Different Measures of Molecular Mobility: Comparison between Calorimetric and Thermally Stimulated Current Relaxation Times Below Tg and Correlation with Dielectric Relaxation Times Above Tg

Stability of the amorphous state has been linked to molecular mobility of the matrix; however different techniques may capture different mobility substates. Our previous work suggested that two calorimetric techniques, Isothermal Microcalorimetry (TAM) and MDSC, measured different aspects of mobility with TAM measuring, in part, some faster modes of relaxation in addition to the modes mobilized at Tg. The aim of this work is to compare the relaxation times obtained using Thermally Stimulated Depolarization Current Spectroscopy (TSDC) with calorimetric mobility measured below Tg and to determine if all measures of relaxation times below Tg are consistent with relaxation times obtained above Tg using Dielectric Spectroscopy (DRS). Model compounds were indomethacin, ketoconazole, nifedipine, flopropione, felodipine. For all compounds, relaxation times obtained using Thermal Windowing-TSDC technique below Tg correlated well with relaxation times (τ) obtained above Tg by DRS. At any given temperature below Tg, relaxation times measured depended upon the technique used and were in the following order TSDC < TAM < MDSC (τ). TSDC captures some faster relaxations not measured by calorimetric techniques, and therefore, different techniques give different measures of relaxation times below Tg. This information is important in understanding the relationships between mobility in the glassy solid and pharmaceutical stability.