Diffusion oxidative degradation

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. 

Although the diffusion coefficient (79abts = 3.2 x 10 cm /s) and solubility (>30 mM) of ABTS are much higher than those of competing redox polymers, the compound was found to suffer from oxidative degradation at potentials exceeding 0.92 V vs SHE in pH 7 buffer. Cyclic voltammetry of ABTS detected two oxidation peaks, one at 530 mV that was... 

However, although the oxidation process was ascribed to either direct reaction on the electrode surface, or mediated by peroxodisulphate and other inorganic reagents electrogenerated at the anode surface, the linear decrease of the Faradic yield down to zero in the investigated range of concentration was interpreted as an indication of a process under diffusive control. This leads to the conclusion that the oxidative degradation of the compounds essentially occurred at the electrode interface. 

Having taken into account the above findings, it seems reasonable to explain the observed retardation of thermal oxidative degradation of st-PE-n-MMT by the capability of exfoliated MMT nanolayers to hinder the diffusion of oxygen throughout the partly cross-linked and carbonized nanocomposite matrix. 

Jellinek et al. [312—314] discussed the role of diffusion in the kinetics of polymer oxidative degradation. It should be ascertained in the investigation of the oxidation of polymer films whether the observed oxidation rate is actually due to the chemical oxidation reaction and that it is not influenced by the relatively slow diffusion of oxygen into the film. When the polymer is very finely powdered, diffusion effects would be expected to be negligible. 

Stress in plastic composites and profiles can decrease the free energy of activation of oxidation processes and, hence, speed up the oxidation degradation of the materials in the stressed areas. Besides, stress decreases local densities (specific gravities) of composites and thereby increases porosity and provides space for oxygen from air to diffuse in and oxidize the material from inside. ... 

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