A microscopic look at the Johari-Goldstein relaxation in a hydrogen-bonded glass-former

Abstract

Understanding the glass transition requires getting the picture of the dynamical processes that intervene in it. Glass-forming liquids show a characteristic decoupling of relaxation processes when they are cooled down towards the glassy state. The faster (βJG) process is still under scrutiny, and its full explanation necessitates information at the microscopic scale. To this aim, nuclear γ-resonance time-domain interferometry (TDI) has been utilized to investigate 5-methyl-2-hexanol, a hydrogen-bonded liquid with a pronounced βJG process as measured by dielectric spectroscopy. TDI probes in fact the center-of-mass, molecular dynamics at scattering-vectors corresponding to both inter- and intra-molecular distances. Our measurements demonstrate that, in the undercooled liquid phase, the βJG relaxation can be visualized as a spatially-restricted rearrangement of molecules within the cage of their closest neighbours accompanied by larger excursions which reach out at least the inter-molecular scale and are related to cage-breaking events. In-cage rattling and cage-breaking processes therefore coexist in the βJG relaxation.

Experimental evidence of mosaic structure in strongly supercooled molecular liquids

“…It is then evident that, at least for T ≤ 131.4 K, TDI is sensitive to the β JG -process both at the inter and intra-molecular lengthscales. This provides broader validity to similar observations for another monoalcohol 12 .…”

“…Nuclear γ -resonance time-domain-interferometry (TDI) is one of the few techniques able to probe directly microscopic density fluctuations at the timescale where the β JG -process decouples from the structural one. TDI is based on an interferometric analysis of the X-rays scattered by the sample and, as such, provides information that is both time and q (and therefore space) resolved ( q being the momentum exchanged in the scattering process) 8 , 12 . TDI has been here employed to study 1-propanol, a model glass-former with a genuine β JG -process 17 rather coupled to the α -relaxation.…”

“…While these ideas are not necessarily alternative to each other, unambiguous experimental input is required as a guide among these rather different views. Combining new X-ray scattering data on 1-propanol with data available in the literature 8 – 10 , 12 and here re-analyzed, evidence shall now be provided of two striking features related to the molecules participating in the β JG -relaxation: (i) their mean-squared displacement satisfies the Lindemann criterion for structural instability and (ii) their number matches the threshold for site percolation. They, therefore, form a mobile percolating, infinite cluster pervading the whole sample.…”

“…This conclusion is in apparent contrast with the restricted re-orientational dynamics of the process 7 and with its mild Arrhenius T -dependence, usually interpreted as the signature of a hindered, simply activated process 2 . The few available neutron and X-ray scattering experiments also describe it, above T g , as an essentially restricted if not localized molecular motion occurring mainly within the transient cages formed by the nearest-neighbors 8 – 12 . Sub- T g experiments 13 and atomistic simulations 14 , 15 support instead of the idea that the β JG -relaxation is rather cooperative and characterized by string-like excitations occurring via hopping between adjacent atomic sites.…”

Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation

“…Concerning ISF, timescale separation is possible exploring high wavevector q, in order to be more sensitive to local and restricted dynamics, but the relative strength is also affected by this choice [41]. As recently shown [42], the microscopic density correlation function is dominated at short times by spatial fluctuations of some molecules within the cage formed by the nearest neighbours, that is, by rattling motions within the cage, while at longer times part of the relaxation strength is related to large spatial fluctuations extending at least up to the intermolecular distance, that is, outside the cage. The faster type of motion has been identified as the secondary, while the slower and more intense is the primary one.…”