Photo-oxidative degradation mechanism

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

Hydrogen abstraction from the poly(vinyl chloride) backbone by the free radicals (R or P ) leads to the formation of two types of polymer alkyl radicals  

Polymer alkyl radicals are very reactive with molecular oxygen and produce polymer peroxy (POj) radicals [529, 552, 559, 1410, 1513, 1797, 2182]  

Formation of PO2 radicals can be measured by ESR spectroscopy [1064, 1771, 1883]. The typical ESR spectrum of a polymer peroxy (PO ) radical is an asymmetric single line spectrum (Fig. 3.34). 

The propagation reactions by peroxy radicals should generate mostly radical (3.56), since a tertiary C—H bond is about 25 times more reactive than a secondary C—H bond toward alkylperoxy radicals at 25°C [771, 2182] so that the main oxidation products of poly(vinyl chloride) are assumed 

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Through FTIR and UV-Vis spectroscopy methods there were identified diverse products resulted during photo-oxidative degradation, such as carbonyls, hydroxyl entities and peroxides and hydroperoxides, and photo-oxidative degradation mechanisms were given in Schemes 3 and 4 [12] (reproduced with kind permission from Elsevier—license no. 3842460646409). 

Scheme 4 Photo-oxidative degradation mechanism [12] (reproduced with kind permission from Elsevier—license no. 3842460646409)...

What is mechanical degradation Why is it less commonly encountered relative to thermal and photo-oxidative degradation ... 

General mechanism of patterned resist polymer photo-oxidative degradation... 

W.K. Busfield, T. Paulina, Photo-oxidative degradation of mechanically stressed polyolefins. Polym. Degrad. Stab. 51, 185-196 (1996)... 

Light radiation increases the brittleness of polyolefins and seriously lowers their physical properties, mainly mechanical properties. Such degradation is more pronounced in the presence of oxygen. All modifications produced by photo-oxidative degradation are similar to those produced by thermal degradation and, are accelerated by an increase in light intensity [19] and temperature [20]. 

The literature describes the mechanism of photo-oxidative degradation in isotactic and atactic polypropylene well (Figure 5.25). However, the initiation step - the formation of primary radicals - has not been precisely explained. Photo-oxidative degradation in polypropylene, like thermal-oxidative degradation, also leads to the formation of hydrogen peroxides, ketones, esters, and acids. Figure 5.26 [653]. Simultaneously, molecular weight decreases. Table 5.2. 

The mechanical properties are changed by water absorption and as a consequence of photo-oxidative degradation. In actual practice, the mechanical properties are often maintained for years, even if the parts are severely weathered optically. Table 5.9 shows the reduction in tensile strength of carbon-black stabilized polyamides after twelve years exposure to various climate zones [14]. 

Photo-oxidative degradation of polyethylene results in chain scission and crosslinking. On photoirradiation in air, gel formation is discernible after 2h of irradiation however, after 15h of photoirradiation nearly 50% gel fraction can be found in irradiated polyethylene samples. The effects of cross-linking on the formation of oxidation products and the mechanical properties of polyethylene have been studied [2117, 2128]. 

Even very low amounfs of the ketone chromophoric groups in polypropylene can be responsible for the initiation of polymer photo-oxidative degradation which may further occur by mechanisms similar to those described for polyethylene (cf section 3.1.2). 

Photodegradation and photo-oxidative degradation and the Norrish Type II mechanisms ... 

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],... 

Polystyrene can form a charge-transfer (CT) complex with molecular oxygen [36, 1386, 1610, 1611, 1781, 1793, 1794, 2301]. The absorption band of the CT complex extends from 350 nm toward longer wavelengths [1610]. High oxygen pressure promotes the formation of the CT complex [1794] and the rate of photo-oxidation, but does not influence the mechanism of photo-oxidative degradation [1610, 1793]. The concentration of the CT complex depends on two factors ... 

The mechanism of photo-oxidative degradation of polystyrene was also investigated using low molecular compounds such as benzene 3-phenyl-pentane [1356] 2-phenylbutane [1360, 1361] 1,3-diphenylpropane-l-one [719, 725] 1,3-diphenylpropane 1,3-diphenylbutane 2,4-diphenylpentane [1293] cumene cumene hydroperoxide [759] acetophenone hexanoic acid 3-heptanone y-valerolactone 2-phenyl-2-propanol [1250, 1397] benzal-dehyde benzoic acid l-3-diphenyl-propanone-3 l,3-diphenyl-buten-2,3-on-l (dypnone) l,3-diphenylpropen-2,3-on-l (chalcone) l-phenyl-butanedion-2,3 l-diphenylpropanedion-1,2 l-phenyl-butanedion-1,3 phenylacetic aldehyde l,5-diphenyl-pentanone-3 4-phenyl-butanone-2 [1250]. 

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] ... 

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

The photo-oxidative degradation occurs via a free radical mechanism with the formation of polymer peroxy (POj) and polymer oxy (PO ) radicals and polymeric hydroperoxides (POOH). 

The mechanism of photo-oxidative degradation of polyamides has also been investigated using low molecular weight compounds such as JV-hexyl-hexanamide 1,6-hexamethylenebis(hexanamide) iV,JV -bis(hexyl)adipamide N-alkylamides and N-acylamides [587, 1221, 1348, 1683, 1880, 1959]. 

Ozone accelerates the photodegradation of polyacrylamide in water solution [726, 727]. Photo-oxidative degradation of polyamides causes a decrease of their mechanical properties [1395, 1396, 2305]. 

In spite of considerable study, the mechanism of wool photo-oxidative degradation has not been fully elucidated. 

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