Comparative study of the Debye-Waller factor anomaly at the glass transition of isotopically substituted orthoterphenyls

We report on neutron-scattering measurements of the Debye-Waller factor of three isotopically substituted glassy orthoterphenyls: fully protonated, fully deuterated and selectively deuterated at the lateral phenyl rings. From the comparison of the observed anomaly in the glass transition region (β-process) found in the three systems, we can exclude any intramolecular phenyl-ring dynamics as the dominant motional mechanism for the β-process. The data thus support mode-coupling theory proposing a center-of-mass motion as precursor process of the glass transition.