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

[Image: see text] 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 PI(3)-TESPT (PI of 2900 g/mol) used in rubber compounding is equal to that of the reference sample with TESPT (S(TESPT)). For S(PI-TESPT) samples, the amounts of PI(6)-TESPT (PI of 5500 g/mol) and PI(14)-TESPT (PI of 13,700 g/mol) used were calculated as molar ethoxyl groups which are nearly equivalent to those of PI(3)-TESPT. At the same wt % (parts per hundred, phr) of elemental sulfur in rubber compounds, despite the order of cross-linking density being S(TESPT) (sample prepared with TESPT) > S(PI(3)-TESPT) > S(PI(6)-TESPT) > S(PI(14)-TESPT), the exhibited tensile strength is of the order of S(PI(3)-TESPT) > S(PI(6)-TESPT) > S(TESPT) ≈ S(PI(14)-TESPT). The better mechanical properties of S(PI(3)-TESPT), as opposed to those of S(TESPT), 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 S(PI(3)-TESPT) are inferior to those of S(TESPT), these properties can be improved by adding more elemental sulfur to increase the cross-linking density.