Aldehydes, ketones and carboxylic acids along or at the end of polymer chains are generated by oxygenated species in photolysis of photo-oxidation. The initiation of photo-oxidation reactions is due to the existence of chromophoric groups in the macromolecules. Photo-oxidation can occur simultaneously with thermal degradation and each of these effects can accelerate the other.
The photo-oxidation reactions include chain scission, cross linking and secondary oxidative reactions. The following process steps can be considered:
- Initial step: Free radicals are formed by photon absorption.
- Chain Propagation step: A free radical reacts with oxygen to produce a polymer peroxy radical (POO•). This reacts with a polymer molecule to generate polymer hydroperoxide (POOH) and a new polymer alkyl radical (P•).
- Chain Branching: Polymer oxy radicals (PO•) and hydroxy radicals (HO•) are formed by photolysis.
- Termination step: Cross linking is a result of the reaction of different free radicals with each other.
where PH = Polymer
P• = Polymer alkyl radical
PO• = Polymer oxy radical (Polymer alkoxy radical)
POO• = Polymer peroxy radical (Polymer alkylperoxy radical)
POOH = Polymer hydroperoxide
HO• = hydroxy radical
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