Decoupling of the dc conductivity and (α-) structural relaxation time in a fragile glass-forming liquid under high pressure (original) (raw)
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The Journal of Chemical Physics, 2008
Shear-mechanical and dielectric measurements on the two monohydroxy (mono-alcohol) molecular glass formers 2-ethyl-1-hexanol and 2-butanol close to the glass transition temperature are presented. The shear-mechanical data are obtained using the piezoelectric shear-modulus gauge method covering frequencies from 1mHz to 10kHz. The shear-mechanical relaxation spectra show two processes, which follow the typical scenario of a structural (alpha) relaxation and an additional (Johari-Goldstein) beta relaxation. The dielectric relaxation spectra are dominated by a Debye-type peak with an additional non-Debye peak visible. This Debye-type relaxation is a common feature peculiar to mono-alcohols. The time scale of the non-Debye dielectric relaxation process is shown to correspond to the mechanical structural (alpha) relaxation. Glass-transition temperatures and fragilities are reported based on the mechanical alpha relaxation and the dielectric Debye-type process, showing that the two glass-transition temperatures differ by approximately 10K and that the fragility based on the Debye-type process is a factor of two smaller than the structural fragility. If a mechanical signature of the Debye-type relaxation exists in these liquids, its relaxation strength is at most 1% and 3% of the full relaxation strength of 2-butanol and 2-ethyl-1-hexanol respectively. These findings support the notion that it is the non-Debye dielectric relaxation process that corresponds to the structural alpha relaxation in the liquid.
Physical Review E, 2000
The effect of pressure variation on dynamics of ␣ relaxation process in poly͓͑phenyl glycidyl ether͒-co-formaldehyde͔ has been investigated both under isothermal (Tϭ293 K) and isobaric (Pϭ0.1, 60, 120, 180, and 240 MPa͒ conditions using broad band dielectric spectroscopy (10 Ϫ2 to 10 6 Hz). The ␣ relaxation is analyzed by means of the Havriliak-Negami relaxation function which has two shape parameters ͑␣ and ␥͒ to characterize non-Debye behavior. As a result we found that the shape parameters of the dielectric function collected for different pressures fall on master curves constructed by plotting ␣ and ␣␥ against the logarithm of relaxation time. The scaling of shape parameters for different pressure stems from pressure independence of fragility. This provides strong experimental evidence supporting correlation of non-Debye behavior with non-Arrhenius relaxation under high pressure. From an analysis of the shape parameters of relaxation function, in terms of the Schonhals and Schlosser model, we drew conclusions that the molecular mobility of PPG is controlled in the same way by temperature and pressure. The relaxation times exhibit a clear non-Arrhenius behavior under isothermal and isobaric condition.
Non-Debye response for the structural relaxation in glass-forming liquids: Test of the Avramov model
The Journal of Chemical Physics, 2007
The experimentally observed characteristic features of the ␣-relaxation process in glass-forming liquids are the non-Arrhenius behavior of the structural relaxation times and the non-Debye character of the macroscopic relaxation function. The Avramov model in which relaxation is considered as an energy activation process of surmounting random barriers in liquid energy landscape was successfully applied to describe the temperature and pressure dependences of the macroscopic relaxation times or viscosity. In this paper, we consider the dielectric spectrum associated with Avramov model. The asymmetrical broadening of the loss spectra was found to be related directly to dispersion of the energy barrier distribution. However, it turns out that temperature dependence of the spectrum broadening as predicted by the Avromov model is at odds to experimental observation in glass-forming liquids.
IEEE Transactions on Dielectrics and Electrical Insulation, 2001
The dynamics of the epoxy compound polyphenyl-glycidyl-ether-coformaldehyde was investigated by wideband dielectric spectroscopy (lo-' to lo1' Hz) by varying either pressure or temperature. The measurements were undertaken in two different conditions: under isobaric condition (atmospheric pressure) varying temperature down to the supercooled and glassy phases and under isothermal conditions by changing the pressure from 0.1 to 270 MPa. The analysis of the structural relaxation time behavior either varying the temperature or the pressure was found to agree well with a previously derived expression for T ( T , P ) based on a cooperative model. In particular it was found that with a unique set of parameters it is possible to describe the 2-dimensional surface T(T, P ) over a wide range of temperature and pressure.
