The applicability of the conventional Hoffman-Weeks (HW) linear extrapolation for the determination of the equilibrium melting temperatures of semicrystalline polymers is critically reviewed. It is shown that the linear extrapolation of observed melting temperatures cannot, in general, provide a reliable estimate of the equilibrium melting temperature. A more rigorous nonlinear HW analysis is proposed in this dissertation, which yields more accurate estimates of the equilibrium melting temperatures for semicrystalline polymers. The proposed nonlinear HW analysis is successfully applied to the cases of isotactic polypropylene and poly(ethylene oxide). The predicted initial lamellar thickness as a function of the crystallization temperature matches well with experimental results and/or literature values. Results based on the nonlinear HW analysis are consistent with those obtained from the analysis of the temperature dependence of the crystal growth rates. The general applicability of the Lauritzen Hoffman (LH) secondary nucleation theory is also addressed for isotactic polypropylene and poly(ethylene oxide). While the LH theory provides an excellent account of the temperature dependence of spherulitic growth rates and ratio of nucleation constants for different regimes, it appears not to yield a meaningful value for the substrate length, L, for poly(ethylene oxide). In a second project, the effects of structural and topological constraints on the morphology, melting and crystallization behavior of ethylene/styrene copolymers are investigated. During cooling from the melt, the longest ethylene sequences crystallize into lamellae in the primary crystallization process, while the shorter ethylene sequences are suggested to form fringed micelles in the secondary crystallization process. Kinetic studies indicate that secondary crystallization is characterized by an Avrami exponent of ½ which is consistent with a one dimensional, diffusion controlled growth. The increase in the melting temperature of secondary crystals with crystallization time is tentatively explained by a decrease in the molar conformational entropy of the remaining amorphous fraction as a result of secondary crystallization, although the possible role of an increase of crystal lateral dimensions with time cannot be rigorously ruled out. / Ph. D.

Differential scanning calorimeter (DSC) and temperature modulated differential scanning calorimeter (TMDSC) were used to study the isothermal crystallization kinetics and the melting behaviors at heating rates of 2, 10, 50, and 80

No description available.

Identification of fungal isolates is critical in studying Aspergillus flavus ecology and for developing methods to reduce aflatoxin contamination. In our efforts to track biocontrol applications of the atoxigenic A. flavus K49 (NRRL 30797), we have developed a rapid and accurate classification system for A. flavus based on PCR product melting temperatures (Tm). Using 18 primers and a total of 59 Aspergilli strains, including all 49 representatives of the Georgian peanut Vegetative Compatibility Groups (VCGs), a decision tree Tm flowchart was generated. The decision tree can classify all 59 strains using only 9 of the SSR primers and an average of 3.4 primers for each definitive classification. To confirm the effectiveness of the decision tree for strain identification, unknown samples isolated from experimental fields inoculated with various A. flavus strains in Stoneville, MS were analyzed. Ninety-six percent of the samples could be placed into a VCG using Tm(s) coupled with the decision tree. This dynamic system is an excellent tool for researchers studying biodiversity of A. flavus.

Simultaneous differential scanning calorimetry (DSC), small-angle (SAXS) and wide-angle X-ray scattering (WAXS) measurements were adopted for more precise determination of the equilibrium melting temperatures (Tm*) of a and b phases in bulk-crystallized syndiotactic polystyrene. On the basis of Kratky-Porod approximation, a new method for determining crystalline lamellar thickness from SAXS profiles obtained at high temperatures where there are only limited number of discrete crystalline lamellae dispersed in the melt matrix was developed. This method is shown to be reliable as it gave comparable results obtained from the conventional 1D correlation function method for SAXS profiles obtained at lower temperatures where lamellae are closely stacked. Results of the subsequent Gibbs-Thomson analysis indicated that the trigonal a phase is the entropically favored high temperature phase with Tm* = 355 oC whereas the b phase is enthalpically favored at lower temperatures, with Tm* = 314 oC. Compared to previous held contention in the temperature-dependent phase stability of these two phases, the current phase stability assignment is more consistent with both the density and the symmetry of the corresponding crystal structures. It also explains various observations reported previously on the competition between the two polymorphs during crystallization and during melting.

