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      Dielectric studies of molecular motions in amorphous solid and ultraviscous acetaminophen.

      1 , ,

      Journal of pharmaceutical sciences

      Wiley

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          Abstract

          The dielectric permittivity and loss spectra of glassy and ultraviscous states of acetaminophen have been measured over the frequency range 10 Hz-0.4 MHz. The relaxation spectra show an asymmetric distribution of times expressed in terms of the Kohlrausch exponent, beta, which remains constant at 0.79+/-0.02 over the 305-341 K range. The dielectric relaxation time increases on cooling according to the Vogel-Fulcher-Tammann equation. However, the values of the parameters are considerably different from the values deduced from earlier work by other researchers using the heat capacity of ultraviscous acetaminophen and relating it to its molecular mobility. The calorimetric glass softening temperature of 296 K obtained from differential scanning calorimetry is close to the value measured from dielectric relaxation. The equilibrium permittivity of ultraviscous acetaminophen decreases on heating like that of a normal dipolar liquid, as anticipated from the Curie law. But, its value decreases rapidly with time when it begins to crystallize. The equilibrium permittivity of this crystal phase is approximately 3.1 at 300 K and increases with temperature, which indicates a partial, orientational-disordering of its structure. The results show limitations of the procedures used in the modeling of the kinetics of molecular motions, that is, estimating physical stability, using thermodynamic considerations based on thermal analyses of the amorphous solid phase of acetaminophen.

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          Author and article information

          Journal
          J Pharm Sci
          Journal of pharmaceutical sciences
          Wiley
          0022-3549
          0022-3549
          Oct 2005
          : 94
          : 10
          Affiliations
          [1 ] Department of Materials Science and Engineering, McMaster University, Hamilton, Ont, L8S 4L7, Canada. joharig@mcmaster.ca
          Article
          S0022-3549(16)31869-X
          10.1002/jps.20455
          16136559

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