Secondary relaxation processes in polybutadiene studied by²H nuclear magnetic resonance and high-precision dielectric spectroscopy

Using ²H NMR and high-precision dielectric spectroscopy, we identified in polybutadiene (PB) at low temperatures (20-100 K) a third relaxation process (γ-process) in addition to the α- and β-processes. PB samples with different molecular weights (M_w) exhibit a strongly varying relaxation strength (Δε_γ) though no clear-cut relationship is observed between Δε _γ and M_w. Concerning the temperature dependence of the γ-process, the set of samples can be divided into two subsets characterized by different activation energies, where one subset of time constants agrees well with those obtained recently from quasi-elastic light scattering in the melt. From our NMR experiments we conclude that at T < T_g a highly hindered motion (<8°) is responsible for the γ-process, which is difficult to explain by typical conformational changes.