Diffusion photo-oxidative degradation

Oxygen diffuses only into amorphous regions of a polymer making them more susceptible to photo-oxidative degradation. 

The decrease in the diffusion coefficients for different gases through UV irradiated polymeric membranes (films) can be interpreted by the formation of chemical crosslinks and the decrease of free volumes. In the case of photo-oxidative degradation the decrease in diffusion through an irradiated membrane can be the result of physical interactions between primary hydroperoxide photoproducts, and can be used as a sensitive criterion to characterize the photo-oxidative evolution of polymers [1717]. 

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. 

Degradation of Polymers. From the results of dynamic analysis, the degree of the deterioration of polymers varies with time and the depth from the surface, and is influenced by the photochemical reaction constants (ki, k2) and diffusion coefficient (D.). Figure 9 shows the theoretical result of the influence of photo-oxidation on the distribution of oxygen in polymers. Deriving the time dependence of surface deterioration, we obtain equation 13. 

It was shown that the kinetics of photo-oxidation of polyethylene is characterized by the superposition of two phenomena. The first corresponds to an exponential increase in the concentration of the carbonyl groups with time and is observed when the kinetics are controlled by the diffusion of oxygen. The second one is not controlled by diffusion, but corresponds to a linear increase of the carbonyl concentration with time and takes place in degraded samples. This is explained in terms of chain rupture in the amorphous regions of the polymer, allowing free access of oxygen [567]. 

There are many similar attributes of thermal-, photo- and radiafion-degradafion. They progress on the similar mechanisms based on the homolytic or heterolytic scissions and the formation of degradation initiators for loop process of oxidative degradation. The discrepancies between these three ageing ways consist of the concentration of primary free radicals and their distribution in the material. The depth of degradation is only some microns, but the diffusion of chain promoters, hydroperoxides, towards the inner layers of materials leads to a parabolic distribution of oxidation products around the symmetry axe. 

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