The origin of delayed polymorphism in molecular crystals under mechanochemical conditions

Control over ball milling transformations is needed before the transformative potential of mechanochemical processing can be realized. Many parameters are known to affect the outcome of mechanochemical polymorphism, yet the energy of ball milling is itself often overlooked. We here demonstrate how milling energy can exert significant influence over the polymorphic outcome of ball mill grinding and be used to control the overall reaction profile. Milling energy exerts its effect on the reaction profile by changing the rate at which structural defects form in crystalline phases. These defects destabilize a crystal to drive the system step-by-step towards polymorphic transformation. Our results demonstrate decisively that careful design and interpretation of ball milling experiments are necessary to obtain control over mechanochemical polymorphism