Photooxidation polymers

Ketone photolysis in an inert atmosphere has been widely studied--ft 5-Q. Apart from polymer photooxidation studies, however, little work has been done on their degradative irradiation in an oxidizing medium51 53. 

Zweig, A., Henderson, W.A. 1975. Singlet oxygen and polymer photooxidations. I. Sensitizers, quenchers and reactants. J. Polym. Sci. Part A 1, 717-736. 

The effect of ultraviolet irradiation in air on the wettability of thin films of amorphous polymers has been studied. With poly(vinyl chloride), poly(methyl methacrylate), poly(n-butyl methacrylate), poly (ethylene terephthalate), and polystyrene the changes in contact angles for various liquids with irradiation time are a function of the nature of the polymer. A detailed study of polystyrene by this technique and attenuated total reflectance spectra, both of which are sensitive to changes in the surface layers, indicates that the contact angle method is one of the most sensitive tools for the study of polymer photooxidation in its early stages. The method is useful in following specific processes and in indicating solvents to be used in the separation and isolation of photooxidation products. 

The antioxidant action of nitroxyl radicals is due to their ability to react with alkyl radicals as in Reaction 9. Because of their reactivity with alkyl radicals, nitroxyls are better photooxidation inhibitors than the inhibitors of other classes. The rate constant for the reaction of nitroxyls with hydroxyl radical (Equation 10) is 10 L/mol. sec. This reaction can be of particular importance for the inhibition of polymer photooxidation. 

Under thermal conditions, hydroxylamine ethers can reversibly decompose (Reaction 15). The radicals formed disproportionate to eliminate olefins and yield hydroxylamine (Reaction 16). In the presence of sufficiently effective acceptors of alkyl radicals (e.g., oxygen), the reaction rate of peroxy radical formation is much higher than that of hydroxylamine formation. Thus, in the process of polymer photooxidation, nitroxyl radicals regenerate and can break multiple oxidative chains. 

A further point of discussion, however, is whether the often cited reactions (6, 7, 8) of TMP derivatives (and their conversion products) with peroxy radicals may sufficiently compete with the propagation steps (5) in Scheme I. Indeed, TMP-derivatives including NOR are known to be rather weak scavengers of peroxy radicals (7) in the liquid phase. Based on considerations which take into account that rapid randomization of macroradicals is largely restricted in the solid polymer, Carlson and Wiles, however, concluded that fast radical scavenging would in fact not be needed for efficient inhibition of long chain polymer photooxidation processes (2, 8, 9). 

At the present time, a discussion of the results of our model investigations in terms of possible consequences for polypropylene must be completely speculative. Apart from the differences expected between liquid phase and solid polymer photooxidation kinetics, differences in the chemical structure between our model substance, isooctane, and the structural unit of polypropylene have to be also considered. With respect to the number of CH,-groups per structural unit, isooctane and polypropylene differ by a ratio of 5 1. 

Computer Modeling Studies of Polymer Photooxidation and Stabilization... 

We have recently found that this free radical oxidation of the methyl groups is in fact not a major pathway in the photooxidation of poly(phenylene oxide). Instead, the oxidation apparently occurs through an electron-transfer mechanism on the backbone of the polymer not chemically involving the methyl groups at all. In this paper, we present evidence inconsistent with the free radical mechanism and supporting this novel pathway for polymer photooxidation. 

Oxygen atoms might react with polymers either by a direct reaction or by an energy-transfer process. Since solar radiation on Earth has wavelengths longer than about 300 nm, this process does not seem important as an initiation step of polymer photooxidation by solar radiation. 

Luminescent Species in Polymer Photooxidation. The problems associated with establishing a mechanism for the photooxidation and weathering of synthetic polymers are great, and any method that provides additional information Is useful. In addition to traditional methods such as product analysis. Infrared spectroscopy (both conventional and ATR), and UV-vlslble absorption spectroscopy, luminescence methods have recently been employed. 

Table I. Elementary Reactions in Polymer Photooxidation and Corre onding Rates- —...

Still other mechanisms have been postulated, e.g., the interaction of HALS with a,P,-unsaturated carbonyl compounds and the formation of charge-transfer complex between HALS and peroxy radicals. However, despite extensive publications in the field, a complete knowledge of the process occurring in polymer photooxidation in the presence of HALS is still not available. 

Negative influences of pigments on polymer light stability are due to sensitization of singlet oxygen formation, initiation of polymer photooxidation by photo-exdted pigment chromophore formation of other photo-excited species with adverse effects on... 

The relaxation (migration in the film) of SEs makes it inevitable that the SE interacts with SE, other quasiparticles such as TE and polarons, and chemical and structural defects. Yet, as it was demonstrated by Rothberg as early as 1994 for different polymers, photooxidation of the material strongly quenches the PL. This is caused by the irreversible destruction of the polymer backbone as well as by the induced defects, such as carbonyl groups, acting as quenching sites for SEs consequently leading to the observed decrease of the photoluminescence [95-97]. 

High oxygen concentration (atmospheric conditions) If polymer photooxidation is carried out in the presence of surplus oxygen then termination occurs almost exclusively by the recombination of polymer peroxy radicals (POO ), as shown by equation 2.134. Under these conditions the rate of combination of polymer radicals (P ) with oxygen (equation 2.131) is relatively fast and so [P ] [POO ]. It follows from equation (2.137) that ... 

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