Oxygen diffusion effects polymer

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. 

H. Nishide, H. Kawakami, T. Suzuki, Y. Azechi, Y. Soejima and E. Tsuchida, Effect of Polymer Matrix on the Oxygen Diffusion via a Cobalt Porphyrin Fixed in a Membrane, Macromolecules 24, 6306 (1991). 

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 solution oxidation technique allows the study of polyolefin autoxi-dation under conditions where the temperature, concentration of reactants, and rates of radical initiation can be controlled. The results should be considered as a useful prelude to any fundamental understanding of the autoxidation processes which occur in neat polymers where the effects of very high viscosity, partial crystallinity, and oxygen diffusion rates are included. The objective of our work was to determine the kinetics and stoichiometry of the inhibited autoxidation of polypropylene in solution. A relatively detailed study of the oxidation of polypropylene inhibited by 2,6-di-terf-butyl-4-methylphenol [butylated hydroxytoluene (BHT)] has been made for comparison with data obtained in polypropylene oxidations inhibited by a variety of other stabilizers which include commercial polyfunctional antioxidants. Singly hindered phenols appeared to be superior in the inhibited-solution oxidation of polypropylene, and the application of this finding to stabilization technology was investigated briefly. 

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. 

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. 

Latex-modified mortar and concrete also have an excellent oxygen diffusion resistance, since they have an impermeable structure as stated above. Figure 4.64 represents the effect ofthe polymer-cement ratio on the oxygen diffusion coefficient of latex-modified mortars. Such excellent oxygen diffusion resistance is found to contribute to the prevention of the rust formation in the corrosion of reinforcing bars in reinforced concrete structures. 

Figure 4.64 Effect of polymer-cement ratio on oxygen diffusion coefficient of latex-modified mortars.

The CCL is a composition of carbon threads, catalyst particles, polymer electrolyte and voids for gas transport. An effective oxygen diffusion coefficient in this system is poorly known. Fortunately, modern catalyst layers are usually thin (typically less than 10 /xm). This enables us to assume that the variation of oxygen concentration across the CCL is not large (the limit of fast oxygen diffusion). In this limit, Eikerling and Komyshev obtained analytical solution in parametric form. However, their parametric solution is cumbersome and difficult to deal with. A transparent result, which reveals important features of CCL function can be obtained as follows. 

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... 

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