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 analys
is, UV, luminescence and FTIR spectroscopy coupled with crosslinking and hy
droperoxide analysis in order to understand the nature of the processes inv
olved. High temperature oxidation of non-commercial unstabilised material r
esults in chain scission and severe crosslinking giving rise to extensive d
iscolouration. FTIR analysis shows complex degradation processes with disti
nct features associated with each phase. There is a solvent soluble clear p
hase showing oxidation due primarily to the aliphatic part with a predomina
nt absorption associated with terminal carboxylic acid groups at 1713 cm(-1
). Anhydrides and alpha,beta -unsaturated carbonyl species are also formed
in this matrix. There is also a solvent insoluble phase, which is predomina
ntly 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 autocatalyt
ic formation of high concentrations of primary hydroperoxides during the ea
rly stages of oxidation. These species are unstable and breakdown rapidly l
eaving a steady-state concentration of more stable hydroperoxides. The lumi
nescence also shows a rapid initial disruption of the polystyrene excimers
coupled with the formation of long wavelength emitting polyconjugated chrom
ophores, possibly, stilbene type in nature. Colour is evident is both cross
linked and uncrosslinked phases. The former shows evidence for the presence
of unsaturated carbonyl products and vinyl absorption. Phosphorescence ana
lysis also indicated the presence of initial acetophenone chromophores, whi
ch are associated with polystyrene end groups formed by chain breakage at t
he aliphatic links. These species can act as initial active sensitive sites
for further breakdown, possibly via a thermally induced hydrogen atom abst
raction process. The end-chain aliphatic radicals are the sites for initial
rapid hydro-peroxidation. The presence of phenolic antioxidants and phosph
ites appear to exhibit a powerful effect in synergistically inhibiting the
discolouration and oxidation processes. This is evident through thermal ana
lysis (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 SEES are destroyed. This is in complete contrast to the effec
ts observed in control unstabilised SEES material. Mechanisms are proposed
and discussed for each phase oxidation. (C) 2000 Elsevier Science Ltd. All
rights reserved.