Fast Secondary Relaxation Research Articles

Gamma relaxation in Dy-based metallic glasses and its correlation with plasticity

The discovery of secondary relaxation in metallic glass (MG) breaks the longstanding cognition that MG, as a simple glassy system, exhibits only α relaxation and contributes significantly to the understanding of the structure and mechanical properties of MG. In this work, two independent fast secondary relaxation peaks corresponding to γ and β′ relaxation were detected by dynamic mechanical measurement in several Dy-based MGs at temperatures far below the glass transition temperature. Compared with β′ relaxation, the novel γ relaxation is activated at lower temperatures and possesses smaller activation energy approximately 2/3 of that for β′ relaxation. By exciting γ relaxation, loosely packed regions evolve into vein-like patterns in the fracture surface, leading to unusual plasticity at cryogenic temperatures. Furthermore, a holistic relaxation spectrum including boson peak, fast γ and β′ relaxation, slow β relaxation and primary α relaxation in MG was depicted.

Intra- vs Intermolecular Cross-Links in Poly(methyl methacrylate) Networks Containing Enamine Bonds

The molecular dynamics of a copolymer composed of methyl methacrylate (MMA) and (2-acetoacetoxy)ethyl methacrylate (AEMA) monomers and the influence on it of intra- to intermolecular cross-links of AEMA units with ethylenediamine (EDA) was studied by combining dielectric relaxation experiments and thermal investigations. The dielectric spectra of the non-cross-linked copolymer show three dynamical processes: a slow relaxation (α) and a faster (β), both dominated by the MMA dynamics, and an even faster secondary relaxation (γ) reflecting the AEMA dynamics. Already for low cross-linking densities, the γ process is very much affected and eventually disappears, increasing the cross-linking density. The secondary β relaxation however was nearly unaffected by cross-linking. The effect of cross-linking on the α relaxation was very pronounced with an important increasing of the glass transition temperature Tg. There was also an increase of the dynamic heterogeneity and the relaxation intensity when increasing the cross-linking density (up to the maximum explored, 9 mol % EDA). The quality of the average time scale and Tg value have similarities in behavior for intra- and intermolecular cross-linking, but clear differences in the dynamic heterogeneities where observed. These differences can be interpreted in connection with the sparse internal structure of the collapsed single chains obtained by intramolecular cross-linking.

Dielectric relaxation study of poly (ether imide) by thermally stimulated depolarization currents

AbstractThis work presents a detailed study, by thermostimulated depolarization currents, of the molecular mobility of poly (ether imide) in a broad temperature range covering the amorphous solid state, the glass transition, and the rubber state. Taking advantage of the very wide distribution of motional modes of the PEI glass transition (the broadest of all known glass formers), we carried out an in‐depth study of physical aging involving different modes, which provided a more comprehensive view of the aging process. We also analyzed in detail the mobility below Tg where, in addition to the fast secondary relaxation, we identified a Johari–Goldstein. In the rubber state, on the other hand, we detected a mobility but showed that it corresponds to space charge motions rather to dipolar reorientations.

Unravelling Main- and Side-Chain Motions in Polymers with NMR Spectroscopy and Relaxometry: The Case of Polyvinyl Butyral.

Polyvinyl butyral (PVB) is an amorphous polymer employed in many technological applications. In order to highlight the relationships between macroscopic properties and dynamics at a microscopic level, motions of the main-chain and of the propyl side-chains were investigated between Tg − 288 °C and Tg + 55 °C, with Tg indicating the glass transition temperature. To this aim, a combination of solid state Nuclear Magnetic Resonance (NMR) methods was applied to two purposely synthesized PVB isotopomers: one fully protonated and the other perdeuterated on the side-chains. 1H time domain NMR and 1H field cycling NMR relaxometry experiments, performed across and above Tg, revealed that the dynamics of the main-chain corresponds to the α-relaxation associated to the glass transition, which was previously characterized by dielectric spectroscopy. A faster secondary relaxation was observed for the first time and ascribed to side-chains. The geometry and rate of motions of the different groups in the side-chains were characterized below Tg by 2H NMR spectroscopy.

Dynamics in the plastic crystalline phase of cyanocyclohexane and isocyanocyclohexane probed by 1H field cycling NMR relaxometry.

