Polymer photooxidative degradation

Thermal, Thermooxidative, and Photooxidative Degradation. LLDPE is relatively stable to heat. Thermal degradation starts at temperatures above 250°C and results in a gradual decrease of molecular weight and the formation of double bonds in polymer chains. At temperatures above 450°C, LLDPE is pyrolyzed with the formation of isoalkanes and olefins. 

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

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. 

However, these polymers do degrade when subjected to terrestrial ultraviolet radiation, and this has been attributed to the presence of small amounts of impurities which absorb light and initiate oxidative chain reactions within the polymer. In the oxidation process, compounds containing peroxy and keto groups are formed, and these absorb in the long wavelength region and accelerate the breakdown of the polymer chains. Obviously a detailed study of such reactions in polymers will be difficult because of the frequently unknown nature of the trace impurities which initiate the reactions and of the multiplicity of products formed in the photooxidation. 

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechamsm (see Polymers, ENViRONiNffiNTALLY degradable). 

Fig. 2 a Infrared spectra of monoalkylated (dotted line) and dialkylated polyfluorenes (solid line) show that in the case of monoalkylated polymers keto defects are already present after polymer synthesis, b Besides polymer synthesis, keto defects can also be generated by, e.g., photooxidative degradation. Here, the corresponding infrared spectra after photooxidative degradation of dialkylated polyfluorenes are shown (modified from [16,17])... 

Even in the case of simple aromatic dopants (naphthalene, anthracene, pyrene), fluorescence intensity is found to decrease with successive laser pulses and/or increasing laser fluence. The decrease may be ascribed to thermal desorption of the dopant, as indicated by Fukumura et al. [62], and/or to photooxidation/degradation of the dopants. In the present case, film transmission decreases in parallel with the dopant fluorescence, thus indicating the formation of photoproducts that absorb stronger than the precursor. (The photoproduct(s) are not detected in the fluorescence experiments, evidently because of its (their) low emission yield). NapH and related compounds are known to generate (Ag) efficiently [81]. This has a long lifetime within polymers and it can attack the aromatic to form the corresponding endoperoxide (Nap-02). Such processes have been shown to occur in related polymer systems. 

Linden, L. A., Rabek, J. R, Kaczmarek, H., Kaminska, A., and Scoponi, M. "Photooxidative Degradation of Polymers by HO and HO2 Radicals Generated During the Photolysis of H2O2 FeCl3 and Fenton Reagants." Coordination Chemistry Reviews 125 (1993) 195-218. 

As an introduction, the mechanism responsible for the outdoor photooxidative degradation of many synthetic polymers can be summarized in Figure 29 (110,111). Some absorbing chromophore absorbs sunlight to produce an excited electronic state, which may, in addition to the usual physical fates, suffer chemical reactions leading ultimately to loss of mechanical properties of the polymer. Such reactions Include direct molecular dissociation, such as the Norrlsh Type II process referred to earlier (see Section II), production of O2 ( g) generally through triplet-state sensitization... 

Prediction of moisture diffusion parameters in polymers undergoing photooxidative degradation is considered to be of somewhat lesser significance than sorption predictions for three reasons. First, it seems probable that the occurrence of anomalous sorption-diffusion behavior may be expected in most polymer... 

Since hydroperoxides play a determining role in the photooxidative degradation of polymers, decomposition of hydroperoxides into more stable compounds, before the hydroperoxides undergo photolytic cleavage, would be expected to provide an effective means of UV protection. Metal complexes of sulfur-containing compounds such as dialkyldithiocarbamates (XIX), dialkyldithiophosphates (XX) and thiobisphenolates (XXI) are very efficient hydroperoxide decomposers even if used in almost... 

The principle remaining feature, the sulfide links between the "A" and segments remains to be eliminated from consideration. Much recent interest by others in the photooxidation of monomeric sulfides supports this possibility (7). The alternative that photooxidative degradation in these polymers is a relatively random process should also be considered. 

Resistance to photooxidative degradation can be improved by incorporating UV absorbers or opaque pigments into the polymer film. 

Low-molecular-weight HALS, a hydroperoxide inhibitor, has been shown to considerably reduce photooxidation of PBT [107]. It can be expected that the combination of a UV absorber with HALS will be particularly effective. There is little data on stabihzation of thermoplastic polyester elastomers, but the stabilizers used to protect polymers that degrade by free-radical chain oxidations should also be efficient for these types. Thus, UV absorbers of the benzotriazole and benzophenone types may be used for their screening capabihties, and synergistic effects can be expected with the addition of HALS to reduce the rate of chain oxidations. 

Balaban L, Majer J, Vesely K. Photooxidative degradation of polypropylene. J Polym Sci Part C Polym Symp 1969 22 1059-1071. 

Exposure of the molded HDPE articles to sunlight and air also attacks the polymer over time, especially at wavelengths less than 400 nm. Photooxidation resembles thermooxidation in that it is a complex chain of radical transformations. Such exterior aging of the polymer results in development of surface cracks, brittleness, changes in color, and a deterioration of mechanical and dielectrical properties. Photooxidation degradation is prevented by small amounts of light stabilizers, such as 2-4% carbon black or, for colorless articles, esters of salicylic acid or derivatives of benzotriazole or benzophone and others in the 0.1-0.5% range. 

The influence of the photooxidative degradation on the elongation at break of polymers is also very dramatic. This effect has been attributed to the degradation of the macromolecules but also to the formation of physical and chemical defects on the surface. ... 

KAC 00] Kaczmarek H., Kaminska A., Van Herk A., Photooxidative degradation of poly(alkyl methaciylate)s , European Polymer Journal, vol. 36, pp. 767-777, 2000. 

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