Probing anisotropic mechanical behaviour in carbamazepine form III

Abstract

Nanoindentation measurements of the mechanical properties of the (020), (002) and (101) crystal faces in carbamazepine (CBZ) form III revealed that the (020) face had a greater elastic modulus and hardness than the (002) and (101) faces, which had similar modulus and hardness values. Atomic force microscopy (AFM) imaging of the indents showed that whilst no surface plastic displacement was observed around the residual indents on the (020) face, indents on both (002) and (101) faces nucleated cracks in multiple orientations and showed slip bands parallel to (020). The computed compliance matrix of CBZ form III was in good agreement with the experimental results and predicted anisotropic stiffness, with E₍₀₂₀₎ > E₍₀₀₂₎ > E₍₁₀₁₎. Modelling also revealed that the molecular flexibility of CBZ results in a less stiff material than that of an equivalent rigid compound. Indentation in one direction results in the opening of the CBZ-butterfly wings whilst in the other two results in their closing. Molecular dynamics simulations confirmed the primary (020) slip plane, consistent with the AFM images of the indents. It was also found that shear on the primary (020) slip plane, requires the breaking and forming of CBZ hydrogen bonded dimers. The slip plane, (020), was identified as having the lowest structural rugosity and hydrogen bonding density but not the lowest attachment energy.