Cross-linked polymer membranes

Using this concept the enzyme myoglobin was successfully encapsulated by a photo-cross-linked polymer membrane and the pH-dependent activity was reported [9], This principle offers a high potential for biotechnological apphcations. Similarly, pH-sensitive swelling of the cross-linked membrane can also be used for controlled dmg release systems. 

Covalently cross-linked ionomer membranes or blend membranes are expected to have dimensional and chemical stability to reduce methanol crossover. Cross-linking agents such as divinylbenzene, sulfonyl -imidazolide, 4,4 -diaminodiphenylsulfone can imbibe in the polymer main chain during polymerizations. Cross-linked polymer membranes showed reduced methanol crossover however, it is questionable whether the cross-linking bridges are stable in the strongly acidic environment of the fuel cell. 

Jung, D.H., Myoung, Y.B., Cho, S.Y., Shin, D.R., and Peck, D.H. (2001) A performance evaluation of direct methanol fuel cell using impregnated tetraethyl-orthosilicate in cross-linked polymer membrane. International Journal of Hydrogen Energy, 26 (12), 1263-1269. 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

PVA must be cross-linked in order to be useful for a wide variety of applications. A hydrogel can be described as a hydrophilic, cross-linked polymer, which can sorbe a great amount of water by swelling, without being soluble in water. Other specific features of hydrogels are their soft elastic properties, and their good mechanical stability, independent of the shape (rods, membranes, microspheres, etc.). 

Physical entrapment This is often used in sandwich membranes, the enzyme is trapped in a cross linked polymer or adsorpted onto a gel matrix when constructing a sensor. It relies on weak hydrogen bonding and electrostatic interactions between the... 

We are interested in the application of porous polymers for the preparation of adsorbents [191,192], ion exchangers [193,194], and membranes [60,198,199]. For these applications, cross-linked polymers are fundamentally applied. 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

Physical immobilization methods do not involve covalent bond formation with the enzyme, so that the native composition of the enzyme remains unaltered. Physical immobilization methods are subclassified as adsorption, entrapment, and encapsulation methods. Adsorption of proteins to the surface of a carrier is, in principle, reversible, but careful selection of the carrier material and the immobilization conditions can render desorption negligible. Entrapment of enzymes in a cross-linked polymer is accomplished by carrying out the polymerization reaction in the presence of enzyme the enzyme becomes trapped in interstitial spaces in the polymer matrix. Encapsulation of enzymes results in regions of high enzyme concentration being separated from the bulk solvent system by a semipermeable membrane, through which substrate, but not enzyme, may diffuse. Physical immobilization methods are represented in Figure 4.1 (c-e). 

Lee outlines three different physical methods that are commonly utilized for enzyme immobilization. Enzymes can be adsorbed physically onto a surface-active adsorbent, and adsorption is the simplest and easiest method. They can also be entrapped within a cross-linked polymer matrix. Even though the enzyme is not chemically modified during such entrapment, the enzyme can become deactivated during gel formation and enzyme leakage can be problematic. The microencapsulation technique immobilizes the enzyme within semipermeable membrane microcapsules by interfacial polymerization. All of these methods for immobilization facilitate the reuse of high-value enzymes, but they can also introduce external and internal mass-transfer resistances that must be accounted for in design and economic considerations. 

The bacterial cell wall is a cross-linked polymer of polysaccharides and pentapeptides. Penicillins interact with cytoplasmic membrane-binding proteins (PBPs) to inhibit transpeptidation reactions involved in cross-linking, the final steps in cell wall synthesis. 

T. Uragami, F. Yoshida and M. Sugihara, Studies on syntheses and permeabilities of special polymer membranes. 59. Active transport of organic ions through cross-linked chitosan membrane, Separation Sci. Technol., 1988, 23, 1067-1082. 

Chemistry on soluble polymer matrices has recently emerged as a viable alternative to solid-phase organic synthesis (SPOS) involving insoluble cross-linked polymer supports. Separation of the functionalized matrix is achieved by solvent or heat precipitation, membrane filtration, or size-exclusion chromatography. Suitable soluble polymers for liquid phase synthesis should be crystalline at room temperature, with functional groups on terminal ends or side chains, but must not be not cross-linked they are therefore soluble in several organic solvents. 

There are presumably a variety of reasons why soluble polymers are less often used as supports or ligands for catalysis. The most likely reason is that there is a perception that recovery of a soluble polymer is e3q>erimentally more difficult than recovery of an insoluble cross-linked polymer. This perception stems from the mistaken belief that a soluble polymer can only be isolated as a viscous, intractable gooey material. However, as is demonstrated in the examples below, this is not true. Indeed, many soluble polymers can be easily isolated as tractable solids. Moreover, even in cases where the soluble polymer is not a tractable soUd or where a solid/liquid separation is deemed less desirable, soluble polymers can often be separated from low molecular weight products either on the basis of size (membrane fQtration) or on the basis of their selective solubility in one phase of a biphasic mixture. 

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