Radiation cross-linked experiments

Experiments on cross-linking by irradiahon have revealed that in many cases the cross-link densities equivalent to those obtained by conventional methods of cross-linking require relatively high radiation doses. Therefore, a great deal of experimental work has been done on the use of additives, which promote cross-linking by irradiation. Such compounds are called radiation cross-link promoters or prorads. 

The pulse mode experiments were conducted by placing the sample between the quartz plates and exposing it to 30 second bursts of UV radiation. Dynamic rheological tests were then applied to the sample and this process was repeated until the sample had passed its gel point and became highly cross-linked. 

For an un-cross-linked material that undergoes predominantly cross-linking when exposed to ionizing radiation, solvent extraction experiments reveal that at certain absorbed dose (the critical dose), a percentage of the material, is converted into insoluble gel. Beyond that point, the percentage of gel increases as a function of irradiation dose. In general, both the degrees of... 

In the case of rubber this cross linking is called vulcanization. Experiments conducted in 1946 at the Clinton Pile (D6) demonstrated a slight curing action of pile irradiation on natural rubber. Charlesby has shown that the degree of cross linking is directly proportional to radiation dose. Approximately 50 X 106 r. produces one cross link/90 isoprene units. 

Comparable experiments with the nucleic acids have confirmed that cycloaddition of their pyrimidine bases also occurs with ultraviolet light and effectively cross-links the chains, a process obviously quite inimical to the functioning of the DNA (see Section 25-13B). A remarkable and not well understood aspect of photobiology is the repair and defense mechanism both plants and animals possess to minimize the damaging effects of radiation. 

Sol-gel analysis is an important tool for distinguishing between polymers that can be cross-linked by irradiation and polymers that are not cross-linkable by radiation techniques. These experiments are typically done for wall-to-wall gels and offer the possibility to determine the irradiation conditions for microgel formation. Charlesby and Pinner first obtained a simple expression relating the soluble part of a polymer sample, the sol fraction (5) or sol content (in contrast to the insoluble part, the gel fraction (g) or gel content) to the absorbed dose D of radiation [10, 12],... 

Experiments using no emulsifier were conducted in tbe same stainless steel autoclave equipment described above (Machi et nf.. 1975). Stable latices were obtained, believed to be achieved by hydroxyl end groups and adsorbed hydroxyl ions. As with a number of the experiments with emulsifiers the polyethylene had a considerable cross-linked gel content. Finally, the same group of workers studied tbe radiation-induced emulsion polymerization of ethylene in a flow system (Kodama et al, 1974). Both potassium inyristate and ammonium perfluorooctanoate were used as emulsifiers. At longer residence times (above 0.2 hr) the rate of polymerization was essentially constant. As with the batch system it was assumed that the number of particles remained constant. In this region the rate was found to be proportional to tbe 0.3 power of the potassium myristate concentration and the 0.5 power of the dose rate, not too different from the batch systems. The kinetics was developed and estimates of tbe propagation rate constants obtained. Despite other similarities between the two systems, these were quite different, however, from those extracted from the batch experiment. 

An advantage of radiochemical induced cross-linking is the possibility to combine polymers with different properties. For that purpose, the polymers were separately dissolved in water. The solutions were mixed and irradiated. (Gottlieb et al. 2005) describes the synthesis of temperature-sensitive hydrogel blends of PVME (as thermo-sensitive polymer) and the radiation-cross-linkable polymer PVP (a polymer that is applied in pharmaceutics). The experiments show that the gelation dose of the blend is between the gelation doses of the two pure polymers. 

The two main classes of NMR experiment applied to the study of the radiation chemistry of polymers are concerned with measurement of either the chemical structure of the polymer after irradiation, or the changes in the physical properties brought about by cross-linking or scission. In Sections 3-5 of this review the use of NMR to identify changes in chemical structure is discussed. The changes in the NMR line shape and relaxation times induced by radiation, and the use of these methods to determine radiochemical yields, are discussed in Section 2. 

Figure 10.8 Continuous and intermittent stress relaxation at 130°C on radiation cured c s-1,4-poiybutadiene in the presence of air. The experiment separates oxidative cross-linking and chain scission. The quantity /(() is the time-dependent retractive force. From A. V. Tobolsky, Properties and Structure of Polymers, Wiley, New York, 1960.

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