Radiation attacks

The oxidation of hydrocarbons, including hydrocarbon polymers, takes the form of a free-radical chain reaction. As a result of mechanical shearing, exposure of ultraviolet radiation, attack by metal ions such as those of copper and manganese as well as other possible mechanisms, a hydrocarbon molecule breaks down into two radicals... 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

Simulation of frank SSBs and base damage as expressed by ALS has been achieved with the RADACK (RADiation attACK) procedure (Begusova et al. 2001b). This takes into account that the various nucleobases and the hydrogens of the sugar moiety react with different rate constants. The effect is shown in Fig. 12.4, where B-DNA is represented in a space-filling model with only the reactive atoms represented, with the same atoms but with sizes according to their OH cross-section or with the non-reactive atoms re-added. It is the last structure that- OH "encounters" in the RADACK procedure. 

Light radiation attacks hair proteins, the cell membrane complex hpids, and the hair pigments. The emphasis in this section is on the photochemical degradation of the proteins of hair with some discussion on the degradation of the cell membrane complex lipids and proteins. Later in this chapter, in the discussion on hair pigments, the effects of light on melanins are considered. 

It is possible there could be a radiation attack that would not become known until the local medical community began to find multiple patients with signs of radiation sickness— nausea and vomiting, anemia and leukopenia (low white blood cells), hair loss, and possibly radiation burns. An attack with lower doses of radiation might show up only years later with an increase in cancer in the community. 

Increased accumulation of anthocyanins is observed in many plants in response to stress. This includes fall coloration of leaves, response to ultraviolet radiation, attack by pathogens, deficiency of some minerals, mechanical damage, water stress, and many other factors (Hrazdina, 1982). 

Radiation Effects. Polytetrafluoroethylene is attacked by radiation. In the absence of oxygen, stable secondary radicals are produced. An increase in stiffness in material irradiated in vacuum indicates cross-linking (84). Degradation is due to random scission of the chain the relative stabiUty of the radicals in vacuum protects the materials from rapid deterioration. Reactions take place in air or oxygen and accelerated scission and rapid degradation occur. 

Ultraviolet radiation causes cleavage of the aryl ether linkage (23). DMPPO undergoes oxidation when exposed to ultraviolet light and oxygen by direct attack on the aromatic ring to produce a variety of ring-cleaved and quinoidal stmctures (24). 

Such structural changes are a consequence of chemical reactions of which the most common are oxidation, ozone attack, dehydrochlorination and ultraviolet attack. (Reactions due to high-energy radiation or to high temperature are not considered here as causing natural aging.) Over the years many materials have been introduced as antioxidants, antiozonants, dehydrochlorination stabilisers and ultraviolet absorbers—originally on an empirical basis but today more and more as the result of fundamental studies. Each of these additive types will be eonsidered in turn. 

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