Oxygen diffusion effects oxidation

Although tga studies of oxidation are relevant to combustion, it must be emphasized that studying degradation imder conditions of rapid weight loss from liquid samples at over 200°C has no relevance for degradation of solid polymers in service use. Furthermore, unless exceedingly thin sample are used, such studies are almost certainly influenced by oxygen diffusion effects discussed below. 

In low density reactant compacts, the reaction is believed to involve gas phase oxygen diffusion whereas under conditions of improved contact, in high-density material, the mobile species is identified as Fe2+. The metal catalyzes decomposition of the oxidant (KMn04), an effect that is inhibited by small quantities of certain additives (e.g. NaF). There is a large and specialist literature devoted to self-heating reactions. 

It should be noted that temperature can have a significant effect on the degradation as it controls both the rate of oxygen diffusion into the material and the rates of reaction of the products of the irradiation. The diffusion of oxygen is a limiting factor (as it is with heat ageing) and oxidation is directly connected to the dose rate effect. 

Chemical effects also occur in crystalline oxides that is, impurity atoms diffuse at varying rates through oxides. In all cases, cation diffusion is much faster than oxygen diffusion, but similar cations, such as Ca and Mg, can behave very differently in different oxides. This is due to differences in chemical potential and activity, and mobility differences. 

Heat ageing is not normally carried out in a vacuum or an inert atmosphere but in air or oxygen so that the ageing effect is caused by a combination of heat and oxygen. At high temperatures, the rate of oxidation of the rubber may be faster than the rate of oxygen diffusion so that uneven... 

The liquid phase oxidation of cyclohexane involves both physical and chemical mechanisms. Upon entering the liquid, oxygen diffuses towards the bulk of the liquid. Reaction takes place simultaneously with diffusion. It is possible that the relative speed of reaction and diffusion can affect the course of reaction and the final produce distribution. It is obvious, however, that the reaction must be fast before such an effect is appreciable. 

Bohne et al. (2004) Molybdenum oxides, titanium oxides Phase transformation upon oxidation + + n.a. Ion implantation, effect of oxygen diffusivity on subsurface phase formation... 

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