Physical cross-linking

Characteristic Physical adsorption Ionic binding Chelation Covalent bonding Cross-linking Physical entrapment Membrane entrapment Whole cells... 

Both chemical and physical methods may be used to immobilize biocatalysts while retaining or modifying their activity, selectivity, or stability. Among the techniques used for immobilization of enzymes are physical adsorption, covalent bonding, ionic binding, chelation, cross-linking, physical entrapment, microencapsulation, and retention in permselective membrane reactors. The mode of immobilization employed for a particular application depends not only on the specific choice of enzyme and support, but also on the constraints imposed by the microenvironment associated with the application. 

Demonstration of the fact that presented model describes strain electromagnetic anisotropy realization in all densely cross-linked physical states is showed in Figurel. 

T. Nicolai, F. Prochazka, and D. Durand, Comparison of Polymer Dynamics between Entanglements and Covalent Cross-links Physical Review Letters 82,863-866,1999. 

The ideal, classical, rabbeiy properties are displayed by pofymers cross-linked by valence bonds (main-chain bonds or sulphur bridges, see Introduction, Figure 0.1) these are termed chemical cross-links. Physical cross-links are also important in many useful rubbery materials. In pl ical cross-linking the chains are not chemically attached one to another, but are effectively pinned together in one of three ways ... 

An ideal robber consists of flexible cross-linked polymer chains undergoing violent liquid-like motions. No matter what the type of cross-linking, physical or chemical, the elastomers all have this in common the macromolecules between cross-links undergo extremely rapid molecular movement. 

Interpenetrating Networks. IPNs consist of an interwoven matrix of two polymers. A typical method for producing these alloys involves cross-linking one of the monomers in the presence of the other. The need for a chemical similarity between the two types of molecules is thus reduced, because cross-linking physically traps one with the other. The result is a structure composed of two different intertwined plastics, each retaining its own physical characteristics. 

PZDMA. Because of the large numbers of ion pairs in PZDMA molecules and the strong electrostatic interaction between ion pairs, aggregates consisting of several pairs called multiplets could be formed, restricting the mobility of adjacent polymer chains [47]. Moreover, the PZDMA also can form a filler network, which is similar to a black carbon network. Thus, a considerable PZDMA and cross-links of NR form a relatively developed primary network (cOTitaining covalent crosslink points, ionic cross-links, physical adsorption and filler-filler joints). As a result, the fact that the 1 min-cured sample with 20 phr ZDMA show the highest G and S is a result of the developed primary network . 

Figure 2 Influence of cross-linking physical (thermal or UV treatments) or chemical (pre- or post-treatments by formaldehyde) treatments on the mechanical properties (i.e. strength and deformation at break) of wheat gluten...

A polymer gel is a network of flexible cross-linked chains. Structures of this type can be obtained by chemical or physical processes. Some gels are cross-linked chemically by covalent bonds (chemical gel), whereas other gels are cross-linked physically by weak forces, such as hydrogen bonds, van der Waals forces, or hydrophobic and ionic interactions (physical gel). Physical gelation processes are usually reversible and are called sol-gel transitions. The final gel structures and properties are sensitive to the preparation meth-... 

No polymer is ever 100% crystalline at best, patches of crystallinity are present in an otherwise amorphous matrix. In some ways, the presence of these domains of crystallinity is equivalent to cross-links, since different chains loop in and out of the same crystal. Although there are similarities in the mechanical behavior of chemically cross-linked and partially crystalline polymers, a significant difference is that the former are irreversibly bonded while the latter are reversible through changes of temperature. Materials in which chemical cross-linking is responsible for the mechanical properties are called thermosetting those in which this kind of physical cross-linking operates, thermoplastic. 

The polymeric products can be made to vary widely in physical properties through controlled variation in the ratios of monomers employed in thek preparation, cross-linking, and control of molecular weight. They share common quaHties of high resistance to chemical and environmental attack, excellent clarity, and attractive strength properties (see Acrylic ester polymers). In addition to acryHc acid itself, methyl, ethyl, butyl, isobutyl, and 2-ethylhexyl acrylates are manufactured on a large scale and are available in better than 98—99% purity (4). They usually contain 10—200 ppm of hydroquinone monomethyl ether as polymerization inhibitor. 

The physical properties of polyurethane adhesives result from a special form of phase separation which occurs in the cross-linked polyurethane stmcture. The urethane portions of polyurethanes tend to separate from the polyol portion of the resin, providing good shear strength, good low temperature flexibiUty, and high peel strength. Catalysts such as dibutyltin dilaurate [77-58-7], stannous octoate [1912-83-0], l,4-diazabicyclo[2.2.2]octane... 

The physical properties of the final foam can be varied broadly by controlling the degree of cross-linking in the final polymer as well as the stmcture... 

The physical properties of any polyisoprene depend not only on the microstmctural features but also on macro features such as molecular weight, crystallinity, linearity or branching of the polymer chains, and degree of cross-linking. For a polymer to be capable of crystallization, it must have long sequences where the stmcture is completely stereoregular. These stereoregular sequences must be linear stmctures composed exclusively of 1,4-, 1,2-, or 3,4-isoprene units. If the units are 1,4- then they must be either all cis or all trans. If 1,2- or 3,4- units are involved, they must be either syndiotactic or isotactic. In all cases, the monomer units must be linked in the head-to-tail manner (85). 

The tailoring of PE properties in commercial processes is achieved mostiy by controlling the density, molecular weight, MWD, or by cross-linking. Successful control of all reaction parameters enables the manufacture of a large family of PE products with considerable differences in physical properties, such as the softening temperatures, stiffness, hardness, clarity, impact, and tear strength. 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

Polyolefins. Interest has been shown in the plasticization of polyolefins (5) but plasticizer use generally results in a reduction of physical properties (12), and compatibiHty can be achieved only up to 2 wt %. Most polyolefins give adequate physical properties without plasticization. There has been use of plasticizers with polypropylene to improve its elongation at break (7) although the addition of plasticizer can lower T, room temperature strength, and flow temperature. This can be overcome by simultaneous plasticization (ca 15 wt % level) and cross-linking. Plasticizers used include DOA. 

As the quinone stabilizer is consumed, the peroxy radicals initiate the addition chain propagation reactions through the formation of styryl radicals. In dilute solutions, the reaction between styrene and fumarate ester foUows an alternating sequence. However, in concentrated resin solutions, the alternating addition reaction is impeded at the onset of the physical gel. The Hquid resin forms an intractable gel when only 2% of the fumarate unsaturation is cross-linked with styrene. The gel is initiated through small micelles (12) that form the nuclei for the expansion of the cross-linked network. 

Monomers such as aUyl methacrylate and diaUyl maleate have appUcations as cross-linking and branching agents selected especiaUy for the different reactivities of their double bonds (90) some physical properties are given in Table 8. These esters are colorless Uquids soluble in most organic Uquids but htde soluble in water DAM and DAF have pungent odors and are skin irritants. 

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