Poly , permeability

Fouling Industrial streams may contain condensable or reactive components which may coat, solvate, fill the free volume, or react with the membrane. Gases compressed by an oil-lubricated compressor may contain oil, or may be at the water dew point. Materials that will coat or harm the membrane must be removed before the gas is treated. Most membranes require removal of compressor oil. The extremely permeable poly(trimethylsilylpropyne) may not become a practical membrane because it loses its permeability rapidly. Part of the problem is pore collapse, but it seems extremely sensitive to contamination even by diffusion pump oil and gaskets [Robeson, op. cit., (1994)]. 

Fried, J.R., Sadat-Akhavi, M., and Mark, J.E., Molecular simulation of gas permeability Poly(2,6-dimethyl-l,4-phenylene oxide), J. Membrane Sci., 149, 115, 1998. 

D. Fritsch and N. Avella. Synthesis and properties of highly gas permeable poly(amide-imide)s. Macromo/. Chem. Phys., 197(2) 701-714, February 1996. 

L. C. Costa (Ionics) Asymmetric semi-permeable poly(aryletherketone) membranes and method of producing same. US Patent 5089192, February 1992. 

The commercial success of pervaporation has been a disappointment to many process developers. Current pervaporation sales worldwide are probably less than US 10 million almost all are for dehydration of ethanol or isopropanol solutions using water-permeable poly (vinyl alcohol) or equivalent membranes. A smaller market also exists for the separation of volatile organics from water using silicone-rubber membranes. 

The preceding analysis indicates die value of considering the separate solubility and mobility constituent contributions to the permeability and selectivity of a given ihembrane. These data also suggest approaches for modifying existing membrane materials at the two extremes of permeability shown in Fig. 20.3-9. The mose selective film (Kapton ) represented in Table 20.3-1 is also the least permeable, and the most permeable [poly(phenylene) oxide] is also the least selective a rather unfortunate statement of Muiphy s Law. 

Moon JH, McDaniel W, MacLean P et al (2007) Live-cell-permeable poly(p-phenylene ethynylene). Angew Chem Int Ed 46 8223-8225... 

Liu L, Sheardown H. Sheardown glucose permeable poly(dimethyl siloxane) poly(V-isopropylacrylamide) interpenetrating networks as ophthalmic biomaterials. Biomaterials 2005 26 233. ... 

Positron annihilation lifetime spectroscopy (PALS) is an efficient tool for measuring free volume and sizes of free volume elements in polymeric materials. This is particularly inq)ortant for studies of membrane materials, since free volume determines the permeation rate of small molecules. Free volume was studied by means of PALS in polymers characterized by extremely high permeability poly(l-trimethylsilyl-l-propyne) and copolymers of 2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole and tetrafluoroethylene. The results obtained were compared with those observed for conventional glassy polymers. For the first time, the size distribution of free volume has been determined for these membrane materials. 

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Fig. 15. Oxygen permeability versus 1/specific free volume at 25 °C (30). 1. Polybutadiene 2. polyethylene (density 0.922) 3. polycarbonate 4. polystyrene 5. styrene-acrylonitrile 6. poly(ethylene terephthalate) 7. acrylonitrile barrier polymer 8. poly(methyl methacrylate) 9. poly(vinyl chloride) 10. acrylonitrile barrier polymer 11. vinyUdene chloride copolymer 12. polymethacrylonitrile and 13. polyacrylonitrile. See Table 1 for unit conversions.

Barrier Layers. Depending on composition, barrier layers can function simply as spatial separators or they can provide specified time delays by swelling at controlled rates or undergoing reactions such as hydrolysis or dissolution. Suitable barrier materials include cellulose esters and water-permeable polymers such as gelatin and poly(vinyl alcohol) (see Barrier polymers). 

Hard lenses can be defined as plastic lenses that contain no water, have moduli in excess of 5 MPa (500 g/mm ), and have T well above the temperature of the ocular environment. Poly(methyl methacrylate) (PMMA) has excellent optical and mechanical properties and scratch resistance and was the first and only plastic used as a hard lens material before higher oxygen-permeable materials were developed. PMMA lenses also show excellent wetting in the ocular environment even though they are hydrophobic, eg, the contact angle is 66°. 

Much of the success of the poly(ethylene terephthalate) bottle has arisen from the control of the biaxial orientation that occurs during manufacture to give a product both strong and of low gas permeability. 

Copolymers of acrylonitrile and vinylidene chloride have been used for many years to produce films of low gas permeability, often as a coating on another material. Styrene-acrylonitrile with styrene as the predominant free monomer (SAN polymers) has also been available for a long time. In the 1970s materials were produced which aimed to provide a compromise between the very low gas permeability of poly(vinylidene chloride) and poly(acrylonitrile) with the processability of polystyrene or SAN polymers (discussed more fully in Chapter 16). These became known as nitrile resins. 

Table 15.4 illustrates that though the nitrile resins had a gas permeability much higher than has poly(acrylonitrile) the figures for oxygen and carbon dioxide are much lower than for other thermoplastics used for packaging. 

Ionic liquids have already been demonstrated to be effective membrane materials for gas separation when supported within a porous polymer support. However, supported ionic liquid membranes offer another versatile approach by which to perform two-phase catalysis. This technology combines some of the advantages of the ionic liquid as a catalyst solvent with the ruggedness of the ionic liquid-polymer gels. Transition metal complexes based on palladium or rhodium have been incorporated into gas-permeable polymer gels composed of [BMIM][PFg] and poly(vinyli-dene fluoride)-hexafluoropropylene copolymer and have been used to investigate the hydrogenation of propene [21]. 

Figure 2 Light permeability of polyolefins after quenching (1-4) and of nonquenched samples (l -4 ) l,l -poly-propylene (PP) 2,2 -high-pressure polyethylene (HPPE) 3,3 -low-pressure polyethylene (LPPE) 4,4 -medium-pres-sure polyethylene (MPPE). Film thickness-150 fic moulding time-10 minutes, moulding pressure HPPE-160°C LPPE, MPPE, PP-190-200X.

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