Kia L Ngai

• University of Pisa

The generalized Langevin equation for cooperative dynamics of Guenza predicts the dynamics of chains in entangled polymers is cooperative with anomalous properties, and at variance with the tube‐reptation model. The center‐of‐mass mean‐square displacements at shorter times are subdiffusive with with β < 1, contradicting the common assumption of rep...

The purpose of this paper is to demonstrate the existence of a number of universal properties present in a wide variety of ionically conducting materials in the liquid, glassy and crystalline states. This remarkable finding deserves attention and explanation. Moreover these universal properties are at the same time anomalous. Progress in research a...

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 relaxat...

An advance was made by Guiselin et al. [arXiv:2103.01569 (2021)] in molecular dynamics simulations of the equilibrium dynamics of supercooled liquids near the experimental glass transition by utilizing the giant equilibration speedup provided by the swap Monte Carlo algorithm. The found emergence of a power law in relaxation spectra at lower temper...

The studies of molecular dynamics in the vicinity of liquid–glass transition are an essential part of condensed matter physics. Various experimental techniques are usually applied to understand different aspects of molecular motions, i.e., nuclear magnetic resonance (NMR), photon correlation spectroscopy (PCS), mechanical shear relaxation (MR), and...

According to the Coupling Model (CM), secondary relaxation belonging to a special class has strong connection to the α-relaxation. These Johari-Goldstein (JG) β-relaxations are predicted to be ubiquitous, and its relaxation time is approximately equal to the primitive relaxation time and the surface diffusion time. These predictions cannot be teste...

Traditionally the study of dynamics of glass-forming materials has been focused on the structural α relaxation. However, in recent years experimental evidence has revealed that a secondary β relaxation belonging to a special class, called the Johari-Goldstein (JG) β relaxation, has properties strongly linked to the primary α relaxation. By invoking...

The frequency dispersion of structural α-relaxation obtained from broadband dielectric spectroscopy measurements is relatively narrow in many polar glass-formers. On the other hand, it becomes much broader when probed by other techniques, including photon correlation spectroscopy (PCS), nuclear magnetic resonance (NMR), and mechanical shear modulus...

In this Letter we report significant differences in the dielectric behavior of four nonpolymeric and sizable glass-forming molecules with related chemical structures. They belong to the recently constituted class of sizable glass-formers [Jedrzejowska et al. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2020, 101, 010603], fo...

The density scaling behavior of dynamics and thermodynamics is investigated in ionic liquids. By comparing the scaling exponent of different dynamic quantities (dc-conductivity, viscosity, and diffusion) of a series of homologous ionic liquids, we discover that longer alkyl chain in the cation provides lower scaling value. Additionally, a systemati...

Since 1998 the primitive relaxation time τ 0(T,P) of the Coupling Model (CM) and the Johari-Goldstein (JG) β-relaxation time τJG(T,P) are shown approximately equal in many glass-formers. The CM relation between τ 0(T,P) and τα(T,P) at any T and P is exact. Additionally from the CM relation τα(T,P)/τ 0(T,P) is exactly invariant to variations of T an...

In this Rapid Communication we report the unusual dynamics of planar, rigid, and anisotropy glass-forming molecules of unusually large size by dielectric spectroscopy by using two examples. The size of the molecules is much larger than the dipolar moiety located at the end of the longer axis of each molecule. The observed dynamics deviates strongly...

Invariance of the relation between α relaxation and β relaxation in metallic glasses to variations of pressure and temperature Dielectric relaxation experiments performed at ambient and elevated pressures P in molecular, ionic, and polymeric glass formers have established that the relation of the Johari-Goldstein (JG) β-relaxation time τ β (T, P) t...

The 2003 seminal paper entitled "Is the Fragility of a Liquid Embedded in the Properties of Its Glass?" by Tullio Scopigno, Giancarlo Ruocco, Francesco Sette, and Giulio Monaco, reported that the properties of the structural $\alpha$-relaxation of glass-formers are already present in the faster caged dynamics. Their important discovery has far-reac...

Advance was made recently in identifying the nature of the Debye relaxation observed by dielectric spectroscopy in monohydroxy alcohols by other experimental techniques. Consistent with the experimental findings, the transient chain model give a good description of the origin of the Debye relaxation. Notwithstanding, a general property of the Debye...

The dynamics in highly asymmetric mixtures with large difference in Tg of the two components are complex. At low values of the lower-Tg2 component only the α1-relaxation and its Tgα1 were found together with a fast relaxation. Three different interpretations of the fast relaxation were given. The first one interpreted the fast relaxation as the JG...

Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature Tg near 136 K, not much known are the spectral dispersion of the structural α-relaxation and the temperature dependence of its relaxation time τα,bulk(T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (J...

The research of dynamics of protein is predominantly focused on the hydrated or solvated state, which is close to the physiological condition of biological relevance. Eclipsed by the hydrated and solvated counterparts, the dynamics of dry proteins are seldom studied in earnest. Studied occasionally, the data of dry proteins were not addressed, poss...

The physical and chemical instability of amorphous pharmaceuticals during storage has become a major problem for the pharmaceutical industry, as most of the pharmaceuticals including some life saving drugs are also found to be poorly water soluble. Molecular mobility is a key factor to understand the various crucial parameters that determine the st...

Cangialosi and co-workers found two steps in the enthalpy recovery deep in the glassy state of high and low molecular weight bulk polystyrene (PS). We attribute the first step to the segmental α-relaxation and the second step to the sub-Rouse modes and explain the two-step enthalpy recovery by the dynamic properties of the two processes in bulk PS...

Coupling-Model-based theoretical explanation of the minor change of JG β-relaxation achieved by ultrastability in contrast to the dramatic change in α-relaxation.

Nowadays the study of physiochemical stability of amorphous pharmaceuticals is of great interest in the field of medicinal application due to its enhanced water solubility and bioavailability than its crystalline counterpart. Molecular relaxations play an important role in understanding the physical stability of amorphous drugs. Hence herein, we in...

An approximate relation between the Johari-Goldstein (JG) β-relaxation time τJG and the α−relaxation time τα was derived and predicted to be approximately invariant to variations of pressure P and temperature T at constant τα. This prediction was verified in several glass-formers where τα and τJG were determined directly without fitting by empirica...

We studied the conductivity relaxation originating from a glass-former composed of cations and anions, and the relation to the structural a-relaxation at temperatures above and below the glass transition temperature. The material chosen was amorphous amlodipine besylate (AMB), which is also a pharmaceutical with a complex chemical structure. Measur...

Secondary relaxations persistent in the glassy state after structural arrest are especially relevant for the properties of the glass. A major thrust in research in dynamics of glass-forming liquids is to identify what secondary relaxations exhibit a connection to the structural relaxation and are hence more relevant. Via the Coupling Model, seconda...

In this paper, a stable amorphous solid dispersion of an antihypertensive drug, amlodipine besylate (AMB) was prepared by entrapping it in a polymer matrix, polyvinyl pyrrollidone, in different weight ratios (AMB/PVP 05:95, 10:90, 20:80, 30:70). The glass forming ability of all binary dispersions were studied by means of differential scanning calor...

Molecular dynamics (MD) simulations of nano-porous lithium disilicate (Li2Si2O5) systems have been performed in previous work [Junko Habasaki, J. Chem. Phys., 145, 204503 (2016)], where importance of caged ion dynamics in porous materials was shown. The shape of the self-part of the Van Hove function and the non-Gaussian parameter clearly show a he...

In a series of papers on binary glass-forming mixtures of tripropyl phosphate (TPP) with polystyrene (PS), Kahlau et al. [J. Chem. Phys. 140, 044509 (2014)] and Bock et al. [J. Chem. Phys. 139, 064508 (2013); J. Chem. Phys. 140, 094505 (2014); and J. Non-Cryst. Solids 407, 88–97 (2015)] presented the data on the dynamics of the two components studi...

Secondary relaxations are fundamental for their impact in the properties of glasses and for their inseparable connection to the structural relaxation. Understanding their density dependence and aging behavior is key to fully address the nature of glasses. Ultrastable glasses establish a new benchmark to study the characteristics of secondary relaxa...

Recently Fakhraai and coworkers measured surface diffusion in ultrastable glass produced by vapor deposition, ordinary glass with and without physical aging, and ultrathin films of the same molecular glass-former, N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD). Diffusion on the surfaces of all these glasses is greatly enhanced compared with...

Molecular dynamics (MD) simulations are one of the methods of the computational science. One can study the structure and dynamics of the system in the computer by solving the equation of motion. Utilization of MD simulations has spread over many fields, such as biophysics, drug designs, as well as fundamental research areas in chemistry and physics....

Please visit Springer https://www.springer.com/jp/book/9783319423890 Typical ionic salts such as NaCl have high melting point. However, different from the classical salt, some newly developed molten salts are liquids even at room temperature. They form a new class of chemical compounds, and are now called the room-temperature ionic liquids (RTIL)...

This chapter describes some models that are often used in trying to understand experimental data and fundamental questions in ion diffusion in ionically conducting materials. The basics of linear response theory are introduced first, with the definition of the linear response function, the Kramers-Kronig relations, and the Fluctuation-Dissipation t...

