Chemical bonds physical change compared

Copolymerizations are of considerable theoretical importance. They enable us compare the reactivity of various monomers, the reactivity of active centres derived from various monomers, as well as reactivity changes caused by substituents and physical effects. The practical importance of copolymers is great. They form a considerable part of the industrially produced plastics. One of the advantages of copolymers is the chemical bond between their constituent units, i. e. separation of components cannot occur. 

The chemical composihons of the zeolites such as Si/Al ratio and the type of cation can significantly affect the performance of the zeolite/polymer mixed-matrix membranes. MiUer and coworkers discovered that low silica-to-alumina molar ratio non-zeolitic smaU-pore molecular sieves could be properly dispersed within a continuous polymer phase to form a mixed-matrix membrane without defects. The resulting mixed-matrix membranes exhibited more than 10% increase in selectivity relative to the corresponding pure polymer membranes for CO2/CH4, O2/N2 and CO2/N2 separations [48]. Recently, Li and coworkers proposed a new ion exchange treatment approach to change the physical and chemical adsorption properties of the penetrants in the zeolites that are used as the dispersed phase in the mixed-matrix membranes [56]. It was demonstrated that mixed-matrix membranes prepared from the AgA or CuA zeolite and polyethersulfone showed increased CO2/CH4 selectivity compared to the neat polyethersulfone membrane. They proposed that the selectivity enhancement is due to the reversible reaction between CO2 and the noble metal ions in zeolite A and the formation of a 7i-bonded complex. 

All these immobilization techniques run the risk of altering activity compared to the native enzyme. Improved activity is occasionally reported, but this is the exception. The immobilization techniques fisted above are in approximate order of loss in activity. Physical entrapment normally causes no change. Adsorption will distort the shape of the molecule compared to the native state. The effect of covalent bonding depends on the location of the bond relative to an active site. If remote from the site, it may have no effect. The chemical nature of the support can affect activity. Crosslinking requires two covalent attachments per enzyme molecule and is thus likely to distort the shape of the enzyme to the point that catalytic activity is lost. Such distortions are even more likely, but not inevitable, for coagulated or flocculated enzymes. On the positive side, immobilization tends to stabilize enzymes against deactivation. 

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