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  • 1
    ISSN: 0392-6737
    Keywords: Phase equilibria, phase transitions, and critical points of specific substances ; Dielectric loss and relaxation ; Amorphous materials ; glasses ; Conference proceedings
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Summary Dielectric and ultrasonic measurements have been performed in the temperature and in the frequency domain to investigate the α (or primary) relaxation in the supercooled state of a «fragile» molecular liquid. This relaxation is strictly connected with the phenomenon of the glass transition and therefore an understanding of the former is a prerequisite for the explanation of the latter. The dynamic susceptibilities relative to these probes have been analysed with the Cole-Davidson function employing (where required) a Vogel-Fulcher-Tamman law to relate the main relaxation time to temperature. The results obtained are consistent with the dynamic behaviour of liquids of high fragility.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 117 (2002), S. 2161-2171 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We propose a detailed investigation of the schematic mode-coupling approach to experimental data, a method based on the use of simple mode-coupling equations to analyze the dynamics of supercooled liquids. Our aim here is to clarify different aspects of this approach that appeared so far uncontrolled or arbitrary, and to validate the results obtained from previous works. Analyzing the theoretical foundations of the approach, we first identify the parameters of the theory playing a key role and obtain simple requirements to be met by a schematic model for its use in this context. Then we compare the results obtained from the schematic analysis of a given set of experimental data with a variety of models and show that they are all perfectly consistent. A number of potential biases in the method are identified and ruled out by the choice of appropriate models. Finally, reference spectra computed from the mode-coupling theory for a model simple liquid are analyzed along the same lines as experimental data, allowing us to show that, despite the strong simplification in the description of the dynamics it involves, the method is free from spurious artifacts and provides accurate estimates of important parameters of the theory. The only exception is the exponent parameter, the evaluation of which is hindered, as for other methods, by corrections to the asymptotic laws of the theory present when the dynamics is known only in a limited time or frequency range. © 2002 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 4477-4477 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3417-3427 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By using simple mode coupling equations, we investigate the depolarized light scattering spectra of two so-called "fragile" glassforming liquids, salol (phenylsalicylate) and CKN (Ca0.4K0.6(NO3)1.4), measured by Cummins and co-workers. Nonlinear integrodifferential equations for the time evolution of the density-fluctuations autocorrelation functions are the basic input of the mode coupling theory. Restricting ourselves to a small set of such equations, we fit the numerical solution to the experimental spectra. It leads to a good agreement between model and experiment, which allows us to determine how a real system explores the parameter space of the model, but it also leads to unrealistic effective vertices in a temperature range where the theory makes critical asymptotic predictions. We finally discuss the relevance and the range of validity of these universal asymptotic predictions when applied to experimental data on supercooled liquids. © 1997 American Institute of Physics.
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  • 5
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    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6574-6585 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent experimental studies of the glass transition of molecular liquids have exploited light scattering techniques in order to support the dynamical model proposed by the mode coupling theory. In the framework of the dipole-induced-dipole (DID) formalism and the Stephen's approximation, we have checked this theory with several memory functions in the microscopic region, where phononlike excitations dominate, i.e., in the frequency window of 5–130 cm−1 accessible by a classical Raman spectrometer. The fitting procedure compares the experimental susceptibility spectra of one of the simplest fragile molecular liquid, m-toluidine, to the theoretical ones and estimates, in each case, the T dependence of the different control parameters as well as the crossing point of the transition line of type B. The agreement observed for spectra from a temperatures above the melting point down to the glass transition temperature Tg suggests, on the one hand, that information about the dynamical behavior of the supercooled liquid are contained in this frequency region and, on the other hand, that vibrational contributions are incorporated in this formalism, independently of the form of the relaxation kernel. Finally, the two-peak shape in the microscopic range of the susceptibility spectra is related to the relaxation of a linear combination of the Fourier components of the two density correlators. © 1995 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2250-2257 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the first calorimetric characterization of isothermal glass transitions obtained by changing the pressure at