160
5 Vibrational Up-Pumping in Polymorphic Materials
latter has been suggested to play a role in the shock-induced processes of FOX-7
[20].
It is generally accepted that energetic materials that adopt layered crystal packing
(e.g. TATB) display lower sensitivities, although no underlying mechanism for this
effect has been deduced. Two common theories include:
1 Increased hydrogen bonding, which forms upon layering, leads to dissipation of
energy through the layers [22]
2 Slip planes associated with layered materials [23, 24]
Some have also proposed that a higher inter-layer compressibility of layered materials effectively decreases the energy input from an impact. However, in accordance
with Eq. 3.1, a larger compressibility in fact does quite the opposite, and leads to a
larger proportion of the impact energy co4nverting into heat, rather than work. This
has the effect of enhancing the direct excitation of the molecules in the system. In line
with the theory proposed in Chap. , higher compressibility also leads to hardening
of vibrational modes and thus enhances up-pumping. Instead, the lower compressibility of hydrogen-bonded layers has instead been proposed to account for the lower
sensitivity of layered materials. [22]
The polymorphs of FOX-7 therefore offer an opportunity to test the up-pumping
model developed in this thesis, on the effects of how layered structures influence
impact sensitivity.
5.2 Aims
The work in this chapter seeks to use the model employed in Chaps. 3 and 4 to
investigate two polymorphic systems: HMX and FOX-7. The former contains two
polymorphs that are well-known to exhibit very different impact sensitivities, while
the impact sensitivities of the latter have yet to be experimentally reported. This
chapter therefore aims to:
• Determine whether the up-pumping model is sensitive to polymorphic modifications
• Experimentally determine the impact sensitivity of the γ -polymorph of FOX-7
• Explore the effects of increased layering on the sensitivity of energetic materials
• Rationalise the sensitivity of FOX-7 polymorphs based on the up-pumping model.
5.3 Materials
Synthesis of FOX-7. Preparation of 1,1-diamino-2,2-dinitroethene (DADNE, FOX-7)
followed from Scheme 5.1 [26]. 2-methyl-4,6-pyrimidinedione (6.0 g, 0.05 mol) was
5 Vibrational Up-Pumping in Polymorphic Materials
latter has been suggested to play a role in the shock-induced processes of FOX-7
[20].
It is generally accepted that energetic materials that adopt layered crystal packing
(e.g. TATB) display lower sensitivities, although no underlying mechanism for this
effect has been deduced. Two common theories include:
1 Increased hydrogen bonding, which forms upon layering, leads to dissipation of
energy through the layers [22]
2 Slip planes associated with layered materials [23, 24]
Some have also proposed that a higher inter-layer compressibility of layered materials effectively decreases the energy input from an impact. However, in accordance
with Eq. 3.1, a larger compressibility in fact does quite the opposite, and leads to a
larger proportion of the impact energy co4nverting into heat, rather than work. This
has the effect of enhancing the direct excitation of the molecules in the system. In line
with the theory proposed in Chap. , higher compressibility also leads to hardening
of vibrational modes and thus enhances up-pumping. Instead, the lower compressibility of hydrogen-bonded layers has instead been proposed to account for the lower
sensitivity of layered materials. [22]
The polymorphs of FOX-7 therefore offer an opportunity to test the up-pumping
model developed in this thesis, on the effects of how layered structures influence
impact sensitivity.
5.2 Aims
The work in this chapter seeks to use the model employed in Chaps. 3 and 4 to
investigate two polymorphic systems: HMX and FOX-7. The former contains two
polymorphs that are well-known to exhibit very different impact sensitivities, while
the impact sensitivities of the latter have yet to be experimentally reported. This
chapter therefore aims to:
• Determine whether the up-pumping model is sensitive to polymorphic modifications
• Experimentally determine the impact sensitivity of the γ -polymorph of FOX-7
• Explore the effects of increased layering on the sensitivity of energetic materials
• Rationalise the sensitivity of FOX-7 polymorphs based on the up-pumping model.
5.3 Materials
Synthesis of FOX-7. Preparation of 1,1-diamino-2,2-dinitroethene (DADNE, FOX-7)
followed from Scheme 5.1 [26]. 2-methyl-4,6-pyrimidinedione (6.0 g, 0.05 mol) was