Fast local motions and their transitions in polybutadiene across the glass transition

We investigated the local dynamics of polybutadiene over a wide temperature range, from the glassy state to the liquid (rubbery) state, using quasielastic neutron scattering (QENS) combined in a recently developed modified mode distribution analysis. This approach enabled high-precision separation of QENS spectra into vibrational components and multiple relaxation processes without relying on specific model functions. Among these processes, the subpicosecond fast process was analyzed using a local diffusion model, which characterizes the dynamics via two physically meaningful parameters: the local diffusion coefficient and the harmonic potential stiffness. Temperature-dependent analysis revealed a distinct dynamic transition near the glass transition, which is interpreted as the disappearance of cooperative diffusion involving multiple segments. Furthermore, an additional relaxation mode, termed the extra-fast process, was identified and interpreted as a higher-order mode within the local diffusion model. These findings suggest a unified physical framework for the evolution of local motions across the glass transition and establish a robust methodology for quantifying polymer dynamics at subnanometer and subpicosecond scales.