The dependence of dielectric properties on crosslinking density of rubbers

The dielectric properties of natural rubber, synthetic polyisoprene, and polybutadiene samples crosslinked with sulfur and of natural rubber samples crosslinked simultaneously with sulfur and dicumyl peroxide were studied. In the investigated systems, the number of polar groups in unit volume and the crosslinking density may be changed independently in relatively wide ranges. The measurements were performed in the frequency range from 101 to 105 Hz at various temperatures, and macroscopic parameters describing the dielectric properties of the samples were evaluated. The position and intensity of the dielectric dispersion peak in the main transition zone is only slightly dependent on the crosslinking density of the samples. These two values are mainly influenced by the amount of combined sulfur, or more generally, by the number and the dipole moments of polar groups in the sample. The influence of crosslinking density on the width of the absorption curve is very strong; the width increases with increasing crosslinking density. The possibility of explaining this effect in terms of the dependence of the free volume distribution function on crosslinking density is discussed.