Exploration of structure – property relationships in nitropyrazole-based energetic materials using co-crystallisation and high pressure
Energetic materials are chemical compositions that can rapidly release large amounts of energy when suitably triggered by impact, shock, spark or friction. Key properties of energetic materials include detonation velocity, oxygen balance, thermal and chemical stability, and sensitivity to initiation by external stimuli. To develop strategies for the design of energetic materials with desirable properties usually refer high energetic performance and low sensitivity, reliable structure-property relationships should be established. Therefore, the main objective of this thesis is to characterise a variety of energetic materials and correlate their structures with their energetic properties. Nitropyrazole-based energetic materials were chosen as the subject of this study due to growing interest in their potential as replacements for existing energetic materials. Among the pyrazole family of energetic compounds, 3,4,5-TNP (3,4,5-trinitropyrazole) is of particular interest due to its superior detonation performance and low sensitivity to impact, friction and spark. To better understand the properties of 3,4,5-TNP, Chapter 3 explores the high pressure behaviour of 3,4,5-TNP. Energetic materials experience elevated pressures under operational conditions and this can alter the crystal structures and form novel polymorphs. The first part of the study focused on a high-pressure investigation of 3,4,5-TNP using a combination of Raman spectroscopy, neutron powder diffraction (NPD), and single crystal X-ray diffraction (SXRD). On compression up to 7.3 GPa, three new high-pressure phases of 3,4,5-TNP were identified and characterised. Form II was observed at 0.7 GPa and is characterised by an abrupt shortening of hydrogen-bonding interactions. Form III was observed at 2.2 GPa and is characterised by an abrupt conformational change affecting all of the NO2 torsional angles in the asymmetric unit of 3,4,5-TNP. Form IV was observed at 5.3 GPa via a single crystal to single crystal transition. The structure of Form IV was solved and refined from single crystal X-ray diffraction data – it adopts the monoclinic crystal system with space group Cc. The neutron powder diffraction patterns recorded for a polycrystalline sample are consistent with Form IV at pressures above 5.3 GPa. Form IV appears to be more sensitive to initiation as demonstrated by its spontaneous initiation at elevated pressures during both neutron experiments. This increased impact sensitivity of Form IV compared to Form I was rationalised using a computational technique based-on a vibrational up-pumping model. This work, therefore, highlights that pressure-induced polymorphism has a high probability of significantly altering the sensitivity of energetic materials. Chapters 4 and 5 explore the use of co-crystal/salt-formation to tune the energetic properties of a variety of nitropyrazole-based energetic materials. Co-crystallisation/salt formation is a useful branch of crystal engineering that can be used for modifying key energetic properties such as sensitivity, thermal/chemical stabilities, processability, and energetic performance. The main motivation of multicomponent crystallisation studies was to establish structure-property relationships of novel co-crystals and salts of nitropyrazoles and to understand the extent to which these techniques are applicable for tailoring the properties of nitropyrazoles. As a result of broad co-former investigation, pyridine, substituted pyridines, and morpholine were found to be suitable co-formers for nitropyrazoles. 1,3-dinitropyrazole (1,3-DNP), 3,5-dinitropyrazole (3,5-DNP) and 3,4,5-trinitropyrazole (3,4,5-TNP) were crystallised with a selection of co-formers including pyridine, substituted pyridines, and morpholine resulting in 9 salts and 3 co-crystals. Structural characterisation of these compounds was performed using SXRD and NPD. The energetic parameters of the co-crystals/salts including detonation velocity, detonation pressure and oxygen balance were predicted using the EXPLO5 program. The thermal behaviour of the energetic salts and co-crystals were determined using differential scanning calorimetry. The thermal properties, impact sensitivities and energetic performance parameters were found to be altered upon co-crystallisation/salt formation. These material properties were attempted to correlate with structural features including crystal density, crystal-packing motif and intermolecular interactions. Non-covalent interactions were analysed in detail and hydrogen bonding appeared as the main factor increasing the stabilities of these salts and co-crystals. In Chapter 5, six halogen containing salts and co-crystals were investigated in the same group for highlighting the importance of halogen bonding in influencing energetic properties of materials. It is demonstrated that in addition to hydrogen bonding, halogen bonding contributes to the stability of these multicomponent structures and increase their crystal densities. In summary, it is seen that co-crystallisation/salt formation is demonstrated to be an efficient technique to tailor the energetic properties of nitropyrazole-based energetic materials.