Radiation cations

In doped PMMA, solute ions were efficiently formed on exposure to ionizing radiation cations were formed for most cases, but anions were formed only when the solute had a high electron affinity [86]. Kira et al. irradiated solid PMMA containing excess biphenyl and a small amount of a second solute by electron-pulses and observed the absorption spectrum of the biphenyl radical-cation produced by the following reactions [109] ... 

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

The y-radiation-induced polymerization requires an extremely high purity reaction system. Trace amounts of water can terminate a cationic reaction and inhibit polymerization. Organic bases such as ammonia and trimethylamine also inhibit polymerization. The y-radiation-induced polymerization of a rigorously dried D obeys the Hayashi-WilHams equation for completely pure systems (150). 

Sodium nitrite has been synthesized by a number of chemical reactions involving the reduction of sodium nitrate [7631-99-4] NaNO. These include exposure to heat, light, and ionizing radiation (2), addition of lead metal to fused sodium nitrate at 400—450°C (2), reaction of the nitrate in the presence of sodium ferrate and nitric oxide at - 400° C (2), contacting molten sodium nitrate with hydrogen (7), and electrolytic reduction of sodium nitrate in a cell having a cation-exchange membrane, rhodium-plated titanium anode, and lead cathode (8). 

Complexation of the initiator and/or modification with cocatalysts or activators affords greater polymerization activity (11). Many of the patented processes for commercially available polymers such as poly(MVE) employ BE etherate (12), although vinyl ethers can be polymerized with a variety of acidic compounds, even those unable to initiate other cationic polymerizations of less reactive monomers such as isobutene. Examples are protonic acids (13), Ziegler-Natta catalysts (14), and actinic radiation (15,16). 

The diffusion coefficients of cations in metal oxides are usually measured through the use of radioactive isotopes. Because of the friable nature of oxides it is exU emely difficult to use the sectioning technique employed for metal samples. The need for this can be avoided by the application of radioisotopes which emit radiation having a well established absorption law in matter. Isotopes which emit y radiation are very useful when the cation has a relatively high diffusion coefficient because of the long-range peneU ation of y rays. The absorption law is... 

Figure 14-9 also shows a flowchart for analysis of wet and dry precipitation. The process involves weight determinations, followed by pH and conductivity measurements, and finally chemical analysis for anions and cations. The pH measurements are made with a well-calibrated pH meter, with extreme care taken to avoid contaminating the sample. The metal ions Ca, Mg, Na, and are determined by flame photometry, which involves absorption of radiation by metal ions in a hot flame. Ammorda and the anions Cl, S04 , NO3 , and P04 are measured by automated colorimetric techniques. 

Radical cations can be derived from aromatic hydrocarbons or alkenes by one-electron oxidation. Antimony trichloride and pentachloride are among the chemical oxidants that have been used. Photodissociation or y-radiation can generate radical cations from aromatic hydrocarbons. Most radical cations derived from hydrocarbons have limited stability, but EPR spectral parameters have permitted structural characterization. The radical cations can be generated electrochemically, and some oxidation potentials are included in Table 12.1. The potentials correlate with the HOMO levels of the hydrocarbons. The higher the HOMO, the more easily oxidized is the hydrocarbon. 

Radiation curing of epoxies with cationic initiators is well known [20—28]. UV-visible light has been the predominant radiation source the process has been limited to thin coatings due to the low penetration of the visible-UV light [22,23], Thermal and mechanical properties of these materials are low and the curing is incomplete. Several studies have shown that commercially available epoxies with various cationic initiators can be polymerized with EB curing [20,29-34]. 

An analogous mechanism should also produce polymers on irradiation of epoxies. Crivello s recent mechanistic suggestions [29] are consistent with the mechanisms given above. One can conclude that radiation-induced polymerization of epoxies can proceed via several mechanisms. However, further work is needed to determine the relative contributions of the different mechanisms, which might vary from one epoxy to another. As part of the Interfacial Properties of Electron Beam Cured Composites CRADA [37], an in-depth study of the curing mechanism for the cationic-initiated epoxy polymerization is being undertaken. 

Janke, C.J., Dorsey, G.F., Havens, S.J. and Lopata, V.J., Toughened epoxy resins eured by eleetron beam radiation. 28th International SAMPE Technical Conference, S71, 1996. Janke, C.J., Electron beam curable cationic epoxy resin systems and composites. 1st Annual Electron Beam Curing of Composites Workshop, September 18-19, Oak Ridge, TN, Ref. ID, 1996. 

Poly(acrylamide-diallylethylamine-HCl) (cationic polyacrylamide pAM-HCl) was prepared by gamma radiation-initiated copolymerization of acrylamide with di-allylethylamine-HCI in aqueous solution at the optimum composition for copolymerization of acrylamide with diallylamine derivatives [61]. 

T. Siyam, "Studies on Gamma Radiation-Induced Preparation of Cationic and Amphoteric Copolymers. Ph.D. Thesis, Fac. Sci. Cairo Univ., Cairo, Egypt... 

To avoid homopolymer formation, it is necessary to ensure true molecular contact between the monomer and the polymer. Even if this is initially established, it needs to be maintained during the radiation treatment while the monomer is undergoing conversion. Several methods are used for minimizing the homopolymer formation. These include the addition of metal cations, such as Cu(II) and Fe(II). However, by this metal ion technique, both grafting and homopolymerization are suppressed to a great extent, thus permitting reasonable yield of graft with little homopolymer contamination by the proper selection of the optimum concentration of the inhibitor [83,90,91]. 

Several authors have discussed the ion exchange potentials and membrane properties of grafted cellulose [135,136]. Radiation grafting of anionic and cationic monomers to impart ion exchange properties to polymer films and other structures is rather promising. Thus, grafting of acrylamide and acrylic acid onto polyethylene, polyethylene/ethylene vinyl acetate copolymer as a blend [98], and waste rubber powder [137,138], allows... 

The most radiation-stable poly(olefin sulfone) is polyethylene sulfone) and the most radiation-sensitive is poly(cyclohexene sulfone). In the case of poly(3-methyl-l-butene sulfone) there is very much isomerization of the olefin formed by radiolysis and only 58.5% of the olefin formed is 3-methyl-l-butene. The main isomerization product is 2-methyl-2-butene (37.3% of the olefin). Similar isomerization, though to a smaller extent, occurs in poly(l-butene sulfone) where about 10% of 2-butene is formed. The formation of the olefin isomer may occur partly by radiation-induced isomerization of the initial olefin, but studies with added scavengers73 do not support this as the major source of the isomers. The presence of a cation scavenger, triethylamine, eliminates the formation of the isomer of the parent olefin in both cases of poly(l-butene sulfone) and poly(3-methyl-1-butene sulfone)73 indicating that the isomerization of the olefin occurred mainly by a cationic mechanism, as suggested previously72. 

Applications of electron propagator methods with a single-determinant reference state seldom have been attempted for biradicals such as ozone, for operator space partitionings and perturbative corrections therein assume the dominance of a lone configuration in the reference state. Assignments of the three lowest cationic states were inferred from asymmetry parameters measured with Ne I, He I and He II radiation sources [43]. 

Benztriazole, derivatives 281 ff 2-(2-Hydroxy-5-niethylphenyl)- 282 2-(2-Hydroxy-3-(l -methylpropyl)-5-tert-butylphenyl- 283 Berberine reagent 44,213 Beryllium cations 144,145,311,312 Besthorn s hydrazone reagent 347 Beta-blockers 74, 299, 301, 426—428 Beta-fronts 126 Beta-radiation 12 Betulae, Extr. 279 Betulic acid 59... 

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