Influence of ozone on styrene―ethylene―butylene―styrene (SEBS) copolymer

摘要

Non-commercial and commercial SEBS copolymer materials have been subjected to severe ozone treatment for different periods of time and concentrations in an ozone cabinet in order to obtain a deeper understanding on the mechanism of ozone attack in this type of material. The polymer materials were subsequently analysed by FTIR (ATR method), fluorescence and phosphorescence spectroscopy. Hydroperoxide analysis and determination of gel content was also carried out. Original functionalities in the SEBS based on aliphatic vinyl and aromatic (styrene) structures were observed to decrease in intensity and these were consistent with the concurrent formation of ozonide groups. Immediate exposure of SEBS to ozone resulted in the rapid and consistent formation of a variety of carbonyl and unsaturated carbonyl products based on aliphatic esters, ketones, and lactones as well as aromatic carbonyl associated with the styrene phase. These were followed by a more gradual formation of ether, hydroxyl and terminal vinyl groups with time and concentration. Also, of interest was the evident formation of a strong enol tautomer of a β-diketone functionality. These functional group changes were specific and concentrated on the very surface layer of the SEBS only. Whist there was strong evidence for hydroxyl group formation hydroperoxide analysis showed minimal evidence for active peroxides although growth was consistent with ozone dosage for the less ozone resistant materials. No crosslinking was also found in this material. Early decreases in in-chain vinyl groups by FTIR analysis were also consistent with an observed decrease in fluorescence functionalities in the SEBS associated with primarily trans-stilbene groups whereas longer periods of exposure showed new fluorescence functionalities. Phosphorescence analysis showed the formation of acetophenone end groups on the styrene chains associated with chain scission within the aliphatic rubber-styrene interphase region. Commercially ozone resistant SEBS materials were found to contain lower levels of fluorescent trans-stilbenic chromophores indicating this to be the weak link at the interphase in non-commercial ozone susceptible samples. Mechanistic routes for these processes are proposed and discussed.