379 research outputs found
CHANGES IN THE DYNAMICS OF SUPERCOOLED SYSTEMS REVEALED BY DIELECTRIC SPECTROSCOPY
No full textThe dynamics of monoepoxy, diepoxy, and triepoxy glass-formers from below to above the glass transition temperature, T-g, has been investigated through the temperature behavior of relaxation times, strengths, and conductivity, determined in a wide frequency range (10(2)-2x10(10) Hz). In all systems the main and secondary relaxations define a splitting temperature T(S)similar to 1.3xT(g); moreover, a crossover temperature T(B)similar to T-S is recognized, marking the separation between two different Vogel-Fulcher regimes for the structural dynamics. The strengths behavior reflects the distribution of the overall energy between the relaxation processes and no peculiar behavior is revealed at T-S. A strong increase characterizes the strength of the secondary relaxation on crossing the glass transition from the lower temperatures. Conductivity data have been analyzed to test the dynamics in terms of the Debye-Stokes-Einstein (DSE) diffusion law. The prediction of the DSE model is well verified for mono- and diepoxide up to the high viscosity regime, while a fractional DSE law with exponent similar to 0.81, accounting for a decoupling between translational and rotational motions, replaces the DSE relation in triepoxide for temperatures below T-S. The change of the structural dynamics, the splitting between main and secondary relaxation and the breakdown of the DSE behavior, all occur within a narrow temperature range around T-S; this finding argues in favor of the existence of a change of the dynamics in the supercooled liquid state well above the glass transition temperature. (C) 1999 American Institute of Physics. [S0021-9606(99)50444-3]
Comment on "Decrease in the configurational entropy during a melt's polymerization" [Chem. Phys. 305 (2004) 231]
No full textPressure dependence of structural relaxation times in terms of the Adam-Gibbs model
No full textA new equation describing the behavior of the structural relaxation lime, tau (T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S-c, in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient alpha and the isothermal bulk modulus K-o. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of tau versus temperature with the fragility parameter independent from pressure. The equation for tau (T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the Values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of tau (max) is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena
Effect of elastic properties modification on the vibrational density of states: A joint Brillouin and Raman scattering study
No full textThe modification of the vibrational density of states in a reactive epoxy-amine mixture during isothermal polymerization have been characterized by Brillouin and Raman scattering measurements at different temperatures. During the reaction the system undergoes the glass transition passing from the liquid to the glassy phase. The relative variation of the boson peak and the elastic moduli have been investigated. We find that the intensity variation and the shift of the boson peak are only an apparent effect due to the elastic medium transformation. A master curve of the spectra in the boson peak region can always be obtained both as a function of the reaction temperature and as a function of the reaction time. Moreover, the Brillouin light scattering results give evidence of the validity of a Cauchy-like relation for the real part of the elastic moduli measured at finite frequencies also in relaxing liquids
Hopping charge transport in conducting polymers studied by d.c. conduction and dielectric response analysis
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Connecting Irreversible to Reversible Aggregation: Time and Temperature
No full textWe report molecular dynamics simulations of a gel-forming mixture of ellipsoidal patchy particles with different functionality. We show that in this model, which disfavors the formation of bond-loops, elapsed time during irreversible aggregation-leading to the formation of an extended network-can be formally correlated with equilibrium temperature in reversible aggregation. We also show that it is possible to develop a parameter-free description of the self-assembly kinetics, bringing reversible and irreversible aggregation of loopless branched systems to the same level of understanding as equilibrium polymerization
Stress-induced modification of the boson peak scaling behavior
No full textThe scaling behavior of the so-called boson peak in glass-formers and its relation to the elastic properties of the system remains a source of controversy. Here the boson peak in a binary reactive mixture is measured by Raman scattering (i) on cooling the unreacted mixture well below its glass-transition temperature and (ii) after quenching to very low temperature the mixture at different times during isothermal polymerization. We find that the scaling behavior of the boson peak with the properties of the elastic medium - as measured by the Debye frequency - holds for states in which the elastic moduli follow a generalized Cauchy-like relationship, and breaks down in coincidence with the departure from this relation. A possible explanation is given in terms of the development of long-range stresses in glasses. The present study provides new insight into the boson peak behavior and is able to reconcile the apparently conflicting results presented in literature
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