Determining the structural relaxation times deep in the glassy state of the pharmaceutical Telmisartan

By using the dielectric relaxation method proposed recently by Casalini and Roland (2009 Phys. Rev. Lett. 102 035701), we were able to determine the structural alpha-relaxation times deep in the glassy state of the pharmaceutical, Telmisartan. Normally, deep in the glassy state tau(alpha) is so long that it cannot be measured but tau(beta), which is usually much shorter, can be directly determined. The method basically takes advantage of the connection between the alpha-relaxation and the secondary beta-relaxation of the Johari-Goldstein kind, including a relation between their relaxation times tau(alpha) and tau(beta), respectively. Thus, tau(alpha) of Telmisartan were determined by monitoring the change of the dielectric beta-loss, epsilon '', with physical aging time at temperatures well below the vitrification temperature. The values of tau(alpha) were compared with those expected by the coupling model (CM). Unequivocal comparison cannot be made in the case of Telmisartan because its beta-loss peak is extremely broad, and the CM predicts only an order of magnitude agreement between the primitive relaxation frequency and the beta-peak frequency. We also made an attempt to analyze all isothermal and aging susceptibility data after transformation into the electric modulus representation. The tau(alpha) found in the glass state by using the method of Casalini and Roland in the modulus representation are similar to those obtained in the susceptibility representation. However, it is remarkable that the stretching parameter beta(KWW-M) = 0.51 in the electric modulus representation gives more precise fits to the aging data than in the susceptibility representation with beta(KWW) = 0.61. Our results suggest that the electric modulus representation may be useful as an alternative to analyze aging data, especially in the case of highly polar glassformers having a large ratio of low frequency and high frequency dielectric constants, such as the Telmisartan studied.