Broadband dielectric measurements on the pharmaceutical indomethacin (IMC) were performed at ambient and elevated pressure. Data on molecular dynamics collected at ambient pressure are in good agreement with that published in the literature. In the glassy state, there is a well-resolved secondary relaxation with Arrhenius activation energy E(a) = 38 kJ/mol. This commonly observed relaxation process (labeled gamma) is of intramolecular origin because it is pressure-insensitive. Closer analysis of the ambient pressure dielectric spectra obtained in the vicinity of the T(g) indicated the presence of one more secondary relaxation (beta), which is slower than that commonly observed. Application of the CM predictions enabled us to classify it as a true JG relaxation. Pressure measurements confirmed our supposition concerning the origins of the two secondary relaxations in IMC. Moreover, we have found that IMC under pressure does not crystallize, even at very high temperatures of T > or = 372 K. This finding was discussed in the framework of the two-order parameter model proposed by Tanaka (Konishi, T.; Tanaka, H. Phys. Rev B 2007, 76, 220201), as well as the JG relaxation proposal by Oguni (Hikima T.; Hanaya M.; Oguni M. J. Mol Struct. 1999, 479, 245). We also showed that the shape of the alpha-relaxation loss peak is the same when comparing dielectric spectra with the same tau(alpha) but obtained at ambient and elevated pressure. Additionally, we found out that the fragility of IMC decreases with increasing pressure. In addition, the pressure coefficient of the glass transition temperature, dT(g)/dP, was determined, and it is 255 K/GPa. Finally, we discuss the possibility of preparation of the amorphous state with higher density than by cooling of the liquid.

Dielectric spectroscopy and differential scanning calorimetry (DSC) were applied to study the molecular dynamics and thermal properties of a low-molecular-weight glass-forming liquid, salol (phenyl salicylate), confined in anodic aluminum oxide membranes of different pore diameters (100−13 nm). On increasing the geometrical confinement, the glass transition temperature shifts toward lower temperatures, while at the same time broadening of the shape of the structural relaxation is observed. This was attributed to the interplay between surface and confinement effects leading to the transition from Vogel−Fulcher− Tammann-like to Arrhenius-like dependence of the structural relaxation times. We have noticed that the temperature of such crossover agrees with the endothermic process detected by DSC. Combined dielectric and calorimetric data have indicated that this phenomenon is related to the decoupling of the dynamics of molecules attached to the pore walls and those at the center. The enhancement of the structural relaxation of the core molecules increases with decreasing pore size possibly due to changes in the packing density. This finding gives a new insight into the behavior of glass-forming liquids under confinement and helps in the understand of the characteristic shift of the dynamic glass transition temperature with decreasing of the pore diameter. ■ INTRODUCTIONManipulation with the physicochemical properties of the materials at the nanoscale, for instance confined polymers, gives an opportunity to obtain unique morphologies that can find promising applications in nanotechnology as miniaturized sensors, magnetic labels, tissue implants, and so on. 1−3 Therefore, the effects at the nanoscale is a very active research area. For example, under confinement on the nanometer scale, the properties of various materials are affected mostly by the finite size and their interactions with the interfaces or confining surfaces. Numerous studies have shown that the melting/ freezing temperature, solid−solid transition, surface free energy, glass transition, and molecular mobility 4,5 are strongly affected by one or two-dimensional confinement. These changes are hotly discussed in the context of varying pore sizes 6−8 or film thicknesses. 9,10 In addition, the strength and the type of interactions between the confined molecules and pore walls (or a substrate) play a key role and have an important impact on the basic physical properties of different materials and potential applications. 11 Despite the intensive studies, the behavior of glass-forming liquids under confinement is still very puzzling. It is very difficult to rationalize or generalize it, because of the variety of theoretical concepts and experimental results that scatter a lot depending on the confining environment or surface interactions. 12,13 According to literature data, the glass transition temperature T g can decrease, increase, or even remain unaffected under nanoconfinement. 6,14−16 The influence of the spatial restriction on T g can be discussed in t...

Dielectric spectroscopy (DS) was used to investigate the relaxation dynamics of supercooled and glassy ibuprofen at various isobaric and isothermal conditions (pressure up to 1750 MPa). The ambient pressure data are in good agreement with that reported previously in the literature. Our high pressure measurements revealed validity of temperature-pressure superpositioning (TPS) for the alpha-peak. We also found that the value of the fragility index decreases with compression from m = 87 +/- 2 at atmospheric pressure to m = 72.5 +/- 3.5 at high pressure (p = 920 MPa). The drop of fragility observed in our experiment was discussed in the framework of the two-order-parameter (TOP) model. In addition, we have also studied crystallization kinetics in a liquid state of examined drug at ambient and high pressure. We found out that, for the same structural relaxation time/same viscosities, the samples prepared by compression of liquid at high temperatures have significantly elongated induction times as well as overall crystallization times (sample 2: t(0) approximately = 4 h, t(1/2) approximately = 37.5 h; sample 3: t(0) approximately = 5.6 h, t(1/2) approximately = 49 h) compared to that held at lower temperature and ambient pressure (sample 1: t(0) approximately = 1.2 h, t(1/2) approximately = 12.2 h). A possible explanation of this finding is also given.

Dielectric relaxation studies for model glass-forming liquids confined to nanoporous alumina matrices were examined together with high-pressure results. For confined liquids which show the deviation from bulk dynamics upon approaching the glass transition (the change from the Vogel-Fulcher-Tammann to the Arrhenius law), we have observed a striking agreement between the temperature dependence of the α-relaxation time in the Arrhenius-like region and the isochoric relaxation times extrapolated from the positive range of pressure to the negative pressure domain. Our finding provides strong evidence that glass-forming liquid confined to native nanopores enters the isochoric conditions once the mobility of the interfacial layer becomes frozen in. This results in the negative pressure effects on cooling. We also demonstrate that differences in the sensitivity of various glass-forming liquids to the "confinement effects" can be rationalized by considering the relative importance of thermal energy and density contributions in controlling the α-relaxation dynamics (the E(v)/E(p) ratio).