Cross-linking process of cis-polybutadiene rubber with peroxides studied by two-dimensional infrared correlation spectroscopy: a detailed tracking

Abstract

cis-Polybutadiene rubber (cis-BR) is one of the typical unsaturated rubbers in mass production and is widely used. However, the detailed mechanism of its cross-linking with peroxides is still unclear so far. In this study, in situ FTIR spectra combined with the powerful PCMD2D and 2D correlation spectroscopy was used to track the detailed cross-linking process. The temperature region of cis-BR cross-linking determined by PCMW2D was within 165–195 °C. The temperature with a maximum cross-linking rate was determined at 183 °C via PCMW2D, which is identical with DSC. The generation of –˙CH– macromolecular free radicals through losing α hydrogens was observed when below 165 °C. An abnormal increase of double bonds with trans-1,4-structure during the cross-linking (165–195 °C) was observed. An obvious enhancement of –CH₂– groups was also found, which indicated that a large number of the double bonds with cis-1,4- and 1,2-structure involved in cross-linking is transformed into –CH₂– groups. A 5-step process for the whole cross-linking was inferred from the sequential order of group motions. The first step is DCP decomposition and free radical release. The second step is the generation of trans-1,4-structure due to the internal rotation of the cis-1,4-structure induced by free radicals at α position. The third step is the free radical addition of double bonds with a 1,2-structure, and the fourth is the free radical addition of double bonds with a cis-1,4-structure. The final step is the cross-linking via double coupling of two macromolecular free radicals. In the last step, the free radicals from the cis-1,4-structure also can be probably terminated by a chain transfer.