Neutron scattering from vibrations and relaxations in amorphous polymers

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Abstract
Recent neutron scattering investigations of amorphous polymers in the meV range are reviewed. They reveal several new results: (i) the low-frequency vibrations of the boson peak shift to lower frequencies with increasing temperature, (ii) a change in the atomic motional behaviour occurs at a crossover frequency of several tenths of a meV, corresponding to a relaxation time of about two picoseconds and (iii) the fast picosecond relaxation, supposed to be a typical feature of the undercooling liquid, has been found in some polymers far below the glass transition. A possible explanation for these findings is a crossover from more or less damped vibrations at high frequencies to low-barrier relaxations at lower-frequencies. The concept can be quantified in terms of the vibration-relaxation model, a simplified classical version of the soft-potential model for glasses.