Chain-length-dependent relaxation scenarios in an oligomeric glass-forming system: from merged to well-separated alpha and beta loss peaks

We have studied the relaxation dynamics of a homologous series of propylene glycol based dimethyl ethers in the supercooled regime by means of broadband dielectric spectroscopy. The system is chosen in order to minimize changes of the intermolecular interactions with varying molecular weight, M. A gradual transformation from a scenario of well-separated to one of merged alpha and beta loss peaks was observed with decreasing M. The results give strong evidence for the currently debated excess wing being due to an underlying beta relaxation. The study suggests that the main difference between glass formers with and without excess wings is the relaxation time at the merging temperature.     

Excess wing in the dielectric loss of glass formers: A johari-goldstein beta relaxation?

Dielectric loss spectra of glass-forming propylene carbonate and glycerol at temperatures above and below T(g) are presented. By performing aging experiments lasting up to five weeks, equilibrium spectra below T(g) have been obtained. During aging, the excess wing, showing up as a second power law at high frequencies, develops into a shoulder. The results strongly suggest that the excess wing, observed in a variety of glass formers, is the high-frequency flank of a beta relaxation.     

Pressure evolution of the excess wing in a type-B glass former

Glass formers are defined as "type B" when they exhibit a distinct Johari-Goldstein (JG) relaxation, but lack an excess loss ("excess wing," EW) in their structural relaxation peak. By studying the dielectric spectra of a well-known type-B glass former under high pressure, we unequivocally show the existence of an EW, simultaneously with the JG relaxation. Moreover, at very high pressures (0.6 GPa), the EW becomes a distinct relaxation peak, although correlated with the structural relaxation. The implication is that the EW, rather than the higher frequency relaxation ascribed to the JG process, is perhaps a universal feature of glass formers, albeit not always discernible at ambient pressure. Our findings may reconcile all opposing points of view present in the literature, as well as indicate that the type-A or type-B classification of glass formers should be modified or even discontinued.     

Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers: a leading model to describe the onset of Johari-Goldstein processes and their relationship with fully cooperative processes

The interrelation of dynamic processes active on separated time-scales in glasses and viscous liquids is investigated using a model displaying two time-scale bifurcations both between fast and secondary relaxation and between secondary and structural relaxation. The study of the dynamics allows for predictions on the system relaxation above the temperature of dynamic arrest in the mean-field approximation, that are compared with the outcomes of the equations of motion directly derived within the Mode Coupling Theory (MCT) for under-cooled viscous liquids. By varying the external thermodynamic parameters, a wide range of phenomenology can be represented, from a very clear separation of structural and secondary peak in the susceptibility loss to excess wing structures.     

Statistical mechanical approach to secondary processes and structural relaxation in glasses and glass formers A leading model to describe the onset of Johari-Goldstein processes and their relationship with fully cooperative processes

The interrelation of dynamic processes active on separated time-scales in glasses and viscous liquids is investigated using a model displaying two time-scale bifurcations both between fast and secondary relaxation and between secondary and structural relaxation. The study of the dynamics allows for predictions on the system relaxation above the temperature of dynamic arrest in the mean-field approximation, that are compared with the outcomes of the equations of motion directly derived within the Mode Coupling Theory (MCT) for under-cooled viscous liquids. By varying the external thermodynamic parameters, a wide range of phenomenology can be represented, from a very clear separation of structural and secondary peak in the susceptibility loss to excess wing structures.     

Depolarized light scattering study of glycerol

We have obtained depolarized (anisotropic) light scattering spectra of the glass forming material glycerol in a wide temperature (160 to 430 K) and frequency (0.4 GHz to 48 THz) range and compared them with light scattering and dielectric spectroscopic data from the literature. The excess wing, which is usually discussed concerning dielectric data, is strongly pronounced in light scattering of glycerol. The temperature evolution of the wing, together with that of the fast relaxation spectrum, leads to a susceptibility minimum that persists in the spectra down to Tg 185 K and flattens with decreasing the temperature below ~290 K. The -peak in light scattering is significantly wider and weaker in amplitude than in the dielectric loss spectra, and thus cannot be interpolated by the asymptotic laws of the mode coupling theory (MCT). However, similar trends were reported in the literature for solutions of MCT models for rotational dynamics. Furthermore we can identify in both the light scattering and the dielectric data two distinct regimes of temperature evolution with a blurred crossover in the range of 300–320 K. The high-temperature regime is characterized by a nearly temperature independent shape of the -peak and of the susceptibility minimum, and a nearly temperature independent strength of the fast dynamics. In contrast, at low temperatures the excess wing emerges and the relaxation strength of the fast dynamics changes strongly with the temperature.     

