Fast process of amorphous polystyrene below and above the glass transition temperature T g as studied by quasielastic neutron scattering

Abstract

The dynamics of atactic polystyrene (PS) has been studied by a quasielastic neutron scattering technique in a temperature range of 21–475 K covering the glass transition temperature Tg (=373 K). The so-called fast process in picosecond order is observed similarly to other glass-forming materials in supercooled states so far reported. However, the onset temperature of the fast process is ∼170 K below Tg in PS, which is far below Tg in contrast to the previous observations. This result implies that the fast process is not necessarily the β process predicted by the recently noted mode coupling theory. By comparing the result of hydrogenated PS with that of partially deuterated polystyrene (PS-d5) where only phenyl rings were preferentially deuterated, the origin of the fast process occurring far below Tg has been assigned to librational motion of phenyl rings, which is coupled with main chain motion near Tg. Analysis of the spectra by curve fit has suggested that the nature of the fast process changes at a temperature ∼30 K above Tg. This change also involves the appearance of a slow relaxation process in the energy range of several tens μeV. The origin of the slow process is discussed in terms of conformational transitions of the main chain.