Charles A. Wight

Model-free and model-fitting approaches to kinetic analysis of isothermal and nonisothermal data

The model-free and model-fitting kinetic approaches have been applied to data for nonisothermal and isothermal thermal decompositions of HMX and ammonium dinitramide. The popular model-fitting approach gives excellent fits for both isothermal and nonisothermal data but yields highly uncertain values of the Arrhenius parameters when applied to nonisothermal data. These values cannot be meaningfully compared with the values derived from isothermal measurements, nor they can be used to reasonably predict the isothermal kinetics. On the other hand, the model-free approach represented by the isoconversional method yields similar dependencies of the activation energy on the extent of conversion for isothermal and nonisothermal experiments. The dependence derived from nonisothermal data permits reliable predictions of the isothermal kinetics. The use of the model-free approach is recommended as a trustworthy way of obtaining reliable and consistent kinetic information from both nonisothermal and isothermal data.

Kinetics of the Thermal and Thermo-Oxidative Degradation of Polystyrene, Polyethylene and Poly(propylene)

Full Paper: The thermal degradations of polystyrene (PS), polyethylene (PE), and poly(propylene) (PP) have been studied in both inert nitrogen and air atmospheres by using thermogravimetry and differential scanning calorimetry. The model-free isoconversional method has been employed to calculate activation energies as a function of the extent of degradation. The obtained dependencies are interpreted in terms of degradation mechanisms. Under nitrogen, the thermal degradation of polymers follows a random scission pathway that has an activation energy L200 kJ N mol ‐1 for PS and 240 and 250 kJ N mol ‐1 for PE and PP, respectively. Lower values (L150 kJ N mol ‐1 ) are observed for the initial stages of the thermal degradation of PE and PS; this suggests that degradation is initiated at weak links. In air, the thermoxidative degradation occurs via a pathway that involves decomposition of polymer peroxide and exhibits an activation energy of 125 kJ N mol ‐1 for PS and 80 and 90 kJ N mol ‐1 , for PE and PP respectively.

Isothermal and non-isothermal kinetics of thermally stimulated reactions of solids

This review covers both the history and present state of the kinetics of thermally stimulated reactions in solids. The traditional methodology of kinetic analysis, which is based on fitting data to reaction models, dates back to the very first isothermal studies. The model fitting approach suffers from an inability to determine the reaction model uniquely,and this does not allow reliable mechanistic conclusions to be drawn even from isothermal data. In non-isothermal kinetics, the use of the traditional methodology results in highly uncertain values of Arrhenius parameters that cannot be compared meaningfully with isothermal values. An alternative model-free methodology is based on the isoconversional method. The use of this model-free approach in both isothermal and non-isothermal kinetics helps to avoid the problems that originate from the ambiguous evaluation of the reaction model. The model-free methodology allows the dependence of the activation energy on the extent of conversion to be determined. Th...

Thermal decomposition kinetics of PBAN-binder and composite solid rocket propellants

Abstract Thermogravimetric analysis at heating rates between 0.5 and 10°C min −1 has been used to study the decomposition kinetics of the PBAN binder and three propellants based on ammonium perchlorate (AP) with PBAN, HTPB, or BAMO-AMMO binders. Thermal runaway was observed for the PBAN and HTPB propellants at heating rates faster than 4.5 and 3°C min −1 , respectively. The multistep decomposition kinetics were analyzed by an advanced isoconversional method that showed that the effective activation varies with the extent of decomposition of the studied systems. For PBAN the activation energy increases from 100 to 200 kJ mol −1 throughout the polymer decomposition. In propellants the major mass loss step primarily relates to AP decomposition accompanied by some degradation of the polymer binder. For the PBAN and HTPB propellants this step shows respective increases in the activation energy from 60 to 180 kJ mol −1 and from 100 to 230 kJ mol −1 . The major decomposition step of the BAMO-AMMO propellant has a fairly constant activation energy of 120 kJ mol −1 .

Inhomogeneous broadening of infrared and Raman spectral bands of amorphous and polycrystalline thin films

Infrared and Raman spectroscopy are used to examine effects of solid structure on vibrational line shapes in thin solid films of CO2 and N2O prepared by vapor deposition. A new method is described for forming films consisting of weakly‐interacting microcrystals by deposition of the solute in an argon matrix followed by complete evaporation of the argon. The infrared absorption bands of these films exhibit inhomogeneous spectral broadening in the range between the transverse and longitudinal frequencies of the bulk crystal. The origin of this broadening is shown to be the dependence of vibrational frequencies on size and shape of the microcrystals due to long‐range dipole interactions. In contrast, the spectra of polycrystalline films formed by direct vapor deposition are the same as for single crystals because the long‐range dipole interactions effectively couple vibrational motions across the grain boundaries. An analysis shows that inhomogenously broadened spectra are obtained whenever the separation be...