No description available.

The ignition temperature of the aluminium copper oxide (Al-CuO) thermite was measured using differential thermal analysis (DTA) at a scan rate of 50 °C/min in an inert nitrogen atmosphere. Thermite reactions are difficult to start as they require very high temperatures for ignition, e.g. for the Al-CuO thermite comprising micron particles it is ca. 940 °C. It was found that the ignition temperature is significantly reduced when the binary Si-Bi2O3 system is used as sensitiser. Further improvement is achieved when nano-sized particles are used. For the composition CuO + Al + Bi2O3 + Si (65.5:14.5:16:4 wt %), when all components except the aluminium fuel are nano-sized, the observed ignition temperature is reduced to ca. 615 °C and results in a thermal runaway. / Dissertation (MSc)--University of Pretoria, 2011. / Chemical Engineering / unrestricted

<p> The thaw rates of the active layer above the permafrost zone from a series of sites along the Hudson Bay coastline at lat. 56° have been examined with respect to temperature and moisture gradients, the characteristics of the surface layer and the bulk thermal properties for each profile. The thermal properties have been examined using firstly a Fourier approach with the parameter of degree days and using secondly a graphical approach employing thermal relationships obtained in the laboratory analyses by Kersten (1949).</p> <p> It was found that thaw rates are controlled by the interaction of a number of environmental factors of which vegetation appears to be the most important.</p> <p> The two approaches to the derivation of thermal properties give quite different results, such that the graphical approach is deemed to be unsuitable to field application.</p> / Thesis / Bachelor of Science (BSc)

Cette thèse de doctorat s’inscrit dans le cadre de l’étude in silico des relations qui lient la séquence d’une protéine à sa structure, sa stabilité et sa fonction. Elle a pour objectif de permettre à terme la conception rationnelle de protéines modifiées qui restent actives dans des conditions physico chimiques non physiologiques. Nous nous sommes plus particulièrement penchés sur la stabilité thermique des protéines, qui est définie par leur température de fusion Tm au delà de laquelle leur structure n’est thermodynamiquement plus stable. Notre travail s’articule en trois grandes parties : la recherche de facteurs favorisant la thermostabilité des protéines parmi des familles de protéines homologues, la mise sur pied d’une base de données de protéines de structure et de Tm déterminées expérimentalement, de laquelle sont dérivés des potentiels statistiques dépendant de la température, et enfin la mise au point de deux outils bioinformatiques visant à prédire d’une part la Tm d’une protéine à partir de la Tm de protéines homologues et d’autre part les changements de thermostabilité d’une protéine (Tm) engendrés par l’introduction d’une mutation ponctuelle. La première partie a pour objectif l’identification des facteurs de séquence et de structure (e.g. fréquence de ponts salins, d’interactions cation-{pi}) responsables des différentes stabilités thermiques de protéines homologues au sein de huit familles (chapitre 2). La spécificité de chaque famille ne nous a pas permis de généraliser l’impact de ces différents facteurs sur la stabilité thermique des protéines. Cependant, cette approche nous a permis de constater la multitude de stratégies différentes suivies par les protéines pour atteindre une plus grande thermostabilité. La deuxième partie concerne le développement d’une approche originale pour évaluer l’influence de la température sur la contribution de différents types d’interactions à l’énergie libre de repliement des protéines (chapitres 3 et 4). Cette approche repose sur la dérivation de potentiels statistiques à partir d’ensembles de protéines de thermostabilité moyenne distincte. Nous avons d’une part collecté le plus grand nombre possible de protéines de structure et de Tm déterminées expérimentalement, et d’autre part développé des potentiels tenant compte de l’adaptation des protéines aux températures extrêmes au cours de leur évolution. Cette méthode originale a mis en évidence la dépendance en la température d’interactions protéiques tels les ponts salins, les interactions cation-{pi}, certains empilements hydrophobes ... Elle nous a en outre permis de mettre le doigt sur l’importance de considérer la dépendance en la température non seulement des interactions attractives mais également des interactions répulsives, ainsi que sur l’importance de décrire la résistance thermique par la Tm plutôt que la Tenv, température de l’environnement de l’organisme dont elle provient (chapitre 5). La dernière partie de cette thèse concerne l’utilisation des profils énergétiques dans un but prédictif. Tout d’abord, nous avons développé un logiciel bioinformatique pour prédire la thermostabilité d’une protéine sur la base de la thermostabilité de protéines homologues. Cet outil s’est avéré prometteur après l’avoir testé sur huit familles de protéines homologues. Nous avons également développé un deuxième outil bioinformatique pour prédire les changements de thermostabilité d’une protéine engendrés par l’introduction d’une mutation ponctuelle, en s’inspirant d’un logiciel de prédiction des changements de stabilité thermodynamique des protéines développé au sein de notre équipe de recherche. Ce deuxième algorithme de prédiction repose sur le développement d’une grande base de données de mutants caractérisés expérimentalement, d’une combinaison linéaire de potentiels pour évaluer la Tm, et d’un réseau de neurones pour identifier les coefficients de la combinaison. Les prédictions générées par notre logiciel ont été comparées à celles obtenues via la corrélation qui existe entre stabilités thermique et thermodynamique, et se sont avérées plus fiables. Les travaux décrits dans notre thèse, et en particulier le développement de potentiels statistiques dépendant de la température, constituent une nouvelle approche très prometteuse pour comprendre et prédire la thermostabilité des protéines. En outre, nos travaux de recherche ont permis de développer une méthodologie qui pourra être adaptée à l’étude et à la prédiction d’autres propriétés physico chimiques des protéines comme leur solubilité, leur stabilité vis à vis de l’acidité, de la pression, de la salinité ... lorsque suffisamment de données expérimentales seront disponibles.

