Characterization of Asymmetric Membranes

Although models and techniques such as those described in preceding sections permit characterization of dense-film transport properties, a need exists for improved characterization of the transport behavior of asymmetric membrane properties. Currentiy, performance tests such as gas permeability and selectivity using standardized feed streams provide useful tools for quality control but are ambiguous for fundamental characterizations of the type possible for dense membranes. The following discussion presents information pertinent to the characterization of porosity and moiphology of asymmetric membranes and to analysis of their flux deciine as a function of time in service. These are two of the most important additional characteristics that asymmetric structures introduce into the description of transport behavior of the separator module. 

5-1 Porosity and Morphology Characterizations Void Fraction Determinations 

Estimates of the overall void volume in an asymmetric membrane can be made by equilibrating a known dry weight of sample, tv, in a suitable liquid which will penetrate into the membrane pores but will not swell the polymer substrate. After centrifugation to remove superficial fluid and fluid held in the bore of hollow-fiber membranes, determination of the weight gain of the sample provides an estiinale of the void fraction present in the membrane using Eq. (20.5-1)  

This technique clearly assumes that the polymer is not swollen appreciably by the liquid. Care should be exercised to verify this fact using a dense thin-flim sample of the polymer immersed in the candidate liquid. In addition, the apparent value of 1V2, determined roughly as the difference between the centrifuged sample weight and the diy weight, should be corrected to account for actual molecular sorption into the polymer mass. This sorption, which occurs in addition to the capillary uptake, can produce an overesti-mation of the void volume percent in the fiber by as much as 5% if unaccounted for. The simplest means of accounting for such sorption involves adjustment of the value of using sorption data for die liquid in the dense film for which capillary effects are not present. 

As a result of these shortcomings, porosity characterizations generally involve both gas permeability measurements to provide estimates of the average pore size and liquid displacement to measure maximum pore size in the membrane surface. The techniques rely on a number of assumptions for their results to have physical meaning. Depending on the structure of the asymmetric membrane, some of the assumptions may be satisfied only marginally. In such a case, the characterizations are primarily useful as empirical indices for comparison of dilferent samples, and the fundamental meaning of the numbers derived from such analyses is questionable. 

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Additionally we may conclude that low-pressure integral permeability is a quick and easy to use tool for crack (defect) detection and leads to detailed structural characterization of asymmetric membranes. Furthermore, permeability experiments give a rough but reliable measure of the maximum pore size of the structure under study. 

Wang, G-M., Chen, C-H., Ho, H-O., Wang, S-S., and Sheu, M-T. (2006), Novel design of osmotic chitosan capsules characterized by asymmetric membrane structure for in situ formation of delivery orifice, Int. J. Pharm., 319, 71-81. 

Three new methods to characterize the pore structure and pore size distribution in the top layer of asymmetric membranes have been developed or refined in our laboratory during the past few years a) the gas ad-sorption/desorption method, b) thermoporometry and o) selective permeation (fractional rejection). 

Ismail, A.F. and Hassan, A.R. 2006. Formation and characterization of asymmetric nanofiltration membrane Effect of shear rate and polymer concentration. J. Memb. Sci. 270 57-72. 

HOU Hourdet, D., Muller, G., Vincent, J.C., Avrillon, R., and Robert, E., Characterization of polyimides in solution. Relation with the preparation of asymmetric membranes, in Polyimides and other High-Temperature Polymers, Elsevier Sci. Publ., Amsterdam, 507, 1991. 

Kusuki Y, Shimazaki H, Tanihara N, Nakanishi S, Yoshinaga T (1997) Gas permeation properties and characterization of asymmetric carbon membranes prepared by pyrolyzing asymmetric polyimide hollow fiber membrane. J Membr Sci 134 (2) 245-253... 

Vu, D. Q. (2001). Formation and characterization of asymmetric carbon molecular sieve and mixed matrix membranes for natural gas purification. University of Texas, Austin, TX. 

Rahimpour, A., M. Jahanshahi, N. Mortazavian, S. S. Madaeni, and Y. Mansourpanah. 2010. Preparation and characterization of asymmetric polyethersulfone and thin-film composite polyamide nanofiltration membranes for water softening./4pp/. Surf. Sci. 256 1657-1663. 

Once synthesized several factors influence the particular leaflet of the membrane lipid bilayer where the lipids reside. One is static interactions with intrinsic and extrinsic membrane proteins which, by virtue of their mechanism of biosynthesis, are also asymmetric with respect to the membrane. The interaction of the cytoplasmic protein, spectrin with the erythrocye membrane has been the subject of a number of studies. Coupling of spectrin to the transmembrane proteins, band 3 and glycophorin 3 via ankyrin and protein 4.1, respectively, has been well documented (van Doit et al, 1998). Interaction of spectrin with membrane lipids is still somewhat conjectural but recent studies have characterized such interactions more precisely. O Toole et al. (2000) have used a fluorescine derivative of phosphatidylethanolamine to investigate the binding affinity of specttin to lipid bilayers comprised of phosphatidylcholine or a binary mixture of phosphatidylcholine and phosphatidylserine. They concluded on the basis... 

The types of polymeric membranes that have attracted much interest for analytical applications and are nowadays in common use are characterized as nonporous membranes such as low-density polyethylene (LDPE), dense PP and PDMS silicone rubbers, and asymmetric composite membranes... 

Preparation and Characterization of Tablet Coatings Tablet cores were prepared by compressing the drug without any excipient using a hydraulic press operated at llOMPa. A stainless steel die with a diameter of 1.2cm was used to produce 400-mg drug tablet cores. Asymmetric-membrane tablet coatings were applied... 

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