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

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 (Tc‐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 Tc which is situated typically 30 to 150 degrees above the caloric glass transition temperature Tg. We find our neutron scattering data to be compatible with the predictions of theory leading to a value of Tc that is situated well above Tg i.e. in the range 285–295 K as compared to Tg = 243 K. This agrees well with a value of Tc = 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 Tc.

Dynamics in the Glassy State and Near the Glass Transition of Amorphous Polymers as Studied by Neutron Scattering

“…Such a fast process was first found for some polymers by Kaji et al [85][86][87]. Since then, similar fast processes have been observed in inorganic and organic glass-forming molecules [75][76][77][78] as well as in many glass-forming polymers [32,35,36,39,51,79,[88][89][90][91][92][93][94][95]. The dynamic properties are rather similar to those of the b-process predicted by the MCT, so that it was often discussed on the basis of MCT [77,[96][97][98].…”

“…These works revealed anomalous but common features of the dynamics near T g . One of the most outstanding results is the finding of a very fast motion of picosecond order appearing below T g in various glass-forming materials in an energy range below about 2 meV [75][76][77]. It is obvious that the so-called a-process is mainly concerned with the glass transition phenomena.…”

“…Stimulated by this theory, many experimental works have been performed on inorganic [73,74] and organic [75][76][77][78] glass-forming materials, including polymers [79][80][81][82][83], using neutron scattering, light scattering, and dielectric relaxation techniques. These works revealed anomalous but common features of the dynamics near T g .…”

The significance of the amorphous potential energy landscape for dictating glassy dynamics and driving solid-state crystallisation

“…Furthermore, to highlight that the results presented in this work are not simply confined to small-molecule or hydrogen bonded systems like sorbitol, and as originally suggested based on previous reports, 18,19 Fig. 1 also contains results from BLS, 36 neutronscattering mean-squared-displacement (MSD), 37 and deuteron nuclear magnetic resonance ( 2 H-NMR) spin-lattice relaxation 38 measurements on ortho-terphenyl (OTP), and neutron-scattering MSD, 39 positron spectroscopy (PS), 40,41 quasi-elastic light scattering (QELS), 39 and dielectric spectroscopy 42 for polymethyl methacrylate (PMMA), which exhibit the same temperature-dependent behaviour as described for sorbitol, although with different values for T ga and T gb . It is important to comment that as noted previously, 18,19 these data represent the detection of T ga and T gb across timescales spanning over 15 orders of magnitude, indicating that a fundamental link between the processes must exist.…”

Primary relaxation in a hard-sphere system

“…y' is defined by the occurrence of a fold bifurcation in Eq. (11) for fq. The fractal short-time power behavior 4q(t) fq »et is the so-called von Schweidler law.…”