Adhesion of Explosive Particles to Functionalized Surfaces

Adhesion of Explosive Particles to Functionalized Surfaces

Darby J. Hoss, Sanjoy Mukherjee, Bryan W. Boudouris, and Stephen P. Beaudoin

Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive West Lafayette, IN 47907-2100, USA

            The improved elucidation of the adhesion behavior of energetic materials will improve munitions formulation and the performance of collection materials (e.g., swabs or traps) used to detect trace explosive residue. Resolving the role that surface chemistry plays in the adhesion of energetic materials is a critical step towards the rational design of these materials. In particular, the use of specific chemical functional groups at the interface allows for the modulation of the adhesion force, and this key parameter will directly impact the performance and reliability of munitions and collection materials. In order to evaluate the effects of specific functional groups on adhesion, atomic force microscopy (AFM) pull-off force measurements were performed using nitrate-based energetic (and non-energetic) particles against self-assembled monolayers (SAMs) of representative chemical functionalities, and these SAMs on gold substrates were selected to evaluate surface chemistries due to their reproducibility and facile production. Gold films prepared by the template-stripped method produced sufficiently smooth surfaces for repeatable force measurements. Microparticles composed of either: (1) 2,4,6-Trinitrotoluene (TNT), (2) pentaerythritol tetranitrate (PETN), (3) 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), and (4) silica were evaluated against methyl, carboxyl, amine, and hydroquinone surface end-groups. In addition to the experimental results, stabilization energies for the optimized most-stable configurations for a coupled receptor-analyte system were determined using density functional theory (DFT).

            For PETN and RDX, the adhesion force of the functional groups determined by AFM can be ordered as follows in terms of the strength of the adhesion force: amine > carboxyl > hydroquinone > methyl. This trend is in agreement with the trend calculated in DFT simulation. For TNT, the adhesion force of the functional groups determined by AFM can be ordered: amine > carboxyl > hydroquinone > methyl. However, DFT calculations showed the highest interaction with hydroquinone. This is due to a possible π-π stacking geometry between the hydroquinone and TNT that is readily accessible in the computational calculations; however, experimentally, and in practical applications, this well-aligned π-π stacking interaction is unlikely to occur. The electron donating amine functional group shows enhanced adhesion specific to the electron withdrawing nitro sites of the energetics relative to the other functional groups. Additionally, the methyl groups yielded the lowest adhesion force across all particles. Overall, this study suggests that electron donating functional groups are the most promising class of chemical moieties to increase adhesion to common energetics. Ultimately, these results will facilitate the rational design of energetic particle collection materials and munitions through chemical-tailoring in order to enhance performance.