The Infrared Spectrum of Vulcanized Rubber and the Chemical Reaction between Rubber and Sulfur
Abstract In an earlier paper we described an infrared spectroscopic investigation of the vulcanization of rubber, and drew attention to a band at 10.4 µ (960 cm.−1) which appeared in the spectrum of both straight and accelerated vulcanization. We have investigated further this phenomenon, and find that the intensity of this band correlates well with the proportion of sulfur chemically incorporated, but not with the degree of vulcanization as reflected in the physical properties of the finished product. We were unable to explain this band in a convincing manner in terms of C—S linkages, but more recently have found a band in the same position (10.4 µ) and another at 11.2 µ (890 cm.−1) in certain thermally degraded rubbers. Since in this case the two new bands are obviously due, respectively, to the development of new CHR:CHR′ and CRR′:CH2 groups, it seemed probable that the 10.4 µ band might also be caused by new groupings of the former type. An investigation of the spectrum of an ebonite (in which the rubber-sulfur reaction has been carried to an extreme) confirmed this idea. In this spectrum, absorptions in the region of 6 µ showed that a considerable number of bonds remained, but the characteristic absorption at 12.0 µ arising from CRR′: CHR″groups in rubber had largely disappeared. On the other hand, the band at 10.4 µ (although shifted to slightly longer wave lengths) had increased greatly in intensity.