Philosophical Magazine B-physics of Condensed Matter Statistical Mechanics Electronic Optical and Magnetic Properties, 2002
We present a comparative analysis of dielectric spectroscopy and optical Kerr eOE ect measurements of relaxation processes in glass-forming phenyl glycidyl ether. In particular we focus our attention on the structural a-relaxation process and the most important ®nding is the existence of a constant scaling factor between the characteristic times obtained by the two experiments in the high-temperature region.
Temperature Dependence of Structural Relaxation in Glass-Forming Liquids and Polymers
Entropy
Understanding the microscopic mechanism of the transition of glass remains one of the most challenging topics in Condensed Matter Physics. What controls the sharp slowing down of molecular motion upon approaching the glass transition temperature Tg, whether there is an underlying thermodynamic transition at some finite temperature below Tg, what the role of cooperativity and heterogeneity are, and many other questions continue to be topics of active discussions. This review focuses on the mechanisms that control the steepness of the temperature dependence of structural relaxation (fragility) in glass-forming liquids. We present a brief overview of the basic theoretical models and their experimental tests, analyzing their predictions for fragility and emphasizing the successes and failures of the models. Special attention is focused on the connection of fast dynamics on picosecond time scales to the behavior of structural relaxation on much longer time scales. A separate section disc...
The Journal of Chemical Physics, 2001
Previous dielectric relaxation measurements of glycerol and propylene carbonate and new results on propylene glycol performed below the conventional glass transition temperatures T g after long periods of aging all show that the excess wing ͑a second power law at higher frequencies͒ in the isothermal dielectric loss spectrum, develops into a shoulder. These results suggest that the excess wing, a characteristic feature of a variety of glass-formers, is the high frequency flank of a Johari-Goldstein -relaxation loss peak submerged under the ␣-relaxation loss peak. With this interpretation of the excess wing assured, the dielectric spectra of all three glass-formers measured at temperatures above T g are analyzed as a sum of a ␣-relaxation modeled by the Fourier transform of a Kohlrausch-Williams-Watts function and a -relaxation modeled by a Cole-Cole function. Good fits to the experimental data have been achieved. In addition to the newly resolved -relaxation on propylene glycol, the important results of this work are the properties of the -relaxation in this class of glass-formers in the equilibrium liquid state obtained over broad frequency and temperature ranges.
The Journal of Chemical Physics, 2013
In the frame of fractional-kinetic approach, the model of the structural α-relaxation in the presence of the secondary β-relaxation processes is suggested. The model is based on the rigorous bond between β-processes with α-process and leads to the generalized and justified expression for the complex dielectric permittivity (CDP). It allows to form a new sight on the problem of the fitting of multipeak structure of the dielectric loss spectra in glass-forming materials. The consistency of the CDP expressions obtained is based on a good fit of experimental data for binary methanol-water mixtures.
Physical review. E, Statistical, nonlinear, and soft matter physics, 2001
From temperature studies at ambient pressure, it was pointed out for several glass-forming liquids that the alpha-relaxation time (tau) can be related to the dc-ionic conductivity (sigma) through the phenomenological fractional Debye-Stokes-Einstein (DSE) equation. In the present paper we test the validity of fractional DSE equation for relaxation data obtained from pressure variable experiments. To this end we carried out broadband dielectric measurements (10 mHz-10 MHz) in a wide range of pressures (0.1-300 MPa). The material under study were N,N-diglycidyl-4-glycidyloxyaniline and N,N-diglycidylaniline. As a result we found that the fractional DSE equation is also obeyed for pressure pathways.