Linear polysulfides : their characterisation and degradation pathways
Linear polysulfides (LPs) are widely used in high performance sealants. The typical structure of the polymer is: H(SC2H40CH2OC2H4S)nH. We have completely characterised the complex components that make up commercially used LPs, by means of electrospray ionisation mass spectrometry (ESI- MS), combined with collision induced dissociation (CID). ESI has proved to be an invaluable mass spectroscopic technique in the characterisation of linear polysulfides. The individual spectra are well-resolved, enabling conclusions to be reached about the presence of variant mers (usually associated with additional oxymethylene or oxyethylene units or a monosulfide linkage, or some combination of these), and about the identity of end-groups and nature of the repeat unit. We have also been able to use ESI-MS to study the degradation reactions of LP pre-polymers. We have obtained spectra of LPs using matrix assisted laser desorption ionisation time-of-flight mass spectrometry, MALDI-TOF-MS), but these are not as informative as spectra obtained by ESI-MS. Field desorption mass spectrometry (FD-MS) has also been successful in characterising LP samples, but this technique is less generally accessible and much more time-consuming than either ESI-MS or MALDI-TOF-MS. LPs degrade thermally and photochemically by two competing degradation pathways: i) a free radical autoxidation mechanism, and ii) a hydrolysis mechanism. UV photolysis or pyrolysis of LP pre-polymers in the presence of air or oxygen resulted in the development of carbonyl groups detectable by IR and 13C NMR spectroscopy, while 2-D NMR studies and the highly characteristic field positions of the NMR resonances show the carbonyl group to be due to a formate ester. All thin film samples of cured LP, irrespective of the LP structure and curing agent, including ZL-2264 (with an extra CH2 linkage in the repeat unit), also produce this carbonyl absorption at 1725 cm-1 when exposed to UV irradiation. The formate ester arises as a result of a conventional free radical mechanism of autoxidation; attack at the C-H bond adjacent to the ether oxygen atom leads to a hydroperoxide and hence an alkoxyl radical, followed by scission to give the formate ester. The study of liquid exudates formed when TBBP- and HDDA- cured LP cast blocks are exposed to prolonged periods of heat or UV irradiation also show the development of carbonyl bands at 1724 cm-1 in their IR spectra, again indicating that degradation is occurring at least in part via a free radical mechanism. ESI-MS studies of degraded pre-polymer samples and liquid exudates show that, in parallel with the degradation route involving oxygenation of a methylene group, there is a hydrolysis mechanism involving initial cleavage of the formal group to release formaldehyde, followed by secondary reactions to give other products detected in the ESI-MS spectrum of the photolysate/pyrolysate. This mechanism is supported by the extremely slow degradation of ZL-2264, which has no formal group. The hydrolysis mechanism is the principal route for the degradation of LP pre- polymers and TBHP- and HDDA- cured LP block samples, while LPs cured using MnO2 and NaB03 degrade almost exclusively via the free radical autoxidation pathway.