Membrane casting

Fig. 9. SEM photographs of cellulose acetate membranes cast from a solution of acetone (volatile solvent) and 2-meth5l-2,4-pentanediol (nonvolatile solvent). The evaporation time before the stmcture is fixed by immersion ia water is shown (24).

Fig. 11. Schematic of Loeb-Sourirajan membrane casting machine used to prepare reverse osmosis or ultrafiltration membranes. A knife and trough is used to coat the casting solution onto a moving fabric or polyester web which enters the water-filled gel tank. After the membrane has formed, it is washed...

Supported Tube. There are three types of supported tubular membranes cast in place (integral with the support tube), cast externally and inserted into the tube (disposable linings), and dynamically formed membranes. 

The most common supported tubes are those with membranes cast in place (Fig. 17). These porous tubes are made of resin-impregnated fiber glass, sintered polyolefins, and similar materials. Typical inside diameters are ca 25 mm. The tubes are most often shrouded to aid in permeate collection and reduce airborne contamination. 

FIGURE 6-2 Steps in the preparation of an ainperometric enzyme electrode with simple enzyme immohilization hy trapping between an inner cellulose acetate and outer collagen membrane, cast on the electrode body. (Reproduced with permission from reference 1.)... 

More recently, many workers have concentrated on controlling the size and shape of particles, with an eye on industrial applications, where such features must be well defined and controllable. Many shapes have been made, including fibres, spheres, plates, as well as membranes cast on... 

There exist a maximum allowable thickness of the supported gel layers above which it is not possible to obtain crack-free membranes after calcination. For Y-alumina membranes this thickness depends on a number of (partly unknown) parameters and has a value between 5 and 10 /im. One of the important parameters is certainly the roughness and porosity of the support system, because unsupported membranes (cast on teflon) are obtained crack-free up to 100 )xm. The xerogel obtained after drying was calcined over a wide range of temperatures. At 390°C the transition of boehmite to y-AljOj takes place in accordance with the overall reaction... 

Poly(phenylquinoxaline) membranes cast from m—cresol were sulfonated by reactions in 50% sulfuric acid solution for 2 h followed by drying at 300 °C for 90 min. The attachment site for the sulfonic acid group is believed to be ortho to the ether linkage, as in Figure 26, but even this has yet to be confirmed. 

The diffusion coefficients of potassium and sodium chlorides in the membrane we have found to be independent of solution concentration within experimental error. The value of sodium chloride agrees well with those found by others bearing in mind the differences in the polymer and in the membrane casting procedures (5,13). 

It has been shown (, , 2.) that a membrane casting dope is a strongly structurlzed polymer solution, and that the morphology of the membrane surface layer can be correlated to the structure of the casting solution. The latter parameter affects the nature and details of the phase inversion process occuring in the upper part of the cast solution, in an incipient skin. Thus the solution structure is one of the factors responsible for the skin properties, and consequently for the performance of the ultimately formed asymmetric membrane. 

The viscosities of the membrane casting dopes were measured by a Hoeppler viscometer as previously described (6). The solution densities were obtained by the modified Gibson and Loeffler (,9) dllatometer. From these data the solution viscosities at different temperatures were calculated. The activation energies of viscous flow obtained from the logri-1/T plots were corrected by the corresponding values of the solvents used. 

The shape of the presented curve seems to be significant because an increase of the AR values is known (6,11) to indicate the enlargement of the supermolecular aggregation within the membrane casting dope. 

The reverse osmosis results for membranes cast from the solutions listed in Table 1 are given in Figure 2. The shrinkage... 

Further, the observed changes in trends of both the AE curve (Figure 1) and the membrane performance data (Figure 3) that happen in the same range of PA concentrations, lead to the conclusion that a certain stable level of supermolecular organization reached in the membrane casting dope creates conditions for the membranes with optimal skin properties to be formed. 

In order to explain the specific role of PA in the membrane casting dope one has to take into consideration the structure of PA. The molecule of PA forms (15,16) a tetrahedral POi, group which is linked to the other molecules by hydrogen bonds. In 86 wt.% PA two P0 groups are connected by one hydrogen bond (Figure 6) enabling formation of a chainlike structure of PA. 

