Interfacial interactions in CL-20/alloy-metal/polymer energetic composites: a molecular dynamics study

The interfacial interactions between energetic filler crystals, metallic fuels, and polymer binders govern the mechanical integrity, processing characteristics, and safety performance of polymer-bonded explosives (PBXs). Despite extensive prior investigations of individual binary interfaces, a systematic comparative dataset spanning all three interface classes under a unified computational protocol has not been available, and the effect of alloying additions to aluminum fuels on interfacial adhesion remains unexplored. To address these gaps, molecular dynamics simulations were performed to comprehensively characterize interfacial adhesion and mechanical properties across 23 binary material combinations involving $\epsilon$ -hexanitrohexaazaisowurtzitane (CL-20), three metallic phases (Al, Al-2.5 at.% Li, Al-5.0 at.% B), and five polymer binders (fluororubber F2603, butadiene rubber BR, ethylene-vinyl acetate EVA, ethylene-propylene-diene monomer EPDM, and microcrystalline wax). The results reveal a three-tier hierarchy of interfacial interaction strength: metal/wax interfaces exhibit the strongest cohesion (cohesive energy density > 4.5 $\times$ 10 $_{}^9$ kJ $·$ cm $_{}^{-3}$ ), dominated by van der Waals forces; CL-20/metal interfaces show intermediate binding ( $\sim$ 1.8 $\times$ 10 $_{}^9$ kJ $·$ cm $_{}^{-3}$ ) with mixed van der Waals-electrostatic character; and CL-20/polymer interfaces represent the mechanically weakest links (< 6.2 $\times$ 10 $_{}^8$ kJ $·$ cm $_{}^{-3}$ ). Alloying aluminum with Li or B produced no significant enhancement in interfacial adhesion compared to pure Al. Among polymer binders, EVA exhibited the optimal balance of CL-20 interfacial adhesion and composite stiffness, while EPDM offered superior ductility.

All molecular dynamics simulations employed the COMPASS III force field as implemented in the Forcite Plus module of Materials Studio 2023 (BIOVIA, Dassault Systèmes). Interface models were constructed using the Build Layers tool with a $6\times 6\times 3$ Al supercell and a $2\times 2\times 3$ $\epsilon$ -CL-20 supercell. Each model was geometry-optimized (convergence threshold $1\times {10}^{-4}$ kcal $·$ mol $_{}^{-1}·$ Å $_{}^{-1}$ ) and equilibrated under NVT ensemble conditions at 295 K for 500 ps. Cohesive energy density with van der Waals and electrostatic decomposition, binding energy, and mechanical properties-including elastic modulus, bulk modulus, shear modulus, Poisson's ratio, and the $C_{12}-C_{44}$ anisotropy index calculated via the Voigt-Reuss-Hill approximation-were computed for all material combinations and averaged over 51 equilibrated trajectory frames.