Polymer oxygen diffusion, activation energy

Kcal/mol, and that the rate of oxydation the radicals is independent of oxygen pressure over a wide range of oxj en pressures. These two facts seem to support our picture that the mechano-radicals are on the fre surfece, because small concentration of ox3 n at low pressure is enou fm oxidation reaction of radicals only when all the radicals are on the surface and also such small value of the activation energy is presumably too small for the diffusion of oxygen into solid polymers. 

Labile structures initiating polymer decay [215] are formed during the process of thermooxidation in the air. Thermooxidation rate is defined by the rate of oxygen diffusion into polymer. Constant of destruction rate in the air compared with inert medium increases, and activation energy decreases [216]. However, in some cases active energy increases this is connected with the contribution of physical phenomena of heat and mass transition together with chemical processes into the total kinetics of destruction. 

Wilson [666, 667] and Bauman and Maron [47] show that it is possible to express the reaction rate as a function of film thickness, diffusion constant and solubility of oxygen in the film. When the thickness of the film is reduced to less than a certain value, the chemical reaction and not the diffusion becomes the controlling factor. The activation energy of the oxidation reaction amounts to 16—35 kcal mole-1, or even more, whereas the activation energy of the diffusion of gases in polymer films [39] is of the order of only 10 kcal mole-1. Control by diffusion is facilitated at higher temperatures by the decrease of oxygen solubility in the films. 

The straight-line portion of the Arrhenius curve above about 100 K observed in both cases is attributed to quenching of the phosphorescence emission by oxygen, and the slope of this curve accurately reflects the activation energy associated with the permeability of the oxygen quencher. This effect can be used (23,24) as a means of measuring the rates of oxygen diffusion in a variety of polymers. 

Figure 7-12. Compensation effect between the action constant A) and the activation energy i) of diffusion of nitrogen, , oxygen, O, and carbon dioxide, , through various polymers above their glass transition temperatures.

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