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Raman Spectroscopic and Ultrasonic Measurements to Monitor the HMX ( ) Phase Transition
The HMX {beta}-{delta} solid-solid phase transition, which occurs as HMX is heated near 170 C, is clearly linked to increased reactivity and sensitivity to initiation. Thermally damaged energetic materials (EMs) containing HMX therefore may present a safety concern. Information about the phase transition is vital to a predictive safety model for HMX and HMX-containing EMs. We report work in progress on monitoring the phase transition with real-time Raman spectroscopy and ultrasonic measurements aimed towards a better understanding of physical properties through the phase transition. HMX samples were confined with minimal free volume.in a cell with constant volume. The cell was heated at a controlled rate and real-time Raman spectroscopic or ultrasonic measurements were performed. Raman spectroscopy provides a clear distinction between the two phases because the vibrational transitions of the molecule change with confirmational changes associated with the phase transition. Ultrasonic time-of-flight measurements provide an additional method of distinguishing the two phases because the sound speed through the material changes with the phase transition. Ultrasonic attenuation measurements also provide information about microstructural changes such as increased porosity due to evolution of gaseous decomposition products
Energy Transfer Between Coherently Delocalized States in Thin Films of the Explosive Pentaerythritol Tetranitrate (PETN) Revealed by Two-Dimensional Infrared Spectroscopy
Pentaerythritol
tetranitrate (PETN) is a common secondary explosive
and has been used extensively to study shock initiation and energy
propagation in energetic materials. We report 2D IR measurements of
PETN thin films that resolve vibrational energy transfer and relaxation
mechanisms. Ultrafast anisotropy measurements reveal a sub-500 fs
reorientation of transition dipoles in thin films of vapor-deposited
PETN that is absent in solution measurements, consistent with intermolecular
energy transfer. The anisotropy is frequency dependent, suggesting
spectrally heterogeneous vibrational relaxation. Cross peaks are observed
in 2D IR spectra that resolve a specific energy transfer pathway with
a 2 ps time scale. Transition dipole coupling calculations of the
nitrate ester groups in the crystal lattice predict that the intermolecular
couplings are as large or larger than the intramolecular couplings.
The calculations match well with the experimental frequencies and
the anisotropy, leading us to conclude that the observed cross peak
is measuring energy transfer between two eigenstates that are extended
over multiple PETN molecules. Measurements of the transition dipole
strength indicate that these vibrational modes are coherently delocalized
over at least 15–30 molecules. We discuss the implications
of vibrational relaxation between coherently delocalized eigenstates
for mechanisms relevant to explosives