Irradiation during

The absorbed light may act as calalv. i for a spontaneous reaction, but in other cases it may supply energy to make possible a reaction which, without light, would be thermodynamically impossible. In some cases, such a reaction reverses itselfby thermal reaction (e.g. if left in the dark) and, hence, during irradiation a phoiostationary state is reached. 

Figure Bl.16.1. CIDNP spectrum (250 MHz top) observed during irradiation of chloranil with sabinene...

Photoresist appHcations in the microelectronics industry have also been disclosed (340). Thermally stable ben2yl sulfonate esters based on 2-methyl-3-nitroben2otrifluoride [6656-49-1] can serve as nonionic photoacid generators to promote a cascade of reactions during irradiation of the resist. 

In air, PTFE has a damage threshold of 200—700 Gy (2 x 10 — 7 x 10 rad) and retains 50% of initial tensile strength after a dose of 10" Gy (1 Mrad), 40% of initial tensile strength after a dose of 10 Gy (10 lad), and ultimate elongation of 100% or more for doses up to 2—5 kGy (2 X 10 — 5 X 10 rad). During irradiation, resistivity decreases, whereas the dielectric constant and the dissipation factor increase. After irradiation, these properties tend to return to their preexposure values. Dielectric properties at high frequency are less sensitive to radiation than are properties at low frequency. Radiation has veryHtde effect on dielectric strength (86). 

In contrast, the photochemistry of uracil, thymine and related bases has a large and detailed literature because most of the adverse effects produced by UV irradiation of tissues seem to result from dimer formation involving adjacent thymine residues in DNA. Three types of reaction are recognizable (i) photohydration of uracil but not thymine (see Section 2.13.2.1.2), (ii) the oxidation of both bases during irradiation and (iii) photodimer formation. 

Cava and Muller " independently showed that epimerization at C-13, which can occur during irradiation of 17-keto steroids, is not observed with diazoketones (94) or (96). 

The hydroperoxide groups thus formed undergo decomposition during irradiation producing macroxy radicals that offer sites for grafting. 

These hydroperoxide groups undergo radiolytic cleavage during irradiation of the aqueous polymer monomer mixture. 

The system should be relatively free of oxygen during irradiation. Oxygen apparently combines with a photochemical intermediate to form cyclohexanone.3 Under the conditions recommended in the procedure, oxygen is prevented from entering the system by the water trap which also serves as a monitor for gas evolution. 

Ultraviolet absorption curves obtained at intervals during irradiation of dilute aqueous solutions until constant (ca. 40 minutes) showing three (left) and two (right) isosbestic points... 

Fig. 16. Change of optical density (JD) of aqueous solutions of poly (potassium propynoate) for different wavelengths in the process of oxidation during irradiation with white light...

Fig. 21. Kinetic curves of photosensitized oxygen absorption by thermally treated PAN samples during irradiation with white light. ( ) PAN (450 °C,...

Compound A was affected most by the irradiation, Compound B least, and Compound C intermediate. The effect of the irradiation on cohesion increased with increasing irradiation dose and temperature. The iso-butylene-isoprene copolymer in Compound A and Compound C degrades during irradiation (9), becoming softer after irradiation. Since the seam of a can is formed before irradiation, some softening of the compound in the seam is not detrimental to the integrity of the seam. 

Irradiation Conditions. The gamma (cobalt-60) radiation facility and the source calibration are described by Holm and Jarrett (4). Irradiation temperature was 21 (initial) - 40°C (final). The gamma source was calibrated with the ferrous sulfate/cupric sulfate dosimeter for a dose rate of 8 X 102 rads per second. Pouches were fabricated from multilayered materials and then irradiated while empty. The container used to hold the multilayered materials and the empty pouches during irradiation was a large size, flexible package that was sealed under vacuum prior to the irradiation. 

When exposed to daylight, the sulfide and selenide halides HgsY2X2 are blackened within a few minutes. This black color reversibly disappears when the sample is heated to 90 to 120°C, or stored in the dark for several days 204, 375-377). The nature of this phototropic behavior has now been widely investigated by analytical, spectroscopic, structural, magnetic, EPR, and radiotracer investigations 205, 233, 375-377, 379, 380, 382). During irradiation of the compounds, electrons belonging to or I ions are excited to upper states. The result-... 

UV-Vis spectroscopy may also provide valuable information if small molecules are studied. However, the photochemical sensitivity of many sulfur-containing molecules may trigger changes in the composition of the sample during irradiation. For instance, this phenomenon has been observed in Raman spectroscopy using the blue or green hnes of an argon ion laser which sometimes decompose sensitive sulfur samples with formation of Sg [2, 3]. Reliable spectra are obtained with the red hnes of a krypton ion or a He-Ne laser as well as with the infrared photons of a Nd YAG laser. 

The yield or efficiency of cross-linking is also dictated by the hydrostatic pressure to which the polymer is subjected during irradiation [76]. The increase in G(X) is due to an enhancement of chain-reaction cross-linking furthered by a closer proximity of the polymer chains. 

Radiation cross-linking of PVC in the absence of additives has been reported by several authors [20,21,267,268]. PVC by itself is not readily cross-linkable by EB radiation. In the study of radiation chemistry of PVC, it is known that the oxidation takes place in the presence of atmospheric oxygen during irradiation [269] and hydroperoxide groups are produced on oxidation. The formation of carbonyl group along with the elimination of HCl from PVC on irradiation in air has also been reported [270,271]. 

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