Sulfur vulcanisation Raman spectroscopy

Over the last decade the development of advanced analytical techniques, such as Fourier transform (FT) Raman and solid-state NMR spectroscopy, have been impressive, resulting in a great deal of progress in the field of the sulfur vulcanisation of unsaturated elastomers [22-25]. 

The FT-Raman spectra of the sulfur vulcanisates of the various model olefins do not contain the characteristic disulfide signal at 510 cm"1, but do contain the typical higher sulfide bands at 490, 460 and 440 cm"1 (Table 6.2). In addition, a new band at about 475 cm 1 is observed for the vulcanisates of 2-methyl-2-pentene and 3-hexene, which has not yet been assigned (hexasulfide ). Results of HPLC analysis have shown that the vulcanisate of 2,3-dimethyl-2-butene consists mainly of a mixture of disulfide to pentasulfide with about 15 mole% of disulfide [79]. This illustrates that FT-Raman spectroscopy is not very sensitive for the identification of disulfides. Because of an overlap of signals, FT-Raman does not provide detailed, quantitative information on the presence of the individual higher sulfides (S>2). 

NMR spectroscopy and solid-state and 13C NMR relaxation-time experiments. However, the sensitivity of solid-state 13C NMR is not as high as that of Raman and IR spectroscopy. For instance, solid-state 13C NMR of sulfur-vulcanised EPDM could only be performed when the ENB unsaturation of EPDM was fully isotopically enriched with 13C NMR [124]. 

As a result of the high polarisability of C—S and S—S bonds, Raman spectroscopy is especially suitable for studying sulfur vulcanisation of elastomers. However, conventional Raman studies of elastomers are limited on account of sample fluorescence (often due to impurities). 

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