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Comment on “Decrease in the configurational entropy during a melt's polymerization” [Chem. Phys. 305 (2004) 231]
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Glass transition of an epoxy resin induced by temperature, pressure and chemical conversion: a rationale based on configurational entropy
No full textA comparative dielectric study is reported on the dynamics of a glass-forming epoxy resin when the glass transition is approached through different paths: cooling, compression, and polymerisation. Deep similarities are observed in dynamic properties. A unified reading of the experimental behaviour of the structural relaxation time of chemically reacting and stable systems close to the glass transition is given in the framework of the Adam-Gibbs theory of configurational entropy
Brillouin light scattering from shear waves in an epoxy resin through the glass transition
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Influence of temperature and pressure on dielectric relaxation in a supercooled epoxy resin
No full textIsothermal and isobaric dieletric measurements of a supercooled epoxy resin have been compared. A simple scaling relates isobaric and isothermal spectra corresponding to the same frequency of the main loss peak. Thus, the main and secondary processes retain a relative weight that is the same under isothermal and isobaric conditions. It is inferred that both pressure and temperature, equivalently, are able to take effect on the relaxation processes, without changing the relaxation mechanism itself. Careful analysis of the structural relaxation time behavior revealed that the traditional free volume equation, where only the macroscopic volume controls the pressure evolution of free volume, is not a suitable description of the data, as well as a Vogel-Fulcher (VF) type pressure dependent function. Based on a derivative method, a different function for describing the bidimensional surface tau(T,P) has been proposed, which accounts for the observed behavior through a nonlinear correction of the critical temperature T-0 in the VF law. The function we propose predicts pressure dependencies of the glass transition temperature and fragility which are appealing in view of a comparison with experimental results in this and many other systems. Interesting hints for interpreting the phenomenological results can be obtained within the Adam-Gibbs theory
Pressure dependence of structural relaxation time 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
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