Poly cross-linking

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

SiHcone PSAs are blends or interpenetrating networks (IPNs) composed of a tackifyiag MQ resia cured ia a cross-linked poly(siloxane) aetwork. 

Corrosion. Aqueous solutions of citric acid are mildly corrosive toward carbon steels. At elevated temperatures, 304 stainless steel is corroded by citric acid, but 316 stainless steel is resistant to corrosion. Many aluminum, copper, and nickel alloys are mildly corroded by citric acid. In general, glass and plastics such as fiber glass reinforced polyester, polyethylene, polypropylene, poly(vinyl chloride), and cross-linked poly(vinyl chloride) are not corroded by citric acid. 

Figure 1. Sketch of the primary structure (vide infra) of cross-linked polystyrene (a) and of a typical cross-linked poly-vinyil co-monomer-functional co-monomer-cross-linker (b) [14].

Me+Rh(CO)2l2 (R425 = Reillex cross-linked poly-4-vinylpyridyne-divinylbenzene [3] R425 Me = methyl... 

Macromolecular Substitution Route. The current surge in poly-phosphazene research Is mainly a result of the development in the mid 1960 s (2-4) of a substitutive route to the synthesis of organo phosphazene high polymers. Before that time, only a sporadic interest in the subject existed because the known polymers, cross linked poly(dihalophosphazenes), (1,5) were insoluble and hydrolytically unstable. 

Simple physical entanglements can be sufficient to produce a structurally stable gel if the polymer has a sufficiently great molecular weight and if the polymer is of only modest hydrophilicity. In this case, the polymer will swell in water without dissolving, even in the absence of covalent cross-links. Poly(2-hydroxyethyl methacrylate) (PHEMA) is a prominent example of this type of hydrogel when uncross-linked, it will dissolve in 1,2-propanediol but only swell in water. 

Figure 10 Polymer-water interaction parameter (%) of cross-linked poly(/V,/V-alkyl-substituted acrylamide) as a function of temperature. (From Ref. 32.)...

Figure 13 Typical swelling and deswelling rates of cross-linked poly(acryloyl pyrroli-dine-co-styrene) between 27°C and 37°C. AS15 ( ) AS20 (A). The numbers indicate the content of styrene in the feed composition in moles during polymerization. Membrane thickness is 0.5 mm in the dried state. (From Ref. 34.)...

Figure 14 The reversibilities of insulin permeation through polymer membranes in a two-compartment diffusion cell AH20 ( ), AS 15 (A), AS20 ( ), H ( ). Numbers indicate the content of styrene or HEMA in feed compositions in moles. H represents a cross-linked poly(2-hydroxyethyl methacrylate) (HEMA). (From Ref. 34.)... 

Figure 17 Permeability of uranine ( ), dextran 4.4K (O), and dextran 150K (A) through cross-linked poly(/V-isopropyl acrylamide-co-butyl methacrylate, 95 5 mol%) membrane. Error bars represent standard deviation in the slope of the curve of the receiver concentration of solute as a function of time at steady state. (From Ref. 37.)...

Experimental determinations of the contributions above those predicted by the reference phantom network model have been controversial. Experiments of Rennar and Oppermann [45] on end-linked PDMS networks, indicate that contributions from trapped entanglements are significant for low degrees of endlinking but are not important when the network chains are shorter. Experimental results of Erman et al. [46] on randomly cross-linked poly(ethyl acrylate)... 

Degradation of unsaturated and cross-linked poly(3HO) derivatives 72... 

Brandi et al. [71] using culture fluid of Acidovorax delafieldii and cyclic 3HB oligomers were in agreement with the presence of endo-hydrolase activity of poly(3HB) depolymerases. Similar results were obtained by de Koning et al. [72] who demonstrated that covalently cross-linked poly(HAMCL) was hydrolyzed completely by P. fluorescens. It is assumed that most - if not all - extracellular poly(HA) depolymerases have endo- and exo-hydrolase activity. Depending on the depolymerase the hydrolysis products are only monomers, monomers and dimers, or a mixture of oligomers (mono- to trimers). 

Pyrolysis-Gas Chromatography-Mass Spectrometry. In the experiments, about 2 mg of sample was pyrolyzed at 900°C in flowing helium using a Chemical Data System (CDS) Platinum Coil Pyrolysis Probe controlled by a CDS Model 122 Pyroprobe in normal mode. Products were separated on a 12 meter fused capillary column with a cross-linked poly (dimethylsilicone) stationary phase. The GC column was temperature programmed from -50 to 300°C. Individual compounds were identified with a Hewlett Packard (HP) Model 5995C low resolution quadruple GC/MS System. Data acquisition and reduction were performed on the HP 100 E-series computer running revision E RTE-6/VM software. 

