Ionizing radiation initiation

The use of high-energy radiation in polymer chemistry is one of the major industrial applications of radiation. Main advantages of ionizing radiation initiation include (Charlesby 1960, 1987 Chapiro 1962) the following ... 

The rate coefficient of propagation, fcp, strongly depends on the nature of the monomer (Brandrup et al. 1999). O Table 23.6 shows the fcp and fct rate coefficients for a few monomers that are often polymerized by ionizing radiation initiation. 

Copolymerization is effected by suspension or emulsion techniques under such conditions that tetrafluoroethylene, but not ethylene, may homopolymerize. Bulk polymerization is not commercially feasible, because of heat-transfer limitations and explosion hazard of the comonomer mixture. Polymerizations typically take place below 100°C and 5 MPa (50 atm). Initiators include peroxides, redox systems (10), free-radical sources (11), and ionizing radiation (12). 

High temperatures, eleetromagnetie radiation, ionizing radiation, or highly reaetive ehemieal initiators often generate aetive eenters. 

Evaluate the initial direet reading instrument survey data for the presenee of eombustible gases, organie and inorganie gases, partieu-lates, vapors, and ionizing radiation. 

The growth in popularity of radiation as the initiating system for grafting arises from the improvement in availability and cost of ionizing radiation. This is due to the introduction of more powerful nuclear reactors. Apart from its inexpensiveness, radiation is a very convenient method for graft initiation because it allows a considerable degree of control to be exercised over such structural... 

Another mode of initiation of polymerization being vigorously studied in numerous laboratories involves the use of ionizing radiation. In the primary process a molecule (not necessarily the monomer) is ionized and thus a positive ion and an electron are formed. The latter is eventually captured by another molecule yielding a negative ion. On recombination of these ions, enough... 

Total Ionization—The total electric charge of one sign on the ions produced by radiation in the process of losing its kinetic energy. For a given gas, the total ionization is closely proportional to the initial ionization and is nearly independent of the nature of the ionizing radiation. It is frequently used as a measure of absorption of radiation energy. 

REMPI provides high detection sensitivity for free radicals similar to that of LIF.4 In the REMPI method, one or more photons typically from a focused laser radiation initially excite the free radicals to an intermediate excited electronic state. The radicals are further excited and ionized by another photon in the same laser pulse (one-color REMPI) or by a photon of different wavelength from another laser beam overlapping in space and time... 

Protonic acids are less suitable because the conjugate base is too active a nucleophile. HC1, for example, will not initiate polymerization because chloride ion adds immediately to the carbenium ion before the latter can propagate. Other initiators which have been studied include electroinitiation, photoinitiation and ionizing radiation. 

From a chemist s viewpoint, the most important act of ionizing radiation (usually X-rays, y-rays or high energy electrons) is electron ejection. Initially the ejected electrons have sufficient energy to eject further electrons on interaction with other molecules, but the electrons ultimately become thermalised and then are able to interact "chemically". We consider first various reaction pathways for these electrons, and then consider the fate of the "hole" centres created by electron ejection. [We refer to electron-gain and electron-loss centres rather than to radical-anions and -cations since, of course, the substrate may comprise ions rather than neutral molecules. 

The quantity k3 may be considered as an instrumental constant to be determined in a blank experiment—that is, without added solute. In this case, the current is given by I(t)/I(0) = (1 - vt/d) exp( - k3 t), from which k can be determined. With the solute added, the current initially decays exponentially (fast decay) from which is determined h + k2 + k3, while the ratio of the initial plateau to the initial current gives k2/(k] + k2 + k ). The detachment rate k2 is now obtained from the last two numbers, and then the attachment rate fe, is also obtained since k3 is already predetermined. In short, both attachment (kj and detachment (k2) rates are obtainable from the time dependence of the cell current following a brief pulse of ionizing radiation. 

Figure 32.6 Chernobyl air plume behavior and reported initial arrival times of detectable radioactivity. Plume A originated from Chernobyl on April 26, 1986 Plume B on April 27-28 and Plume C on April 29-30. The numbers indicate initial arrival times 1, April 26 2, April 27 3, April 28 4, April 29 5, April 30 6, May 1 7, May 2 and 8, May 3. (From United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). 1988. Sources, Effects and Risks of Ionizing Radiation. United Nations, New York. 647 pp.)... 

The most probable fate of a photon with an energy higher than the binding energy of an encountered electron is photoelectric absorption, in which the photon transfers its energy to the electron and photon existence ends. As with ionization from any process, secondary radiations initiated by the photoelectron produce additional excitation of orbital electrons. 

Ionizing radiation. Further studies on the effects of ionizing radiation such as 60Co y-ray radiation and ion beams on polysilanes have been reported.295 304-306 Both positive and negative resist properties can be shown, depending on the side-chain type, branch density, and the presence or absence of radical initiators. 

The theory of polymerizations by ionizing radiations gives the rate of initiation Rt (ions cm"3 s 1) as... 

The DP of polymers formed by ionizing radiations is more difficult to interpret than that of polymers formed by chemical initiation, because after quite a low degree of conversion the degradation of the polymers by the radiation may become significant (Stannett et al. 1964 Deffieux et al. 1982). It is not known to what extent this fragmentation and to... 

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