Cross-linking agent effect

On curing, amino resins not only react with the nucleophilic sites (hydroxyl, carboxyl, amide) on the other film formers in the formulation, but also self-condense to some extent. Highly alkylated amino resins have less tendency to self-condense (33,34) and are therefore effective cross-linking agents, but may require the addition of a strong acid catalyst to obtain acceptable cure even at bake temperatures of 120—177°C. 

An excellent review of composite RO and nanofiltration (NE) membranes is available (8). These thin-fHm, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-fHm composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-kniniscible solvent. 

Figure 16.14. Effect of cross-linking agent on impact strength of 75/25 styrene-acrylonitrile/ nitrile...

An aqueous ethanol solution of acrylamide, 2,2 -methylenebisacrylamide as cross-linked agent and third acrylamide derivative, is dispersed in an n-alkane. Then three monomers are polymerized to spherical porous gels. The effect of the composition of the third monomer on the exclusion limits of the gel in size-exclusion chromatography has been investigated (82). 

Fig. 29. Effect of the extent of crosslinking of sulfonated cation exchangers (quantity of cross-linking agent) on diffusion coefficient of tetracycline in sorbent grains 1) SDMDMA, 2) SHMDMA, i) SEDMA, 4) Dowex-50W...

Cross-linking agents have been proposed for the improvement of chitin fibres in the wet state. Epichlorohydrin is a convenient base-catalysed crosslinker to be used in 0.067 M NaOH (pH 10) at 40 °C. The wet strength of the fibres was considerably improved, whereas cross-hnking had neghgible effect on the dry fibre properties. Of course, the more extended the chemical modification, the more unpredictable the biochemical characteristics and effects in vivo. Every modified chitin or modified chitosan fibre should be studied in terms of biocompatibiUty, biodegradabiUty and overall effects on the wounded tissues. 

Standard alkylating and cross-linking agents such as dimethylsulfate or TV-mustards, respectively, have only one opportunity to partition between various nucleophiles since their reactions are irreversible. In contrast, QMs have the potential to partition between nucleophiles multiple times as long as the resulting adducts are formed reversibly. Continual capture and release of QMs consequently can extend their effective lifetime almost indefinitely and is ultimately limited by only the competitiveness of possible irreversible reactions. For DNA, the strongest nucleophiles act reversibly so terminal quenching remains an infrequent event. 

Mentzer Jr., W.C., and Lubin, B.H. (1979) The effect of cross-linking agents on red-cell shape. Semin. Hematol. 16, 115-127. 

Singh, V., Mavila, A.K., and Kar, S.K. (1993) Comparison of the cytotoxic effect of hormonotoxins prepared with the use of heterobifunctional cross-linking agents N-succinimidyl 3-(2-pyridyldithio)propionate and N-succinimidyl 6-[3-(2-pyridyldithio)propionamido]-hexanoate. Bioconjugate Chem. 4, 473M82. 

Polyacrylamide shows many advantages over starch gel as a medium for high resolution electrophoresis and because of its synthetic nature its pore size can be more easily controlled. The gel is formed by the polymerization of the two monomers, acrylamide and a cross-linking agent, N, iV-methylene-bis-acrylamide (Figure 3.26). The proportion of the two monomers and not their total concentration is the major factor in determining the pore size, the latter having more effect on the elasticity and... 

Yasuda, R. and Minato, K. (1995). Chemical modification of wood by non-formaldehyde cross-linking agents. Part 3. Mechanism of dimensional stabilization by glyoxal treatment and effect of the addition of glycol. Wood Science and Technology, 29(4), 243-251. 

It is noteworthy that, unlike the previously described induces of PTPC opening, GD3 is able to exert its effect on mitochondria even in the presence of submicromolar calcium concentration (Kristal and Brown, 1999a), a property shared only by the most potent PT inducers, such as the thiol cross-linking agent phenylarsine oxide and by the peptide mastoparan. This low calcium requirement is important because intracellular calcium concentration during CD95-mediated apoptosis was not described to rise above 0.4 pM (Oshimi and Miyazaki, 1995). 

Self-Associative Polymer Network. In self-associative polymer networks (often called one component systems), the hydrogen bonding recognition units that are covalently attached to the polymer backbone have an appreciable tendency for self-association, that is, self-dimerize, which leads to interchain cross-linking of the polymers. As a result, the system is inherently cross-linked and does not require any external cross-linking agents for network formation (Fig. 5.11a). Because the cross-linking is based on dimerization phenomena, to achieve effective... 

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