Dana M. Dattelbaum

Pressure-induced phase change in poly(tetrafluoroethylene) at modest impact velocities

Although poly(tetrafluoroethylene) (PTFE) is an unusually ductile polymer, it undergoes an abrupt ductile-brittle transition at modest impact velocities. No previous explanation for this behavior has been found after an extensive literature search. In this paper, we examine the role of a pressure-induced phase transition in PTFE in the dynamic failure of Taylor cylinder samples. There is a known phase transition in PTFE with a marked decrease in volume and compressibility that occurs at 0.5–0.65GPa at 21°C, with the transition pressure inversely related to temperature. Varying the temperature of the samples in the experiment revealed that the phase transition is probably involved in sample failure because the ductile/brittle transition velocity increased for decreasing temperature, despite the material fracture toughness decreasing. Additionally, Taylor tests were carried out on samples of poly(chlorotrifluoroethylene) PCTFE to investigate the behavior of a similar material to PTFE but without a pressure-...

A molecular dynamics simulation study of crystalline 1,3,5-triamino-2,4,6-trinitrobenzene as a function of pressure and temperature

Quantum chemistry-based dipole polarizable and nonpolarizable force fields have been developed for 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Molecular dynamics simulations of TATB crystals were performed for hydrostatic pressures up to 10 GPa at 300 K and for temperatures between 200 and 400 K at atmospheric pressure. The predicted heat of sublimation and room-temperature volumetric hydrostatic compression curve were found to be in good agreement with available experimental data. The hydrostatic compression curves for individual unit cell parameters were found to be in reasonable agreement with those data. The pressure- and temperature-dependent second-order isothermal elastic tensor was determined for temperatures between 200 and 400 K at normal pressure and for pressures up to 10 GPa on the 300 K isotherm. Simulations indicate considerable anisotropy in the mechanical response, with modest softening and significant stiffening of the crystal with increased temperature and pressure, respectively. For most properties the polarizable potential was found to yield better agreement with available experimental properties.

Probing the localized-to-delocalized transition

Detailed understanding of the transition between localized and delocalized behaviour in mixed valence compounds has been elusive as evidenced by many interpretations of the Creutz–Taube ion, [(NH3)5Ru(pz)Ru(NH3)5]5+. In a review in 2001, experimental protocols and a systematic model to probe this region were proposed and applied to examples in the literature. The model included: (i) multiple orbital interactions in ligand-bridged transition metal complexes, (ii) inclusion of spin-orbit coupling which, for dπ5–dπ6 complexes, leads to five low-energy bands, two from interconfigurational (dπ→dπ) transitions at the dπ5 site and three from intervalence transfer transitions, (iii) differences in time scale between coupled vibrations and solvent modes which can result in solvent averaging with continued electronic asymmetry defining ‘class II–III’, an addition to the Robin–Day classification scheme, and (iv) delineation of coupled vibrations into barrier vibrations and ‘spectator’ vibrations. The latter provide direct insight into localization or delocalization and time scales for electron transfer. In this paper, the earlier model is applied to a series of mixed-valence molecules.

Brillouin-scattering determination of the acoustic properties and their pressure dependence for three polymeric elastomers

The acoustic properties of three polymer elastomers, a cross-linked poly(dimethylsiloxane) (Sylgard 184), a cross-linked terpolymer poly(ethylene-vinyl acetate-vinyl alcohol), and a segmented thermoplastic poly(ester urethane) copolymer (Estane 5703), have been measured from ambient pressure to approximately 12 GPa by using Brillouin scattering in high-pressure diamond anvil cells. The Brillouin-scattering technique is a powerful tool for aiding in the determination of equations of state for a variety of materials, but to date has not been applied to polymers at pressures exceeding a few kilobars. For the three elastomers, both transverse and longitudinal acoustic modes were observed, though the transverse modes were observed only at elevated pressures (>0.7 GPa) in all cases. From the Brillouin frequency shifts, longitudinal and transverse sound speeds were calculated, as were the C(11) and C(12) elastic constants, bulk, shear, and Youngs moduli, and Poissons ratios, and their respective pressure dependencies. P-V isotherms were then constructed, and fit to several empirical/semiempirical equations of state to extract the isothermal bulk modulus and its pressure derivative for each material. Finally, the lack of shear waves observed for any polymer at ambient pressure, and the pressure dependency of their appearance is discussed with regard to instrumental and material considerations.

