Material selection synthetic membranes

A variety of containment strategies employ floating solid objects to control the rate of gaseous emissions from surface impoundments. These include synthetic membrane covers, rafts, and hollow plastic spheres. Synthetic membrane covers are feasible where the out-gassing of volatiles due to biological activity is not expected. Selection of the liner material must be... 

Lloyd, D.R. and Meluch, T.B. 1985, Selection and evaluation of membrane materials for liquid separations. In Materials Science of Synthetic Membranes, Lloyd, D.R., Ed. American Chemical Society Washington, DC. 

Many synthetic membranes are known to be useful for separation of water and various sizes of solutes from aqueous solutions by selective separation, for examples reverse osmosis, ultrafiltration, dialysis and so on 1 7). The permeability is much dependent on both of chemical and physical structures of the membranes. The choice of the barrier materials for membranes and the control of their morphology are important to get effective permselective membranes. 

Synthetic membranes for molecular liquid separation can be classified according to their selective barrier, their structure and morphology and the membrane material. The selective barrier- porous, nonporous, charged or with special chemical affinity -dictates the mechanism of permeation and separation. In combination with the applied driving force for transport through the membrane, different types of membrane processes can be distinguished (Table 2.1). 

Material science aspects of synthetic polymeric membranes are presented In this survey. The objective Is to place each of the subsequent chapters of this volume Into proper perspective. Therefore, frequent reference Is made to the accompanying chapters and, where necessary, to alternative Information sources. By way of Introduction, this chapter considers In turn material selection, material characterization and evaluation, membrane preparation, membrane characterization and membrane evaluation. Membrane module design and manufacture, transport phenomena and process performance are Introduced In the discussion only as they pertain to membrane materials science. Following this Introduction, the various chapters of this volume are previewed. 

Membrane science can arbitrarily be divided Into seven Intimately related categories material selection, material characterization and evaluation, membrane preparation, membrane characterization and evaluation, transport phenomena, membrane module design and process performance. This chapter and those to follow emphasize the materials science aspects of synthetic polymeric membranes that Is, the selection, characterization and evaluation of membrane materials as well as the preparation, characterization and evaluation of membranes. Transport phenomena, membrane module design and process performance enter the discussion only as these topics pertain to materials science. 

The various chapters in this book address the topics of material selection, characterization and evaluation as well as membrane preparation, characterization and evaluation. At the expense of neglecting membranes for applications such as controlled release and impermeable barriers, this book focuses on synthetic membranes for separation processes as well as active membranes and conductive membranes. While many of the concepts developed herein can be extrapolated to other applications, the Interested reader is referred elsewhere for specific details (for example, controlled release (25-30), coating and packaging barriers (31-33), contact lenses (34,35), devolatilization (36), ion-selective membrane electrodes (37-42) and membranes in electrochemical power sources (43)). 

MATERIALS SCIENCE OF SYNTHETIC MEMBRANES Table III Selected Homopolymers Investigated for ... 

Lloyd DR, Meluch TB, Selection and evaluation of membrane materials for hquid separations. In Lloyd DR, Ed., Material Science of Synthetic Membranes, ACS Symp. Ser. No 269, Washington DC, ACS, 1985. 

Membranes are used for a wide variety of separations. A membrane serves as a barrier to some particles while allowing others to selectively pass through. The pore size, shape, and electrostatic surface charge are fundamental to particle removal. Synthetic polymers (cellulose acetate, polyamides, etc.) and inorganic materials (ceramics, metals) are generally the principal materials of construction. Membranes may be formed with symmetric or asymmetric pores, or formed as composites of ultra thin layers attached to coarser support material. Reverse osmosis, nanofiltration, ultrafiltration, and microfiltration relate to separation of ions, macromolecules, and particles in the 0.001 to 10 pm range (Rushton et al. 1996). 

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.)... 

Membrane material selection is dependent upon the mode of therapy employed. Convective therapies such as hemofiltration require a high hydraulic permeability and a large pore size, which might permit large molecules such as cytokines to pass through the fiber wall. Synthetic membranes are well suited for this role and are desired for most continuous, convective techniques (Jones, 1998). 

A major part of the commercially available synthetic membranes is made up of hydrophobic polymeric materials. The latter have inert surface, thus resulting in high biofouling and decrease in flux. Surface photografting technique appears as a viable solution to overcome these disadvantages. In the following, some examples of photosynthesized membranes are summarized and concern the selective permeation and the molecular imprinting technique. 

The membrane processes that have just been briefly described can be implemented, in principle, with any material (ceUulosic, synthetic polymer or inorganic) or in any format (hollow fibre, spiral wound, etc.). The system design process selects the most appropriate material and format according to the process operating parameters. 

Glasses exist that fnnction as selective electrodes for many different monovalent and some divalent cations. Alternatively, a hydrophobic membrane can be made semiper-meable if a hydrophobic molecnle called an ionophore that selectively binds an ion is dissolved in it. The selectivity of the membrane is determined by the structnre of the ionophore. Some ionophores are natnral products, such as gramicidin, which is highly specific for K+, whereas others such as crown ethers and cryptands are synthetic. Ions such as, 1, Br, and N03 can be detected using quaternary ammonium cationic surfactants as a lipid-soluble counterion. ISEs are generally sensitive in the 10 to 10 M range, but are not perfectly selective. The most typical membrane material used in ISEs is polyvinyl chloride plasticized with dialkylsebacate or other hydrophobic chemicals. 

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