Cross-link determination

A hydrogel placed in an excess of water will absorb the liquid until it reaches a maximum. This ability is typically reported as the percent water in a fully swollen gel. The hydrophilicity of the polymer and the degree of cross-linking determine the degree to which the gel will absorb. Some hydrogels contain as much as 99% water. An acrylic acid gel will have a higher equilibrium moisture than a polyvinyl alcohol gel. This characteristic is not unlike the hydrodynamic volume factor described above. 

FIGURE 4. Molecular model of the peptide backbone of silicatein a (221 amino acid residues, constrained by three intramolecular disulfide cross-links), determined as described in the text. Locations of the putative catalytically active serine (at position 26) and histidine (at position 165) in juxtaposition on both sides of the active-site (substrate binding) cleft are identified. These features are very similar to those in the homologous protease (hydrolytic enzyme)... 

Polyrotaxane networks 163 have been developed as prototype recyclable cross-linked polymers <2004AGE966>. These systems, which comprise a poly(crown ether)polyurethane and a bisammonium disulfide salt, are cross-linked via mechanical bonds ([3]rotaxane-like substructures) and are capable of undergoing reversible assembly and disassembly as they rely on thiol-disulfide reactions. Macroscopically, cross-linking determines gelation while de-cross-linking enables recovery of the starting materials. 

Determination of the extent of cross-linking Determination of the extent of non-cross-linked gelatin... 

Butyl rubber is a copolymer of isobutylene and I -2% isoprene. As a result the polymer chains contain internal double bonds which are expected to participate in cross-linking reactions. However, the overall molecular mass is expected to fall on irradiation due to the predominance of main-chain scission through the isobutylene units. Thus the radiation chemistry of the isoprene units within butyl rubber is accessible to study via solution NMR. In a comprehensive study Hill identified the primary free radical species by electron spin resonance spectroscopy at low temperatures, and the products of their subsequent reaction by C solution-state NMR. A number of new cross-link structures were identified and the mechanisms of cross-linking determined. Initial reaction involves addition of radicals either directly to the isoprene double bonds or to allyl radicals. Further addition of hydrogen atoms results in a mixture of fully-saturated and unsaturated cross-link structures. Cross-links of both H- and Y-type were identified and the yields of products agreed closely with the yields determined from measurement of changes in molecular weight on irradiation. 

Sephadex is one of the most popular materials for gel chromatography. It is widely used by biochemists for separating proteins, nucleic acids, enzymes, and carbohydrates. Most often, water or aqueous solutions of buffers are used as the moving phase. Chemically, Sephadex is a polymeric carbohydrate that has been cross-linked. The degree of cross-linking determines the size of the "holes" in the polymer matrix. In addition, the hydroxyl groups on the polymer can adsorb water, which causes the material to swell. As it expands, "holes" are created in the matrix. Several different gels are available from manufacturers, each with its own set of characteristics. 

As mentioned above, the net result of irradiation of PBD is cross-linking. G-values for cross-linking, determined as described immediately above, range from... 

A silicone rubber sample is cross-linked by exposure to gamma radiation. The average number of chain atoms between cross-links determined by a mechanical test is 850. What volume (cm ) will be occupied by 1 g of rubber when it is equilibrated in n-octane at 20°C The formula weight for dimethyl siloxane Si(CMe3),0 is 78. The solid rubber has a density of... 

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