Abstract
By using the dielectric relaxation method proposed recently by Casalini and Roland(2009 Phys. Rev. Lett. 102 035701), we were able to determine the structuralα-relaxationtimes deep in the glassy state of the pharmaceutical, Telmisartan. Normally, deep in the glassy stateτα is so long that it cannotbe measured but τβ, which is usually much shorter, can be directly determined. Themethod basically takes advantage of the connection between theα-relaxation andthe secondary β-relaxation of the Johari–Goldstein kind, including a relation between their relaxation timesτα andτβ, respectively.Thus, τα of Telmisartan were determined by monitoring the change of the dielectricβ-loss,ε′′,with physical aging time at temperatures well below the vitrification temperature. The values ofτα were compared with those expected by the coupling model (CM). Unequivocalcomparison cannot be made in the case of Telmisartan because itsβ-loss peak is extremely broad, and the CM predicts only an order ofmagnitude agreement between the primitive relaxation frequency and theβ-peak frequency. We also made an attempt to analyze all isothermal and agingsusceptibility data after transformation into the electric modulus representation. Theτα found in the glass state by using the method of Casalini and Roland in themodulus representation are similar to those obtained in the susceptibilityrepresentation. However, it is remarkable that the stretching parameterβKWW − M = 0.51 in the electric modulus representation gives more precise fitsto the aging data than in the susceptibility representation withβKWW = 0.61. Our results suggest that the electric modulus representation may be useful as analternative to analyze aging data, especially in the case of highly polar glassformers havinga large ratio of low frequency and high frequency dielectric constants, such as theTelmisartan studied.
Published Version
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