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Two solubility limited pharmaceuticals bezafibrate and fenofibrate, belonging to BCS class II drugs, were compared, by investigating molecular dynamics of the former and compared with the neat and as a probe in an apolar host polystyrene of mole. wt. 800 (PS800) of the latter by broadband dielectric spectroscopy (BDS), other supporting thermal and spectroscopic experimental means and density functional theory (DFT) calculations. The BDS experiments from frozen glassy state to vibrant molten state of the title drugs were done to explore the role of parameters describing their complex molecular dynamics and the stabiliy of amorphous phase, such as glass transition temperature (Tg), temperature dependence of α-relaxation times (τα), width of structural dispersion and its role on the occurrence of Johari-Goldstein (JG) β-relaxation, fragility index (m) and apparent activation energy of secondary processes. Realizing that most of the pharmaceuticals have narrow α-dispersion which limits the visibility of JG β-relaxation as a separate process in the dielectric window, to resolve the conundrum, we have further explored the amorphous dynamics of fenofibrate as a probein an apolar host polystyrene of molecular weight 800 (PS800) having higher Tg than the polar probe. The Tg of bezafibrate and PS800-fenofibrate mixture were determined to be 298 K and 275 K respectively, whereas the fractional exponent of Kohlrausch–Williams–Watts function βKWW of bezafibrate was determined to be 0.68 which is closer to that of fenofibrate, whereas that of PS800-fenofibrate mixture was found to be much broader with a value of 0.39, all in the vicinity of Tg. Even though βKWW decreased in binary mixture of fenofibrate when compared to its pure form due to concentration fluctuations and other aspects, but cold crystallization tendency was absent and Tg is elevated than that of fenofibrate. Due to higher Tg than that of fenofibrate, amorphous bezafibrate is more stable than fenofibrate. Finally, using quantum computational calculations using DFT, the relevant flexible parts of the two drug molecules, whose motions manifests the secondary γ-relaxation was identified by calculating potential energy barriers and dipole moments on relaxed dihedral angle scans, as no experimental technique is available for this purpose.

Orientationally disordered (OD) mixed crystals formed between cyanoadamantane (CN-A) and chloroadamantane (Cl-A) have been thermodynamically characterized by means of thermal analysis and X-ray diffraction. The dynamics of these OD mixed crystals has been studied by means of broad band dielectric spectroscopy. Results evidence that dynamics of the α-relaxation process associated with the overall molecular tumbling slows down when molecules of CN-A are substituted by molecules of Cl-A.

We investigated the microscopic origin of the excess wing through isothermal and isobaric dielectric relaxation measurements for the Quinaldine/tristyrene mixture. Our results show that the excess wing, characteristic of the high frequency side of the structural loss peak in neat Quinaldine, becomes a well resolved Johari-Goldstein secondary relaxation on mixing with the apolar tristyrene. Analyzing the temperature and pressure behavior of the two processes, a clear correlation has been found between the structural relaxation time, the Johari-Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). These results support the idea that the Johari-Goldstein relaxation acts as a precursor of the structural relaxation and therefore of the glass transition phenomenon. (c) 2007 Elsevier B.V. All rights reserved.

Measurements of polarization fluctuations were performed for an epoxy glass former. The voltage noise produced by polarization fluctuations of the sample filling a capacitor was acquired via a home-made very high input impedance current-voltage converter, in series with an ultra low noise pre-amplifier, achieving high sensitivity and accuracy in the range 0.1-1000 Hz. The temperature and frequency dependence of polarization noise was investigated above and below the glass transition temperature T-g. The sample was driven to the glassy state with different cooling rate and then isothermally aged, while the noise spectral density was measured at different times and compared with the Johnson Nyquist noise determined by the sample impedance measured by conventional dielectric spectroscopy. At thermodynamic equilibrium the polarization noise agreed with the predictions of the fluctuation-dissipation theorem linking noise spectral density to susceptibility. On the contrary, a strong violation of the theorem was observed after a fast cooling below Tg: an intense polarization noise was detected, with a power spectral density following an inverse power law frequency behavior, whose intensity was decreasing with aging time. At the same time, the amplitude of polarization fluctuations showed a non-Gaussian distribution, whose width reduced during the aging process, up to recover the Gaussian statistics on approaching the equilibrium. (c) 2006 Elsevier B.V. All rights reserved.

The different nature of multiple secondary (β- and γ-) relaxations for the glass-formers dipropyleneglycol-dibenzoate and benzoin-isobutylether has been investigated by varying temperature and pressure in the equilibrium liquid and glassy states, as well as by monitoring the structural recovery after a rapid quenching of the liquid to form a glass. For both systems the behavior of the slower β-relaxation mimicked that of the α-relaxation: its time scale was strongly affected by pressure variation and its behavior below T g was sensitive to thermal history and to aging. On the contrary the γ-relaxation time was negligibly dependent on pressure and on aging. The correlation between the dynamic properties of the α-process and those of the slower β-process indicates that the latter was likely the genuine Johari–Goldstein (JG) intermolecular relaxation. Moreover, a good agreement was found between the β-relaxation time and the independent relaxation time τ 0 of the coupling model (CM), leading to predictive quantitative relations between the JG and the structural relaxation, supported also by the experimental data of many other systems, in both liquid and glassy state.

The relaxational dynamics of poly(propyleneglycol) (PPG) and poly(propyleneglycol)-diglycidylether (PPGDE) compounds, the latter with different average number of repeating units, was studied by broadband dielectric spectroscopy (10 mHz to 10 GHz) from above to below the glass transition temperature in the range 115–350 K. The dielectric response of all systems showed a high temperature single relaxation while below a crossover temperature TB it split in a main (α-) and a secondary (β-) relaxation. The effect on the main and secondary relaxation of different groups terminating the polymer chain was analyzed: in particular, a decrease of the secondary relaxation time was observed for PPGDE with respect to PPG with the same molecular weight. Moreover the change of the Vogel temperature T0 paralleled what previously found for similar systems. The relationship between main and secondary relaxation and the microscopic nature of this latter were analyzed in the framework of the coupling model.