Polymer photooxidation initiation

Oxygen atoms might react with polymers either by a direct reaction or by an energy-transfer process. Since solar radiation on Earth has wavelengths longer than about 300 nm, this process does not seem important as an initiation step of polymer photooxidation by solar radiation. 

Negative influences of pigments on polymer light stability are due to sensitization of singlet oxygen formation, initiation of polymer photooxidation by photo-exdted pigment chromophore formation of other photo-excited species with adverse effects on... 

It is of some interest to consider photooxidation initiation from the olefin groupings actually, as is well known, polyolefins, as obtained from the polymerization, contain terminal double bonds as a consequence of the polymerization withZiegler-Natta catalysts (34,35) as well as intemal double bonds (10. 36). It is worth noting that the role of the initiation processes considered so far is probably important only for the very early stages of the polymer oxidation in the subrequent stages, the predominant role in photooxidation initiation should be played by the oxidation products (hydroperoxides, ketones, etc.) (see Section II.2). 

Initial photooxidation and several other factors will diminish the molecular weight of polymers thereby releasing low molecular weight portions of pol3nneric chains which eventually can be biodegraded. The total environmental effect on polymers is therefore put together by... 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

The photoimaging process occurs via a photooxidation process photo-initiated by residual transition metal impurities in the presence of oxygen and terminated by coupling of polymer-bound radicals. Photoinduced cross-linking thus requires generation of a critical, and large, concentration of free radicals. 

Despite the numerous papers devoted to photooxidation of hydrocarbon polymers [21], the initiation step has not been clearly established yet even for polyethylene or polystyrene which were the most studied [22,23]. Difficulties which follow from solution of this problem consist in the necessity of analysis of small amounts of decomposing unstable structures and products which are thereby formed. Moreover, photoinitiation does not include one reaction only but the overall complex of many chemical and physical processes, which importance depends on experimental conditions. 

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. 

Under irradiation with polychromatic light at X > 300 nm and 60 °C, representative of outdoor exposures, polystyrene (PS) homopolymer, copolymers and blends do not directly absorb the incident radiation. It is well known that the photooxidation of these polymers results from light absorption by chromopho-ric impurities [1,2]. Photooxidation generates modifications of the chemical structure of the material, which results in the formation of oxidized groups, the development of discoloration and the loss of the initial mechanical properties. 

Blends. The type I reaction produces free radicals which, in the presence of oxygen, initiates photooxidation which also results in a decrease in the polymer molecular wei t. An indication of the relative importance of the type I reaction in these systems can be estimated from the amount of chain scission induced in a blend of the copolymers with homopolymer polystyrene. For these experiments, one part of 5% vinyl ketone copolymer was blended with four parts of styrene homopolymer to retain an overall ketone monomer concentration of 1%. 

None of these products contribute towards the initiation of photooxidation of the polymer other than having some u.v. antioxidant action. On long-wavelength irradiation (365 nm) photoinitiation occurs via impurity chromophore whereas with light of wavelengths shorter than 300 nm direct excitation of the diphenylcarbonate unit occurs. 

The reason of increased photooxidative stability is not only stabilizing activity of bis-aroilenbenzimidazoles derivatives but also the effect of these compounds on supermolecular structure of polymer matrix. As it is seen from the Table 6 physico - mechanical indexes of polymer do not practically depend on the method of additives introduction. Data of Table 6. agree with data of Table 5. In all cases the greatest degree of conservation of polymer initial properties is registered at the concentration of XLY compound - 1 mass %. 

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