Terminal relaxations in linear and three-arm star polyisoprenes

Dynamic mechanical spectra were obtained for linear and three-arm star-branched polyisoprenes (PI) in the terminal region of the viscoelastic spectrum. Terminal relaxation times, defined from the peak frequency of the dispersion in the loss modulus, were found to be more sensitive to changes in temperature for the stars than for the linear polyisoprene. From Arrhenius plots of the ratio of the relaxation times for the star and linear polymers, an activation energy describing the excess temperature dependency of the stars was determined. The results were 2.3 and 3.2 kcal/mol for star PI with respective arm molecular weights equal to 18 and 44 times the molecular weight between entanglements. Unlike the linear PI, the star-branched polymers were thermorheologically complex. These results are very similar to the behavior of polybutadiene, hydrogenated polybutadiene, and hydrogenated polyisoprene. Nevertheless, they are unexpected, in that the existing hypothesis for the excess temperature dependence and thermorheological complexity of branched polymers should not apply to polyisoprene.