“Stubborn” triaminotrinitrobenzene: Unusually high chemical stability of a molecular solid to 150 GPa

We report an unexpectedly high chemical stability of molecular solid 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under static high pressures. In contrast to the high-pressure behavior of the majority of molecular solids, TATB remains both chemically stable and an insulator to 150 GPa--well above the predicted metallization pressure of 120 GPa. Single crystal studies have shown that TATB exhibits pressure-induced Raman changes associated with two subtle structural phase transitions at 28 and 56 GPa. These phase transitions are accompanied by remarkable color changes, from yellow to orange and to dark red with increasing pressure. We suggest that the high-stability of TATB arises as a result of its hydrogen-bonded aromatic two-dimensional (2D) layered structure and highly repulsive interlayer interaction, hindering the formation of 3D networks or metallic states.

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.

Adhesive properties of some fluoropolymer binders with the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB).

Adhesion between binders and explosive crystals is of critical importance for the mechanical performance of plastic-bonded explosives (PBXs). The surface properties of several prospective binders have been determined from static advancing contact angle measurements. The surface energies have been used to calculate theoretical work of adhesion to 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a common insensitive high explosive. The fluorinated terpolymer Oxy-461™, and Kel-F™ chlorotrifluoroethylene-vinylidene fluoride copolymers show the greatest potential for wetting TATB surfaces, and should promote the best adhesion to TATB in PBX formulations. In general, none of the fluoropolymer binders investigated here exhibit markedly superior adhesion to TATB. Thus, bulk physical properties are likely to be more important when choosing a binder.

A molecular dynamics simulation study of the pressure-volume-temperature behavior of polymers under high pressure.

Isothermal compression of poly (dimethylsiloxane), 1,4-poly(butadiene), and a model Estane (in both pure form and a nitroplasticized composition similar to PBX-9501 binder) at pressures up to 100 kbars has been studied using atomistic molecular dynamics (MD) simulations. Comparison of predicted compression, bulk modulus, and U(s)-u(p) behavior with experimental static and dynamic compression data available in the literature reveals good agreement between experiment and simulation, indicating that MD simulations utilizing simple quantum-chemistry-based potentials can be used to accurately predict the behavior of polymers at relatively high pressure. Despite their very different zero-pressure bulk moduli, the compression, modulus, and U(s)-u(p) behavior (including low-pressure curvature) for the three polymers could be reasonably described by the Tait equation of state (EOS) utilizing the universal C parameter. The Tait EOS was found to provide an excellent description of simulation PVT data when the C parameter was optimized for each polymer. The Tait EOS parameters, namely, the zero-pressure bulk modulus and the C parameter, were found to correlate well with free volume for these polymers as measured in simulations by a simple probe insertion algorithm. Of the polymers studied, PDMS was found to have the most free volume at low pressure, consistent with its lower ambient pressure bulk modulus and greater increase in modulus with increasing pressure (i.e., crush-up behavior).

High-pressure elastic properties of a fluorinated copolymer: Poly(chlorotrifluoroethylene-co-vinylidene fluoride) (Kel-F 800)

The acoustic properties and their pressure dependence have been determined to 18.5 GPa for nearly amorphous poly(chlorotrifluoroethylene-co-vinylidene fluoride) (Kel-F 800) using high-pressureBrillouin scattering. At all measuredpressures, both longitudinal and transverse acoustic modes were observed allowing for calculation of the pressure-volume isotherm for this predominantly amorphous material. Analysis of the room temperature isotherm using semi-empirical equation of state fitting forms to 5.5 GPa yielded a zero-pressure bulk modulus,Ko , and pressure derivative, Ko ′, of 2.8 GPa and 30.6, respectively, which are consistent with the results from dilatometry measurements at very low pressures. Furthermore, the C11 and C12 elastic moduli for the isotropic polymer were determined at each pressure interval and, subsequently, examined to provide the pressure dependence of the bulk, shear, and Young’s moduli. These results are discussed in relation to polymer mechanics at pressures far exceeding those of previous, static compression experiments.

Electromagnetically induced localized ignition in secondary high explosives: Experiments and numerical verification

In order to more fully understand the factors that influence the thermal “hot-spot” initiation of high explosives, we have chosen a model system for study that uses the internal and localized dissipation of long wavelength electromagnetic energy (microwaves). High purity organic crystals generally interact weakly with microwave energy. Therefore, the addition of electromagnetically absorbing inclusions of silicon carbide provides a tractable system for the study of hot-spot ignition phenomena. Previously, we developed a simple analytic model that illuminates the important physical factors. In this work, we verify the relationships of the analytic model using a series of experiments and a numerical model.