellulose. Several manufacturers currently offer 96-well plates with proprietary coatings for LC-MALDI-MS applications. This approach was described in more detail above. As mentioned previously, all of these sample preparation methods were originally developed for pre-purified samples or fractions, not for analytes eluting from an HPLC or capillary electrophoresis column. 

The properties of supported enzyme preparations are governed by the properties of both the enzyme and the carrier material. The interaction between the two provides an immobilized enzyme with specific chemical, biochemical, mechanical and kinetic properties. The support (carrier) can be a synthetic organic polymer, a biopolymer or an inorganic solid. Enzyme-immobilized polymer membranes are prepared by methods similar to those for the immobilized enzyme, which are summarized in Fig. 22.7 (a) molecular recognition and physical adsorption of biocatalyst on a support membrane, (b) cross-linking between enzymes on (a), (c) covalent binding between the biocatalyst and the membrane, (d) ion complex formation between the biocatalyst and the membrane, (e) entrapment of the biocatalyst in a polymer gel membrane, (f) entrapment and adsorption of biocatalyst in the membrane, (g) entrapment and covalent binding between the biocatalyst and the membrane, (h) entrapment and ion complex formation between the biocatalyst and the membrane, (i) entrapment of the biocatalyst in a pore of an UF membrane, (j) entrapment of the biocatalyst in a hollow-fiber membrane, (k) entrapment of biocatalyst in microcapsule, and (1) entrapment of the biocatalyst in a liposome. 

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