Chemical cross-link density

From a fit of Equation (10) to spatially resolved relaxation curves, images of the parameters A, B, T2, q M2 have been obtained [3- - 32]. Here A/(A + B) can be interpreted as the concentration of cross-links and B/(A + B) as the concentration of dangling chains. In addition to A/(A + B) also q M2 is related to the cross-link density in this model. In practice also T2 has been found to depend on cross-link density and subsequently strain, an effect which has been exploited in calibration of the image in Figure 7.6. Interestingly, carbon-black as an active filler has little effect on the relaxation times, but silicate filler has. Consequently the chemical cross-link density of carbon-black filled elastomers can be determined by NMR. The apparent insensitivity of NMR to the interaction of the network chains with carbon black filler particles is explained with paramagnetic impurities of carbon black, which lead to rapid relaxation of the NMR signal in the vicinity of the filler particles. 

Conventionally, cross-link density is determined by measurements of the modulus, the glass transition temperature T, and by solvent uptake in swelling experiments. In these procedures, the chemical cross-link density cannot be discriminated from network-... 

Overall cross-linking density data (Pt) of sulfur vulcanized (V)-EPTM and ENB-EPDM were obtained from stress-strain measurements performed under equilibrium conditions and u g the Mooney-Rivlin and Guth-Einstein uations. Chemical cross-linking density data (Pg) were obtained through the emiarical relationship found by... 

Elastomers are cross-linked macromolecules above the glass-transition temperature. They are entropy elastic and free of viscous flow. For most applications, the rubber is blended with filler material such as silicates and carbon black before vulcanization. Carbon black is an active filler which introduces physical cross-links of macromolecular chains in addition to the chemical cross-links formed during the vulcanization process. The chemical cross-link density is temperature independent, while the strength of the physical cross-links varies with temperature. 

The chemical cross-link density is a central quantity of interest in the elastomer industry, which, in a car tyre for instance, may vary in a well-defined fashion across the tyre tread. Both types of cross-links together determine the moduli of shear and elasticity, G and E, respectively. At small elongations A = L/Lq the tensile stress [Pg.440]

By studying chemical cross-link density, stress-relaxation behaviour, and the nature of peroxide reaction products attempts have been made to obtain data on the initiator efficiency and on the scission-cross-link ratio. Widely different results have however been quoted and there is disagreement on the methods of interpreting the data available (see Baldwin and VerStrate, 1972). It would appear that one cause of the disparity is due to physical entanglements. Typical figures for the scission cross-link ratio are of the order 0 5-0 7. 

It is curious to note, that sulfur-containing epoxy polymers the greatest deformability is reached at the greatest chemical cross-linking density realized at = 1.20 [49] (see Fig. 7.9). This nontrivial observation can be explained within the frameworks of the fractal analysis. As it is known [17],... 

Hie increase of absorption mention above in the intermediate region of the FIR spectrum with increasing chemical cross-linking density of PS is accompanied by the increase of the mechanical losses at 140-160 K, or in y-rdaxation [136] the same was observed radiation cross-linking of PS [137]. 

In addition to the above techniques, inverse gas chromatography, swelling experiments, tensile tests, mechanical analyses, and small-angle neutron scattering have been used to determine the cross-link density of cured networks (240—245). Si soHd-state nmr and chemical degradation methods have been used to characterize cured networks stmcturaHy (246). H- and H-nmr and spin echo experiments have been used to study the dynamics of cured sihcone networks (247—250). 

Ring S. In O-ring appHcations, the primary consideration is resistance to compression set. A fluorocarbon elastomer gum is chosen for O-ring apphcations based on its gum viscosity, cross-link density, cure system, and chemical resistance so that the best combination of processibiUty and use performance is obtained. Sample formulations for such uses are given in Table 4. 

The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex, depending on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

There has been interest, particularly in Japan, in the production of cross-linked low-density polyethylene foam. Some processes, such as the Furukawa process and the Hitachi process, use chemical cross-linking techniques whilst others, such as the Sekisui process, involve radiation cross-linking. 

Although the elastomer phase is essentially in particulate form, the tensile strength of the blend can be increased five-fold by increasing the cross-link density from zero to that conventionally used in vulcanisation processes, whilst tension set may be reduced by over two-thirds. Since the thermoplastic polyolefin phase may be completely extracted by boiling decalin or xylene, there is apparently no covalent chemical bonding of elastomer and thermoplastic phases. 

Liu, S.-H., Cheng, H.-H., Huang, S.-Y., Yiu, P.-C., and Chang, Y.-C. (2006) Studying the protein organization of the postsynaptic density by a novel solid phase- and chemical cross-linking-based technology. Mol. Cell. Proteomics 5, 1019-1032. 

Film dissolution behavior Cross-link density gel nature chemical vs. physical. 

Telechelic polymers rank among the oldest designed precursors. The position of reactive groups at the ends of a sequence of repeating units makes it possible to incorporate various chemical structures into the network (polyether, polyester, polyamide, aliphatic, cycloaliphatic or aromatic hydrocarbon, etc.). The cross-linking density can be controlled by the length of precursor chain and functionality of the crosslinker, by molar ratio of functional groups, or by addition of a monofunctional component. Formation of elastically inactive loops is usually weak. Typical polyurethane systems composed of a macromolecular triol and a diisocyanate are statistically simple and when different theories listed above are... 

A product of this type will have over 50% of its weight derived from maleic anhydride. This very high content of reactive double bonds will lead to a very brittle solid when it is cross-linked with styrene. Without further modification, this solid material will have very high tensile moduli, probably over 600 kpsi, but a very low tensile elongation, way below 1 %. Such a brittle material obviously has only very limited applications. Thus, for most general-purpose applications, it is necessary to incorporate some chemically inert components to soften the polymer backbone. This will reduce the cross-linking density and improve the physical properties of the cured solid. 

Compared with chemical cross-linking of PE, radiation curing produces a different product in many respects. The chemical cross-linking is done at temperatures near 125°C (257°F), where the polymer is in the molten state. Consequently, the cross-link density in the chemically cross-linked polyethylene is almost uniformly distributed, while there are relatively few cross-links in the crystalline fraction of the radiation cross-linked PE. The crystalline fraction of the radiation-processed polyethylene is greater than that in the chemically cured product. ... 

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