Coupling of Caged Molecule Dynamics to JG β-Relaxation: I

The paper (Sibik, J.; Elliott, S. R.; Zeitler, J. A. J. Phys. Chem. Lett. 2014, 5, 1968-1972) used terahertz time-domain spectroscopy (THz-TDS) to study the dynamics of the polyalcohols, glycerol, threitol, xylitol, and sorbitol, at temperatures from below to above the glass transition temperature Tg. On heating the glasses, they observed the dielectric losses, ε″(ν) at ν = 1 THz, increase monotonically with temperature and change dependence at two temperatures, first deep in the glassy state at TTHz = 0.65Tg and second at Tg. The effects at both temperatures are most prominent in sorbitol but become progressively weaker in the order of xylitol and threitol, and the sub-Tg change was not observed in glycerol. They suggested this feature originates from the high-frequency tail of the Johari-Goldstein (JG) β-relaxation, and the temperature region near 0.65Tg is the universal region for the secondary glass transition due to the JG β-relaxation. In this paper, we first use isothermal dielectric relaxation data at frequencies below 106 Hz to locate the "second glass transition" temperature Tβ at which the JG β-relaxation time βJG reaches 100 s. The value of Tβ is close to TTHz = 0.65Tg for sorbitol (0.63Tg) and xylitol (0.65Tg), but Tβ is 0.74Tg for threitol and 0.83Tg for glycerol. Notwithstanding, the larger values of Tβ of glycerol are consistent with the THz-TDS data. Next, we identify the dynamic process probed by THz-TDS as the caged molecule dynamics, showing up in susceptibility spectra as nearly constant loss (NCL). The caged molecule dynamics regime is terminated by the onset of the primitive relaxation of the coupling model, which is the precursor of the JG β-relaxation. From this relation, established is the connection of the magnitude and temperature dependence of the NCL and those of βJG. This connection explains the monotonic increase of NCL with temperature and change to a stronger dependence after crossing Tβ giving rise to the sub-Tg behavior of ε″(ν) observed in experiment. Beyond the polyalcohols, we present new dielectric relaxation measurements of flufenamic acid and recall dielectric, NMR, and calorimetric data of indomethacin. The data of these two pharmaceuticals enables us to determine the value of Tβ = 0.67Tg for flufenamic acid and Tβ = 0.58Tg or Tβ = 0.62Tg for indomethacin, which can be compared with experimental values of TTHz from THz-TDS measurements when they become available. We point out that the sub-Tg change of NCL at Tβ found by THz-TDS can be observed by other high frequency spectroscopy including neutron scattering, light scattering, Brillouin scattering, and inelastic X-ray scattering. An example from neutron scattering is cited. All the findings demonstrate the connection of all processes in the evolution of dynamics ending at the structural α-relaxation. © 2015 American Chemical Society

Interpretation of Voltage Measurements in Cutting Torches

Anode-cathode and nozzle-cathode voltages, plenum pressure and gas mass flow measurements in a low current (30 A) cutting torch, operated with oxygen gas, are used as inputs for an electrical model coupled to a simplified fluid model, in order to infer some properties of the plasma-gas structure that are difficult to measure. © 2006 American Institute of Physics.Fil:Kelly, H. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Minotti, F.O. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Coupling of Caged Molecule Dynamics to JG β-Relaxation II: Polymers

