A 2H NMR and dielelectric spectroscopy study of the slow β-process in organic glass formers

Abstract

We study the slow β-process of several organic compounds by recording dielectric and 2H NMR spectra below the glass transition temperature T g. For the neat systems toluene, polybutadiene, cis-decalin and ethanol as well as for the binary glasses chlorobenzene/ cis-decalin and toluene/2-picoline, the β-process manifests itself in very similar changes of the 2H NMR spectrum when varying the spatial resolution by increasing the interpulse delay t p in the solid-echo pulse sequence. These findings indicate that for all considered glasses, in particular, for both components of the binary mixtures, molecular dynamics involved in the β-process are highly comparable. A line-shape analysis reveals that the secondary relaxation is mainly caused by a highly restricted, step-wise reorientation of essentially all molecules.For the neat glasses 2-picoline, glycerol and polystyrene (PS) which do not show a β-peak in dielectric spectroscopy, there is no evidence for molecular reorientation in the 2H NMR spectra at T< T g.