Dielectric investigation of the temperature dependence of the nonexponentiality of the dynamics of polymer melts

Using broad band dielectric spectroscopy (10(-5)-10(9) Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the alpha relaxation of polymer melts in the glass transition temperature T(g) range. The alpha relaxation process is described in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter beta to characterize the nonexponentiality of the relaxation. At high temperatures, beta remains nearly insensitive to temperature changes, whereas in the vicinity of T(g) a nearly linear increasing of beta with temperature is found. The temperature range where the change of the beta(T) behavior occurs is located for all the polymers investigated around 1.2T(g). Moreover, our results indicate a common value of beta approximately equal to 1/3 at the temperature where the relaxation time diverges. The beta(T) behavior near T(g) is discussed in terms of a "rugged landscape" phase space which allows us to rationalize both the beta(T) behavior observed as well as the similarities of our findings near T(g) with the results reported in simulations on Ising spin glasses and other model systems.     

Spectra of molecular light scattering in guaiacol

The propagation velocity and absorbance of longitudinal hypersound weremeasured in a wide temperature range by scattered light spectra in pure guaiacol. The Rayleigh line wing spectrum was studied and the relaxation time of anisotropy fluctuations was determined. The temperature dependence and value of the molecular viscosity of guaiacol were determined from the data on the Rayleigh line wing width.     

跨音速全位势方程的机翼气动性能计算

采用守恒型的跨音速全位势方程计算了机翼的气动性能,包括表面压力分布,力和力矩系数。计算中未计及粘性效应。坐标变换采用Joukowsky变挟。计算采用Jameson超松弛逐次线迭代格式(SLOR)和逐次加密网格技术。以ONERA M6机翼作为算例。计算结果表明与其他采用全位势方程的计算结果以及实验结果符合很好。     

Correlation studies on dielectric and thermodynamic parameters in the binary mixtures of N-methyl aniline and alcohols

The dielectric and thermodynamic studies on polar binary mixtures of N- methyl aniline with alcohols — propan-1-ol and propan-2-ol have been carried out, at different temperatures and mole fractions. LF impedance analyzer, Microwave bench and Abbe’s refractometer are used respectively in radio, microwave (X-Band) and high frequency regions to determine the dielectric data. The experimental data is used to correlate the dielectric and thermodynamic parameters — static permittivity, high frequency permittivity, Kirkwood effective correlation factor, corrective Kirkwood correlation factor, excess permittivity, excess Helmholtz free energy, dipole moment, excessive dipole moment, relaxation time, excess inverse relaxation time and the excess thermodynamic parameters. These parameters are used to interpret the molecular interactions between the molecular species of the liquid mixtures.     

Frequency and temperature amplitude dependence of complex heat capacity in the melting region of polymers

Amplitude and frequency dependence of reversible melting of polycaprolactone (PCL) and an ethylene octene copolymer (EOM) were studied using temperature-modulated differential scanning calorimetry (TMDSC) (2?10^?1 Hz <f < 0.05 Hz) and shear spectroscopy (dynamic mechanical analysis, DMA) (5?10^?4 Hz < f < 100 Hz). It was found that the excess heat capacity of PCL is constant for temperature amplitudes in the range 5 mK < A_T < 2 K. The excess heat capacity decreases with frequency of temperature perturbation and tends to zero at about 0.1 Hz and 100 Hz for PCL and EOM, respectively. The constant excess heat capacity and the frequency dependence support the idea that reversible melting is related to a relaxation process on the surface of the polymer crystals. The occurrence of such a relaxation process was shown by shear modulus measurements in the same frequency and temperature region. The relaxation process is, in the melting region, much slower than main relaxation (glass transition). At low temperatures, a crossover can be seen, indicating the independence of both processes because of spatial separation. The main relaxation is related to the melt, while the other is related to the crystal surface.     

Beta relaxation versus high frequency wing in the dielectric spectra of a binary molecular glass former

The binary molecular glass former 2-picoline in tri-styrene is investigated by means of broadband dielectric spectroscopy with the aim of understanding the role of secondary relaxation processes that emerge during the glass transition. It is shown that the "high frequency wing," which is seen in neat picoline, becomes a separate process in the mixture and exhibits all the features of a Johari-Goldstein relaxation. In particular, the previously found relation between activation energy and Tg is recovered. In addition, below Tg the width parameter of this secondary relaxation is shown to be governed by a common temperature dependence, and the time scale is characterized by an isokinetic point. Above Tg pronounced deviations from an Arrhenius behavior are observed.     

Does the arrhenius temperature dependence of the Johari-Goldstein relaxation persist above T(g)?

Dielectric spectra of the polyalcohols sorbitol and xylitol were measured under isobaric pressures up to 1.8 GPa. At elevated pressure, the separation between the alpha and beta relaxation peaks is larger than at ambient pressure, enabling the beta relaxation times to be unambiguously determined. Taking advantage of this, we show that the Arrhenius temperature dependence of the beta relaxation time does not persist for temperatures above T(g). This result, consistent with inferences drawn from dielectric relaxation measurements at ambient pressure, is obtained directly, without the usual problematic deconvolution the beta and alpha processes.     

