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We investigated the stretched dynamics of the structural relaxation in molecular glass formers by using dielectric and thermal (or enthalpic) relaxations. The dielectric stretching exponents βdie are determined by the Havriliak–Negami function, while the enthalpic βTNMH is quantified by using the Tool–Narayanaswamy–Moynihan–Hodge formalism. We found βTNMH is anticorrelated with the degree of freedom, a molecule addressed by the concept of beads. Referring to the reported relation of βdie to the dipole moment μ, we proposed a combined parameter of μ2*beads, which can rationalize the difference in stretching exponents obtained by dielectric and enthalpic relaxations. For the majority of glass-forming molecules, the difference is trivial, but for those molecules with both unusually high dipole moments and flexibility, a large difference is obvious. The interplay of the degree of freedom and dielectric dipole–dipole interaction in molecular dynamics is addressed. [ABSTRACT FROM AUTHOR]

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 (JG) β-relaxation is a question that has not been answered. By studying the structural α-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures Tg close to 136 K, we deduce the properties of the α-relaxation and the temperature dependence of τα,bulk(T) of bulk water. The frequency dispersion of the α-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of τα,bulk(T) at low temperatures as well as at high temperatures from neutron scattering and GHz–THz dielectric relaxation, and hence, there is no fragile to strong transition. The Tg-scaled VFT temperature dependence of τα,bulk(T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG β-relaxation in bulk water is supported by its equivalent relaxation observed in water confined in spaces with lengths of nanometer scale and having Arrhenius T-dependence of its relaxation times τconf(T). The equivalence is justified by the drastic reduction of cooperativity of the α-relaxation in nanoconfinement and rendering it to become the JG β-relaxation. Thus, the τconf(T) from experiments can be taken as τβ,bulk(T), the JG β-relaxation time of bulk water. The ratio τα,bulk(Tg)/τβ,bulk(Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch α-correlation function, exp[−(t/τα,bulk)1−n], having (1−n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with Tg higher than that of water show the presence of a secondary ν-relaxation from the water component. The ν-relaxation is strongly connected to the α-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time τν(T) is longer than τβ,bulk(T), but τν(T) becomes about the same as τβ,bulk(T) at sufficiently high water content. However, τν(T) does not become shorter than τβ,bulk(T). Thus, τβ,bulk(T) is the lower bound of τν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τβ,bulk(T) but not τα(T) that is responsible for the dynamic transition of hydrated globular proteins. [ABSTRACT FROM AUTHOR]

We have investigated the enthalpic and dielectric relaxations of four groups of quinoline analogs having similar structural properties (i.e., rigidity, stiffness, and bulkiness) but a different steric character and the nature of intermolecular interactions and flexibility. The dielectric fragility index (md) and the enthalpic one (mH), determined by the Tool–Narayanaswamy–Moynihan–Hodge formalism, are comparable. Generally, for the four sets of molecules of similar structures, both the interactions and flexibility are found to be critical in making the large span of fragility (i.e., from 59 to 131) and glass forming ability. By contrast, individual impacts of the interaction and flexibility can only explain fragility partly among each group of isomers. We found that the molecules with high fragility are of relatively low liquid density, reflecting the joint impact of the interactions and flexibility. An interesting result is observed among the isomers that the molecules which are fragile have enhanced glass forming ability. The results are unveiling the joint impacts of molecular structure (flexibility) and intermolecular interaction on the molecular dynamics. [ABSTRACT FROM AUTHOR]

The dielectric relaxation of two long-chain glass forming monohydroxy alcohols, 2-butyl-1-octanol and 2-hexyl-1-decanol, is studied at low temperature. Remarkable broadening from the pure Debye relaxation is identified for the slowest dynamics, differing from the dielectric spectra of short-chain alcohols. The broadening of the Debye-like relaxation in the two liquids develops as temperature increases, and the approaching of the Debye-like and structural relaxation widths is shown. Similar results are observed in the dielectric spectra of dilute 2-ethyl-1-hexanol in either 2-hexyl-1-decanol or squalane. The results of the liquids and mixtures reveal a correlation between the broadening and the Debye-like relaxation strength. Molecular associations in monohydroxy alcohols are discussed with the modification of the Debye relaxation. [ABSTRACT FROM AUTHOR]