Reported is a study of the crystallization behavior of syndiotactic polystyrene (sPS) and its copolymers (with 4-bromostyrene as the comonomer or with atactic polystyrene arms grafted on the comonomer sites) via three sets of experiments. The first involves the study of structural identification of negatively birefringent spherulites by means of polarized light microscopy (PLM) and scanning electron microscopy (SEM). Results indicated that the optically positive and optically negative spherulites have same morphological features. Differences in the optical texture are due entirely to differences in orientation of the (anisotropic) sheaf-like precursors: the rigid nature of crystalline lamellae renders incomplete development of spherical symmetry even at the axialitic size of tens of microns. In the second part, we propose a modified approach for more precise determination of the Tm* value by taking advantage of the dual-mode distribution of crystalline lamellae in analyzing small-angle X-ray scattering (SAXS) profiles. This method should be generally applicable to other semi-crystalline polymers with dual-mode distribution in lamellar thickness. Results from wide-angle X-ray diffraction (XRD) suggest the presence of ?'-to-?" phase transformation at ca. 264 oC; no indications for the previously proposed ?-to-? transformation are identified. We therefore conclude that the ?' form is truly metastable; the ?"-form is the entropically favored high temperature phase (with Tm* = 300 oC) whereas the more ordered ?' phase (with Tm* = 288 oC) is enthalpically favored at lower temperatures. In the third set of experiments, identification of effects of copolymerization has been studied via a combination of PLM, differential scanning calorimetry (DSC), XRD, SAXS, and transmission electron microscopy (TEM). Results show that the equilibrium melting temperatures (determined via either Hoffman¡VWeeks or Gibbs¡VThomson plots) of the copolymers are significantly lower than that of the corresponding sPS homopolymer. The PLM observations indicate that the axialitic growth rates in copolymers are drastically lower than that of the corresponding homopolymer at comparable backbone length and supercooling. Both XRD and TEM results indicate preferred formation of the ?" phase upon melt crystallization in the bulk state; however, the ?" phase (instead of ?' phase that is the more commonly observed for sPS homopolymers in the bulk state) is dominant in thin films.