Limits to the polarization for spin-exchange optical pumping of 3He

Based on measurements of the temperature dependence of 3He relaxation in a wide range of spin-exchange optical pumping cells, we report evidence for a previously unrecognized surface relaxation process. The relaxation rate was found to be linearly proportional to the alkali-metal density with a slope that exceeds the spin-exchange rate, which limits the polarization for current applications, including neutron spin filters, polarized targets, and polarized gas magnetic resonance imaging. We find that the magnitude of this excess relaxation can vary widely between cells, and that the variation is larger for cells of higher surface to volume ratio. We have observed 3He polarization as high as 81%, but further improvements require understanding the origin of this relaxation.     

Measuring the configurational heat capacity of liquids

A high electric field impedance experiment on supercooled molecular liquids is employed to transfer energy to the slow modes by absorption from the field and detect the increase of their "configurational temperature", T(cfg), via the change of the relaxation times. This allows us to determine the configurational heat capacity, which accounts for most of the excess heat capacity for stronger liquids, but for only half of the heat capacity step in the case of more fragile systems. It is also observed that T(cfg) gradually approaches the phonon temperature on the structural relaxation time scale.     

GdBaCo2O5+δ体系的滞弹性内耗研究

GdBaCo₂O_5+δ体系的低频内耗研究表明：体系中存在一个由额外氧运动引起的弛豫内耗峰；额外氧δ对这个弛豫内耗峰的大小、峰形及峰位有较大影响，反映了额外氧状态随δ而变化.当δ=0.005，体系中额外氧含量很少而接近零时，相应的内耗峰消失；δ达到一定数量后，出现弛豫内耗峰.由δ=0.278，0.407，0.421，0.515样品的弛豫内耗峰分析可得到体系随δ不同存在着三种不同的额外氧形态.此外，δ=0.421及0.515的样品在360K附近存在一个相变内耗峰，它对应着体系中的金属—绝缘体转变. 关键词： 2O_5+δ')" href="#">GdBaCo₂O_5+δ 额外氧 内耗峰     

Relaxation processes in supercooled confined water and implications for protein dynamics

We show that the viscosity-related main (alpha) relaxation of confined water vanishes at a temperature where the volume required for the cooperative alpha relaxation becomes larger than the size of the geometrically confined water cluster. This occurs typically around 200 K, implying that above this temperature we observe a merged alpha-beta relaxation, whereas below it only a local (beta) relaxation remains. This also means that such confined supercooled water does not exhibit any true glass transition, in contrast to other liquids in similar confinements. Furthermore, it implies that deeply supercooled water in biological systems, such as membranes and proteins, generally shows only a local beta relaxation, a finding of importance for low temperature properties of biological materials.     

Mechanical properties of polymer mixtures: Effect of compatibility

The effect of compatibility on the mechanical properties of polymer mixtures is discussed. From solution thermodynamics we conclude that for compatible mixtures of high molecular weight polymers the excess internal energy of mixing must be negative. For most polymer mixtures, this results in a negative excess volume of mixing. The excess entropy of mixing is therefore also negative. These general conclusions point to a stiffening of the solid lattice and a reduction in chain mobility. In dynamic mechanical behavior, the relaxation of molecular moieties responsible for the specific interaction should be shifted to intermediate temperatures, and the relaxation of other moieties should be shifted to higher temperatures if the potential well is inter-molecular. In finite deformation behavior craze initiation stress and shear yielding stress should both be higher than a linear combination of the properties of the pure components.     

Dielectric relaxation and hydrogen bonding studies of 1,3-propanediol–dioxane mixtures using time domain reflectometry technique

The complex permittivity, static dielectric constant and relaxation time for 1,3-propanediol, 1,4-dioxane and their mixtures have been studied using time domain reflectometry (TDR). The excess permittivity, excess inverse relaxation time and Kirkwood correlation factor have also been determined at various concentrations of dioxane. Hydrogen bonded theory was applied to compute the correlation terms for the mixtures. The Bruggeman model for the nonlinear case has been fitted to the dielectric data for mixtures.     

Dielectric behaviour of propylene glycol-water mixtures studied by time domain reflectometry

Dielectric relaxation measurements on water mixtures of propylene glycol across the entire concentration range were carried out using time domain reflectometry at 25°C over the frequency range from 10 MHz to 4 GHz. For all the mixtures, only one dielectric loss peak was observed in this frequency range. The relaxation in these mixtures can be described by a single relaxation time using the Debye model. A plot of the calculated relaxation time of the mixtures gives a straight line against the mole fraction of water, X_w. It is reasoned that the diameter of the water cluster is nearly the same as the length of propylene glycol. Further, a plot of the dielectric relaxation strength δ? against X_w suggests that there is a changing pattern of dielectric behaviour from below X_w = 0.5 to higher values of X_w. The excess permittivity, the excess inverse relaxation time and the activation free energy have been determined, to confirm the formation of hydrogen bonded homogeneous and heterogeneous cooperative domains, the dynamics of solute-solvent interaction and the hindrance to molecular rotation in the hydrogen bonded glass forming propylene glycol-water system.