The dielectric relaxation of a substituted monohydroxy alcohol, 3-methylthio-1-hexanol, is studied in the highly viscous regime near the glass transition. The Debye relaxation is detected in the dielectric spectra showing the slowest and strongest relaxation dynamics. The calorimetric and dielectric measurements of the liquid and the mixtures with a Debye liquid (2-ethyl-1-hexanol) and a non-Debye liquid (2-ethylhexylamine) reproduce the dynamic characters of the relaxations in monohydroxy alcohols. The Debye relaxation strength and time of 3-methylthio-1-hexanol do not change much compared with 2-ethyl-1-hexanol, while the structural relaxation strength shows a considerable enhancement accompanied by an increase in relaxation time, indicative of a reduction in the dynamic separation between the Debye and structural relaxations. The experimental results allow for the examination of the structural models proposed for the Debye relaxation. [ABSTRACT FROM AUTHOR]

The dielectric strength of the Debye relaxation in thebinary mixtures of two isomeric monohydroxy alcohols, 2-ethyl-1-butanol(2E1B) and 4-methyl-2-pentanol (4M2P), is studied at low temperaturenear glass transition. Enhanced dielectric strength is exhibited inthe mixtures, remarkably different from the mixing behaviors of thestructural (α-) relaxation of generic liquids. A similar resultis observed when analyzing the dielectric data of the binary mixturesof 2-ethyl-1-hexanol and 2-methyl-1-butanol reported in an early study.The unusual behavior of the dielectric strength in the mixtures revealsa new feature of the Debye relaxation in monohydroxy alcohols. Yet,the calorimetric measurements of the glass transition temperaturein 2E1B–4M2P mixtures show a distinct negative deviation fromthe ideal mixing law. The explanation of the Debye relaxation is discussedwith the results. [ABSTRACT FROM AUTHOR]

• IL-TiO 2 hybrid particles are prepared as a new dispersed-phase material of ERFs. • The IL-TiO 2 ERF shows greater sensitivity to electric field and enhanced ER response. • The IL-TiO 2 ERF presents two dielectric relaxation processes due to the modification by ionic liquid. An ionic liquid functionalised silane agent, 1-(3-triethoxysilylpropyl)-3-methylimidazolium chloride (TESPM-IL), was prepared and used to fabricate ionic liquid (IL)-grafted titanium dioxide (IL-TiO 2) spheres via a sol-gel process. Thermogravimetric analysis and Fourier transform infrared spectroscopy confirmed that TESPM-IL was attached to TiO 2 spheres. Morphologies of the IL-TiO 2 spheres were observed by scanning electron microscopy and transmission electron microscopy, indicating the rough surface and smaller size of the IL-TiO 2 particles caused by the modification of TESPM-IL. The IL-TiO 2 particles were applied as a new dispersed-phase material of electrorheological (ER) fluids. The ER effect of IL-TiO 2 particles was studied and compared with that of pure TiO 2 particles. It was found that, upon the introduction of TESPM-IL, both yield stress and storage modulus of the IL-TiO 2 ER fluid were enhanced significantly, thus improving the ER effect of TiO 2 - or inorganic-based ER materials effectively. Dielectric analysis was also used to observe the interfacial polarization process of the particles, which confirmed the effect of the IL layer on the surface of the TiO 2 particles. [ABSTRACT FROM AUTHOR]

• The relaxation dynamics of phosphates depends on the strength of intermolecular interaction/coupling. • Results of the mixtures reaffirm that intermolecular coupling is small in neat-phosphates. • The deduction comprehend the success/failure of ultrastable glass formation by vapor deposition. • The enhancement of diffusion at the surface compared to the bulk can be estimated by ratio τ α (T g)/τ β (T g). The dielectric study of tri-alkyl phosphates had elucidated that the dynamics of relaxations depends on the strength of intermolecular interaction/coupling, which in turn depends on the chemical structure of phosphates. In the same spirit, we examine these phosphates of about 10 mol% in mixtures with the o -terphenyl host. The spectra coming exclusively from the rotational dynamics of phosphates revealed two secondary processes in addition to the primary relaxation. The slower β-relaxation is new in the mixtures, and is identified as the intermolecular Johari-Goldstein-Extended (JGX) β-relaxation with relaxation time τ β (T g) having strong connection to τ α (T) of the α-relaxation. The ratio τ α (T g)/ τ β (T g) decreases monotonically with increasing length of the alkyl chains in higher members. The results of the mixtures reaffirm the small intermolecular coupling in the pure phosphates. The deduction is attempted to explain the success of ultrastable glass formation in TMP and failure in TEP and TBP. [ABSTRACT FROM AUTHOR]