Cross-linking efficiency

The general properties of the resins are much as to be expected. They have very good heat resistance but are mechanically much weaker than the corresponding organic cross-linked materials. This weakness may be ascribed to the tendency of the polymers to form ring structures with consequent low cross-linking efficiency and also to the low intermolecular forces. 

Of the polymers that can be cross-linked, the cross-link efficiency varies considerably. In general the relative efficiency of peroxides vulcanization of polymers [50,51] follows as ... 

The effect of antioxidants such as hindered phenohcs, secondary amine, and thioester on the radiation cross-linking efficiency of LDPE has been reported [260]. Amount of cross-linking at a given dose decreases with aU the antioxidants, the thioester being the most effective. IR absorption spectroscopy has been used to demonstrate dose-rate dependence of trani -vinylene unsaturation in irradiated Marlex 50 PE [261]. When the irradiated polymer is stored in vacuum a decrease is observed in trani-vinylene absorbance over a period of several weeks. After high dose-rate irradiation the decay is preceded by an initial increase. These phenomena have been ascribed to the reaction of trapped radicals. 

The cross-linking efficiency of the more branched polychloroprene latex (mercaptan modified) has been found to be higher than the less branched sulfur-modified one. The latex dispersion is found to display higher rate of cross-linking than the coagulated and subsequently dried mbber films [386] due to higher concentration of radical in the former. 

For optimum cross-linking efficiency, a combination of accelerators is used. Table 6 shows the increase in load-bearing capability of vulcanizates based on different rubbers as the ratio of two accelerators is changed (14). The term 300% modulus represents the strain at 300% stress and is not a true modulus because rubber gives nonlinear stress—strain behavior. For polymers with primary allylic carbon atoms, the use of two accelerators gives significandy higher 300% modulus than either accelerator used alone. When the mbber polymer consists of secondary allylic carbon atoms, the modulus is level until the sulfenamide OBTS becomes the principle accelerator. 

Peroxides decompose when heated to produce active free radicals which in turn react with the mbber to produce cross-links. The rate of peroxide cure is controlled by temperature and selection of the specific peroxide, based on half-life considerations (see Initiators, free-radical Peroxy compounds, organic). Although some chemicals, such as bismaleimides, triallyl isocyanurate, and diallyl phthalate, act as coagents in peroxide cures, they are not vulcanization accelerators. Instead they act to improve cross-link efficiency (cross-linking vs scission), but not rate of cross-link formation. 

Studies performed on elastomers have shown that the cross-linking efficiency is relatively low. G(X) values of 1 and approximately 3 were found for natural rubber and polybutadiene, respectively, when irradiated at... 

The copolymer of ethylene and trichlorofluoroethylene (ECTFE) behaves upon irradiation like ETFE, including improvement of cross-linking efficiency with prorads. 

Ionizing radiation on poly(alkyl acrylates) results in cross-linking efficiencies, which are sensitive to the structure of the alkyl ester group."... 

We note from Table VIII a strong interest in halogenated resists, particularly those substituted with chlorine. The addition of chlorine to the aromatic structure of polystyrene has a marked effect on cross-linking efficiency. Monodisperse polystyrene, for example, has a sensitivity on the order of 50 p C/cm2, yet with as little as 20% chloromethyl groups substituted on the ring, the sensitivity is improved to 2 C/cm2 for comparable molecular weight and distribution. 

Cobalt-60 y-Radiolysis Studies. Samples of PIPTBK powder were sealed under vacuum and exposed to 60Co 7-radiation at the National Bureau of Standards in Washington D.C. The samples received 0, 6, 12, and 20 Mrads and the number-average and weight-average molecular weight of each polymer, relative to polystyrene, were determined in THF by GPC. The scission efficiency and the cross-linking efficiency were determined from... 

In Equations 4 and 5, G (s) is the scission efficiency G 0c) is the cross-linking efficiency Mn and Mw are the number-average and weight-average molecular weights after exposure to a dose of D (Mn)0 and (Mw)0 are the initial... 

Soxhlet extraction, with toluene as solvent, of the soluble fraction from the cross-linked gels produced by irradiation, was used as a method to determine the radiation cross-linking efficiency, G(X), for the homopolymers and blends under investigation. Samples were extracted to constant weight and two or three replicate extractions were carried out. 

Intermolecular cross- linking efficiency is remarkably high, compared with other bifunctional cross- linking reagents (data not shown). 

This type of reaction inhibits the repair by endonuclease III from E. coli, and Lys-120 is responsible for this cross-linking reaction. Under the conditions of these experiments, the cross-linking efficiency is 20%. It is expected that such cross-linking reactions are more general and that more examples will be detected. 

Narayanaswamy, M. et al. An assay combining high-performance liquid chromatography and mass spectrometry to measure DNA interstrand cross-linking efficiency in oligonucleotides of varying sequences. Ana/. Biochem. 2008, 374,173-181. 

ECTFE behaves upon irradiation like ETFE, including improvement of cross-linking efficiency wifh fhe addition of prorads. Irradiation at room temperature followed by heat treatment at and above 435 K in nitrogen for 20 min is reported to be most effective [25],... 

In Fig, 11 it is evident that an EPDM containing 0.54 moI/1 of ENBexhibits the same cross-linking efficiency as an EPTM containing 0.05 mol/1 of (III). 

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