STRUCTURE AND DENSITY OF CROSS-LINKS IN NATURAL-RUBBER VULCANIZATES - A COMBINED ANALYSIS BY NMR-SPECTROSCOPY, MECHANICAL MEASUREMENTS, AND RUBBER-ELASTIC THEORY

C-13-NMR MAS solid-state spectroscopy and H-1-NMR relaxation measurements have been applied for quantitative determination of crosslink density in sulfur-accelerated NR vulcanizates. It is shown that the C-13-NMR method is able to distinguish between monosulfidic crosslinks and polysulfidic structures; therefore a quantitative determination of chemical crosslinks was possible. An analysis of H-1-NMR transversal relaxation measurements provided the network densities which were in good agreement with the results of the C-13-NMR measurements. Comparing stress-strain data with the spectroscopically determined crosslink density of the NR vulcanizates and calculated crosslink densities of peroxide-cured vulcanizates, it is shown that the same relation between low-strain modulus and crosslink density is valid independent of the structure of the vulcanizates. The chemical crosslink density determined by a theoretical approach based on the tube concept was in approximate agreement with the NMR results.