Photooxidation of styrene–ethylene–butadiene–styrene (SEBS) block copolymer

Abstract

The photooxidation of hydrogenated poly[styrene- b–butadiene- b–styrene] or poly[styrene- b–(ethylene–co-butylene)- b–styrene] (SEBS), has been studied using a variety of analytical and spectroscopic methods including yellowness, luminescence and FTIR spectroscopy coupled with crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. FTIR and luminescence analysis show complex oxidation processes with distinct features associated with each phase. Rates of photooxidation measured by carbonyl growth and discolouration show that low molecular weight samples are more photostable than those with high molecular weight. Oxidation primarily due to the aliphatic part occurs with a predominant absorption associated with terminal carboxylic acid groups at 1712 cm −1. Anhydrides, ketones, aldehydes, esters and α,β-unsaturated carbonyl species are also formed in this matrix. Unlike thermal oxidation there is no evidence for crosslinking. Thus, end group oxidation is a predominant process at the interphase boundary of the soft aliphatic and hard aromatic segments with the immediate autocatalytic formation of high concentrations of primary hydroperoxides during the early stages of oxidation. These species are unstable and breakdown rapidly leaving a steady-state concentration of more stable hydroperoxides. The luminescence confirms a rapid initial disruption of the polystyrene excimers coupled with the formation of long wavelength emitting polyconjugated chromophores, possibly, stilbene type in nature giving rise to weak colour formation. Phosphorescence analysis also indicated the presence of initial acetophenone chromophores, which are associated with polystyrene end-groups formed by chain breakage at the aliphatic links. These species can act as initial active sensitive sites for further breakdown, possibly via a thermally induced hydrogen atom abstraction process to give benzaldehyde and benzoic acid. These active end-groups show a typical autocatalytic growth and decay process. The end-chain aliphatic radicals then become the sites for initial rapid hydroperoxidation as well as attack of the butene-1 tertiary groups. The presence of phenolic antioxidants and phosphites appear to exhibit a synergistic effect in inhibiting the discolouration and oxidation processes. This is evident through thermal analysis (OIT) and luminescence. In the latter case the consumption of excimer is impaired by the presence of stabilisers while the initial acetophenone end-groups in SEBS are destroyed. This is in complete contrast to the effects observed in control unstabilised SEBS material. Mechanisms are proposed and discussed for each phase oxidation.