(360b) 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.