Microfiltration polycarbonate

Membranes used for the pressure driven separation processes, microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO), as well as those used for dialysis, are most commonly made of polymeric materials. Initially most such membranes were cellulosic in nature. These ate now being replaced by polyamide, polysulphone, polycarbonate and several other advanced polymers. These synthetic polymers have improved chemical stability and better resistance to microbial degradation. Membranes have most commonly been produced by a form of phase inversion known as immersion precipitation.11 This process has four main steps ... 

Fig. 16.6. Atomic force microscope image of a polycarbonate microfiltration membrane (cyclopore), 0.2 p,m pore size.

In micro- and ultrafiltrations, the mode of separation is by sieving through line pores, where microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 mm, and ultrafiltration membranes filter dissolved macromolecules. Usually, a polymer membrane, for example, cellulose nitrate, polyacrilonytrile, polysulfone, polycarbonate, polyethylene, polypropylene, poly-tretrafhioroethylene, polyamide, and polyvinylchloride, permits the passage of specific constituents of a feed stream as a permeate flow through its pores, while other, usually larger components of the feed stream are rejected by the membrane from the permeate flow and incorporated in the retentate flow [10,148,149],... 

Polymeric membranes are prepared from a variety of materials using several different production techniques. Table 5 summarizes a partial list of the various polymer materials used in the manufacture of cross-flow filters for both MF and UF applications. For microfiltration applications, typically symmetric membranes are used. Examples include polyethylene, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane. These can be produced by stretching, molding and sintering finegrained and partially crystalline polymers. Polyester and polycarbonate membranes are made using irradiation and etching processes and polymers such as polypropylene, polyamide, cellulose acetate and polysulfone membranes are produced by the phase inversion process.f Jf f ... 

Efforts to stabilize BLMs by the use of polymerizable lipids have been successful, but the electrochemical properties of these membranes were greatly compromised and ion channel phenomena could not be observed [21]. Microfiltration and polycarbonate filters, polyimide mesh, and hydrated gels have been used successfully as stabilizing supports for the formation of black lipid films [22-25] and these systems were observed to retain their electrical and permeability characteristics [24]. Poly(octadec-l-ene-maleic anhydride) (PA-18) was found to be an excellent intermediate layer for interfacing phospholipids onto solid substrates, and is sufficiently hydrophilic to retain water for unimpeded ion transfer at the electrode-PA-18 interface [26]. Hydrostatic stabilization of solventless BLMs has been achieved by the transfer of two lipid monolayers onto the aperture of a closed cell compartment however, the use of a system for automatic digital control of the transmembrane pressure difference was necessary [27]. 

Dense films of polycarbonate or poly(ethylene terephtalate) can be transformed into porous microfiltration membranes with very narrow pore-size distribution (Fig. 4.7), by exposing them to fission fragments from radioactive decay with subsequent etching in alkaline solutions. The number of pores can be controlled by the length of exposure to the fission segments. The maximum pore... 

A special type of microfiltration membrane may be prepared by track tching various polymeric films (see chapter III). Polycarbonate is often used for this purpose because of its outstanding mechanical properties (see figure II 2S). 

We have used a polymer track membrane (PTM) [64, 65] as a support for reagent immobilization. Such membranes are used for microfiltration in medicine, electronics, biotechnology etc. PTM is made by irradiation of a polymer with high energy ions. The irradiated film is etched and in this way the tracks after passing ion beams are removed. Various polymers can be used as a base film for fabrication of PTM (e.g. polypropylene, polyamide, polycarbonate etc.). The diameter of the pores obtained depends on the type of ions used and on their energy. 

Polymers commonly used to make such microfiltration membranes are polyvinylidene fluoride (PVDF), Nylon 66, polytetrafluoroethylene (PTFE), polysuUbne, cellulose, cellulose acetate/cellulose nitrate, polypropylene (PP), polyester, polycarbonate, etc. Ceramic microfiltration membranes are not uncommon. Polymeric membranes may have the following stmctures. 

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