Inelastic Neutron Scattering Experiments on Van der Waals Glasses — A Test of Recent Microscopic Theories of the Glass Transition

Abstract

Abstract By performing incoherent inelastic neutron scattering experiments on the Van der Waals glass o‐terphenyl we tested predictions made by a recent microscopic theory — the mode coupling theory of the glass transition. In agreement with theory we find — besides the structural relaxation or α‐process which follows a Kohlrausch‐Williams‐Watts behaviour — the existence of a secondary relaxation process which shows very unusual neutron scattering features. This secondary or ß‐process leads to an anomalous decrease of the Debye‐Waller factor which can be interpreted in terms of a critical ( T c ‐T )‐behaviour as predicted by theory. According to mode coupling theory this behaviour should be connected to a critical slowing down of the density fluctuations in the vicinity of a critical temperature T c which is situated typically 30 to 150 degrees above the caloric glass transition temperature T g . We find our neutron scattering data to be compatible with the predictions of theory leading to a value of T c that is situated well above T g i.e. in the range 285–295 K as compared to T g = 243 K. This agrees well with a value of T c = 290 K derived independently by use of a power law, suggested by theory to hold for the behaviour of the viscosity at temperatures higher than T c .