Drastically different mechanical properties from the same compound

Polymorphs of the same compound can exhibit totally different mechanical behaviour – reversible and irreversible bending – new research shows.

Researchers at the Indian Institute of Science were applying and removing external force to crystals of three different polymorphs of 4-bromophenyl 4-bromobenzoate when the different mechanical behaviours became apparent. A crystal of form-I can bend and regain its shape reversibly, exhibiting elasticity. Form-II is brittle, since its intramolecular interactions do not allow for structural changes. Form-III bends under external force, but does not regain its original shape when the force is removed, meaning it exhibits plasticity. This is one of the first examples of a molecular crystal where polymorphism enables one molecule to exhibit mechanical properties ranging from brittle to elastic. 

Through analysing the structures and intramolecular bonding of these three crystalline forms, the researchers identified three molecular characteristics that can lead to obtaining different mechanical behaviours from polymorphs: variable halogen bonding, C-H···π and π···π bonding and flexibility. By identifying and understanding these characteristics, researchers suggest that achieving different mechanical properties for different polymorphs may not be so rare after all.