Membrane polycarbonates

Here three template systems are discussed one that is achieved by assembling monodisperse spheres into a colloidal crystal, the second a membrane (polycarbonate) consisting of cylindrical pores of set dimensions, and the third a biological structure composed of ordered spherical yeast cells. These molds have been cast in most examples, unless blockage of smaller pores prevented the complete filling of larger pores. 

On a native (hydrophobic) polycarbonate membrane, the observed albumin adsorption is dominated by electrostatic interactions. No bioadhesion of bacteria is required in order to protect the permeation properties of the membrane. The evaluation of a polycarbonate membrane coated with polyvinylpyrrolidone indicates that it has a lower affinity for immunoglobulins (IgG) [5] and for albumin [6] than the native one. Hence, the bioadhesion is lower. The protein conformation can be modified by a surface effect related to the hydrophilicity/hydropho-bicity and also to the electronegative/electropositive character. The hydrophihc membrane (polycarbonate/PVP) surface modifies the molecular conformation of albumin, increases its water solubility, and as a result decreases its adsorption onto the membrane. 

In view of the difficulties usually experienced in producing BLM of area 0.008 cm, porous structures have been used to form mini-membranes. Two examples of this are nylon polymer films grown at an interface and polycarbonate films normally employed in organic chemistry for the purpose of filtration (21,22). In the former case, a nylon film was synthesized at the interface between water and an organic solvent. Electron microscopy showed that the polymer structure was close-packed on one with a Swiss-cheese structure on the other. Treatment of the films with lipid solution resulted in functional mini-membranes. Polycarbonate films can be employed in a similar manner. Although such systems are relatively simple to work with, one large imponderable is the overall area of membrane produced for study. 

In the past few years there has been a real surge of new techniques for the preparation of porous materials that are characterized by well-defined cylindrical pores of sizes from a few micrometers, down to the nanometer range. Most notably, porous anodic alumina (PAA) [17] and porous silicon (p-Si) [18,19] that are prepared by electrochemical anodization, and track-etched polymer membranes (polycarbonate, polyimide, polyethylene terephtalate, etc.), represent the most well-known cases of porous membranes that are candidates for filtration applications and also for their use as templates in nanotechnology (nanowire fabrication [20]). The pore diameter range of these membranes is comparable to the typical thickness of polymer brushes that are usually prepared in the laboratory. 

Fig. 4. Diagram of the two-step process to manufacture nucleation track membranes, (a) Polycarbonate film is exposed to charged particles in a nuclear reactor, (b) Tracks left by particles are preferentially etched into uniform cylindrical pores (8).

A total of 15,000—17,000 t of resin is used aimuaHy. Polycarbonate also has many technical uses in instmment panels and devices, especiaHy for membrane switches and insulators. Optical quaHty polycarbonate is the only suitable material for the compact disk market. Since their introduction in 1983, compact disks have shown explosive growth in the consumption of polycarbonate, with utiHty for audio, video, and computer appHcations. Consumption of optical quaHty resin more than doubled between 1988 and 1992, and as of 1995 accounted for about 20,000 t of annual production. 

Track-etched membranes are made by exposing thin films (mica, polycarbonate, etc) to fission fragments from a radiation source. The high energy particles chemically alter material in their path. The material is then dissolved by suitable reagents, leaving nearly cylindrical holes (19). 

A variety of synthetic polymers, including polycarbonate resins, substituted olefins, and polyelectrolyte complexes, are employed as ultrafiltration membranes. Many of these membranes can be handled dry, have superior organic solvent resistance, and are less sensitive to temperature and pH than cellulose acetate, which is widely used in RO systems. 

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.

FIGURE 1 Effect of (sequential) extrusion of MLV dispersions through polycarbonate membrane filters (Unipore) with pore sizes of 1.0, 0.6, 0.4, 0.2, and 0.1 ym on the mean liposome diameter. DXR-containing MLV (phosphatidylcholine/phosphatidylserine/ cholesterol 10 1 4) mean diameter of nonextruded dispersion about 2 ym pH 4. Mean particle size determined by dynamic Light scattering (Nanosizer, Coulter Electronics). (From Crommelin and Storm, 1987.)... 

H. E., and Crommelin, D. J. A., Characterization of liposomes (1987). The influence of extrusion of multilamellar vesicles through polycarbonate membranes on particle size, particle size distribution and number of bilayers, Int. J. Pharm.. 35, 263-274. 

Olson, F., Hunt, C. A., Szoka, F., Vail, W. J., and Papahadjopou-los, D. (1979). Preparation of liposomes of defined size distribution by extrusion through polycarbonate membranes, Biochim. Biophys. Acta. 557, 9-23. 

Thin film nanostructures of the III-VI compound In2Se3 were obtained inside the pores (200 nm) of commercial polycarbonate membrane by automated ECALE methodology at room temperature [157], Buffered solutions with millimolar concentrations of In2(S04)3 (pH 3.0) and Se02 (pH 5.5) were used. The atomic ratio of Se/In in the deposited films was found to be 3/2. Band gaps from FTIR reflection absorption measurements were found to be 1.73 eV. AFM imaging showed that the deposits consist of 100 nm crystallites. 

Within the scope of thermoelectric nanostructures, Sima et al. [161] prepared nanorod (fibril) and microtube (tubule) arrays of PbSei. , Tej by potentiostatic electrodeposition from nitric acid solutions of Pb(N03)2, H2Se03, and Te02, using a 30 fim thick polycarbonate track-etch membrane, with pores 100-2,000 nm in diameter, as template (Cu supported). After electrodeposition the polymer membrane was dissolved in CH2CI2. Solid rods were obtained in membranes with small pores, and hollow tubes in those with large pores. The formation of microtubes rather than nanorods in the larger pores was attributed to the higher deposition current. 

A similar system has been reported based on polycarbonate filters coated with hexadecane, also called hexadecane membranes (HDM) [124, 125]. Thus, this... 

FIG. 20-66 Track-etched 0.4-)lm polycarbonate membrane. Courtesy Milli-pore Corporation.)... 

FIG. 14 Constant height mode gray-scale image of a 5/xm-diameter pore in a polycarbonate membrane obtained with a 3 fim pipette tip. The filling DCE solution contained 10 mM TBATPBCl. The aqueous phase contained 0.4mM TEACl + lOmM LiCl. The scale bar corresponds to 10/xm. The tip scan speed was 10/xm/s. (Reprinted with permission from Ref. 30. Copyright 1998 American Chemical Society.)... 

Efforts to overcome the limitations of the fragile membranes (as delicate as soap bubbles) have evolved with the use of membrane supports, such as polycarbonate filters (straight-through pores) [543] or other more porous microfilters (sponge-like pore structure) [545-548]. 

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