Close examination of explosive-polymer interactions during heating to look for structural changes

What is it about?

In this work, we use optical microscopy while heating several types of explosives composites. These composites are composed of polymers (plastics) and crystalline explosive powder. The explosive we are specifically examining is called HMX and it is known to have a solid-solid phase transformation once it reaches temperatures around 150°C. The microscopy technique used here allows us to study the actual mechanism of the phase change. This phase change is important because it involves a relatively large volume expansion when it happens, which could introduce cracking in the material or other effects. We also use a neutron scattering technique to look at the nano-scale structure of the interface between the crystals and the polymer. All of this information combines to give us new information about the microstructure that could help explain historical results.

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Why is it important?

This combination of techniques is relatively new and allows for direct examination of the crystal-binder interface, which traditionally has been very hard to study. Our hypothesis, based on literature searches of similar materials, was that the interface would serve as a nucleation point for the phase change and probably lower the energy required to transform. However, we actually saw the opposite effect - the introduction of the binder actually stabilized the material. Nitrocellulose (the polymer studied) has some famous stability issues itself, so this was doubly surprising. However, this finding does help interpret large scale safety test data for these materials, indicating that these microstructural features may be more important than previously realized.