Investigations of the Intermolecular Forces between RDX and Polyethylene by Force–Distance Spectroscopy and Molecular Dynamics Simulations

Abstract

The development of novel nanoenergetic materials with enhanced bulk properties requires an understanding of the intermolecular interactions occurring between molecular components. We investigate the surface interactions between 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and polyethylene (PE) crystals on the basis of combined use of molecular dynamics (MD) simulations and force-distance spectroscopy, in conjunction with Lifshitz macroscopic theory of van der Waals forces between continuous materials. The binding energy in the RDX-PE system depends both on the degree of PE crystallinity and on the RDX crystal face. Our MD simulations yield binding energies of approximately 132 and 120 mJ/m(2) for 100% amorphous and 100% crystalline PE on RDX (210), respectively. The average value is about 36% greater than our experimental value of 81 ± 15 mJ/m(2) for PE (∼48% amorphous) on RDX (210). By comparison, Liftshitz theory predicts a value of about 79 mJ/m(2) for PE interacting with RDX. Our MD simulations also predict larger binding energies for both amorphous and crystalline PE on RDX (210) compared to the RDX (001) surface. Analysis of the interaction potential indicates that about 60% of the binding energy in the PE-RDX system is due to attractive interactions between HPE-ORDX and CPE-NRDX pairs of atoms. Further, amorphous PE shows a much longer interaction distance than crystalline PE with the (210) and (001) RDX surfaces due to the possibility of larger polymer elongations in the case of amorphous PE as strain is applied. Also, we report estimates of the binding energies of energetic materials RDX and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) with PE, propylene, polystyrene, and several fluorine-containing polymers using Lifshitz theory and compare these with reported MD calculations.

A perspective on modeling the multiscale response of energetic materials

“…A first experimental validation of the model was accomplished through comparison of binding energies measured using force distance spectroscopy with results from MD simulations of tensile loading of amorphous or crystalline PE adsorbed on RDX surfaces. 9 The binding energies generated from the MD simulations were in reasonable agreement with the experimental values.…”

“…The model to describe the missing RDX-PE interaction terms was developed using QM energies of 250 random configurations of a 43-monomer PE oligomer on a 5x5x1 RDX slab (4200-atoms). 9 The QM data points were used to parametrize an all-atom interatomic interaction potential composed of terms having the Buckingham form. A good degree of transferability of the model was demonstrated in comparison of energies predicted by the model and QM values for several configurations not used in the fitting, including 50 additional energies for the 43-monomer PE oligomer on the RDX slab, and two, three-and four PE oligomers on the surface.…”

“…Construction of a sufficiently large simulation cell that would reflect the same degree of amorphous and crystalline composition of the PE in the experimental system would render the simulations computationally infeasible; thus, the authors of the study used idealized systems in which pure amorphous or crystalline polyethylene was adsorbed on various RDX surfaces. 9 Furthermore, the experimental system (including the interface) was not ideal, yet the computational system included no bulk or surface defects. Also, simulation parameters (e.g., initial states including equilibration and loading conditions, boundary conditions or simulation ensemble choices) could influence the results.…”

Quantitative Scanning Probe Microscopy for Nanomechanical Forensics

“…AFM images have also been used to distinguish various adhesive tapes [21]. Finally, AFM images of different explosive particles and substrates have been used to analyze the morphology of the particles and substrates [22, 23]. …”

“…AFM force-displacement curves were examined on a series of adhesive tapes and interpreted in terms of the force and displacement required to break the probe-tape bond [21]. Finally, AFM force spectroscopy was used to consider the adhesive forces between trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), and pentaerythritol tetranitrate (PETN) explosive particles and metallic and polymeric surfaces [22, 23, 25]. …”