Degradation and stabilisation of styrene–ethylene–butadiene–styrene (SEBS) block copolymer

Abstract

The degradation 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 thermal analysis, UV, luminescence and FTIR spectroscopy coupled with crosslinking and hydroperoxide analysis in order to understand the nature of the processes involved. High temperature oxidation of non-commercial unstabilised material results in chain scission and severe crosslinking giving rise to extensive discolouration. FTIR analysis shows complex degradation processes with distinct features associated with each phase. There is a solvent soluble clear phase showing oxidation due primarily to the aliphatic part with a predominant absorption associated with terminal carboxylic acid groups at 1713 cm −1. Anhydrides and α,β-unsaturated carbonyl species are also formed in this matrix. There is also a solvent insoluble phase, which is predominantly crosslinked aliphatic material due to the formation of hydroperoxides and peracids/peresters. Vinyl groups are also evident in this phase. Thus, end group oxidation is a predominant process 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 also shows a rapid initial disruption of the polystyrene excimers coupled with the formation of long wavelength emitting polyconjugated chromophores, possibly, stilbene type in nature. Colour is evident is both crosslinked and uncrosslinked phases. The former shows evidence for the presence of unsaturated carbonyl products and vinyl absorption. 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. The end-chain aliphatic radicals are the sites for initial rapid hydroperoxidation. The presence of phenolic antioxidants and phosphites appear to exhibit a powerful effect in synergistically 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.