Study of free-volume distributions in polyisoprene by means of positron annihilation lifetime spectroscopy

The o-Ps lifetimes τ3 and their intensities I3 are measured in three samples of polyisoprene (natural rubber) with different concentrations of dicumyl peroxide additives (1 phr, 5 phr, and 10 phr) using positron annihilation lifetime (PAL) technique. From these results, the mean free volume hole sizes and free volume fractions have been deduced. In addition, the probability distributions of the free volumes have been obtained using Laplace inversion of PAL spectra, and found in agreement with T-C theory at the range of free volumes from 50 up to 400 angstroms3. On the other hand, I3 and τ3 are measured for the three samples as a function of temperature in the range between 77 and 315 K. From these results and using Arhenius plot, the activation energies for free volume mobility at Tg are determined as 7.6, 5.3 and 17.2 cal/gm/mole for the 1 phr, 5 phr and 10 phr polyisoprene samples respectively. At T>Tg, the activation energies are found to be 348, 350 and 386 cal/gm/mole. These results have been interpreted in terms of onset of different types of motion.