Towards the understanding and prediction of material changes during micronisation using atomic force microscopy

In this study we aim to explore the potential links between the mechanical properties, micronisation behaviour and surface energy of carbamazepine polymorphs using atomic force microscopy (AFM) measurements of material properties at the nanoscale. Carbamazepine Forms I, II and III were prepared and confirmed using X-ray powder diffraction (XRPD). AFM measurements of indentation hardness, Young's modulus and surface energy were made on the starting material. In addition, the surface energy was measured immediately after micronisation and after storage for four weeks. Carbamazepine polymorphs could be ranked by Young's modulus and hardness. Surface energy measurements showed an increase after micronisation in all cases, and a varying relaxation after storage for four weeks. Form I showed a smaller particle size distribution, indicating more complete micronisation. A promising correlation was observed between the hardness/Young's modulus ratio and the micronisation behaviour, in terms of particle size reduction and surface energy change. The results show potential for the predictive capacity of such an approach, and help to provide a greater understanding of material behaviour and properties during micronisation.