Effects of Substituting Polyisoprenyl Carbanions for Ethoxyl Groups of Bis-[3-(triethoxysilyl)propyl] Tetrasulfide on the Properties of Carbon Black and Silica-Reinforced NR/SSBR

The coupling agent TESPT (bis-[3-(triethoxysilyl)propyl] tetrasulfide) was modified by substituting polyisoprenyl (PI) carbanions for the ethoxyl groups on silicon for increasing the interaction of rubber with its fillers. The modification was carried out by the reaction of TESPT with polyisoprenyllithium, which had been previously prepared by anionic polymerization of isoprene using butyllithium. The success of the substitution was confirmed by Fourier-transform infrared spectroscopy, and the molecular weight of the modified TESPT (PI-TESPT) was determined from gel permeation chromatography measurements. The effects of tethered PI, as well as of its chain length, on the mechanical and dynamic properties of rubber compounds were examined using a universal testing machine and dynamic mechanical analysis (DMA). In rubber sample preparation, the amount of PI3-TESPT (PI of 2900 g/mol) used in rubber compounding is equal to that of the reference sample with TESPT (STESPT). For SPI-TESPT samples, the amounts of PI6-TESPT (PI of 5500 g/mol) and PI14-TESPT (PI of 13,700 g/mol) used were calculated as molar ethoxyl groups which are nearly equivalent to those of PI3-TESPT. At the same wt % (parts per hundred, phr) of elemental sulfur in rubber compounds, despite the order of cross-linking density being STESPT (sample prepared with TESPT) > SPI3-TESPT > SPI6-TESPT > SPI14-TESPT, the exhibited tensile strength is of the order of SPI3-TESPT > SPI6-TESPT > STESPT ≈ SPI14-TESPT. The better mechanical properties of SPI3-TESPT, as opposed to those of STESPT, could be attributed to the extra reinforcement from the PI–rubber chain linkage and better silica dispersion, as suggested by the mixing torque and Payne effect (ΔG′) measurements. While the dynamic properties of SPI3-TESPT are inferior to those of STESPT, these properties can be improved by adding more elemental sulfur to increase the cross-linking density.