Granular swelling agents. These systems typically consist of cross-linked hydrophilic polymers like e.g. cross-linked poly-(N-vinyl pyrrolidone) or hydrophilic but insoluble polymers, e.g. cellulose that swell to a considerable volume within a few seconds on contact with water. These systems are powerful disintegrants and are used particularly in quick-release detergent tablets put in a dispenser tray. 

Hydrogels, 13 729-759. See also Microgels Superabsorbent polymers (SAPs) AMPS polymer, 23 721 applications for, 13 747-753 biodegradable, 13 739-742 bioerodible, 9 63 conducting, 7 524 cross-linked poly (ethylene oxide),... 

Graft polymerization of monomers such as acrylic acid on core particles consisting of magnetic iron oxide embedded in cross-linked poly (vinyl alcohol) (PVA) has been described previously (2). 

The polymers and copolymers discussed here were all prepared by reaction of the homogeneous (linear) or heterogeneous (cross-linked) poly(vinylbenzylchloride) substrate polymer with the potassium or cesium salts of the suitably monofunctionalized donors (Reaction 1). 

Benzyl alcohol can be smoothly converted to benzaldehyde by electrochemi-cally recycled BrO as an oxidizing catalyst in an emulsion system prepared from a mixture of water, amyl acetate, and 2% BU4NHSO4 [60]. A different hypobromite reagent is provided by an electrochemical oxidation of a cross-linked poly-4-vinylpyridine in an MeCN-H20-HBr-(Pt) system [61]. Secondary alcohols can be oxidized by this method to give ketones in high yields. The electrooxidation of A-monoalkyltosylamides (42) in a two-phase system consisting of cyclohexane... 

Figure 18 shows the temperature dependence of the proton conductivity of Nafion and one variety of a sulfonated poly(arylene ether ketone) (unpublished data from the laboratory of one of the authors). The transport properties of the two materials are typical for these classes of membrane materials, based on perfluorinated and hydrocarbon polymers. This is clear from a compilation of Do, Ch 20, and q data for a variety of membrane materials, including Dow membranes of different equivalent weights, Nafion/Si02 composites ° ° (including unpublished data from the laboratory of one of the authors), cross-linked poly ary lenes, and sulfonated poly-(phenoxyphosphazenes) (Figure 19). The data points all center around the curves for Nafion and S—PEK, indicating essentially universal transport behavior for the two classes of membrane materials (only for S—POP are the transport coefficients somewhat lower, suggesting a more reduced percolation in this particular material). This correlation is also true for the electro-osmotic drag coefficients 7 20 and Amcoh...

By contrast, membranes U-1, A-2 and X-2 are all chlorine sensitive, each responding in a unique manner. U-1 is a thin film composite membrane, the active layer consisting of cross-linked poly(ether/urea) polymer. A-2 is a homogeneous aromatic polyamide containing certain polyelectrolyte groups. X-2 is a thin film composite membrane of proprietary composition. 

Phenylcalcium iodide metallates dibenzothiophene in the 3-position however, it has no effect on the corresponding sulfone. Mercuration of dibenzothiophene has been accomplished by adding mercuric acetate to a melt of dibenzothiophene, but the position of mercuration was not established. Mercuric nitrate or bisulfite give no identifiable products. An unsuccessful attempt to metallate dibenzothiophene with cross-linked poly(p-lithiost5Tene) in ether has been recorded. ... 

All the enzymes used in the work described above are quite stable at room temperature and can be used in a free form. They can also be used in an immobilized form to improve the stability and to facilitate the recovery. Many immobilization techniques are available today (25). The recent procedure developed by Whitesides et al using water-insoluble, cross-linked poly(aerylamide-acryloxysuccinimide) appears to be very useful and applicable to many enzymes (37). We have found that the non-crosslinked polymer can be used directly for immobilization in the absence of the diamine cross-linking reagent. Reaction of an enzyme with the reactive polymer produces an immobilized enzyme which is soluble in aqueous solutions but insoluble in organic solvents. Many enzymes have been immobilized by this way and the stability of each enzyme is enhanced by a factor of greater than 100. Horse liver alcohol dehydrogenase and FDP aldolase, for example, have been successfully immobilized and showed a marked increase in stability. 

The 4-sodio and 4-potassio derivatives are known but are less convenient to obtain than the 4-lithio derivative, which is readily available by metallation of dibenzofuran with butyllithium. Metallation can also be achieved with cross-linked poly(p-lithiosytrene) but offers no advantage in yield. The 4,6-dimetallo derivatives of dibenzofuran can be made by treatment of dibenzofuran with butyllithium or butylsodium. Lithiodiben-zofurans and dilithiodibenzofurans are also available by bromine-lithium exchange with butyllithium. " ... 

Sperinde JJ, Griffith LG (1997) Synthesis and characterization of enzymatically-cross- linked poly(ethylene glycol) hydrogels. Macromolecules 30 5255-5264... 

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