Slow dielectric relaxation of cis-polyisoprene near the glass transition temperature

Dielectric measurements were carried out on undiluted cis-polyisoprene (PI) over a wide frequency range from 10(-4) to 10(5) Hz around the glass transition temperature T-g (=200 K). The data indicate that the dielectric loss curves including both the normal and segmental mode processes are superposable in the range above 230 K (T-g + 30 K). Below 230 K, the time-temperature superposition is not applicable to the whole loss curve. The ratio of the relaxation times of the normal and segmental modes is constant above 230 K hut decreases below 230 K with decreasing temperature.