Synchrotron radiation-based quasi-elastic scattering using time-domain interferometry with multi-line gamma rays

Microscopic Observation of the Anisotropy of the Johari-Goldstein-β Process in Cross-Linked Polybutadiene on Stretching by Time-Domain Interferometry

The strain dependence of the Johari-Goldstein (JG)-β relaxation time, as well as the directional dependence, was systematically investigated for stretched cross-linked polybutadiene using time-domain interferometry. We found that the strain dependence of the JG-β relaxation time is directionally dependent, contrary to expectation: the relaxation time of the JG-β motion, whose displacement is perpendicular to the stretching direction, decreases with stretching, whereas the relaxation time of the parallel JG-β motion changes little. This result is distinct from the previously reported strain dependence of the α relaxation time, where the relaxation time increases isotropically with stretching. Thus, the difference in the strain dependence of the relaxation time between the α and JG-β processes suggests a microscopic origin and requires the modification of the conventional dynamic picture for stretched polymers.

Broadband Quasielastic Scattering Spectroscopy Using a Multiline Frequency Comblike Spectrum in the Hard X-Ray Region

We developed a novel quasielastic scattering spectroscopy system that uses a multiline frequency comblike resolution function to overcome the limit on the accessible timescale imposed by the inherent single-energy resolution of conventional spectroscopy systems. The new multiline system possesses multiple resolutions and can efficiently cover a wide time range, from 100 ps to 100 ns, where x-ray-based dynamic measurement techniques are being actively developed. It enables visualization of the relaxation shape and wave-number-dependent dynamic behavior using a two-dimensional detector, as demonstrated for the natural polymer polybutadine without deuteration.

Neutron scattering studies on dynamics of lipid membranes

Neutron scattering methods are powerful tools for the study of the structure and dynamics of lipid bilayers in length scales from sub Å to tens to hundreds nm and the time scales from sub ps to μs. These techniques also are nondestructive and, perhaps most importantly, require no additives to label samples. Because the neutron scattering intensities are very different for hydrogen- and deuterium-containing molecules, one can replace the hydrogen atoms in a molecule with deuterium to prepare on demand neutron scattering contrast without significantly altering the physical properties of the samples. Moreover, recent advances in neutron scattering techniques, membrane dynamics theories, analysis tools, and sample preparation technologies allow researchers to study various aspects of lipid bilayer dynamics. In this review, we focus on the dynamics of individual lipids and collective membrane dynamics as well as the dynamics of hydration water.

Microscopic observation of the effects of elongation on the polymer chain dynamics of crosslinked polybutadiene using quasi-elastic γ-ray scattering

A synchrotron-radiation-based quasi-elastic γ-ray scattering system has been developed that uses time-domain interferometry to observe microscopic polymer dynamics under uniaxial deformation. The stress-producing mechanism of crosslinked polybutadiene has been studied from a microscopic viewpoint. It was found that the mean relaxation time ⟨τ⟩ of the microscopic polymer motion observed over a relatively high temperature (T) range (i.e. T-1 < 0.0045 K-1) increased with elongation on both the intra- and intermolecular scales. Following an extensive strain dependence study, it was found that the strain dependences of both the intra- and intermolecular ⟨τ⟩ changed with the stress dependence. It was therefore suggested that ⟨τ⟩ increased due to the constraint of the local polymer chain motion caused by elongation. The local molecular dynamics of polymer chains under uniaxial deformation could be evaluated at intra- and intermolecular scales separately for the first time using our method.

High-throughput nuclear resonance time domain interferometry using annular slits

Nuclear resonance time domain interferometry (NR-TDI) is used to study the slow dynamics of liquids (that do not require Mössbauer isotopes) at atomic and molecular length scales. Here the TDI method of using a stationary two-line magnetized ⁵⁷Fe foil as a source and a stationary single-line stainless steel foil analyzer is employed. The new technique of adding an annular slit in front of a single silicon avalanche photodiode detector enables a wide range of momentum transfers (1 to 100 nm^-1 by varying the distance between the annular slits and sample) with a high count rate of up to 160 Hz with a Δq resolution of ±1.7 nm^-1 at q = 14 nm^-1. The sensitivity of this method in determining relaxation times is quantified and discussed. The Kohlrausch-Williams-Watts (KWW) model was used to extract relaxation times for glycerol. These relaxation times give insight into the dynamics of the electron density fluctuations of glycerol as a function of temperature and momentum transfers.