Photochemistry of ozone in solid mixtures with argon

The photochemistry of solid ozone and solid solutions of ozone in argon has been investigated at 10–30 K using 266 nm laser irradiation and detection by ultraviolet and infrared absorption spectroscopies. Pure ozone is converted to O2 with a quantum yield of 2.0±0.3 (independent of temperature) due to reaction of each photolytically generated O atom with a second ozone molecule. The quantum yield decreases when O3 is diluted in argon because geminate recombination of O+O2 becomes competitive with the O+O3 reaction. Photodissociation of ozone monomers is thought to occur by direct cage escape, which forms a separated O+O2 pair. This is the first experimental study in which the temperature dependence of the escape probability has been systematically investigated. The apparent activation energy of the process, 0.38±0.08 kJ/mol, is consistent with excitation of librations of ozone in the argon lattice, which brings it into orientations that are favorable for direct cage escape of the O atom.

Simulating accidental fires and explosions

The Utah C-SAFE project is a highly multidisciplinary effort to build an integrated, large scale, high performance simulation framework to simulate accidental fires and explosions involving hydrocarbons, structures, containers, and high-energy materials. C-SAFE will provide a scalable, high performance, problem solving environment (PSE) in which fundamental chemistry and engineering physics are fully coupled with nonlinear solvers, optimization, computational steering, visualization, and experimental data verification. The availability of simulations using this system will help to better evaluate the risks and safety issues associated with fires and explosions. As the article discusses, we validate and document the five-year product, termed Uintah 5.0, for practical application to accidents involving both hydrocarbon and energetic materials.Once upon a time, in the kingdom of Batch, all access to the magical shrine Mainframe was controlled by a guild of wizards called Assemblers. They offered the machine-punched cards with the arcane symbols of their calling. Then, the monks of the benevolent orders Fortran and Cobol settled in the kingdom, and those who learned their language could actually understand what was in columns 7 through 12 of the sacrificial cards. But eventually the knights of Dartmouth, Sirs Kemeny and Kurtz, revolted against the evil card queue and let ordinary citizens submit Basic petitions to the gods through public shrines called Terminals...

Reactions of translationally excited and thermal fluorine atoms with CH4 and CD4 molecules in solid argon

Solid state reactions of fluorine with methane have been studied by ultraviolet laser photolysis of dilute mixtures of the two reagents in solid argon at 13–30 K. Using a combination of EPR and FTIR spectroscopies, three distinct mechanisms of product formation have been identified. At temperatures below 18 K, product formation is dominated by direct photolysis of F2–CH4 heterodimers, resulting in formation of closed-shell complexes HF–CH3F. A small fraction of reaction intermediate, a nonplanar methyl radical trapped in the reaction cage with HF and F, is also formed. This intermediate decays to product on a time scale of 103 s at 13 K after the photolysis period. Above 20 K, photogenerated F atoms undergo thermally activated diffusion in argon, and they react with isolated CH4 molecules to form CH3–HF complexes. The rate constant for this process is ∼10−25 cm3 s−1 at 20 K, and exhibits an activation energy of 1.7 kcal/mol. A third reaction channel involves reaction of F with CH4 to generate isolated pla...

Stabilizing effect of oxygen on thermal degradation of poly(methyl methacrylate)

SUMMARY: The thermal degradation of poly(methyl methacrylate) has been studied under nitrogen and air. The presence of oxygen increases the initial decomposition temperature by 70 C. The stabilizing effect of oxygen is explained by the formation of thermally stable radical species that suppress unzipping of the polymer. This assumption is supported by the experimental fact that introduction of NO into the gaseous atmosphere increases the initial decomposition temperature by more than 100 C.

Infrared line shapes of clusters and microcrystals: Vibrational modes mixed by dipole interactions

Mixing of near‐degenerate vibrational modes by long‐range dipole–dipole interactions can lead to complicated band shapes in the infrared spectra of clusters and microcrystals, particularly when the bands are inhomogeneously broadened as a result of distributions of crystal shapes and orientations in the sample. For the special case in which the clusters have isotropic crystal structures, the equivalence of amplitude vectors for all IR‐active vibrational phonon modes permits the band shapes of dipole‐mixed modes to be predicted from those of isolated modes. This analysis is successfully applied to experimental infrared spectra of thin films containing weakly interacting microcrystals of CO2 or N2O, as well as to CO2 clusters prepared by condensation in the gas phase. The results prove that these gas‐phase clusters of CO2 have an isotropic crystalline structure.