Svoboda R.,University of Pardubice
Journal of Thermal Analysis and Calorimetry | Year: 2014
The article critically reviews the current methodologies for determination of apparent activation energy of structural relaxation, Δh×, in the glass transition range. Tool-Narayanaswamy-Moynihan phenomenological model was used to simulate data for all major types of relaxation behavior, which were consequently evaluated in terms of the tested methodologies (curve-fitting, evaluation of Δh×from intrinsic cycles and evaluation of Δh×from constant heating rate cycles). Advantages and disadvantages of particular methodologies are demonstrated and thoroughly discussed. In addition, effects of various data-distortive effects influencing determination of glass transition activation energy are demonstrated and described. The discussed data-distortive effects include presence thermal gradients, improperly designed temperature programs, incorrectly applied subtractions of the thermokinetic background, or inability of the DSC instrument to perform high cooling/heating rates. Detailed guide for correct determination of Δh×from DSC measurements is introduced. © 2014 Akadémiai Kiadó, Budapest, Hungary.
Svoboda R.,University of Pardubice
Acta Materialia | Year: 2013
Structural relaxation processes were studied by means of differential scanning calorimetry for a number of compositions belonging to the Se-rich sides of Se-Te, Ge-Se and As-Se chalcogenide systems. Based on the results of fitting and non-fitting methods, the relaxation behavior of these selenide glasses is described in terms of the Tool-Narayanaswamy-Moynihan (TNM) equations. The goal of this paper is to introduce an advanced model describing the relationship between the macroscopic relaxation behavior and molecular structure of the glass. Special attention is paid to differences between the particular structural units and molecular arrangements characteristic for each system studied. The TNM parameters as well as their changes are discussed with respect to both the increasing content of dopant in the Se matrix within each chalcogenide system and different structural units employed in the respective systems. A unique novel conception is suggested, showing a connection between the values of TNM parameters and respective molecular structures and their changes during structural relaxation. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Bures F.,University of Pardubice
RSC Advances | Year: 2014
Property tuning in selected examples of D-π-A molecules has been discussed and summarized in this review article. The tuning and structure-property relationships have been demonstrated on the particular A, π and D parts of the push-pull molecule. Special emphasis has been put on the tuning of the FMO levels and optical properties. Further prospective applications of the given chromophore have also been considered. This journal is © 2014 The Royal Society of Chemistry.
Hajek P.,University of Pardubice
Decision Support Systems | Year: 2011
The paper presents the modelling possibilities of neural networks on a complex real-world problem, i.e. municipal credit rating modelling. First, current approaches in credit rating modelling are introduced. Second, previous studies on municipal credit rating modelling are analyzed. Based on this analysis, the model is designed to classify US municipalities (located in the State of Connecticut) into rating classes. The model includes data pre-processing, the selection process of input variables, and the design of various neural networks' structures for classification. The selection of input variables is realized using genetic algorithms. The input variables are extracted from financial statements and statistical reports in line with previous studies. These variables represent the inputs of neural networks, while the rating classes from Moody's rating agency stand for the outputs. In addition to exact rating classes, data are also labelled by four basic rating classes. As a result, the classification accuracies and the contributions of input variables are studied for the different number of classes. The results show that the rating classes assigned to bond issuers can be classified with a high accuracy rate using a limited subset of input variables. © 2010 Elsevier B.V. All rights reserved.
Jandera P.,University of Pardubice
Journal of Chromatography A | Year: 2012
Recent advances in applications of gradient elution in two-dimensional liquid chromatography are reviewed. Benefits of simultaneous programmed elution in two-dimensional liquid chromatography are shown, with emphasis on fast gradient elution in second dimension using various gradient types and profiles. The theory of gradient elution is shortly overviewed with attention to its impacts on two-dimensional separations. Various two-dimensional gradient systems are compared in terms of peak capacity and peak production rate: off-line, stop-and-go and real-time comprehensive approaches. Selection of stationary phases in combination with gradient elution and the effects of gradient elution on improved regularity of two-dimensional space coverage, orthogonality and peak capacity are discussed. Not only two-dimensional reversed-phase systems, but also other HPLC modes such as HILIC, ion-exchange or normal-phase chromatography with organic mobile phases are included. Fraction modulation in gradient elution setups, mobile phase compatibility and solvent effects in the fraction transfer between the first and the second dimension are addressed. Design and optimization of two-dimensional gradient separation systems for particular ordered and non-ordered samples are dealt with in detail. The discussed principles are illustrated by practical examples. © 2012 Elsevier B.V.