constant temperature. This isothermal glass transition is described in the case of m-fluoroaniline, a so-called fragile liquid. The method employed is based on Maxwell's equations and involves measurement of the heat of compression of a sample enclosed in a hydrostatic pressure cell under isothermal and reversible conditions. A discontinuity of the thermodynamic quantity αV, where α is the expansivity and V is the molar volume, defines the glass transition pressure Pg at which ergodicity is broken on the time scale of the experiments (300–1000 s) which depends on the pressure step employed. The step in αV is found initially to decrease rapidly with increasing pressure. The slope (∂Pg/∂T)τ seems smaller than that determined by the usual isobaric methods, implying some distinction in the degrees of freedom involved and associated differences in defining the values of Tg. Because of the high sensitivity and stability of the calorimeter, we can also observe annealing effects in the overcompressed glass at a pressure above Pg; the slow dynamic processes associated are called entropic relaxation because it is the entropy which is measured directly. An approximate relaxation function and a model for the compression data are discussed. Keeping in mind the overall pattern of "strong and fragile'' liquid behavior, isothermal pressure variations appear to offer a useful tool to connect these extremes and may constrain the glass transition of a molecular liquid in a particular range of the P–T diagram. The concept of Kauzmann pressure PK is introduced, but it is noted that PK must become less well defined the further from ambient pressure it lies due to the decrease in ΔαV which is observed with increasing pressure.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 395-398 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Small angle neutron scattering (SANS) has been used to establish the dispersed droplet character of a class of pseudo-three-component oil/water (o/w) microemulsions which can be cooled continuously into the glassy state without either crystallization or microemulsion destabilization. SANS of toluene microemulsions at low volume fractions (∼0.5%) of toluene indicate a droplet radius of 270±10 A(ring). At normal concentrations (∼33 vol % oil), the droplet size cannot be determined precisely but the presence of a droplet state of small polydispersity is established for toluene, p-xylene, benzene, CS2, and CCl4 by the presence of a sharp peak in the scattered neutron intensity vs Q plots. Average interdroplet separations range from 200 to 300 A(ring), implying average radii of ∼125 A(ring). Such radii are consistent with freeze–fracture electron microscope studies of the same or similar systems. Variable temperature studies on toluene microemulsions during both cooling and warming cycles in the temperature range ambient to 100 K, show only minor (∼10%) changes in the interdroplet separation down to 220 K at which temperature diffusion through the viscous matrix phase becomes too slow to permit further emulsion structure changes on the time scale of the cooling experiment. The origin, and some possible exploitations, of this unusual stability against temperature change are discussed.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 5262-5272 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: As a contribution to the understanding of the thermodynamics of the glass transition phenomenon a series of molecules having the same steric character, but differing in the strength and nature of intermolecular interactions, has been investigated. The series is based on systematic changes of substituents on disubstituted benzene ring compounds, the simplest example of which is meta-xylene. Meta-isomers are chosen in each instance because of their greater tendency to supercool. In particular, m-fluoroaniline cannot be crystallized at ambient pressure. The principal measurements performed were of heat capacity and enthalpy change, using the technique of differential scanning calorimetry, and these have been examined in the light of literature data on the liquid viscosities and some recent data for dielectric relaxation. As the strength of hydrogen-bonding interactions between the ring substituents on adjacent molecules increases, the glass transition temperature Tg increases by almost 100 degrees from the lowest value in the series, 122.5 K, for m-fluorotoluene. Empirical rules involving Tb/Tm and Tg/Tm are found wanting. The important thermodynamic characteristic of the glass transition, viz., the change in heat capacity at the glass transition, ΔCp, remains approximately constant until the −OH substituent is introduced, whereupon a new element appears. This is a specific component of ΔCp which appears at temperatures above an initially small jump at Tg. It is well accounted for by the addition of a two-state H-bond breaking component (with the usual H(centered ellipsis)−OH bond energy) to the total excess heat capacity. The liquid ground state (or Kauzmann) temperature TK assessed from thermodynamic data acquired in this study, falls 20%–30% below the glass transition temperature. From the limited transport data available, these liquids appear to be quite fragile in character implying that the phenyl group influence dominates the hydrogen bond factor which has often seemed responsible for decreased fragility. In the case of cresol the hydrogen bonding apparently produces dielectric/shear relaxation anomalies of a character previously only seen in certain aliphatic monoalcohols. © 1999 American Institute of Physics.
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