Probing stress induced phase transformation in aspirin polymorphs using Raman spectroscopy enabled nanoindentation

Nanoindentation has extensively been used to measure the mechanical properties of molecular crystals. However, the possibilities of stress induced polymorphic transformation during indentation are still unexplored. Here, we have adopted a spatially synchronized Raman spectroscopy and nanoindentation technique to probe indentation induced polymorphic transformation in aspirin polymorphs. Spatial hardness maps, generated using an accelerated property mapping technique, showed micro-domain formation in aspirin form II crystals. The variations in Raman bands at different indentation loads provide a reliable understanding of structural modifications during phase transformation in aspirin polymorphs.