Proton Field-Cycling (FC) nuclear magnetic resonance (NMR) relaxometry is applied over a wide frequency and temperature range to get insight into the dynamic processes occurring in the plastically crystalline phase of the two isomers cyanocyclohexane (CNCH) and isocyanocyclohexane. The spin-lattice relaxation rate, R1(ω), is measured in the 0.01-30MHz frequency range and transformed into the susceptibility representation χNMR ″ω=ωR1ω. Three relaxation processes are identified, namely, a main (α-) relaxation, a fast secondary (β-) relaxation, and a slow relaxation; they are very similar for the two isomers. Exploiting frequency-temperature superposition, master curves of χNMR ″ωτ are constructed and analyzed for different processes. The α-relaxation displays a pronounced non-Lorentzian susceptibility with a temperature independent width parameter, and the correlation times display a non-Arrhenius temperature dependence-features indicating cooperative dynamics of the overall reorientation of the molecules. The β-relaxation shows high similarity with secondary relaxations in structural glasses. The extracted correlation times well agree with those reported by other techniques. A direct comparison of FC NMR and dielectric master curves for CNCH yields pronounced difference regarding the non-Lorentzian spectral shape as well as the relative relaxation strength of α- and β-relaxation. The correlation times of the slow relaxation follow an Arrhenius temperature dependence with a comparatively high activation energy. As the α-process involves liquid-like isotropic molecular reorientation, the slow process has to be attributed to vacancy diffusion, which modulates intermolecular dipole-dipole interactions, possibly accompanied by chair-chair interconversion of the cyclohexane ring. However, the low frequency relaxation features characteristic of vacancy diffusion cannot be detected due to experimental limitations.

Influence of short- to medium-range electronic and atomic structure on secondary relaxations in metallic glasses

Unusual secondary relaxations were revealed in metallic glasses (MGs) recently. In this paper, we explore the structural origins of secondary relaxations in La-based MGs alloyed with different species of elements. With the combination of synchrotron X-ray diffraction and ab initio molecular dynamics simulations, solute-atom-centered clusters with a string-like type of medium-range order are found in the MGs, the formation of which leaves dispersed low-electron-density regions. It is found the activation energy of fast secondary relaxation increases with the reduction of low-electron-density regions, while slow secondary relaxation relates to the distance of La in next nearest La-La atomic pairs and the size of the string-like solute-atom-centered clusters. The phenomena are interpreted within the framework of the generalized Maxell model and free volume model. Our results demonstrate fast secondary relaxation as the activation of a small concentration of liquid-like regions with extremely low viscosities preceding slow secondary relaxation, and provide evidence for the correlation of secondary relaxations with short- to medium-range electronic and atomic structure in MGs.

Fast secondary relaxation and plasticity initiation in metallic glasses

在形成玻璃的过冷液体和玻璃态固体中，快于主结构弛豫的动力学行为常统称为二次弛豫。过去大量的实验研究成果揭示高分子玻璃体系中，二次弛豫与其诸多基本物理和力学性能可能存在着密切的关联，如形变、晶化、玻璃转变等现象。 近期，基于大量动态力学分析研究，上海大学和香港城市大学联合课题组在金属玻璃中发现了独特的不同于主弛豫和已发现二次弛豫的快二次弛豫现象。进一步通过分析不同金属玻璃快二次弛豫的动力学特征，他们证明了这种快二次弛豫普遍存在于不同金属玻璃体系中，并建立了其与金属玻璃塑性的关联 。 本文简要阐述了金属玻璃快二次弛豫的基本特征，讨论了新发现与经典“剪切转变区”模型的异同，最后探讨了金属玻璃塑性起源可能的动力学机制。 金属玻璃快二次弛豫的发现对于我们理解金属玻璃动态非均匀性提供了新的视角。同时，对于我们深入探索金属玻璃物理力学性能的动力学起源也提供了重要的实验依据。

Universal secondary relaxation and unusual brittle-to-ductile transition in metallic glasses

It has long been recognized that the relaxation spectrum of glassy solids is intrinsically connected to their disordered structural features and deformation behaviors. However, such connections still remain elusive for metallic glasses. Here, through the extensive study of a variety of metallic glasses over a wide range of temperatures, we provide the compelling evidence for the existence of a universal fast secondary relaxation process, which occurs at a temperature far below the glass transition point with a low activation energy (0.3–0.6 eV). Furthermore, it is demonstrated that the initiation of plasticity in metallic glasses is strongly correlated with the fast relaxation process. By exciting the fast relaxation process, multiple shear banding is triggered as opposed to single shear banding in the metallic glasses, which leads to an unusual brittle-to-ductile transition in malleability, being fundamentally different from the ordinary one commonly observed in crystalline alloys. Furthermore, our results shed the quantitative insights into the atomistic mechanisms that differentiate brittle from plastic metallic glasses at different temperatures.

The Slow Molecular Mobility in Amorphous Ketoprofen and Ibuprofen

The slow molecular dynamics in two active pharmaceutical drugs, ketoprofen and ibuprofen, have been studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). This study allowed finding the main kinetic features of the fast secondary (γ) relaxation, of the Johari-Goldstein relaxation, and of the main (glass transition) relaxation, in particular their distribution of relaxation times. The fragility index of the two glass formers was determined based on data from DSC and from TSDC. The obtained results were compared with those obtained by other experimental techniques, namely, dielectric relaxation spectroscopy.

