Effects of Cross-Linking Density and of Stretching on Dielectric a-Relaxation in Poly(acrylonitrile-co-butadiene) Rubber

Dielectric studies were performed on acrylonitrile–butadiene random copolymer with intermolecular carbon–carbon cross-links by dicumyl peroxide. The effects of cross-linking and of stretching on the a-relaxation were examined on the polymer coated with evaporated-gold electrodes. With increasing cross-linking density the strength of dielectric relaxation decreased, the position of loss maximum shifted to a higher temperature, and the half width of the absorption curve increased. The apparent activation energy was virtually unchanged. On the other hand, the effect of stretching on the a-relaxation could not be observed in the range of the elongation ratio upto 3 examined in these experiments. These results may be interpreted as due to the decrease in free volume, especially in those regions close to the cross-links and hence, to the broadening of the distribution of free volume.