At temperatures below the nominal glass transition temperature Tgα, the structural α-relaxation and the Johari-Goldstein (JG) β-relaxation are too slow to contribute to susceptibility measured at frequencies higher than 1 GHz. This is particularly clear in the neighborhood of the secondary glass transition temperature Tgβ, which can be obtained directly by positronium annihilation lifetime spectroscopy (PALS) and adiabatic calorimetry, or deduced from the temperature at which the JG β-relaxation time τβ reaches 1000 s. The fast process at such high frequencies comes from the vibrations and caged molecules dynamics manifested as the nearly constant loss (NCL) in susceptibility measurements, elastic scattering intensity, I(Q, T), or the mean-square-displacement, «u2(T)», in quasielastic neutron scattering experiment. Remarkably, we find for many different glass-formers that the NCL, I, or «u2» measured in the glassy state changes its temperature dependence at temperature THF near Tgβ. In paper I (Capaccioli, S.; et al. J. Phys. Chem. B 2015, 119 (28), 8800-8808) we have made known this property in the case of the polyalcohols and a pharmaceutical glass former, flufenamic acid studied by THz dielectric spectroscopy, and explained it by the coupling of the NCL to the JG β-relaxation, and the density dependence of these processes. In this paper II, we extend the consideration of the high frequency response to broader range from 100 MHz to THz in the glassy state of many polymers observed by quasielastic light scattering, Brillouin scattering, quasielastic neutron scattering, and GHz-THz dielectric relaxation. In all cases, the NCL changes its T-dependence at some temperature, THF, below Tgα, which is approximately the same as Tgβ. The latter is independently determined by PALS, or adiabatic calorimetry, or low frequency dielectric and mechanical spectroscopy. The property, THF Tgβ, had not been pointed out before by others or in any of the quasielastic neutron and light scattering studies of various amorphous polymers and van der Waals small molecular glass-formers over the past three decades. The generality and fundamental importance of this novel property revitalize the data from these previous publications, making it necessary to be reckoned with in any attempt to solve the glass transition problem. In our rationalization, the property arises first from the fact that the JG β-relaxation and the caged dynamics both depends on density and entropy. Second, the JG β-relaxation is the terminator of the caged dynamics, and hence the two processes are inseparable or effectively coupled. Consequently, the occurrence of the secondary glass transition at Tgβ necessarily is accompanied by corresponding change in the temperature dependence of the NCL, I, or «u2» of the fast caged dynamics at THF =Tg

Revealing the rich dynamics of glass-forming systems by modification of composition and change of thermodynamic conditions

Secondary relaxations have been classified into two types, depending on whether they are related to the structural alpha-relaxation in properties or not. Those secondary relaxations that are related to the a-relaxation may have fundamental importance, and are called the Johari–Goldstein (JG) ß-relaxations. Two polar molecular glass-formers, one flexible and another rigid, dissolved in apolar host with higher glass transition temperature are studied by broadband dielectric spectroscopy at ambient and elevated pressure. The neat flexible glassformer diethylphthalate (DEP) has a resolved secondary relaxation which, unlike the a-relaxation, is insensitive to pressure and hence is not the JG ß-relaxation. In the solution, the JG ß-relaxation of DEP shows up in experiment and its relaxation time tß is pressure and temperature dependent like ta. The result supports the universal presence of the JG ß-relaxation in all glass-formers, and the separation between ta and tß is determined by intermolecular interaction. The rigid glass-former is cyano-benzene (CNBz) and its secondary relaxation involves the entire molecule is necessarily the JG ß-relaxation. The dielectric relaxation spectra obtained at a number of combinations of pressure and temperature at constant ta show not only unchanged is the frequency dispersion of the a-relaxation but also tß. The remarkable results indicate that the JG ß-relaxation bears a strong connection to the alpha-relaxation, and the two relaxations are inseparablewhen considering the dynamics of glass-forming systems. Experimentally, tau_alpha has been found to be a function of the product variables, T/rho^gamma, where rho is the density and gamma is a material constant. From the invariance of the ratio, tau_alphaa/tau_ß, to change of thermodynamic conditions seen in our experiment as well in other systems, it follows that tß is also a function of T/rho^gamma, with the same gamma at least approximately. Since the JG ß-relaxation is the precursor of the a-relaxation, causality implies that the T/rho^gamma-dependence originates from the JG ß-relaxation and is passed on to the alpha-relaxation

Coupling of Caged Molecule Dynamics to JG β-Relaxation III: Van der Waals Glasses

In the first two papers separately on the polyalcohols and amorphous polymers of this series, we demonstrated that the fast dynamics observed in the glassy state at high frequencies above circa 1 GHz is the caged dynamics. We showed generally the intensity of the fast caged dynamics changes temperature dependence at a temperature THF nearly coincident with the secondary glass transition temperature Tgβ lower than the nominal glass transition temperature Tgα. The phenomenon is remarkable, since THF is determined from measurements of fast caged dynamics at short time scales typically in the ns to ps range, while Tgβ characterizes the secondary glass transition at which the Johari-Goldstein (JG) β-relaxation time τJG reaches a long time of ∼103 s, determined directly either by positronium annihilation lifetime spectroscopy, calorimetry, or low frequency dielectric and mechanical relaxation spectroscopy. The existence of the secondary glass transition originates from the dependence of τJG on density, previously proven by experiments performed at elevated pressure. The fact that THF ≈Tgβ reflects the density dependence of the caged dynamics and coupling to the JG β-relaxation. The generality of the phenomenon and its theoretical rationalization implies the same should be observable in other classes of glass-formers. In this paper, III, we consider two archetypal small molecular van der Waals glass-formers, ortho-terphenyl and toluene. The experimental data show the same phenomenon. The present paper extends the generality of the phenomenon and explanation from the polyalcohols, a pharmaceutical, and many polymers to the small molecular van der Waals glass-former