Modeling techniques using molecular dynamics simulation can contribute to the design and improvement of materials, and the methods can be used for predicting properties of them and compare with experiments and theory. Recently, potential models based on the ab initio MO calculations or DFT are used in many cases. However, the potential parameters a...

When more than one kind of mobile ions are mixed in ionic conducting glasses, crystals and melts, there is non-linear decrease of conductivity or diffusivity, which can be as large as several orders of magnitude compared with the transport coefficient of either kind of ions. What is a cause of such a large effect? The phenomenon is known as mixed m...

Recently, the development of nanotechnology and the possibility of obtaining artificially nanostructured materials have led to new strategies to find materials with high ionic conductivity values [1]. When decreasing the size of the solid below typically 100 nm, at least in one dimension, finite size effects may appear and result in enhanced ionic...

When considering the ionic liquids (ILs) in general, and especially the room temperature ionic liquids (RTIL), the interest is naturally focused on the ionic conductivity and ion dynamics. Nonetheless, many ionic liquids are glass-formers, and undergo glass transition at T g on cooling at ambient pressure or at P g on elevating the applied pressure...

For anyone interested in performing molecular dynamics simulations, there are many possible approaches. Nowadays, many kinds of MD programs and those for analysis are available. Choice of the method of the calculations depends on the objective of the researcher and the application of MD depends on scientific background of the researcher. Due to larg...

The importance of porous materials in applications to batteries has attracted the attention of researchers in many fields. Many materials having not only microscopic and mesoscopic pores but also nanoscopic pores are the subjects of studies by researches. They are beneficial to use in electric devices as electrode, insulator, and other components....

Nuclear Magnetic Resonance (NMR) has been shown to be a useful tool for the study of the structure [1–4] and dynamics [3, 5–11] of ionic conductors. While magic-angle spinning techniques are most often used for obtaining structural details, the dynamics of ion are usually explored by measuring spin-relaxation times [12]. Many of these NMR studies o...

Impedance Spectroscopy (IS) is by far the most commonly used technique to investigate experimentally the dynamic response of mobile ions in ionic conductors. Although other techniques, like NMR, light scattering, and neutron scattering, are also used to gain some insights into the ion diffusion process, they are mainly useful to probe high frequenc...

In spite of the vast amount of studies devoted during the past decades to understanding the dynamics of ions in ionically conducting materials, it is still unsettled to remain an open and challenging problem [1–4]. Since the charge transport in these materials is mainly due to the mobile ions, the measurement, analysis, and theoretical interpretati...

By now the study of ion transport and conductivity in ionically conducting materials is a subject of interest to physicists, chemists, materials scientists, and engineers. The interest of physicists is to understand the complex motion of ions leading to steady state diffusion and conductivity. This is a challenging endeavor because of the large num...

In several publications we studied the fast caged dynamics of many glass-formers observed in the glassy state at high frequencies above circa 1 GHz by various techniques, including neutron scattering, Brillouin scattering, ²H NMR spin-lattice relaxation, and THz dielectric spectroscopy. On increasing temperature, the intensity of the caged dynamics...

Significance In 1910 F. A. Lindemann introduced a practical criterion to predict the melting of crystals on the basis of the relative magnitude of thermal atomic fluctuations and the crystal lattice constant. The idea of a critical threshold for the local motions that would trigger melting also in disordered media like proteins has inspired importa...

Successful thermodynamic scaling of the structural alpha-relaxation time or transport coefficients of glass-forming liquids determined at various temperatures T and pressures P means the data conform to a single function of the product variable TVgamma, where V is the specific volume and gamma is a material specific constant. In the past two decade...

Ions in ionically conducting materials show complex but universal trends in the time development of the mean squared displacements (MSD) of ions, which correspond to the complex frequency dependence conductivity spectra or susceptibility. Molecular dynamics simulations enable us to characterize the details of such motions including the initial near...

The authors of the paper, Limited surface mobility inhibits stable glass formation for 2-ethyl-1-hexanol, J. Chem. Phys. 146, 203317 (2017), encountered two problems in considering applicability of the Coupling Model. We show in this Comment that these problems do not exist.

Exhaustive analysis of dielectric relaxation data of van der Waals glass-forming liquids revealed a strong correlation between the width of the frequency dispersion of the α-relaxation and the dielectric strength Δϵ, originating from the dipole-dipole interaction contribution to the intermolecular potential [M. Paluch et al., Phys. Rev. Lett. 116,...

