Solid-State Carbon-13 NMR Studies of Vulcanized Elastomers. IX. TMTD-Vulcanized cis-1,4-Polybutadiene at 75.5 MHz

Abstract

Abstract The results of the present study indicate that, for the most part, the variety of C-S bonded structures formed in TMTD-vulcanized cis-BR qualitatively parallel those found in sulfur-cured vulcanizates of the material. Key exceptions include the demonstrated existence of stable rubber-bound accelerator fragments, and the possible elimination of select intramolecular cyclic species. Cis-to-trans isomerization has again been shown to be the predominant feature of the vulcanization reaction sequence and seems to obey first-order kinetics with respect to the concentration of accelerator. While the kinetic rate of conversion among all intermediate species was too rapid to permit a full account of the process, the spectral data demonstrate further that, subsequent to the formation of rubber-bound accelerator residues, sulfur becomes quickly redistributed to form crosslink structures of low average sulfur rank. In contrast to results obtained from elemental sulfur-cured BR vulcanizates, relatively high concentrations of monosulfidic crosslink structures were found in the TMTD vulcanizates at even the shortest reaction times. Similar to the results of the sulfur study, on the other hand, comparison between chemical and physical crosslink densities of the samples indicate the reaction sequence to have been highly localized, producing spatially inhomogeneous networks of elastically inefficient, clustered crosslinks. This latter result can be taken to imply that network morphology in TMTD-vulcanized BR is critically influenced by the solubility of the active accelerator complex in the rubber matrix. The effect was exaggerated in the present study by the omission of a compatible chelating agent for the zinc ions. From the standpoint of efforts aimed at optimization and control of the performance properties of vulcanized rubber articles, systematic investigation of the dependence of this effect on a wider range of formulation variables is warranted.