The offered Interpretation of the presented results seems to be strongly supported by the most significant feature of the produced membranes - their surprisingly good performances at higher operating pressures. Figure 7 compares the performances of the 513-type membranes cast from the solution cooled down to... 

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

Asymmetric Membrane Studies. In light of the results presented in the preceding two sections, plus those found in the literature (21-26,28), the decision was made to commence the asymmetric membrane studies with SPSF-Na(0.A2). The selection of the sodium salt polymer was based on the desire to limit ion exchange in desalination. The selection of D.S. of 0.A2 represents a compromise of hydrophilic/hydrophobic balance and structural stability. The exploration of asymmetric membranes cast from the pol3rmers of other salts and various D.S. values is planned for the future. 

Membranes cast from THF-formamide, dried for 200 seconds in room temperature air, and gelled for one hour, were tested for desalination performance. The results are listed in Table VIII. The best results were obtained with membranes cast from AO 60 and 70 30 THF-formamide and gelled in isopropyl alcohol (IPA)-see discussion below concerning gelation media. Of these two systems the AO 60 is superior. This... 

Numerous asymmetric membranes were prepared under various conditions and their cross-section was examined by SEM. Typical of the results are those shown in Figure 5 for the membrane cast from 70 30 THF-formamide and gelled in IPA. Close inspection of Figure 5 reveals a thin, relatively dense skin supported by a microporous layer. The support layer contains macrovoids, the cause of which is presently under investigation. 

Figure 5. SEM of the cross-section of a SPSF-Na (0.42) membrane cast from 70 30 THF-formamide solution, dried for 200 s, and gelled in IP A...

Membranes cast upon glass slides also fall into the 3D surface category. Most notable are the nitrocellulose-coated FAST slides offered by Schleicher Schuell (S S) BioScience. A cytokine micro-ELISA product xmder the trade name Provision has been introduced in single-slide and 96-well spacing (64 usable wells) formats. It is called FAST Quant (Harvey, 2003). Historically, nitrocellulose membrane has been used for the sequestering of both proteins and nucleic acids. The adaptation by S S of this microporous (0.2-p pores) material cast into a microarray format has been relatively straightforward. 

And, how is it that membranes cast upon glass substrates are now used to prepare protein microarrays We will address these questions in good time. 

The membrane cast from chloroform-formic acid mixtures had an anisotropic structure with a 0.9-1.2 p active layer and a 40 p porous support layer. At a water flux of 139 1/m2 day (kg/cm2 at 20 °C), the membrane showed 99.4 % rejection of cytochrome C and 72.7% of Vitamine B12. At 3980 1/m2 day water flux level, the rejection for bovine serum hemoglobin (MW, 66000 68000), cytochrome C, and Vitamine Bl2 were, 95.6, 79.4, and 39.8%, respectively. 

Figure 3.8 Scanning electron micrographs of aromatic polyamide (Nomex, Du Pont) Loeb-Sourirajan membranes cast from 22 and 18 wt% Nomex in dimethylacetamide [19]...

Figure 3.10 The porosity of cellulose acetate membranes cast from 15-wt% solutions with various solvents. The same trend of high porosity and rapid precipitation with high solubility-parameter solvents was seen with a number of other membrane materials [25]...

Membrane-casting Techniques. Uhtil recently, PVC membranes have been exclusively formed by solvent casting techniques but which are not well-suited to the fabrication of ISFET devices. Membrane components in tetrahydrofuran are difficult to manipulate on a micro scale and are prone to absorb atmospheric moisture, thus weakening the adhesion at the sensor-ISFET interface. Che innovation which dispenses with the tetrahydrofuran casting stage is based on an in situ photolysis of the model calciun sensor cocktail admixed with monobutyl methacrylate + benzoyl peroxide + benzoin methyl ether at 340 nm (3). Hie resultant matrix adhered well to the ISFET gate and its potentiate trie response compared favourably with the analogous P and P2-MPMA ISE (Table II). 

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