Molecular mobility of supercooled amorphous indomethacin, determined by dynamic mechanical analysis

Purpose. To determine the viscosity and the frequency-dependent shear modulus of supercooled indomethacin as a function of temperature near and above its glass transition temperature and from these data to obtain a quantitative measure of its molecular mobility in the amorphous state. Methods, Viscoelastic measurements were carried with a controlled strain rheometer in the frequency domain, at 9 temperatures from 44 degrees to 90 degrees C. Results, The viscosity of supercooled indomethacin shows a strong non-Arrhenius temperature dependence over the temperature range studied, indicative of a fragile amorphous material. Application of the viscosity data to the VTF equation indicates a viscosity of 4.5 x 10(10) Pa.s at the calorimetric Tg of 41 degrees C, and a T-0 of -17 degrees C. From the complex shear modulus and the Cole-Davidson equation the shear relaxation behaviour is found to be non-exponential, and the shear relaxation time at Tg is found to be approximately 100 sec. Conclusions, Supercooled indomethacin near and above its Tg exhibits significant molecular mobility, with relaxation times similar to the timescales covered in the handling and storage of pharmaceutical products.