Trifunctional cross-links

Collagen cross-links. Besides amide bonds between amino acids in the same a chain, bonds between amino acid side chains of different a chains can form "cross-links". These bonds originate from enzymatically-oxidized side chains of lysine and hydroxylysine residues. The oxidized residues react with other lysine and hydroxylysine residues, forming difunctional products. Reactions of such products with oxidized lysine or hydroxylysine yield trifunctional cross-links (Reiser et al., 1992). 

Collagens from different tissues do not necessarily contain the same cross-links. In dentin, cross-links occur between two collagen molecules with two or three peptide chains involved. The difunctional didehydro-hydroxylysinonorleucine and didehydro-dihydroxylysinonorleucine, and the trifunctional hydroxylysylpyridinoline and lysylpyridinoline have been demonstrated in human and animal dentin (fig. 1). Fredentin, on the other hand, contains more difunctional and fewer trifunctional cross-links than dentin (Linde and Robins, 1988 Walters and Eyre, 1983 Yamauchi et al., 1992). These cross-links connect the non-helical extension... 

Substituted aziridines have been used to form homobifunctional and trifunctional cross-linking agents, although their use has been limited (Ross, 1953 Alexander, 1954). The functional group has found use, however, in the design of the fluorescent probe dansyl aziridine (5-dimethylaminonaphthalene-2-sulfonyl aziridine) (Johnson etal., 1978 Grossman et al., 1981). 

Fig. 3 Application of the Doehlert experimental design to optimize a MIP for propranolol with respect to the type of cross-linker (EDMA or TRIM) and the degree of cross-linking, (a) Three-dimensional representation of response surfaces for the percentage of bound [3H]propanolol to the molecularly imprinted polymer (MIP) and the corresponding non-imprinted control polymer (NIP), (b) Contour plot of the function describing binding of [3H]propanolol to MIPs relative to the degree and the kind (bi or trifunctional) cross-linking. The values were corrected for non-specific binding to the non-imprinted control polymer. Adapted from [31] with kind permission from Springer Science + Business Media...

Phthalic acid and ethylene glycol are both bifunctional. Consequently, only linear polymers are produced from the reaction between these monomers. On the other hand, the reaction between phthalic acid and glycerol leads initially to molecules that are either linear, branched, or both. But since glycerol is trifunctional, cross-linking ultimately takes place between these molecules leading to an insoluble and infusible material. 

If a flexible foam is required, then longer, more flexible polyols and trifunctional cross-linking monomers are used, whereas higher-cross-link densities and short-chain polyols tend to form more rigid foams. 

The effect of chain scissions is thus an increasing function of Tg. Let us consider, for instance, an epoxy network based on the triglycidyl derivative of p-aminophenol (TGAP) cross-linked by diaminodiphenylmethane (DDM) in stoichiometric proportion. The characteristics are (Pascault etal., 2002) Tg = 494 K, F = 23 g.mol", Tgi = 293 K and j = 3 (trifunctional cross-links). Then, it follows that ... 

The interpretation of the constants 2Ci and 2C2 has absorbed much time the results are inconclusive (42). JtJs tempting but generally considered incorrect to equate 2Ci and nRT rflrl). The original derivation of Mooney (46) shows that 2C2 has to be finite, but it does not indicate its value relative to 2Ci. According to Flory (41), the ratio 2C2/2C1 is related to the looseness with which the cross-links are embedded within the structure. Trifunctional cross-links have larger values of ICJlCi than tetrafunctional cross-links, for example (58). 

SANS. The polymers were crosslinked with hi- and trifunctional cross-linking units that reacted via Michael addition. These nanoparticles were then studied as a potential nanocarrier for vitamin B9. The particles were loaded with vitamin Bg and the release was monitored by UV-Vis spectroscopy. Complete release of Bg was achieved in 5-6 hours in neutral pH water. The release exponent was calculated to be approximately 0.5, which suggests a Fickian diffusion mechanism for this SCNP based system. 

Fig. 1.6. Sketches of some simple, perfect networks having (a) tetrafunctional and (b) trifunctional cross-links (both of which are indicated by the dots) [25]. (Reproduced with permission copyright 1982, Rubber Chem. Technol.)...

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