Cross linkage

Vinyl groups strengthen the rigidity of the molecular structure by creating easier cross-linkage of molecules. 

Resin type and nominal percent cross-linkage Minimum wet capacity, mequiv mL Density (nominal), g mL Comments... 

Amberlite IRA-400 1.4 1.11 8% cross-linkage. Used for systems essentially free of organic materials. 

Amberlite IRA-402 1.3 1.07 Lower cross-linkage than IRA-400 better diffusion rate with large organic molecules. 

Dowex 50W-X16 2.4 0.85 Strongly acidic cation exchanger with S-DVB matrix and high cross linkage. 

Dg remains constant over a wide range of resin to liquid ratios. In a relatively short time, by simple equilibration of small known amounts of resin and solution followed by analysis of the phases, the distribution of solutes may be followed under many different sets of experimental conditions. Variables requiring investigation include the capacity and percent cross-linkage of resin, the type of resin itself, the temperature, and the concentration and pH of electrolyte in the equilibrating solution. 

Fig. 2. Total capacity vs cross-linkage for polystyreae sulfonic acid resin in the form where A and B correspond respectively to dry and wet weight capacity in meq/g of resin, and C represents wet volume capacity in meq/mL of wet setded resin.

Prevulcanised compounds, which have undergone the necessary cross-linkage while in the liquid state, are becoming more popular in industry. Articles made from these can often be merely dried to be adequate for a purpose, but to achieve optimum physical properties it is usually necessary to give the articles a short, low temperature (ca 80°C) cure in an oven. 

Properties and Structure. a -Acid glycoprotein (a -AGP) has a molecular mass of about 41,000 and consists of a peptide chain having 181 amino acid residues and five carbohydrate units (14,15). Two cystine disulfide cross-linkages connect residues 5 and 147 and residues 72 and 164. The carbohydrate units comprise 45% of the molecule and contain siaUc acid, hexosamine, and neutral hexoses. In phosphate buffer the isoelectric point of the... 

The use of nylon-11 for powder coatings or dry coatings (qv) has been developed in response to a growing concern for the environment (44) (see Coating PROCESSES, powder technology). Electrostatic deposition allows thin films to be appHed to metal substrates. Once the powder is appHed, it must be melted and coalesced into a continuous plastic film. Eorced draft or irradiant ovens are used for fusion, and because no polymerization or cross-linkage are required for curing, coated objects can be processed quickly and air-cooled (45). 

The reactions indicated above in fact lead only to chain extension. In practice, however, polyamines are used so that the number of active hydrogen atoms exceeds two and so cross-linkage occurs. 

Classical gels had a low degree of cross-linkage and were of a large particle size. This resulted in that modest flow rates could only be applied and the separation time was typically 10 hr, which at that time was perfectly acceptable, keeping in mind that preparation of the column could take up to 2 days or more. After the introduction of Sephadex, new materials have been introduced continuously on the market, and still, 30 years after the introduction of the first commercial material, new media are still introduced, also from the originators of Sephadex. What are the driving forces behind this development and what are the features of these new media ... 

Figure 6.7 Formation of cross-linkage between individual peptide chains in the peptidoglycan layer of S. aureus.

Cross-linkage - bifunctional agents may form covalent bonds with each of two adjacent guanine residues and such inter-strand cross-links will lead to inhibition of DNA replication and transcription. Intra-strand and DNA-protein cross-links may also be formed ... 

An example of a nonlinear polymer derived by cross-linking an initially linear polymer is afforded by vulcanized natural rubber. In the usual vulcanization procedure involving the use of sulfur and accelerators, various types of cross-linkages may be introduced between occasional units (about one in a hundred) of the polyisoprene chains. Some of these bonds are indicated to be of the following type ... 

Polymers of dienes such as butadiene frequently contain a substantial portion of gel which will not dissolve in a good solvent, though it may swell to a volume 20 to 100 or more times that of the polymer itself. This gel, which may comprise up to 90 percent or more of the polymer, consists of a space-network structure formed as a result of a very few cross-linkages provided by occasional (perhaps 1 in 1000 or less) diene units both double bonds of which have entered into the polymerization (see Chap. VI). 

Ic. Cross-Linking of Polymer Chains.—Formation of chemical bonds between linear polymer molecules, commonly referred to as cross-linking, also may lead to the formation of infinite networks. Vulcanization of rubber is the most prominent example of a process of this sort. Through the action of sulfur, accelerators, and other ingredients present in the vulcanization recipe, sulfide cross-linkages are created by a mechanism not fully understood (see Chap. XI). Vulcanized rubbers, being typical network structures, are insoluble in all solvents which do not disrupt the chemical structure, and they do not undergo appreciable plastic, or viscous, flow. 

Hydroxyl containing polymers may be cross-linked with diisocyanates. Fordyce and Ferry cross-linked styrene-maleic anhydride copolymers through the action of glycols. The copolymerization of divinyl with vinyl monomers may be looked upon as a method of cross-linking chain polymers. The cross-linkages are introduced simultaneously with the growth of the linear polymer chains, rather than afterwards, but this difference is secondary. 

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