Investigation of the relevant kinetic processes in the initial stage of a double-arcing instability in oxygen plasmas

A numerical investigation of the kinetic processes in the initial (nanosecond range) stage of the double-arcing instability was developed. The plasma-sheath boundary region of an oxygen-operated cutting torch was considered. The energy balance and chemistry processes in the discharge were described. It is shown that the double-arcing instability is a sudden transition from a diffuse (glow-like) discharge to a constricted (arc-like) discharge in the plasma-sheath boundary region arising from a field-emission instability. A critical electric field value of ∼10^7 V/m was found at the cathodic part of the nozzle wall under the conditions considered. The field-emission instability drives in turn a fast electronic-to-translational energy relaxation mechanism, giving rise to a very fast gas heating rate of at least ∼10^9 K/s, mainly due to reactions of preliminary dissociation of oxygen molecules via the highly excited electronic state O2(B^3) populated by electron impact. It is expected that this fast oxygen heating rate further stimulates the discharge contraction through the thermal instability mechanism.Fil: Mancinelli, Beatriz Rosa. Universidad Tecnológica Nacional. Facultad Regional Venado Tuerto; ArgentinaFil: Prevosto, Leandro. Universidad Tecnológica Nacional. Facultad Regional Venado Tuerto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; ArgentinaFil: Chamorro Garcés, Juan Camilo. Universidad Tecnológica Nacional. Facultad Regional Venado Tuerto; ArgentinaFil: Minotti, Fernando Oscar. Universidad de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Kelly, Hector Juan. Universidad Tecnológica Nacional. Facultad Regional Venado Tuerto; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe; Argentin

Régénérer efficacement les peuplements de pin : des travaux souvent indispensables ! L’exemple des peuplements à pin d’Alep en zone méditerranéenne

La régénération naturelle est souvent un phase critique de la vie des peuplements forestiers, notamment pour les peuplements anciens et les espèces pionnières. C’est ainsi le cas des vieilles pinèdes à pin d’Alep en zone méditerranéenne. Dans cet article, les auteurs testent l’impact de diverses perturbations de la végétation et du sol qui peuvent influer sur cette régénération. Cette étude montre l’importance des travaux sylvicoles pour le renouvellement des peuplements

Supercooled Liquid Dynamics Studied via Shear-Mechanical Spectroscopy

We report dynamical shear-modulus measurements for five glass-forming liquids (pentaphenyl trimethyl trisiloxane, diethyl phthalate, dibutyl phthalate, 1,2-propanediol, and m-touluidine). The shear-mechanical spectra are obtained by the piezoelectric shear-modulus gauge (PSG) method. This technique allows one to measure the shear modulus ($10^{5} -10^{10}$ Pa) of the liquid within a frequency range from 1 mHz to 10 kHz. We analyze the frequency-dependent response functions to investigate whether time-temperature superposition (TTS) is obeyed. We also study the shear-modulus loss-peak position and its high-frequency part. It has been suggested that when TTS applies, the high-frequency side of the imaginary part of the dielectric response decreases like a power law of the frequency with an exponent -1/2. This conjecture is analyzed on the basis of the shear mechanical data. We find that TTS is obeyed for pentaphenyl trimethyl trisiloxane and in 1,2-propanediol while in the remaining liquids evidence of a mechanical $\beta$ process is found. Although the the high-frequency power law behavior $\omega^{-\alpha}$ of the shear-loss may approach a limiting value of $\alpha=0.5$ when lowering the temperature, we find that the exponent lies systematically above this value (around 0.4). For the two liquids without beta relaxation (pentaphenyl trimethyl trisiloxane and 1,2-propanediol) we also test the shoving model prediction, according to which the the relaxation-time activation energy is proportional to the instantaneous shear modulus. We find that the data are well described by this model.Comment: 7 pages, 6 figure