Track-etched polymer

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Conical Nanopores in Tracked-Etched Polymer Films.541... 

Track-etched polymer membranes, which have straight cylindrical pores that are oriented normal to the plane of the membrane, provide a promising platform to design membranes with nano-domains with high aspect ratios oriented in the desired direction. Presently, track-etched membranes are commercially available in polyester and polycarbonate with various thicknesses (6 pm or above), pore sizes (10 nm to microns), and porosities ( 0.05% to... 

Flat-sheet asymmetric-skinned membranes made from synthetic polymers (also copolymers and blends), track-etched polymer membranes, inorganic membranes with inorganic porous supports and inorganic colloids such as Zr02 or alumina with appropriate binders, and melt-spun thermal inversion membranes (e.g., hollow-fiber membranes) are in current use. The great majority of analytically important UF membranes belong to the first type. They are usually made of polycarbonate, cellulose (esters), polyamide, polysulfone, poly(ethylene terephtha-late), etc. 

Comparison between alumina and track-etched polymer nanoporous membranes... 

Track-etched polymer membranes are preferred for NEE fabrication over alumina membranes because track-etched membranes are not brittle and they have smaller pore densities. From an electroanalytical viewpoint, the latter is an important feature since it reduces the interactions between individual nanoelectrode elements (see below). 

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. 

In any ID template such as A AO or track-etched polymers, there is the ability to electrochanically grow either nanowires or nanotubes. This has been achieved by both multistep and single-step methods. The growth mechanism for nanowires and multistep growth mechanism of nanotubes is mostly straightforward however, the growth mechanism for single-step nanotubes is debated in the literature. 

FIGURE 10.19 Cyclic voltammetry curves at different scan rates for a track-etched polymer template in an AuCl4 showing sigmoidal shapes at slow scan rates and peak shapes at fast scan rates. (Reproduced from Hariri, M.B. et ah, J. Electrochem. Soc., 160, D279,2013. With pamission from the Electrochemical Society.)... 

The deposition of nanowires and nanotubes into tanplates was pioneered by Martin. In template deposition, the materials are deposited into nanoporous manbranes, such as anodized aluminum or track-etch polymers. The nanoporous membranes function as nanosized beakers that constrain the crystal growth. Figure 17.10 shows TEM micrographs of Au nanowires with a diameter of 70 nm and polypyrrole nanotubes with an outside diameter of 90 nm and an inside diameter of 20-30 nm that were eleclrodeposited into an alumina nanoporous tanplate. ... 

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