This book discusses the physics of the dynamics of ions in various ionically conducting materials, and applications including electrical energy generation and storage. The experimental techniques for measurements and characterization, molecular dynamics simulations, the theories of ion dynamics, and applications are all addressed by the authors, wh...

Examples of the results of MD simulations of softcore model by MATHEMATICA are shown here. *CDF file is freely available as electronic supplementary materials (ESM) (http://extras.springer.com/) (Example 2 folder) of our book entitled "Dynamics of Glassy, Crystalline and Liquid Ionic Conductors", by J. Habasaki, C. Leon and K.L. Ngai (http://www.sp...

This is an example of the structure of lithium silicate glass obtained by MD simulations. Please try to do MD simulation and manipulation of data by yourself. For the methods, see chapter "Molecular dynamics simulations" of ionic systems at http://link.springer.com/chapter/10.1007%2F978-3-319-42391-3_8 and chaper "Molecular Dynamics Simulation of S...

The CDF file for manipulation of the Levy (stable alpha) distribution is available as the ESM of our book. http://link.springer.com/book/10.1007/978-3-319-42391-3

This paper highlights the benefits resulting from the application of double active pharmaceutical ingredient in ionic liquids as active pharmaceutical ingredients (APIs). Double active pharmaceutical ingredient benzalkonium ibuprofenate with anti-bacterial and anti-inflammatory roles, which offers high solubility, bio-availability, and biological a...

Two transitions were found by ellipsometry in freestanding high molecular weight PS films by Pye and Roth (PR). PR suggested that the upper and the lower transitions both came from the segmental a-relaxation, and the upper transition occurs in 90% of the material. In this paper we use dielectric relaxation data of freestanding films to rule out the...

Metallic glasses (MGs) usually exhibit only slow β-relaxation peak, and the signature of the fast dynamic is challenging to be observed experimentally in MGs. We report a general and unusual fast dynamic mode in a series of rare earth based MGs manifested as a distinct fast β′-relaxation peak in addition to slow β-relaxation and α-relaxation peaks...

The presence of sub-Rouse modes in bulk polymers with length scale and relaxation times in between the segmental α-relaxation and the Rouse modes had long been justified by theoretical consideration, and found in many experimental studies. The sub-Rouse modes had been seen directly in creep compliance measurements of polymer thin films by McKenna a...

Background: The properties of the three dynamic processes, α-relaxation, ν-relaxation, and caged dynamics in aqueous mixtures and hydrated proteins are analogous to corresponding processes found in van der Waals and polymeric glass-formers apart from minor differences. Methods: Collection of various experimental data enables us to characterize t...

From an exhaustive examination of the molecular dynamics in practically all van der Waals molecular glass formers ever probed by dielectric spectroscopy, we found that the width of the α-loss peak at or near the glass transition temperature Tg is strongly anticorrelated with the polarity of the molecule. The larger the dielectric relaxation strengt...

Three recently published papers have discovered a general property of the fast caged dynamics observed in the glassy states of polyalcohols (S. Capaccioli et al., J. Phys. Chem. B 119, 8800 (2015)), amorphous polymers (K. L. Ngai et al., J. Phys. Chem. B 119, 12502 (2015)), and van der Waals molecular glass-formers (K. L. Ngai et al., J. Phys. Chem...

Some molecular glass-formers can crystallize in the glassy state, some of which are van der Waals molecules and some are pharmaceuticals. The molecular mechanism responsible for this glass-to-crystal mode of crystallization is of interest to the glass transition research community as well as to the pharmaceutical industry because the effect is detr...

In the paper K. L. Ngai et al., [J. Chem. 140, 044511 (2014)], the empirical correlation of ductility with the Poisson's ratio, νPoisson, found in metallic glasses was theoretically explained by microscopic dynamic processes which link on the one hand ductility, and on the other hand the Poisson's ratio. Specifically, the dynamic processes are the...

In the paper K. L. Ngai et al. , [J. Chem. 140, 044511 (2014)], the empirical correlation of ductility with the Poisson's ratio, ν Poisson , found in metallic glasses was theoretically explained by microscopic dynamic processes which link on the one hand ductility, and on the other hand the Poisson's ratio. Specifically, the dynamic processes are t...

Secondary relaxations with properties closely related to the α-relaxation have fundamental importance in glass-forming substances including polymers. To distinguish these secondary relaxations from those involving intramolecular degrees of freedom, they are called the Johari-Goldstein (JG) β-relaxations. Acting as the precursor of the α-relaxation,...

Despite the importance of Poisson's ratio in materials science and engineering, its connection to molecular dynamics and relaxation processes in glass-forming systems remains unclear. The existence of such connection made the family of metallic glasses in our previous study [K.L. Ngai et al. J. Chem. Phys. 140 (2014) 044511], where itwas also justi...

