Pore Size Measurement of Track Etched Membranes by Capillary Flow Porosimetry
Capillary flow porometry is ideal for the pore size measurement of track etched membranes because these materials usually have almost a perfect cylindrical pore structure from one side to the other. The results of testing five different track etched membranes with the Porometer 3G series of instruments are described.
Track etched membranes (TEMs) are ultra-thin, well-regulated membranes that have been traditionally used for high specification filtration in laboratory applications. These plastic films have discrete pores that are formed through a combination of charged particle bombardment (or irradiation) and chemical etching. The etching process involves passing the tracked film through a number of chemical baths, creating a clean, regulated membrane with good precision in terms of pore size. This etching process determines the size of the pores, with typical pore sizes ranging from 20 nm to 14 µm. Polymers commonly employed are polycarbonate (PC) and polyethylene terephthalate (PET), but some TEMs are also manufactured using other polymers (e.g., polyvinylidene fluoride).
Typical membrane thicknesses are between 10 and 20 µm, controlled to within +/- 1 µm with roughness not exceeding 50 nm (peak to valley). Pore size is usually between -20 to 0% of the stated pore size, with a typical intra-lot coefficient of variance for pore size between 2-3%. In addition to pore size, pore density (or porosity) can also be controlled, typically ranging from 1 x 10-5 to 6 x 108 pores/cm2 and can vary within certain limits in relation to pore size. The internal shape of the pores can also be closely controlled to allow the formation of a truly cylindrical structure. A scanning electron microscopy (SEM) image of a membrane is shown in Figure 1.
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