Vulcanization of cis- and trans-polyisoprene and their blends:: cure characteristics and crosslink distribution

The cure characteristics of the two polyisoprenes having different configuration; naturally occurring cis-1,4-polyisoprene (natural rubber-NR) and synthetic trans-1,4-polyisoprene (TPI) were studied based on the reactivity and extent of vulcanization. Comparative cure rates for NR, TPI and their blends were determined using the differential scanning calorimetry. Rates of vulcanization of NR are found to be higher than those of TPI regardless the concentration of sulfur used due to higher allylic hydrogens. Adding TPI in NR could either reduce or increase rate of vulcanization of the blends. The extent of vulcanization of each polymer in the compatible NR/TPI blends was investigated using the observed changes in the damping height determined using dynamic mechanical analysis in combination with crosslink density and crosslink distribution determined for the vulcanized polymers and for probe treated vulcanizates employing general swelling method. It is clear that TPI undergoes higher degree of vulcanization when it was added more than 30 wt.% in the mixture of the blends. Drastic increase of the polysulfidic crosslink/monosulfidic plus disulfidic crosslink ratio for NR/TPI blend with TPI content of 30 wt.% for both conventional and efficient systems suggests that vulcanization chemistry in NR is altered by adding TPI. In addition, the degrees of vulcanization of TPI in NR/TPI blends cured using conventional vulcanization system with high concentration of sulfur, are considerably higher than those cured using efficient vulcanization system with low concentration of sulfur. TPI vulcanizates have greater degree of crosslinking than cis-1,4-polyisoprene vulcanizates resulting in lower molecular weight between crosslinks regardless the concentration of accelerator used. On the other hand, the proportion of polysulfidic crosslink for TPI was found to be significantly higher than that of cis-1,4-polyisoprene vulcanizate when they were cured using low accelerator/sulfur ratio. As a result, it shows that the different configuration of elastomers can give rise to different chemistry of sulfur vulcanization in which polysulfidic crosslinks for cis-1,4-polyisoprene are believed to predominantly desulfurized to give monosulfidic crosslink, while, for TPI, polysulfidic structures are more resistant to the decomposition. (C) 2001 Elsevier Science Ltd. All rights reserved.