Abstract
Based on the determination of the solvatochromic parameters of a series of polymers with various functional groups containing oxygen atoms, a method was proposed which allowed for discriminating the oxidation products formed by exposure of polymers to photo- and/or thermo-ageing. The polymers used to determine the solvatochromic parameters contained polar groups such as chain-pendant ester (EVA, PMMA, PVAc), chain ester (PLA), alcohol (EVOH, PVA), chain-pendant acid (PAA), ketone (PK), ether (PEO). The solvatochromic parameters π∗, α and β were determined following procedures reported in the literature, and it was shown that the most apolar polymer PE, displays the lowest π∗ value; the presence of acetate groups in PE makes the π∗ value increase. Polyesters (PLA, PMMA, PVAc) have similar π∗ values. Then, alcohols and ether groups (EVOH, PVA, PEO) have higher π∗ values. Finally, the highest π∗ values are obtained for PAA and PK. Concerning the β value, the highest value was obtained for (PE, EVOH, PEO), which have the highest capacity to accept a proton, and are close to polyethylene in terms of chemical structure. On the contrary, the polymers with the lowest β value are the ones which have acid/ester/ketone functions (PK, PAA, PLA), reflecting their lowest capacity to accept a proton. This method was successfully applied to discriminate the oxidation products formed in thermo- and photo-oxidation of polyethylene which are known to be slightly different due to the Norrish reactions that do not occur in thermooxidation.
Highlights
Based on the determination of the solvatochromic parameters of a series of polymers with various functional groups containing oxygen atoms, a method was proposed which allowed for discriminating the oxidation products formed by exposure of polymers to photo- and/or thermo-ageing
A study, based on the use of profluorescent nitroxide sensors as sensitive probes to detect early stage photodegradation [3], showed that the probes are able to highlight polymer degradation within the oxidation “induction” period. It has been shown in our previous work [4] that the emission properties of a fluorescent molecular probe such as Prodan® depend on the solvent polarity [5, 6, 7] and are modified with the oxidation extent of a polymer during ageing. This has been ascribed to the change of polarity in the polymer matrix that comes from the oxidation products which are formed during ageing
The main objective of the present work is to study the sensitivity of Prodan® to the chemical functions of oxidation products which are formed during photo and thermo-oxidation of polyethylene, in order to detect any change of polarity due to the accumulation of oxidation products and in order to discriminate the oxidation products depending on their chemical groups
Summary
Based on the determination of the solvatochromic parameters of a series of polymers with various functional groups containing oxygen atoms, a method was proposed which allowed for discriminating the oxidation products formed by exposure of polymers to photo- and/or thermo-ageing. The polymers with the lowest β value are the ones which have acid/ester/ketone functions (PK, PAA, PLA), reflecting their lowest capacity to accept a proton This method was successfully applied to discriminate the oxidation products formed in thermo- and photo-oxidation of polyethylene which are known to be slightly different due to the Norrish reactions that do not occur in thermooxidation. Polymers with a wide variety of chemical functions involving polar oxygen groups have been chosen: chain-pendant ester (EVA, PMMA, PVAc), chain ester (PLA), alcohol (EVOH, PVA), chain-pendant acid (PAA), ketone (PK), ether (PEO). Solvents were purchased from either Sigma Aldrich, Acros Organics or VWR Chemicals. 2-dimethylamino-6-propionylnaphtalene (Prodan® >98%) was purchased from TCI, 4nitroanisole (ANS, 97%) and 4-nitroaniline (ANL, >99%) were purchased from Sigma
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