Influence of molecular mobility on the physical stability of amorphous pharmaceuticals in the supercooled and glassy states

We investigated the correlation between molecular mobility and physical stability in three model systems, including griseofulvin, nifedipine, and nifedipine-polyvinylpyrrolidone dispersion, and identified the specific mobility mode responsible for instability. The molecular mobility in the glassy as well as the supercooled liquid states of the model systems were comprehensively characterized using dynamic dielectric spectroscopy. Crystallization kinetics was monitored by powder X-ray diffractometry using either a laboratory (in the supercooled state) or a synchrotron (glassy) X-ray source. Structural (α-) relaxation appeared to be the mobility responsible for the observed physical instability at temperatures above T_g. Although the direct measurement of the structural relaxation time below T_g was not experimentally feasible, dielectric measurements in the supercooled state were used to provide an estimate of the α-relaxation times as a function of temperature in glassy pharmaceuticals. Again, there was a strong correlation between the α-relaxation and physical instability (crystallization) in the glassy state but not with any secondary relaxations. These results suggest that structural relaxation is a major contributor to physical instability both above and below T_g in these model systems.