2H-NMR study of the glass transition in supercooled ortho-terphenyl

The glass forming molecular liquid ortho-terphenyl has been investigated by 2H-NMR techniques providing spin-relaxation times (T1, T2), and spin-alignment data which yield information on the time scale and geometry of ultra-slow molecular reorientation. The main results are as follows: The primary glass transition (α process) is characterized by rotational molecular jumps with a jump size distribution weighted in favor of large jump angles, and by a distribution of correlation times. In addition intramolecular flip–flop jumps of the lateral phenyl rings are found which do not take part in the α process. Apart from this (secondary) intramolecular dynamics no residual small angle reorientation persists below Tg on the time scale (10−4 to 102 s) of the spin-alignment experiment.