Photooxidation mechanism

A study of the relative fluorescence intensities at 460 nm of PET and PET-4,4 -SD yarns after receiving identical irradiation intensities reveals an increase in the formation of the hydroxyterephthaloyl moiety with increasing amounts of 4,4 -SD. This indicates that a photooxidative mechanism involving the second monomer may be an explanation of the increasing degradation rates. 

Figure 11 suggests a photooxidative mechanism which may account for the increased formation of hydroxyterephthaloyl with the presence of 4,4 -SD (32). 

Figure 11. Possible photooxidative mechanism occurring in polyfethylene tereph-thalale-co-4,4 -sulfonyldibenzoate) yarns (34)...

Carter, W.P. (1996) Condensed atmospheric photooxidation mechanisms for isoprene, Atmos. Environ. 30 (24), 4275-4290. 

They also reported (fi) the rate constants for the reactions of CH3S + NO, CH3S + NO2, and CH3S + O2 and estimated the rate-constant ratio to be k(NO)/k(Oj)>2 x 106 and k(N02)/k(02)>5 x 106. On the basis of these rate-constant ratio, photooxidation mechanism for reduced sulfur compounds must be re-examined, because most experimental studies used much NO2. 

Sinturel, C. Lemaire, J. Gardette, J. L. Photooxidation of fire retarded polypropylene II. Photooxidation mechanism, European Polymer Journal, 1999, 35(10), 1783-1790. 

After the examination of the PS photooxidation mechanism, a comparison of the photochemical behavior of PS with that of some of its copolymers and blends is reported in this chapter. The copolymers studied include styrene-stat-acrylo-nitrile (SAN) and acrylonitrile-butadiene-styrene (ABS). The blends studied are AES (acrylonitrile-EPDM-styrene) (EPDM = ethylene-propylene-diene-monomer) and a blend of poly(vinyl methyl ether) (PVME) and PS (PVME-PS). The components of the copolymers are chemically bonded. In the case of the blends, PS and one or more polymers are mixed. The copolymers or the blends can be homogeneous (miscible components) or phase separated. The potential interactions occurring during the photodegradation of the various components may be different if they are chemically bonded or not, homogeneously dispersed or spatially separated. Another important aspect is the nature, the proportions and the behavior towards the photooxidation of the components added to PS. How will a component which is less or more photodegradable than PS influence the degradation of the copolymer or the blend We show in this chapter how the... 

Two different cases may occur. If this radical is formed in a succession of styrene units (1), it reacts in the same way as in PS. If it is formed on a styrene unit linked to an acrylonitrile unit (2), three reaction pathways may be envisaged. The alkoxy radical resulting from the decomposition of the hydroperoxide formed on this polystyryl radical may react by 3-scission. Scissions (a) and (b) yield chain ketones, acetophenone end-groups and phenyl and alkyl radicals as previously observed in the case of PS photooxidation mechanism. Scission (c) leads to the formation of an aromatic ketone and an alkyl radical. This alkyl radical may be the precursor of acrylonitrile units (identified by IR spectroscopy at 2220 cm-1), or may react directly with oxygen and after several reactions generates acid groups, or finally this radical may isomerize to a more... 

The materials analyzed were blends of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in various ratios. The two components are miscible in all proportions at ambient temperature. The photooxidation mechanisms of the homo-polymers PS and PVME have been studied previously [4,7,8]. PVME has been shown to be much more sensitive to oxidation than PS and the rate of photooxidation of PVME was found to be approximately 10 times higher than that of PS. The photoproducts formed were identified by spectroscopy combined with chemical and physical treatments. The rate of oxidation of each component in the blend has been compared with the oxidation rate of the homopolymers studied separately. Because photooxidative aging induces modifications of the surface aspect of the material, the spectroscopic analysis of the photochemical behavior of the blend has been completed by an analysis of the surface of the samples by atomic force microscopy (AFM). A tentative correlation between the evolution of the roughness measured by AFM and the chemical changes occurring in the PVME-PS samples throughout irradiation is presented. 

It is a fact that kinetic schemes containing the same processes of propagation and of termination of the chains as the photooxidation mechanism which we have previously discussed should also make it possible to take account of other modes of slow oxidation of the aldehydes, either in the liquid or in the vapor phase. 

The photooxidation mechanisms of polyolefins continues to attract much interest and controversy. A comparison of the photooxidation rates of linear low density polyethylene with low density polyethylene indicates that catalyst levels in the former have little, if... 

Paulson, S.E. and J.H. Seinfeld Development and Evaluation of A Photooxidation Mechanism for Isoprene, Journal of Geophysical Research-Atmospheres, 97 (1992) 20703-20715. 

Effects of adjustments to the SAPRC-99 toluene photooxidation mechanism on simulations of representative toluene experiments experiments. 

Although the UCR EPA chamber has only been in operation for a relatively short time, it has already obtained usefiil information concerning the performance of current mechanism in predicting the effects of VOCs and NOx on ozone formation. As discussed here and also in our companion paper (Carter, 2004a), the SAPRC-99 mechanism predicts O3 formation reasonably well in low NOx experiments, and in ambient simulation experiments at the high ROG/NOx levels where maximum ozone formation potentials are achieved. However, die new data indicate problems with the mechanisms that were not previously realized. The SAPRC-99 mechanism consistently under predicts O3 formation in the lower ROG/NOx experiments where O3 formation is most sensitive to VOCs, and the problem is even worse for CB4. Other experiments indicate that there are problems with the formulation wifli die current aromatics photooxidation mechanisms. It is possible that the problems with the under prediction at low ROG/NOx ratios may be caused by problems with the aromatics mechanisms. Experimental and mechanism development work to investigate and hopefully resolve these problems is underway... 

The interaction and subsequent oxidative behavior under UV light exposure of nanocomposite using poly(styrene) (PS) as polymer and LDH organomodified by a monomer surfactant as filler were recently investigated [115]. The photooxidation study revealed that the hybrid nanofiller did not modify the photooxidation mechanism of PS. The same products of oxidation were observed with the same proportions. A slightly higher oxidation rate was observed in the case of the sample with 5% of filler. The advantage of this system was its ability to be tailored in order to limit/ control eventual interactions with photostabilizers and antioxidants. 

Some other transition metal complexes, e.g., tiifluoroacetates of palladium and copper [64a], some platinum derivatives [64b], are known to promote aerobic alkane photooxidation. Mechanisms of these transformations are not clear in detail. 

FIGURE 29. Schematic diagram of photooxidation mechanisms [after figure 2, Pure Appl. Chem.,... 

The relationship between fatigue life and carbonyl content can be explained as follows according to the photooxidation mechanism of PE, carbonyl groups result mainly from a Norrish type II reaction, i.e., for each carbonyl formation, there is a scission of a segment of a molecule chain. Such scission creates a defect in the structure which can grow and propagate into a microcrack under application of a load. Under cyclic loading, it is understandable that the number of cycles the sample can sustain will be directly related to the number of defects (such as microcracks, microvoids. ..), as is clearly described by equation (2). ... 

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