Orientational effects on low-energy modes in amorphous poly(ethylene terephthalate) fiber

Inelastic and quasielastic neutron-scattering measurements have been perfor med on an amorphous poly(ethylene terephthalate) (PET) fiber in parallel an d perpendicular scattering geometries., i.e., when the fiber axis f is para llel and perpendicular to the scattering vector Q, respectively. The so-cal led boson peak is observed at around -1.4 meV in neutron energy transfer at low temperatures below about 150 K in both the geometries; as temperature increases, the fast process of picosecond order appears at a certain temper ature below the glass-transition temperature T-g (= 348 K). Although no dra stic differences in the dynamics can be observed, a closer look at the spec tra revealed some interesting features of the low-energy modes. From the me an-square displacements evaluated with two different energy resolutions, it was found that the fast process appears only in the direction perpendicula r to the polymer chain (not the fiber axis f) in the energy region between -1 and -0.2 meV. In the inelastic scattering spectra, we found that the bos on peak is stronger in intensity for the parallel direction while the inten sity of the fast process is larger for the perpendicular one. The quantitat ive analysis based on the recent vibration-relaxation model revealed that t he fast process can mainly be explained by the softening of the vibrational modes; the softening occurs at lower temperatures in the perpendicular geo metry than in the parallel one. On the other hand, the conventional single- lorentzian fit to the fast process, in which the fast process is assumed to be a relaxational process and described by a Lorentzian, showed that the o nset temperature of the fast process is lower in the perpendicular geometry than in the parallel one. These have been tentatively attributed to weaker force constants, for motions perpendicular to a polymer chain, such as tor sional and librational motions, than those along the chain. (C) 1998 Americ an Institute of Physics. [S0021-9606(98)50347-9].