Contribution of molecular flexibility to the elastic-plastic properties of molecular crystal alpha-RDX

A Pal and CR Picu, MODELLING AND SIMULATION IN MATERIALS SCIENCE AND ENGINEERING, 25, 015006 (2017).

DOI: 10.1088/1361-651X/25/1/015006

We show in this work that the mechanical properties of molecular crystals are strongly affected by the flexibility of the constituent molecules. To this end, we explore several kinematically restrained models of the molecular crystal cyclotrimethylene trinitramine in the a phase. We evaluate the effect of gradually removing the flexibility of the molecule on various crystal-scale parameters such as the elastic constants, the lattice parameters, the thermal expansion coefficients, the stacking fault energy and the critical stress for the motion of a dislocation (the Peierls-Nabarro stress). The values of these parameters evaluated with the fully refined, fully flexible atomistic model of the crystal are taken as reference. It is observed that the elastic constants, the lattice parameters and their dependence on pressure, and the thermal expansion coefficient can be accurately predicted with models that consider the NO2 and CH2 groups rigid, and the N-N bonds and the bonds of the triazine ring inextensible. Eliminating the dihedral flexibility of the ring leads to larger errors. The model in which the entire molecule is considered rigid or is mapped to a blob leads to even larger errors. Only the fully flexible, reference model provides accurate values for the stacking fault energy and the Peierls-Nabarro critical stress. Removing any component of the molecular flexibility leads to large errors in these parameters. These results also provide guidance for the development of coarse grained models of molecular crystals.

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