Wenkang Tu

Dielectric relaxation of long-chain glass-forming monohydroxy alcohols

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.

Glass formability in medium-sized molecular systems/pharmaceuticals. I. Thermodynamics vs. kinetics

Scrutinizing critical thermodynamic and kinetic factors for glass formation and the glass stability of materials would benefit the screening of the glass formers for the industry of glassy materials. The present work aims at elucidating the factors that contribute to the glass formation by investigating medium-sized molecules of pharmaceuticals. Glass transition related thermodynamics and kinetics are performed on the pharmaceuticals using calorimetric, dielectric, and viscosity measurements. The characteristic thermodynamic and kinetic parameters of glass transition are found to reproduce the relations established for small-molecule glass formers. The systematic comparison of the thermodynamic and kinetic contributions to glass formation reveals that the melting-point viscosity is the crucial quantity for the glass formation. Of more interest is the finding of a rough correlation between the melting-point viscosity and the entropy of fusion normalized by the number of beads of the pharmaceuticals, suggesting the thermodynamics can partly manifest its contribution to glass formation via kinetics.

Comparative study of dynamics in glass forming mixtures of Debye-type N-ethylacetamide with water, alcohol, and amine

The glass transition and relaxation dynamics in the binary mixtures of a Debye liquid, N-ethylacetamide, with water, monoalcohol, and amine are studied by calorimetric and dielectric measurements in the highly viscous regimes near the glass transition. Calorimetric measurements show the glass transition temperature in the N-ethylacetamide-water mixtures is remarkably enhanced as water is added as high as 70 mol. % before crystallization is detected. A similar increase is also observed in the N-ethylacetamide-rich mixtures with the non-Debye 1,2-propanediamine. However, the dielectric measurements show that the main relaxation in the N-ethylacetamide-water mixtures with water fraction up to 60 mol. % reproduces the dynamic characters of the mixtures constituted by two Debye liquids, N-ethylacetamide and 2-ethyl-1-butanol. The comparison of the calorimetric and dielectric features for the three mixing systems suggests that the Debye relaxation persists in the N-ethylacetamide-water mixtures within the experimentally studied compositions.

Anomaly in dielectric relaxation dispersion of glass-forming alkoxy alcohols

The dielectric relaxations in six primary and secondary alkoxy alcohols with varying molecular size and different separation between -O- and hydroxyl group are studied at temperatures around glass transition. The analyses of the apparent full width at half maximum of the main relaxations of the alkoxy alcohols reveal minima in the temperature dependence of the relaxation dispersions. The stretching exponents for the main relaxations of the alkoxy alcohols are also found not to follow the empirical correlations with other dynamic quantities established for generic liquids. A comparison of the relaxation dispersions in the alkoxy alcohols with those in Debye and non-Debye (generic) liquids is presented. The impacts of the β-relaxations on the apparent main relaxation widths are reviewed for molecular glass formers.

Unveiling the Dependence of Glass Transitions on Mixing Thermodynamics in Miscible Systems

The dependence of the glass transition in mixtures on mixing thermodynamics is examined by focusing on enthalpy of mixing, ΔHmix with the change in sign (positive vs. negative) and magnitude (small vs. large). The effects of positive and negative ΔHmix are demonstrated based on two isomeric systems of o- vs. m- methoxymethylbenzene (MMB) and o- vs. m- dibromobenzene (DBB) with comparably small absolute ΔHmix. Two opposite composition dependences of the glass transition temperature, Tg, are observed with the MMB mixtures showing a distinct negative deviation from the ideal mixing rule and the DBB mixtures having a marginally positive deviation. The system of 1, 2- propanediamine (12PDA) vs. propylene glycol (PG) with large and negative ΔHmix is compared with the systems of small ΔHmix, and a considerably positive Tg shift is seen. Models involving the properties of pure components such as Tg, glass transition heat capacity increment, ΔCp, and density, ρ, do not interpret the observed Tg shifts in the systems. In contrast, a linear correlation is revealed between ΔHmix and maximum Tg shifts.

Revisiting the glass transition and dynamics of supercooled benzene by calorimetric studies

The glass transition and dynamics of benzene are studied in binary mixtures of benzene with five glass forming liquids, which can be divided into three groups: (a) o-terphenyl and m-xylene, (b) N-butyl methacrylate, and (c) N,N-dimethylpropionamide and N,N-diethylformamide to represent the weak, moderate, and strong interactions with benzene. The enthalpies of mixing, ΔH(mix), for the benzene mixtures are measured to show positive or negative signs, with which the validity of the extrapolations of the glass transition temperature T(g) to the benzene-rich regions is examined. The extrapolations for the T(g) data in the mixtures are found to converge around the point of 142 K, producing T(g) of pure benzene. The fragility m of benzene is also evaluated by extrapolating the results of the mixtures, and a fragility m ∼ 80 is yielded. The obtained T(g) and m values for benzene allow for the construction of the activation plot in the deeply supercooled region. The poor glass formability of benzene is found to result from the high melting point, which in turn leads to low viscosity in the supercooled liquid.

Direct Evidence of Relaxation Anisotropy Resolved by High Pressure in a Rigid and Planar Glass Former

Rigid molecular glass-formers with no internal degrees of freedom nonetheless have a single secondary β-relaxation. For a rigid and planar molecule, 1-methylindole (1MID), although a secondary relaxation is resolved at ambient pressure, its properties do not conform to the rules established for rigid molecules reported in early studies. By applying pressure to the dielectric spectra of 1MID, we find the single secondary relaxation splits into two. The slower one is pressure sensitive showing connections to the α-relaxation as observed in other rigid molecules, while the faster one is almost pressure insensitive and dominate the loss at ambient pressure. The two secondary relaxations, identified to associate with the out-of-plane and in-plane rotations of the rigid and planar 1MID, are resolved and observed for the first time by increasing density via elevating pressure.

Probing the Debye dielectric relaxation in supercooled methanol

The explanation of the dielectric dynamics in methanol would offer knowledge of the Debye relaxation in supercooled monoalcohols. However, due to the fast crystallization, it is hard to attain the dynamics of pure methanol in the deeply supercooled region. In this paper we studied the dynamics of methanol - 2-ethyl-1-hexanol mixtures with methanol concentration up to 80 mol% using dielectric and calorimetric measurements. Two main relaxations are detected in the dielectric spectra of the mixtures, and the slower one generally reproduces the Debye relaxation features reported in earlier studies of other monoalcohols. The validity of the ideal mixing law is verified for the relaxation time of the slower dynamics in the mixtures. The results suggest that the Debye relaxation is present in the dielectric spectra of methanol. The temperature dependence of the Debye relaxation time is constructed for the supercooled methanol.

An Attempt to Prepare Metallic Glasses from Quasicrystals

Icosahedrons in supercooled liquids and glasses are considered to be of significance for the glass formation in alloy systems. Starting from the similarity of the local structure of quasicrystals to the icosahedrons in metallic glasses, a scheme is put forward to prepare metallic glasses based on a well-known quasicrystal Zr40Ti40Ni20. A series of (Zr40Ti40Ni20)100–xCox metallic glasses are fabricated, and the optimized glass forming composition is determined at (Zr40Ti40Ni20)92Co8. The results show that the glass-forming ability of the alloys is closely related to the quasicrystalline phases. The mechanism of the enhanced glass-forming ability is discussed.