The typical ionic liquid, 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO3), was examined by molecular dynamics simulations of an all-atomistic model to show the characteristics of networks of cages and/or bonds in the course of vitrification of this fragile glass-former. The system shows changes of dynamics at two characteristic temperatures, TB (or...

Mixtures of methyltetrahydrofuran (MTHF) with tristyrene and high molecular weight polystyrene involve exceptionally large difference in the glass transition temperatures of the two components not realized in other binary mixtures studied before. The extensive study of the molecular dynamics of these mixtures by various experimental techniques by B...

Nonlinear dielectric measurements at high electric fields of glass-forming glycerol and propylene carbonate initially were carried out to elucidate the dynamic heterogeneous nature of the structural α-relaxation. Recently, the measurements were extended to sufficiently high frequencies to investigate the nonlinear dielectric response of faster proc...

Secondary relaxations have been classified into two types, depending on whether they are related to the structural α-relaxation in properties or not. Those secondary relaxations that are related to the α-relaxation may have fundamental importance, and are called the Johari-Goldstein (JG) β-relaxations. Two polar molecular glass-formers, one flexibl...

The article reports the dependence of the conductivity relaxation on temperature T and pressure P in the canonical ionic glass former 0.4Ca(NO_{3})_{2}-0.6KNO_{3}(CKN). At constant conductivity relaxation time τ_{σ}, the entire conductivity relaxation spectra obtained at widely different combinations of T and P superpose almost perfectly, and thus...

The article reports the dependence of the conductivity relaxation on temperature T and pressure P in the canonical ionic glass former 0.4Ca(NO3)2−0.6KNO3(CKN). At constant conductivity relaxation time τσ, the entire conductivity relaxation spectra obtained at widely different combinations of T and P superpose almost perfectly, and thus it is the io...

In metallic glasses a clear correlation had been established between plasticity or ductility with the Poisson ratio and alternatively the ratio of the elastic bulk modulus to the shear modulus, K/G. Such a correlation between these two macroscopic mechanical properties is intriguing and is challenging to explain from the dynamics on a microscopic l...

The presence of a viscoelastic mechanism distinctly different from the segmental a-relaxation and the Rouse modes within the glass-rubber transition zone of polymers had been justified by theoretical considerations, and subsequently experimentally verified in different bulk polymers by various techniques, and in several laboratories. Referred to in...

The nature of the liquid-glass transition remains an unsolved fundamental problem. One aspect is the striking change in dynamics of the structural α-relaxation generally observed at a temperature TB above Tg in all glass-formers. More intriguing in the case of polymers is that the change of dynamics occurs not only in the structural α-relaxation bu...

The cusp-like temperature dependence of the Debye-Waller factor or non-ergodicity parameter fQ(T) at some temperature Tc above Tg found by experiments in several fragile glassformers has been considered as critical evidence for validity of the ideal Mode Coupling Theory (MCT). A comprehensive review of experimental data of fQ(T) and beyond brings o...

The β-relaxation of metallic glasses (MGs) bears nontrivial connections to their microscopic and macroscopic properties. In an effort to elucidate the mechanism of the β-relaxation, we studied by dynamical mechanical measurements the change of its properties on varying the composition of La60Ni15Al25 in various ways. The properties of the β-relaxat...

Using broadband dielectric spectroscopy we investigate the changes in the conductivity relaxation times τσ observed during the physical aging of the protic ionic conductor carvedilol dihydrogen phosphate (CP). Due to the large decoupling of ion diffusion from host molecule reorientation, the ion conductivity relaxation time τσ(Tage,tage) can be dir...

Freestanding high molecular weight polymer thin films when the thicknes is reduced to nanometer scale undergoes large reduction of its dynamic heterogeneity length scale, degree of cooperativity, and the coupling parameter n in the Coupling Model (CM). This finite-size effect make the -relaxation time of the polymer in nano-confinement, τ_^nano (...

From ellipsometry measurements, Pye and Roth [Phys. Rev. Lett. 107, 235701 (2011)] presented evidence of the presence of two glass transitions originating from two distinctly different and simultaneous mechanisms to reduce the glass transition temperature within freestanding polystyrene films with thickness less than 70 nm. The upper transition tem...

Molecular dynamics (MD) simulations were performed to study the heterogeneous and anisotropic dynamics of lithium ions in lithium disilicate crystal. Mean squared displacements of Li ions show changing dynamics over several time regions before the terminal diffusive regime is attained. Even near the melting point of the system, simulation of severa...