Photo-oxidation polymers

In fact the extremely rapid reaction of NOH with hydroperoxides combined with the ready oxidation of hydroxylamines to nitroxides during storage even in the solid state makes unlikely the detection of >N0H from hindered amines in photo-oxidizing polymer. 

G.A. George and M. Ghaemy, The effect of stabilizers on integrated chemiluminescence in the early stages of polypropylene photo-oxidation, Polym. Degrad. Stab. 1991, 34, 37-53. 

Mechanistic studies reveal transformations of N, A -disubstituted PD 11 in autoxidized and photo-oxidized polymers and model hydrocarbons [57,58], Wurster s ion radicals 49, associated with ROO in CTC 41 are formed in the first step after electron transfer from 11 [3,24]. 49 is formed regardless of the oxidizing agent and is stabilized via electron delocalization in mesomeric forms [59] ... 

In situ oxidation of secondary and tertiary HAS into the respective nitroxides NO by various oxidants present in (photo)-oxidized polymers or in the environment (Eq.15) has been accepted as the first step in the HAS transformation mechanism in real polymer systems at ambient or increased temperatures. 

Nitroxides were detected by ESR spectrometry in HAS doped photo-oxidized polymers [13,15,125]. Binuclear HAS, like 28 R = H are transformed into NO stepwise [181]. 

In photo-oxidized polymers, HAS may meet excited chromophores (Ch ) such as residues of polymerization catalysts or carbonyls arising in oxidized PO. Attempts were made to obtain information on the ability of HAS or derived NO to quench Ch [213, 214], The phosphorescence emissions from sensitizers anthraquinone, benzophenone or benzhydrol were not affected by HAS. These data indicate that quenching ability is lacking in the HAS mechanism [214],... 

F. A. Bottino, G. Di Pasquale, E. Fabbri, A. Orestano, and A. Pollicino, Influence of montmorillonite nano-dispersion on polystyrene photo-oxidation. Polymer Degradation and Stability, 94 (2009), 369-74. 

The nickel dithiocarbamates (Vllb) are hydroperoxide decomposers [21]. Unlike the zinc analogues, they are light stable and effective UV screening agents below 340 nm [30]. They react rapidly with hydroperoxides as they are formed in the photo-oxidizing polymer to give sulphur acids which are catalysts for hydroperoxide decomposition [21] and are themselves converted to nickel sulphate (see Scheme 1.5). This chemistry leads to the eventual depletion of the metal thiolate in the polymer and at this point (the end of the induction period), the rate of photooxidation reverts to that of the polymer containing the uncomplexed transition metal ion. 

The formation of epoxy groups in polydienes is very much dependent on the structure of the photo-oxidized polymer for example, in the case of pure polybutadiene, it is very low [572], but in styrene-butadiene rubber (SBR) it is very high [292]. 

Polymers with carboxylic groups (COOH) exhibit a broad band due to the O—H stretching vibration and a strong band due to the C=0 stretching vibration. The marked spectral changes which occur when a carboxylic acid is converted to its salt may be used to distinguish it from other carbonyl (C=0) groups present in the photo-oxidized polymer sample. 

Treatment of photo-oxidized polymer sample with 0.5 N methanolic potassium hydroxide. The sample has to be immersed overnight, then washed and dried in vacuum. The carboxyl group band at 1714cm disappears and a new band appears at 1566 cm belonging to the carboxylic acid group salt [1390]. 

Treatment of photo-oxidized polymer (polyethylene) films by suspending them for 16 h in an atmosphere of ammonia, which causes the formation of carboxyhc ammonium salts with absorption at 1570-1550 cm [26, 854] ... 

Treatment of photo-oxidized polymer (e.g. polyethyleneterephthalate films) with SF4 [364, 1274, 1846] ... 

IR-ATR measurements are very useful for the study of the very beginning of a reaction occurring in the upper surface layer of a photo-oxidized polymer sample such as polyolefins [774], poly(vinyl chloride) [684,686,717,718,941], poly(ethylene terephthalate) [264] and poly(esterurethane) coatings [394]. 

Deconvolution of the ESCA core level bands allows the determination of average binding energies for photo-oxidized polymers (Table 10.23). 

Table 10.23 Average binding energies for selected photo-oxidized polymers [1701]...

Zhang, X., Pi, H., Guo, S. Photostabilizing efficiency of ultraviolet light stabilizers for rigid poly(vinyl chloride) against photo-oxidation. Polym. Eng. Sci. 53, 378-388 (2013)... 

Santos, R.M., Pimenta, A., Botelho, G., Machado, A.V. Influence of the testing conditions on the efficiency and durability of stabilizers against ABS photo-oxidation. Polym. Test. 32, 78-85 (2013)... 

Costa L, Luda MP, TrossarelU L. Ultra high molecular weight polyethylene-II. Thermal-and photo-oxidation. Polym Degrad Stab 1997 58 41-54. 

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