Normal mode relaxation of polyisoprene in blends with vinyl polybutadienes

When probe chains are dispersed in a network or higher molecular weight mat rix,the expectation from the reptation model is that the terminal relaxatio n function will narrow, due to suppression of constraint release. Previous experiments found no support for this prediction, although they relied on t ime-temperature superpositioning to obtain spectra. These studies also made use of polyisoprene (PI)/1,4-polybutadiene blends, whose thermodynamic mis cibility is problematic We report herein measurements acquired with a broad -band dielectric spectrometer of the terminal relaxation of PI in blends wi th polybutadiene having substantial 1,2-content. This mixture is thermodyna mically miscible, thus excluding the possibility of phase separation. Corro borating earlier work, we find that the experimental behavior is qualitativ ely opposite to the prediction. Suppression of constraint release broadens the terminal dispersion, increasing the deviation from the theoretical rela xation function. This suggests that the common assumption that reptation an d constraint release are independent processes is incorrect.