The slow molecular mobility in amorphous captopril and captopril disulfide: A DSC and TSDC study

The slow molecular mobility in the amorphous pharmaceutical drug captopril is studied by differential scanning calorimetry (DSC) and thermally stimulated depolarization currents (TSDC). The general kinetic features of the secondary relaxations and of the main relaxation were determined by TSDC. The dynamic fragility of captopril is obtained by DSC and TSDC, and the values compared with that obtained by dielectric relaxation spectroscopy (DRS). The local motional modes of the fast secondary relaxation were found to be slightly aging sensitive, even at temperatures lower than 100° below Tg, which was ascribed to the intermolecular hydrogen bond network that is reported to exist in the amorphous solid state of captopril. Captopril disulfide, a metabolite product of captopril, was synthetized and characterized by DSC and TSDC. Compared to captopril, it behaves as a stronger glass forming liquid.

Secondary relaxation dynamics in rigid glass-forming molecular liquids with related structures.

The dielectric relaxation in three glass-forming molecular liquids, 1-methylindole (1MID), 5H-5-Methyl-6,7-dihydrocyclopentapyrazine (MDCP), and Quinaldine (QN) is studied focusing on the secondary relaxation and its relation to the structural α-relaxation. All three glass-formers are rigid and more or less planar molecules with related chemical structures but have dipoles of different strengths at different locations. A strong and fast secondary relaxation is detected in the dielectric spectra of 1MID, while no resolved β-relaxation is observed in MDCP and QN. If the observed secondary relaxation in 1MID is identified with the Johari-Goldstein (JG) β-relaxation, then apparently the relation between the α- and β-relaxation frequencies of 1MID is not in accord with the Coupling Model (CM). The possibility of the violation of the prediction in 1MID as due to either the formation of hydrogen-bond induced clusters or the involvement of intramolecular degree of freedom is ruled out. The violation is explained by the secondary relaxation originating from the in-plane rotation of the dipole located on the plane of the rigid molecule, contributing to dielectric loss at higher frequencies and more intense than the JG β-relaxation generated by the out-of-plane rotation. MDCP has smaller dipole moment located in the plane of the molecule; however, presence of the change of curvature of dielectric loss, ε″(f), at some frequency on the high-frequency flank of the α-relaxation reveals the JG β-relaxation in MDCP and which is in accord with the CM prediction. QN has as large an in-plane dipole moment as 1MID, and the absence of the resolved secondary relaxation is explained by the smaller coupling parameter than the latter in the framework of the CM.

Unusual fast secondary relaxation in metallic glass.

The relaxation spectrum of glassy solids has long been used to probe their dynamic structural features and the fundamental deformation mechanisms. Structurally complicated glasses, such as molecular glasses, often exhibit multiple relaxation processes. By comparison, metallic glasses have a simple atomic structure with dense atomic packing, and their relaxation spectra were commonly found to be simpler than those of molecular glasses. Here we show the compelling evidence obtained across a wide range of temperatures and frequencies from a La-based metallic glass, which clearly shows two peaks of secondary relaxations (fast versus slow) in addition to the primary relaxation peak. The discovery of the unusual fast secondary relaxation unveils the complicated relaxation dynamics in metallic glasses and, more importantly, provides us the clues which help decode the structural features serving as the ‘trigger' of inelasticity on mechanical agitations.

The slow molecular dynamics in amorphous probucol

The slow molecular mobility in the active pharmaceutical ingredient probucol was studied by Thermally stimulated depolarisation currents (TSDC) in the temperature region between −130 and 50°C. The distribution of relaxation times was characterized for the secondary relaxations and for the glass transition relaxation. Based on aging results we were able to draw distinctions between the fast secondary relaxation on the one hand, and the slow-β or Johari–Goldstein relaxation on the other. The steepness index of probucol was calculated from the temperature dependent relaxation time of the glass transition and using methods based on the influence of the scanning rate on the glass transition signal (in DSC and TSDC). The obtained values show some scattering but they are in the frontier between the fragile and strong behaviors.

Aging behavior of the localization length in a colloidal glass

The localization length rloc associated with a fast secondary relaxation in glassy Laponite is determined by X-ray photon correlation spectroscopy (XPCS) through a Debye–Waller fit of the non-ergodicity parameter. Quantitative differences are observed between the time dependence (aging) of rloc in spontaneously aged and rejuvenated samples. This behavior is also reflected in the calculated shear modulus which matches well with data obtained by rheological measurements.

