Poly photo-oxidative degradation

In conclusion, singlet oxygen plays a role in the photo-oxidative degradation of polymers containing olefinic unsaturations. Polymers that do not contain these groups, e.g. poly(vinyl chloride), poly(methyl methacrylate), polystyrene, etc., are unreactive [24]. 

The photodegradation and photo-oxidative degradation of different poly-olefins have been subjects of many publications (Table 3.1) and reviews [570, 853, 856, 866, 1999, 2268]. 

Poly(ethylene-co-propylene)s are completely amorphous polymers, and their photo-oxidative degradation has been reviewed in detail [1999]. A number of interesting conclusions have been drawn from the study of photo-oxidative degradation of ethylene-propylene copolymers at 310nm in the presence of air [754, 756-758] ... 

The photodegradation and photo-oxidative degradation of polyacrylonitrile [41, 302, 1200, 1951, 2313] and its copolymers such as poly(acrylonitrile-co-isopropyl ketone) [41], poly(acrylonitrile-co-methyl methacrylate) [820], poly(acrylonitrile-co-2 naphthyl methacrylate [338], poly(acrylonitrile-co-butadiene) [24,2001], poly(acrylonitrile-co-styrene) [232, 747], poly(acrylo-nitrile-co-p-styrene) [232] and poly(a-chloroacrylonitrile-co-methyl methacrylate) [820] have been studied in detail. 

Photo-oxidative degradation of poly(vinyl chloride) occurs by the reaction shown in section 2.1. 

The formed hydroperoxide and ketone groups may further participate in the initiation step of photodegradation however it has been suggested that they do not play an important role in this process [559]. On other side it has shown that a-chloroketones are mainly involved in the photo-oxidative degradation of poly(vinyl chloride) [1641]. 

The initial rates of both thermal and photo-oxidative degradations are linearly related to the concentration of hydroperoxide groups in poly(vinyl chloride) samples, but not to the unsaturated groups and carbonyl concentration. 

Polyene structures formed during the thermal- and/or photo-oxidative degradation of poly(vinyl chloride) react easily with a number of different solvents, which cause their discoloration (Table 3.16). This process is accelerated by irradiation with light (Fig. 3.39) or heating in the presence of various radical sources (Fig. 3.40 and Fig. 3.41). The decoloration mechanism depends upon the type of solvent used [1426, 1673] ... 

Poly(vinylidene fluoride) is very stable against photo-oxidative degradation [1740,1741], No oxidized groups can be observed in films exposed to radiation (>300nm) for long periods of time (1800h). Variations in mechanical properties and of permeabilities for COj or O2 can only be observed after very long exposures [1718],... 

The rate of the photo-oxidative degradation of polystyrene in halomethanes increases in the order methylene chloride < chloroform < carbon tetrachloride, i.e. it increases with increasing electron affinity of the solvent [279]. Photodegradation of poly(a-methylstyrene) in carbon tetrachloride by... 

Polybutadiene based polyblends (Table 3.19) are very important plastics with industrial applications, e.g. impact resistance polymers and impact modifiers for rigid poly(vinyl chloride). These polyblends are very susceptible to photo-oxidative degradation, because of the presence of polybutadiene. Intensive studies of the photo-oxidation of the rubber modified polyblends lead to the following conclusions [761-764, 1936] ... 

The site of hydrogen abstraction under UV irradiation from the poly(ethylene-co-propylene-co-5-ethyhdene-2-norbomene) is controlled by the liability of the various carbon-hydrogen bonds. The mechanism of the photo-oxidative degradation of this polymer can be presented by the following reactions [1911, 1316] ... 

The photo-oxidative degradation of poly(ethylene-co-propylene-co-l,4-hexadiene) (3.133) occurs by a combination of reactions characteristic of the degradation of polyethylene and polypropylene (section 3.1) and polydienes (section 3.12). Quantum yields of different photochemical processes which occur in this terpolymer are shown in Figure 3.68. 

Photo-oxidative degradation of poly(ethylene oxide) (4.2) poly(propylene oxide) (4.5) and poly(tetramethylene oxide) (4.4) and low molecular weight... 

Photo-oxidative degradation of poly(ethylene oxide) completely destroys its ability to crystallize in the form of spheruUtes (Fig. 4.1) [1117, 1774]. 

Three mechanisms have been proposed for the photo-oxidative degradation of poly(2,6-dimethyl-l,4-phenylene oxide) ... 

Coatings based on mixed unsaturated aromatic polyesters which have benzene nuclei, such as poly(l,2-maleate-propylene-o-phthalate) (4.48), are also susceptible to photo-oxidative degradation [1375, 1814]. On exposure to sunlight, gradual changes take place in their chemical and physical properties, starting at the surface to produce microcracks and chalking. 

Another computer model for the photo-oxidative degradation of poly-(ethylene-co-vinyl acetate) has been developed [2017]. 

Scheme 18.4 Photo-oxidation reactions of PECT [11], Reprinted from Polymer, 41, Grossetete,T., Rivaton, A., Gardette, J.-L., Hoyle, C. E.,Ziemer, M., Fagerburg, D. R. and Clauberg, H., Photochemical degradation of poly(ethylene terephtha-late)-modified copolymer, 3541-3554, Copyright (2000), with permission from Elsevier Science...

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