Cryogenic grinding of indomethacin polymorphs and solvates: assessment of amorphous phase formation and amorphous phase physical stability.
The effect of cryogenic grinding on five crystal forms of indomethacin (IMC) was investigated with particular interest in the formation of amorphous phase. Powder X-ray diffraction (PXRD) and differential scanning calorimetry (DSC) demonstrated that amorphous phase formation took place for all three polymorphs (gamma, alpha, and delta) and one solvate (IMC methanolate). In the latter case, a postgrinding drying stage was needed to remove desolvated methanol from the ground amorphous product because methanol destabilized amorphous IMC presumably via a plasticizing effect. The crystal structure of another solvate, IMC t-butanolate, was unaffected by grinding, indicating that amorphous phase formation on grinding does not occur in all cases. Ground amorphous materials possessed similar glass transition temperatures but significant differences in physical stability as assessed by both isothermal and nonisothermal crystallization. It is argued that physical factors, namely residual crystal phase and specific surface area, determine the isothermal and nonisothermal crystallization behavior of ground amorphous samples as opposed to intrinsic differences in the structure of the amorphous phase.