Primary and secondary relaxation process in plastically crystalline cyanocyclohexane studied by 2H nuclear magnetic resonance. II. Quantitative analysis

We analyze the results of our previously reported 2H nuclear magnetic resonance (NMR) experiments in the plastically crystalline (PC) phase of cyanocyclohexane (Part I of this work) to study the fast secondary relaxation (or β-process) in detail. Both, the occurrence of an additional minimum in the spin-lattice relaxation T1 and the pronounced effects arising in the solid-echo spectrum above the glass transition temperature T(g) = 134 K, allow for a direct determination of the restricting geometry of the β-process in terms of the "wobbling-in-a-cone" model. Whereas at temperatures below T(g) the reorientation is confined to rather small solid angles (below 10°), the spatial restriction decreases strongly with temperature above T(g), i.e., the distribution of cone angles shifts continuously towards higher values. The β-process in the PC phase of cyanocyclohexane proceeds via the same mechanism as found in structural glass formers. This is substantiated by demonstrating the very similar behavior (for T < T(g)) of spin-lattice relaxation, stimulated echo decays, and spectral parameters when plotted as a function of <log τ(β)> (taken from dielectric spectroscopy). We do, however, not observe a clear-cut relation between the relaxation strength of the β-process observed by NMR (calculated within the wobbling-in-a-cone model) and dielectric spectroscopy.

The nearly constant loss, Johari‐Goldstein β‐relaxation, and α‐relaxation of 1,4‐polybutadiene

AbstractFrom high‐resolution dielectric spectroscopy measurements on 1,4‐polybutadiene (1,4‐PB), we show that in addition to the structural α‐relaxation and higher frequency secondary relaxations in the spectra, a nearly constant loss (NCL) is observed at shorter times/lower temperatures. The properties of this NCL are compared to those of another chemically similar polymer, 1,4‐polyisoprene. The secondary relaxations in 1,4‐PB include the well‐known Johari‐Goldstein (JG) β‐relaxation and two other higher‐frequency peaks. One of these, referred to as the γ‐relaxation, falls between the JG‐relaxation and the NCL. Seen previously by others, this γ‐relaxation in 1,4‐PB is not the JG‐process and bears no relation to the glass transition. At very low temperatures (&lt;15 K), we confirm the existence of a very fast secondary relaxation, having a weak dielectric strength and an almost temperature‐invariant relaxation time. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 342–348, 2007

Brillouin light and X-ray study of glass-forming polybutadiene

Abstract Brillouin light and X-ray scattering have been used to investigate the dynamic structure factor of supercooled and glassy deuterated polybutadiene in the temperature range between 30 and 330 K. The temperature behaviour of velocity and absorption of longitudinal acoustic modes is reported, giving evidence of two relaxation processes: the structural relaxation and the fast secondary relaxation. A method is presented to obtain information on the shape of the relaxation functions and the activation energy of the fast process from velocity dispersion and absorption. The activation energy of the secondary process is close to that of the rotation around —CD2—CD= bonds of the polybutadiene backbone.

Brillouin light and X-ray study of glass-forming polybutadiene

Abstract Brillouin light and X-ray scattering have been used to investigate the dynamic structure factor of supercooled and glassy deuterated polybutadiene in the temperature range between 30 and 330 K. The temperature behaviour of velocity and absorption of longitudinal acoustic modes is reported, giving evidence of two relaxation processes: the structural relaxation and the fast secondary relaxation. A method is presented to obtain information on the shape of the relaxation functions and the activation energy of the fast process from velocity dispersion and absorption. The activation energy of the secondary process is close to that of the rotation around —CD2—CD= bonds of the polybutadiene backbone.

Glass transition of an epoxy resin. A wideband dielectric investigation

The dynamics of an epoxy resin, poly(phenyl glycidyl ether-co-formaldehyde), has been investigated in the supercooled and glassy phases by wideband dielectric spectroscopy (10/sup -2/ to 3/spl times/10/sup 9/ Hz) and compared with that of the previously investigated epoxy resin, diglycidyl ether of bisphenol-A. The temperature evolution of the dynamics of the system is monitored through the characteristic parameters of the relaxations, namely the relaxation times, the relaxation strengths and the shape parameters. Two transition regions are revealed: the glass transition and the split between structural and fast secondary relaxation, where the onset of the structural relaxation is located also.

Brillouin light scattering of a fragile glass former: O-terphenyl

Abstract We report Brillouin scattering measurements of the fragile glass former o-terphenyl over a wide temperature range covering the normal and the supercooled liquid phase down to the glass temperature. The velocity dispersion and the absorption of the longitudinal sound waves are reported and compared with literature data. The presence of both a residual sound speed dispersion in the hot liquid phase and a strongly asymmetric shape of the Brillouin line width curve in the high-temperature region indicates the existence of two relaxation phenomena: a structural and a fast secondary relaxation. This picture is consistent with the classication of o-terphenyl as a non-associated liquid, as emerges from the analysis of the temperature behaviour of the ratio of the experimental to the classically calculated sound wave absorption.