Short-time dynamics of phenylene-rings in bisphenol based engineering thermoplastics

Abstract

We have recently performed one of the first approaches by means of quasielastic neutron scattering (QENS) to the problem of identifying the molecular motions giving rise to the secondary relaxations of engineering thermoplastics. Preliminary results point to phenylene ring π-flips as the main motion causing the observed quasielastic broadening in the ∼ time scale below the glass transition temperature Tg. Continuing our study of sub-Tg dynamics in these systems by QENS, measurements on polycarbonate (PC) and polysulfone (PSF) with deuterated methyl groups (d6) in the ∼10−13–10−11 s time scale have been performed. The intermediate scattering function shows a smooth second decay in addition to that of vibrations and “fast dynamics” at K. The extrapolation of phenylene π-flip motion to faster times does not explain the decay observed. However, a non-negligible contribution of π-flips at T⩾350 K in PCd6 is noticeable, whereas for PSFd6 some effect can be inferred above ∼450 K. In the temperature region where the π-flips do not contribute to the spectra we have characterised the signal by assuming a temperature dependent distribution of small angle oscillations of phenylene rings leading to an activation energy of 0.18 eV.