Crystallization of Amorphous Pharmaceuticals at Ambient and Elevated …
71
Fig. 10 Temperature
dependence of the heat
capacity C p for crystalline
and amorphous EZB
obtained from TOPEM
measurements near the glass
transition temperature
The second method, which also is used to estimate the time scale of structural
relaxation in the glassy state, is much easier. In this method the dielectric spectrum
collected above T g (spectrum on which α-relaxation is well resolved) is horizontally
shifted to match the one registered at T < T g in which only the high-frequency flank
of α-process is present in the experimental frequency range (see Fig. 11a). The result
of such shifting, so-called master plot, allows to predict the structural relaxation time
of the material from the maximum of the reconstructed α-peak since: τ α = 1/2π f max .
It has to be noted that this approach is strictly limited to the samples in which
the shape of the α-relaxation peak is temperature invariant i.e. TTS is valid. To
check whether the temperature affects the shape of the α-relaxation, the dielectric
loss spectra from the whole registered supercooled liquid region should be shifted
to superimpose on the reference spectrum. Time–temperature superposition (TTS)
plot, presented in Fig. 11b, shows that the temperature does not change the shape of
the structural loss peak of a model API—EZB—measured at temperatures from the
region 341–357 K. Thus, the procedure based on construction of master plot can be
employed to predict its temperature dependence of τ α at T < T g .
Fig. 11 a The procedure of master plot constriction involving shifting the α-relaxation peak to
dielectric spectra registered in at T < T g —on the basis data of EZB. b The master plot of EZB
formed by horizontally shifting of spectra to overlap that at 337 K.
71
Fig. 10 Temperature
dependence of the heat
capacity C p for crystalline
and amorphous EZB
obtained from TOPEM
measurements near the glass
transition temperature
The second method, which also is used to estimate the time scale of structural
relaxation in the glassy state, is much easier. In this method the dielectric spectrum
collected above T g (spectrum on which α-relaxation is well resolved) is horizontally
shifted to match the one registered at T < T g in which only the high-frequency flank
of α-process is present in the experimental frequency range (see Fig. 11a). The result
of such shifting, so-called master plot, allows to predict the structural relaxation time
of the material from the maximum of the reconstructed α-peak since: τ α = 1/2π f max .
It has to be noted that this approach is strictly limited to the samples in which
the shape of the α-relaxation peak is temperature invariant i.e. TTS is valid. To
check whether the temperature affects the shape of the α-relaxation, the dielectric
loss spectra from the whole registered supercooled liquid region should be shifted
to superimpose on the reference spectrum. Time–temperature superposition (TTS)
plot, presented in Fig. 11b, shows that the temperature does not change the shape of
the structural loss peak of a model API—EZB—measured at temperatures from the
region 341–357 K. Thus, the procedure based on construction of master plot can be
employed to predict its temperature dependence of τ α at T < T g .
Fig. 11 a The procedure of master plot constriction involving shifting the α-relaxation peak to
dielectric spectra registered in at T < T g —on the basis data of EZB. b The master plot of EZB
formed by horizontally shifting of spectra to overlap that at 337 K.