Oxygen diffusion effects degradation

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. 

The degradation profile can be detected by measuring a property, such as microhardness, as a function of depth after ageing so that the magnitude of any effect from the limitation of oxygen diffusion could be measured for any temperature and material combination. The effect of a degradation profile on a bulk property will depend on the particular... 

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. 

A microhardness method allows thin test pieces and can also be used to examine degradation as a function of thickness to detect any effect of limitation in oxygen diffusion. 

The assumption can be made that degradation is dependent on total radiation dose but is independent of dose rate (see Section 6.13). However, acceleration levels can be very high and this is a prime reason why, in practice, it is often found that the effect of a given dose decreases with increased dose rate. The limiting factor is the rate of oxygen diffusion. Recommended practice is to test at two or more dose rate levels to determine the magnitude of this effect. 

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. 

The reduction of degradation enhancement due to orientation is better seen when samples are stretched and then the time to fail, under UV radiation, is recorded. The results are shown in Fig. 6 where one should notice the break in scale for the reference (non-oxldlzed) sample. There is a drastic decrease in failure time (F.T.) for low draw ratios 1 < X < 1.7. This can be attributed to stored elastic energy which makes the chemical bonds more reactive toward UV, even at low stress levels. As X increases and the polymer structure becomes more and more oriented, F.T. Increases steeply before reaching a plateau once the orientation process is more or less completed. If we consider that photooxidation is oxygen diffusion controlled (1-5), the orientation effect is to decrease such diffusion by making the structure much more compact so that the degradation will be reduced. 

The lifetime of many polymer products in use is limited by oxidative degradation. Exposed samples are usually non-uniformly oxidised. At the macroscopic level, the heterogeneities can result from oxygen-diffusion-limited effects. If the rate of oxygen consumption exceeds the rate of oxygen permeation, oxidation occurs in the surface layers, whereas the core remains practically unoxidised. The importance of this effect depends on several parameters. First, intrinsic parameters are linked to material geometry (e.g., sample thickness) coupled with the oxygen consumption rate, which depends on the reactivity of... 

Electrode porous catalyst layer and gas diffusion layer Degradation effect on oxygen diffusion polarizations Aoki et al., 2010... 

---