Cross-linking of chains

Polymers will be elastic at temperatures that are above the glass-transition temperature and below the liquiflcation temperature. Elasticity is generally improved by the light cross linking of chains. This increases the liquiflcation temperature. It also keeps the material from being permanently deformed when stretched, which is due to chains sliding past one another. Computational techniques can be used to predict the glass-transition and liquiflcation temperatures as described below. 

When the epoxide is to be used, a basic curing agent such as a tertiary amine, R3N, is added to cause the individual prepolymer chains to link together. This "cross-linking" of chains is simply a base-catalyzed epoxide... 

Some of these polymers are so hydrophiUc that they will dissolve in water. However, a judicious choice of cosubstituents, or the cross-linking of chains, can prevent dissolution. These materials will be considered in detail later under the heading of membranes and hydrogels. 

Figure 1. Cross linking of chain molecules of locust bean gum by borax (6)...

Elastomeric fibers can be made out of natural rubber. Rubber latex, which is a suspension of rubber spheres in water, is mixed with a vulcanizing agent, and the mixture is wet spun. The fibers from the spinneret are extruded into a coagulating bath and then vulcanized to produce the cross-linking of chains. Such fibers are used in woven and knitted fabrics, and more often than not they are mixed with other textile fibers to form fabrics for garments. Most natural rubber fibers, however, have been supplanted by synthetic elastomeric fibers, especially in the area of athletic wear. This is attributable to the poor dyeability, abrasion resis-... 

For PE, the relationship between grafting efficiency and cross-linking of chains can be controlled by varying the concentration ratio of the initiator to the monomer (3). For example, no cross-linking was detected when diethylmaleate (DEM) was grafted to PE as initiated by dicumyl peroxide (DCP) if the ratios (DCP) [DEM] < 0.09. A positive effect on grafting is caused by lower molecular weight of PO (lower melt viscosity) (3). 

Figure 5. Cross-Linking of Chain Molecules of Locust Bean Cum by Borax...

Molecular weights of these products vary - generally from a few thousand to about a million. Although most of the materials listed above are represented as linear polymers, cross-linking of chains can often be effected through suitable choice of side groups. Heavily cross-linked polymers are obtained by reacting tris (hydroxy methyl)phosphine oxide with melamine or urea. These are completely insoluble, infusible and incombustible. 

The cross-linking of chains by heating the polymer with sulfur, graphite, and other additives to modify its properties to produce a material suitable for specific purposes is part of the art of Vulcanization of rubber. The details of Vulcanization vary from polymer to polymer, depending upon, for example, the specific monomer(s) used in the polymerization, the extent of polymerization, and the properties desired. 

Cross-linking of chain-extented polyurethane -----NHCONH----------... 

By random cross-linking of chains carrying reactive functional groups (vulcanization, etc.). 

The smaller and simpler substances which act as starting materials for the synthesis of polymers are known as Monomers. Bifunctional monomers (those having only two reactive sites) polymerise to give linear or branched molecules. Presence of poly functional monomers during the process of polymerisation may cause cross-linking of chains and thus result in the formation of a three-dimensional network. 

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