The mechanical properties of crystalline cyclopentyl polyhedral oligomeric silsesquioxane

The anisotropic elastic constants of crystalline octacyclopentyl polyhedral oligomeric silsesquioxane (CpPOSS) were determined using molecular dynamics. The force field used for these calculations was shown to model accurately the rhombohedral and triclinic crystal structures of octasilsesquioxane and CpPOSS, respectively, as well as the vibrational frequencies of octasilsesquioxane. The moduli for CpPOSS are anisotropic, with a Reuss-averaged bulk modulus of 7.5 GPa, an isotropic averaged Young's modulus of 11.78 GPa, and an isotropic averaged shear modulus of 4.75 GPa. These isotropic averages or, alternatively, the full anisotropic stiffness tensor of the crystal can be used with micromechanical composite models to calculate the effective elastic properties of polymer nanocomposites that contain crystalline aggregates of CpPOSS.