Arriving at the most plausible interpretation of the dielectric spectra of glycerol with help from quasielastic γ-ray scattering time-domain interferometry

Glycerol is one of the glass-forming liquids selected by Robert H. Cole in 1950 to start his study of molecular dynamics by dielectric spectroscopy. Seventy-one years have gone by and remarkably no consensus has been reached on the nature and identity of the relaxation processes observed in the dielectric spectra. The macroscopic dielectric relaxation data allow different interpretations to yield contrasting results, and it is not possible to determine which one is most plausible. Coming to the rescue is the application of the nuclear γ-resonance time-domain interferometry (TDI) to glycerol by Saito et al. [Phys. Rev. E 105, L012605 (2022)10.1103/PhysRevE.105.L012605]. Their microscopic TDI data potentially can decide which interpretation of the dielectric spectra of glycerol is most plausible. The attempt was made by Saito et al., but there is a problem in their analysis of the dielectric data of glycerol and hence their conclusion is untenable. In this paper, we critically compare four major interpretations with the TDI data in an effort to identify the most plausible interpretation of the relaxation processes constituting the dielectric spectra of glycerol.

Microscopic observation of hidden Johari-Goldstein-β process in glycerol

The Johari-Goldstein-β (JG-β) process is widely observed in a variety of glass-forming systems and recognized as an intrinsic process in deeply supercooled and glassy states. However, in some systems, e.g., glycerol, a clear sign of the JG-β process is often not apparent; for example, an isolated JG-β peak may not be observed in the dielectric relaxation spectrum. In this study, we directly investigated the angstrom-scale dynamics of glycerol through quasielastic scattering experiments using time-domain interferometry. The relaxation times of the local motions start to decouple from the timescale of the diffusion process and follow the established behavior of the JG-β process. This finding microscopically indicates the existence of the hidden JG-β process in glycerol. In addition, we succeeded in determining the decoupling temperature of the JG-β process by using the spatial-scale selectivity of the quasielastic scattering technique.

Relationship between Viscosity and Acyl Tail Dynamics in Lipid Bilayers

Membrane viscosity is a fundamental property that controls molecular transport and structural rearrangements in lipid membranes. Given its importance in many cell processes, various experimental and computational methods have been developed to measure the membrane viscosity, yet the estimated values depend highly on the method and vary by orders of magnitude. Here we investigate the molecular origins of membrane viscosity by measuring the nanoscale dynamics of the lipid acyl tails using x-ray and neutron spectroscopy techniques. The results show that the membrane viscosity can be estimated from the structural relaxation times of the lipid tails.

Experimental evidence of mosaic structure in strongly supercooled molecular liquids

When a liquid is cooled to produce a glass its dynamics, dominated by the structural relaxation, become very slow, and at the glass-transition temperature T_g its characteristic relaxation time is about 100 s. At slightly elevated temperatures (~1.2 T_g) however, a second process known as the Johari-Goldstein relaxation, β_JG, decouples from the structural one and remains much faster than it down to T_g. While it is known that the β_JG-process is strongly coupled to the structural relaxation, its dedicated role in the glass-transition remains under debate. Here we use an experimental technique that permits us to investigate the spatial and temporal properties of the β_JG relaxation, and give evidence that the molecules participating in it are highly mobile and spatially connected in a system-spanning, percolating cluster. This correlation of structural and dynamical properties provides strong experimental support for a picture, drawn from theoretical studies, of an intermittent mosaic structure in the deeply supercooled liquid phase.

The Johari-Goldstein relaxation is a precursor of the glass transition in liquids. Caporaletti et al. use time-dependent interferometry data to substantiate its suggested structural appearance as a globally percolating, fluctuating mosaic.

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

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.

Probing Quantum Dynamical Couple Correlations with Time-Domain Interferometry

Time-domain interferometry (TDI) is a promising method to characterize spatial and temporal correlations at x-ray energies, via the so-called intermediate scattering function and the related dynamical couple correlations. However, so far, it has only been analyzed for classical target systems. Here, we provide a quantum analysis, and suggest a scheme that allows us to access quantum dynamical correlations. We further show how TDI can be used to exclude classical models for the target dynamics, and illustrate our results using a single particle in a double well potential.

Structural Relaxation and Viscoelasticity of a Higher Alcohol with Mesoscopic Structure

This work studied the slow dynamics of liquids with mesoscopic structure and its relation to shear viscosity. Quasielastic scattering measurements were made on a liquid higher alcohol, 3,7-dimethyl-1-octanol, using γ-ray time-domain interferometry at a synchrotron radiation facility, SPring-8. The quasielastic scattering spectra were measured to determine the structural relaxation at two wavenumbers of the prepeak and the main peak of the static structure factor. It was found that relaxation at the prepeak is more than 10 times slower than that at the main peak. Compared with the viscoelastic spectrum, which exhibits bimodal relaxation, the relaxations at the prepeak and the main peak were shown to correspond to the slower and faster modes of the viscoelastic relaxation, respectively. This indicates that the dynamics of the mesoscopic structure represented as the prepeak contributes to the shear viscosity through the slowest mode of the viscoelastic relaxation.