超分子结构单元研究黑索金的弹性各向异性

Study of elastic anisotropy for 1,3,5-trinitro-1,3,5-triazacyclohexane by supramolecular structural unit

含能材料的弹性性质微观上体现了分子间的结合力,且与含能材料的化学分解和爆炸相关.因此,弹性性质-晶体结构的关联为设计具有特定性质的新材料和理解含能材料点火起爆提供了理论基础.本文提出超分子结构单元作为最小化学单元来定量表征黑索金(RDX)不同晶面的弹性模量.基于超分子结构单元的弹性模量模型表明,与弹性模量相关的微观因素有:超分子结构单元的分子对数量、分子对的平衡距离、分子间力常数以及分子间非键能与晶面法线的夹角;而弹性模量的各向异性来源于分子间非键能与晶面法线的夹角不同.研究结果表明, RDX的超分子结构单元包含15个RDX分子,以该超分子结构单元计算得到RDX(100),(010),(001),(210)和(021)晶面的弹性模量分别为21.7, 17.1, 20.1, 19.1和15.3 GPa.除RDX(001)晶面外,以上晶面的理论计算值与超声共振谱、脉冲激热散射、布里渊散射和纳米压痕实验值基本吻合. RDX(001)晶面的计算值(20.1 GPa)远高于实验值(15.9—16.6 GPa),原因可能是计算过程中将RDX分子看作刚性体,忽略了RDX(001)晶面在外界载荷作用下发生的分子内六元环和NO2基团的移动和变形.

The relation between elastic property and crystal structure provides a foundation for designing new materials with desired properties and understanding the chemical decomposition and explosion of energetic materials.The supramolecular structural unit is proposed as the smallest chemical unit to quantitatively characterize the elastic anisotropy of 1,3,5-trinitro-1,3,5-triazacyclohexane(RDX).The supramolecular structural unit refers to the nearest-neighbor coordination polyhedron of one molecule.The supramolecular structural unit of RDX is composed of 15 molecules,and analyzed by the total molecular number density and the density of intermolecular interactions.The elastic modulus model is established on the assumption that 1)the RDX molecule is of sphere and rigid-body;2)the intermolecular interaction is regarded as a linear spring,i.e.it is described by a bond-spring model;3)the molecules are close-packed in the series mode.The elastic modulus model based on the supramolecular structural unit demonstrates that the elastic modulus is intrinsically determined by the total molecular number,the equilibrium distance of the molecular pair,the intermolecular force constant,and the angle between the intermolecular non-bonding interaction and the normal to crystal face.The intermolecular force constant is calculated as the second derivative of the intermolecular interaction with respect to the equilibrium centroid distance.The intermolecular interaction is expressed as the summation of van der Waals and electrostatic interactions calculated by COMPASS(condensed-phase optimized molecular potentials for atomistic simulation studies)II forcefield.The calculated elastic moduli are 21.7,17.1,20.1,19.1,and 15.3 GPa for RDX(100),(010),(001),(210),and(021)crystal faces,respectively.The calculation results are consistent with the theoretical values computed by the density functional theory.Excluding RDX(001),the calculated elastic moduli accord with the experimental results measured by the resonant ultrasound spectroscopy(RUS),impulsive stimulated thermal scattering,Brillouin spectroscopy,and nanoindentation methods.The theoretical value(20.1 GPa)of RDX(001)overestimates the experimental values in a range of 15.9–16.6 GPa.The reason can be attributed to the rigid-body approximation for flexible molecules,in which are ignored the motion and deformation of the ring and NO2 groups when the external loads are applied to RDX(001).The results suggest that the supramolecular structural unit can be the smallest chemical unit to quantitatively characterize the elastic anisotropy of RDX and the elastic anisotropy is mainly due to the angle between the intermolecular interaction and the normal to crystal face.