Molecular mobility in liquid and glassy states of Telmisartan (TEL) studied by Broadband Dielectric Spectroscopy

The molecular relaxation in liquid and glassy states of Telmisartan (TEL) has been studied by Broadband Dielectric Spectroscopy (BDS) covering wide temperature and frequency range. Multiple relaxation processes were observed. Besides the primary α-relaxation, two secondary relaxations β and γ (labelled in order of decreasing time scale) have been reported.Well-separated β-process observed above and below glass transition temperature T_g, has activation energy E_β = 81.8 kJ/mol and was identified as intermolecular Johari–Goldstein (JG) process. The γ-relaxation visible in dielectric loss spectra at very low temperatures is most likely non-JG relaxation. The temperature dependence of the relaxation times of α-process, measured over 11 orders of magnitude, cannot be described by a single Vogel–Fulcher–Tamman–Hesse (VFTH) equation. At temperature T_B = 475.8 K the change in relaxation dynamics occurred, consequently a new set of VFTH parameters was required. From low temperature VFTH fits the glass transition temperature T_g was estimated as T_g = 400.3 K and fragility index m = 87 was calculated. Of particular interest was the time scale of molecular motion below the glass transition temperature. Our observation clearly indicates that the α-relaxation times at room temperature most probably would exceed 3 years and amorphous TEL should maintain physically and chemically stable over prolonged storage time.