Browsing by Subject "Differential scanning calorimeter (DSC)"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item The glass transition in confined and heterogeneous systems(Texas Tech University, 2008-08) Zheng, Wei; Simon, Sindee L.; Quitevis, Edward L.; Weeks, Brandon L.; McKenna, Gregory B.The glass transition (Tg) is an interesting but challenging problem. Although this phenomenon has been studied for over half a century, Tg is still not well understood especially at the molecular level. One important feature which seems to be missing in the current understanding is its inherent heterogeneous dynamics. The aim of this work is to study the Tg behavior under various conditions such as for material freeze-dried from dilute solutions, confined in nanopores, and blended with other components, and to examine the relation between the dynamic heterogeneity and the observed Tg behavior. Freeze-dried materials from dilute solutions show different Tg behavior from the bulk; however, the origin of the difference remains unclear. In this work, the residual solvent effect on the calorimetric Tg of freeze-dried polystyrene is investigated. A linear correlation is found between the Tg depression and the residual solvent concentration, in agreement with data in the literature, indicating that the Tg depression observed for polymers freeze dried from dilute solution is due to residual solvent. Confinement at the nanoscale is also found to affect the glass transition behavior. Two hydrogen-bonded liquids, glycerol and propylene glychol, confined in silanized and unsilanized nanopores are studied to elucidate the confinement effects on Tg. Upon confinement, these two materials show similar behavior except that an additional Tg is observed for propylene glycol. We find that the confinement effects strongly depend on the competition between size effects and surface effects. Recently, a self-concentration model was proposed to predict the segmental dynamics of misicible polymer blends. To test this model, in this work, the dynamic properties of athermal blends of poly(á-methyl styrene) with its oligomer is examined. The effective Tgs of the components determined from the calorimetric transition can be described by the self-concentration model. However, the self-concentration value obtained is much lower than the theoretical prediction, indicating weak chain connectivity effects in the athermal mixture. Moreover, compared to the pure materials, the blends exhibit considerably broadened transitions and depressed enthalpy overshoots, presumably resulting from their broader relaxation time distribution. In addition to studies dealing with confined or heterogeneous systems, calorimetric measurements of Tg and the limiting fictive temperature, Tf', as a function of cooling and heating rates are performed for a polystyrene to examine the relationship between the Tg and Tf' and to examine the conversion dependence of the apparent activation energy.Item Thermal analysis of aluminum particle combustion in a simulated solid propellant flame(Texas Tech University, 2006-05) White, Randy; Pantoya, Michelle; Berg, Jordan M.; Levitas, ValeryAluminum particles have been shown to provide performance enhancements when used in propellant systems due to the large amount of heat released during aluminum combustion and the ensuing formation of aluminum oxide. The contribution of aluminum particles and their combustion products to the heat transfer characteristics of impinging flow geometries is not well understood. Experiments were performed to elucidate the role of Al particle combustion on the heat transfer characteristics of a propellant flame. Measurements of temperature and heat flux delivered to an impinged surface were made. A particle-laden oxygen-acetylene flame was used to simulate the burning behavior of a solid propellant in a controlled, reproducible manner. Yttria Stabilized Zirconia (YSZ), Alumina-Titania (AlTi), and Aluminum (Al) powders were examined in order to compare and quantify the heat flux contribution of each type of powder. The main focus was Al, so inert powders YSZ and AlTi were used to compare reacting, non-reacting and melting powders. Copper coupons captured and quenched reacting Al particles from the flame. A scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and a differential scanning calorimeter (DSC) were used to examine the degree of completion of Al oxidation. The area encompassed by the aluminum melting endotherm on the DSC plot was used as quantification for available Al energy for the oxidation reaction. Oxygen-acetylene ratio and standoff distance are varied and the change in available Al energy was observed. This allowed for a calculation of Al reaction percentage as a function of flame and environment conditions. Correlations were made between particle properties in the flow and heat flux delivered to an object's surface. Results show that flames seeded with particles which remain in the solid phase during the combustion process experience minor enhancement to heat flux (1.9%) compared to gas-only flames. Flames seeded with AlTi particles, which melt in the flame zone, show an 80.2% increase in heat flux over gas-only flames. Reacting Al particles provide the greatest increase in heat flux, yielding a 232.7% gain over gas-only flames. DSC results show that Al consumption percentages for the flame with 2.5 oxygen-fuel ratio (OFR) are an average of 8.6% higher than those for the 1.5 OFR flame. Al reaction percentages increase steadily as standoff distance from the torch nozzle increases, with one limitation discussed later in the text.