Photo-Oxidative Degradations of Polymers

Singlet oxygen, the lowest excited state of molecular oxygen, ¹Δg O₂, is known to cause rapid degradation of polymers. Singlet oxygen forms by several processes, including one of two mechanisms: (1) through photosensitization by some impurity; (2) through energy transfer from an excited triplet state of a chromophore. Polyolefins that are unprotected from weathering react very readily with oxygen in the presence of sunlight even at room temperature. This leads to loss of molecular weight and often embrittlement. The mechanism of photo-oxidation of polyethylene was demonstrated to involve carbonyl groups that developed in the polymer via oxidation during processing [564, 565]. These carbonyl groups act as chromophores and their triplet states (n → n*) are quenched by molecular oxygen in the ground triplet state. Electronically excited singlet oxygen molecules form as a result [564].

The Norrish Type II reaction then takes place with a cleavage of carbon-to-carbon bonds, formation of olefins and, subsequently, formation of hydroperoxides:

The hydroperoxides apparently have no photo inductive effect on the overall oxidation of polyethylene [566]. Photolysis of hydroperoxides, however, initiates new vinylidine oxidation. It was shown that the kinetics of photo-oxidation of polyethylene is characterized by the superpo sition of two phenomena. The first corresponds to an exponential increase in the concentration of the carbonyl groups with time and is observed when the kinetics are controlled by the diffusion of oxygen. The second one is not controlled by diffusion, but corresponds to a linear increase of the carbonyl concentration with time and takes place in degraded samples. This is explained in terms of chain rupture in the amorphous regions of the polymer, allowing free access of oxygen [567]. Commercial polypropylene can photo-degrade rapidly. This is due to a presence of chromophore groups that form during polymerization. In addition, chromophores may also form through oxidation by singlet oxygen during extrusion in air. At temperatures of 250–300C, light of 300 nm will initiate oxidation by atmospheric oxygen [568]. The hydroperoxide groups that form are not stable and decompose to alkoxy radicals to form ketones on the backbones and at the ends of the chains. These ketones in turn undergo Norrish TypeI and Type II reactions when irradiated with ultraviolet light [568]:

Photo-oxidation of poly(vinyl chloride) appears to be greatly enhanced by imperfections in the polymer structure that form during polymerization and processing. Formation of peroxide and carbonyl groups is very difficult to prevent. The initiation of photo-oxidation probably results from the free-radical that forms as these groups decompose upon irradiation [568]. In addition, double bonds that form from loss of HCl will sensitize formation of free radicals. These radicals are further oxidized and also promote cross-linking that results in gels. Photo-oxidation of cis-polybutadiene was also shown to involve singlet oxygen [570]. Attacks by singlet oxygen on double bonds with formation of allylic hydroperoxides and shifts of these double bonds according to the ene reaction were confirmed by several other studies [571]. On the other hand, a study of model compounds for photo-oxidation of polyisoprene failed to show formation of endo peroxides by 1,4-cycloaddition [572]. Aromatic polyurethanes exhibit a strong tendency to yellow in sunlight. This may be accounted for by two mechanisms based on formation of different photoproducts. The first one was found to be a photo-Fries reaction caused by a short wave length light [572]. The second one, caused by longer wave length light, is formation of quinonoid structures [573]:

Photo-oxidation of Nylon 6 was also investigated [574] The subject of photo-oxidation of silicones is seldom raised because these polymers are known for their stability to oxidation. Nevertheless, an investigation of photo-oxidative degradation of these materials showed that these materials can be affected by combined attacks of oxygen and ultraviolet irradiation [585]. The groups that are affected are the functional groups that are more fragile than the backbones. Thus, the silicon hydride group was found to be readily photo-oxidized, perhaps due to its low bond energy. The reaction can be illustrated as follows [585]:

The main process that occurs in silicones that contain vinyl groups is photo-scission. Also, the vinyl groups are photo-oxidized into silanols, carboxylic acids, and esters. Part of the silanols are condensed into high molecular weight silicones [585].

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الآخبار الصحية

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عادة بسيطة تدعم الصحة وتحافظ على الوزن المثالي

الغذاء النباتي يقلل مخاطر السرطان.. مع استثناء مهم

الحليب قليل أم كامل الدسم.. أيهما أفضل؟

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أنثروبيك تتحدى البنتاغون.. وترامب يأمر بوقف التعامل معها

ميزة جديدة من "إنستغرام" لحماية المراهقين من الانتحار

الأولى من نوعها.. "تقنية مذهلة" في هاتف سامسونغ الجديد

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المزيد

مواضيع ذات صلة

Oxidation of Step-Growth Polymers

Hydrolytic Degradation of Polymers at Elevated Temperatures

The Termination Reaction

صناعة حمض الكبريتيك

تركيب حامض الكبريتوز

كلوريد الثيونيل Thionyl Chloride

حامض الكبريتوز: Sulphureous Acid

مركبات الكبريت مع الأكسجين

مركبات الكبريت مع الهيدروجين

الكبريت الغروي

الحصول على الكبريت من المناجم Sulphur Mining

الأشكال التأصلية للكبريت