Radio-oxidation

Oxidative degradation can be the most serious problem in the use of plastics at higher temperatures. At ambient temperature oxidation proceeds relatively slowly on its own, but can be stimulated by light (photo-oxidation), ionising radiation (radio-oxidation), certain gaseous and liquid environments and by the presence of transition metals. The rate at which oxidation occurs will therefore depend on the intensity of these agents, on temperature, and on the availability of oxygen, which in turn depends upon its solubility, its rate of diffusion (see Section 4.12.2) and the rate at which it is consumed. 

In the case of radio-oxidation, a second important source of radicals is the polymer radiolysis, i.e. the breakdown of lateral bonds in the polymer chain. PE radiolysis leads to the formation of very reactive radicals which recombine rapidly by hydrogen abstraction. Thus, the balance reaction can be written as follows ... 

Pure radio-oxidation predominates at high dose rates (typically higher than about 0.5 Gy.s ) and is characterized by a TOL quasi-proportional to the reciprocal dose rate. 

In the case of UHMWPE hip component, vitamin E is the only molecule that can be envisaged because of toxicity concerns. The rest of this chapter will hence be devoted to vitamin E. However, a recent work by Gijsman and coll [49] shows that HAES could be envisaged as radio stabilizers in the case of radio oxidation while not interfering with the crosslinking process (Figure 3.2) ... 

Poljraer surfaces can be easily modified with microwave or radio-frequency-energized glow discharge techniques. The polymer surface cross-links or oxidizes, depending on the nature of the plasma atmosphere. Oxidizing (oxygen) and nonoxidizing (helium) plasmas can have a wide variety of effects on polymer surface wettability characteristics (92). 

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

Tesoriere, L. et al.. Increased resistance to oxidation of betalain-enriched human low density lipoproteins, Free Radio. Res., 37, 689, 2003. 

There is bound to be one problem with resin glass polyalkenoate cement. Because the matrix is a mixture of hydrogel salt and polymer, lightscattering is bound to be greater than in the conventional material. Moreover, the zinc oxide-containing glass of class II materials is bound to be opaque. This makes it difficult to formulate a translucent material and is the reason why their use is restricted to that of a liner or base. However, the class II material cited will be radio-opaque because it uses strontium and zinc, rather than calcium, in the glass. 

Thus we shall be concerned with properties that furnish information about the nature of the ligands, the oxidation state of the metal, and the geometry of the field of ligands. Techniques such as radio-isotope tracer studies, neutron-activation analysis, and electron microscopy are powerful methods for locating a metal within constituents of the cell and are particularly suited to heavy-metal rather than organic drugs but since they do not provide information about the chemical environment of the metal they will not concern us here. After each section below we shall give an example, not necessarily from platinum chemistry, where the method has been used with success in biochemistry. 

Lowe, GM, Booth, LA, Young, AJ, and Bilton, RF, 1999. Lycopene and beta-carotene protect against oxidative damage in HT29 cells at low concentrations but rapidly lose this capacity at higher doses. Free Radio Res 30, 141-151. 

Although use of radio and stable isotope labels involving the trio of covalently-bonded nitrogenous functions in 3 and in 78, provided evidence that isocyano is the precursor of the isothiocyano and formamido groups [30, 81], it remains to be shown that a biosynthetic equivalent of the in vitro chemically-proven fusion process between isocyano and free sulfur (e.g., cf. Introduction) exists in the cells of sponges. In marine biota, various ionic forms of sulfur in a number of oxidation states, as well as organo-polysulfides are known. However, any association with the isonitrile group and a sulfated species has yet to be established. 

Davies, M.J., Fu, S., Wang, H., and Dean, R.T. (1999) Stable markers of oxidant damage to proteins and their application in the study of human disease. Free Radio Biol. Med. 27, 1151-1163. 

K.R. Zhang, F.R. Zhu, C.H.A. Huan, and A.T.S. Wee, Effect of hydrogen partial pressure on optoelectronic properties of indium tin oxide thin films deposited by radio frequency magnetron sputtering method, J. Appl. Phys., 86 974—980, 1999. 

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