Photodegradation and photostabilization of poly(p-phenylene sulfide). I: Laser flash photolysis studies of model compounds

In order to understand the mechanism of poly(p-phenylene sulfide) (PPS) photodegradation, eight model compounds related to the polymer and two aromatic disulfides have been studied by nanosecond laser flash photolysis. The model compounds absorb light mostly in the deep UV region and fluoresce very weakly with very short lifetimes. Laser pulse excitation of the linear oligomers in solution produces short-lived triplets that are readily quenched by O, a 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy radical, biphenyl, and 1-methylnaphthalene. In particular, the quenching by O sub 2 occurs with unusually high rate constants \(8-13) x 10 exp 9 M exp --1 s exp --1 \, suggesting dominance of charge transfer in the quenching interaction. Laser pulse excitation of the disulfides gives longer-lived thiophenoxy radicals which are essentially nonquenchable by O. For the PPS oligomers, the weak character of residual absorptions following decay of triplets suggests that the C--S bond photocleavage leading to thiophenoxy radicals is an inefficient process (particularly for linear trimer and tetramer, quantum yield < 0.05). Oxygen-quenching effects establish that the photocleavage in oligomers occur from the singlet manifold and not from the lowest triplet state. Major implications of these findings to PPS photodegradation and photostabilization are two-fold. First, in order to suppress degradation resulting from direct C--S bond cleavage, quenching should be aimed at excited singlets rather than triplets; this, however, would be difficult in view of short singlet lifetimes. Second, upon light absorption by the polymer, macromolecular triplets are expected to be formed in high yields as energy-rich, long-lived species; the quenching of these triplets by O or electron-acceptor-type impurities may lead to singlet O (a reactive oxidant for organosulfur compounds) or labile radical ions (potential intermediates through which C--S bond cleavage may readily occur).