Effect of pressure on fragility and glass transition temperature in fragile glass-former

Pressure-Induced Glass Transition Probed via the Mobility of Coumarin 1 Fluorescent Molecule

The route to form a glass is generally achieved upon cooling where the slowing down might be interpreted as the trapping of molecules in potential wells. On the other hand, isothermal compression induces a glassy state by modifying the molecular packing ending in jamming. Here, we focus on how isothermal compression perturbs the mobility of a probe molecule in three different host liquids up to the pressure-induced glass transition. By use of the fluorescence recovery technique, the diffusion of the fluorescent molecule Coumarin 1 (C1) is measured in poly(propylene glycol) (PPG-1000M and -2700M), in the fragile van der Waals propylene carbonate (PC), and in hydrogen-bonded methanol and ethanol. High pressures up to 6 GPa are obtained with a diamond anvil cell. In PC at a pressure ∼1.3 GPa close to the glass-transition pressure, the diffusion coefficient of C1 follows an Arrhenius behavior with an ∼5 orders of magnitude increase of the diffusive time. No decoupling from the Stokes-Einstein equation is noticed. A similar exponential behavior is measured in ethanol and methanol but extended to different pressure ranges up to 2.5 and 6.2 GPa, respectively. In PPG-1000M a decoupling from the Stokes-Einstein relation is observed between 0.3 and 0.8 GPa that could be related to a modification of the interaction between polymer segments and the probe molecule. These results might indicate that interaction between probe and dynamic heterogeneities become less important under applied pressure, unlike in the temperature-induced glass transition.

Effect of Compression on the Molecular Arrangement of Itraconazole-Soluplus Solid Dispersions: Induction of Liquid Crystals or Exacerbation of Phase Separation?

Predensification and compression are unit operations imperative to the manufacture of tablets and capsules. Such stress-inducing steps can cause destabilization of solid dispersions which can alter their molecular arrangement and ultimately affect dissolution rate and bioavailability. In this study, itraconazole-Soluplus solid dispersions with 50% (w/w) drug loading prepared by hot-melt extrusion (HME) were investigated. Compression was performed at both pharmaceutically relevant and extreme compression pressures and dwell times. The starting materials, powder, and compressed solid dispersions were analyzed using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), small- and wide-angle X-ray scattering (SWAXS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and broadband dielectric spectroscopy (BDS). MDSC analysis revealed that compression promotes phase separation of solid dispersions as indicated by an increase in glass transition width, occurrence of a peak in the nonreversing heat flow signal, and an increase in the net heat of fusion indicating crystallinity in the systems. SWAXS analysis ruled out the presence of mesophases. BDS measurements elucidated an increase in the Soluplus-rich regions of the solid dispersion upon compression. FTIR indicated changes in the spatiotemporal architecture of the solid dispersions mediated via disruption in hydrogen bonding and ultimately altered dynamics. These changes can have significant consequences on the final stability and performance of the solid dispersions.

Molecular dynamics study of the thermal and the density effects on the local and the large-scale motion of polymer melts: scaling properties and dielectric relaxation

Results from a molecular dynamics simulation of a melt of unentangled polymers are presented. The translational motion, the large-scale and the local reorientation processes of the chains, as well as their relations with the so-called "normal" and "segmental" dielectric relaxation modes are thoroughly investigated in wide temperature and pressure ranges. The thermodynamic states are well fitted by the phenomenological Tait equation of state. A global time-temperature-pressure superposition principle of both the translational and the rotational dynamics is evidenced. The scaling is more robust than the usual Rouse model. The latter provides insight but accurate comparison with the simulation calls for modifications to account for both the local chain stiffness and the nonexponential relaxation. The study addresses the issue whether the temperature or the density is a dominant control parameter of the dynamics or the two quantities give rise to comparable effects. By examining the ratio /alphatau//alphaP between the isochronic and isobaric expansivities, one finds that the temperature is dominant when the dynamics is fast. If the relaxation slows down, the fluctuations of the free volume increase their role and become comparable to those of the thermal energy. Detectable cross-correlation between the "normal-mode" and the "segmental" dielectric relaxations is found and contrasted with the usual assumption of independent modes.

Changes in dynamic crossover with temperature and pressure in glass-forming diethyl phthalate

Dielectric relaxation measurements have been used to study the crossover in dynamics with temperature and pressure, onset of breakdown of the Debye-Stokes-Einstein law, and the relation between the alpha and the beta relaxations in diethyl phthalate. The measurements made over 10 decades in frequency and a broad range of temperature and pressure enable the dc conductivity and the alpha- and the beta-relaxations to be studied altogether. The isobaric data show that the alpha-relaxation time tau(alpha) has temperature dependence that crosses over from one Vogel-Fulcher-Tammann-Hesse form to another at T(B) approximately 227 K and tau(alpha) approximately 10(-2) s. The dc conductivity sigma exhibits similar crossover at the same T(B). At temperatures above T(B), tau(alpha) and sigma have the same temperature dependence, but below T(B) they become different and the Debye-Stokes-Einstein law breaks down. The breadth of the alpha relaxation is nearly constant for T<T(B), but decreases with increasing temperature for T>T(B). The time dependence of tau(beta) is Arrhenius, which when extrapolated to higher temperatures intersects tau(alpha) at T(beta) nearly coincident with T(B). Isothermal measurements at various applied pressures when compared with isobaric data show that the shape of the alpha-relaxation depends only on tau(alpha), and not on the T and P combinations. At a constant temperature, while tau(alpha) increases rapidly with pressure, the beta-relaxation time tau(beta) is insensitive to applied pressure. This behavior is exactly the same as found in 1,1(')-bis (p-methoxyphenyl) cyclohexane. The findings are discussed in the framework of the coupling model.

Influence of molecular structure on the dynamics of supercooled van der Waals liquids

Dielectric spectroscopy was carried out on the van der Waals liquid, 1,1(')-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC) in the supercooled state at pressures up to 218 MPa. The excess wing in this type-A glass former exhibits a response to pressure and temperature changes that is identical to that of the primary structural relaxation peak, indicating that the two processes reflect correlated molecular motions. Under no conditions was a distinct secondary peak observed in BMMPC, unlike the structurally very similar BMPC [1,1(')-bis(p-methoxyphenyl)cyclohexane]. However, the pressure dependences of both the glass temperature and fragility for the two materials are very close. The fragility is a decreasing function of pressure, although there is no concomitant narrowing of the relaxation peak. The pressure dependence of the relaxation times could be described as a simple volume-activated process, with the activation volume at the glass transition having the same magnitude as the molar volume.

Temperature and pressure dependence of the alpha relaxation and configurational entropy of a prototype glass former

The alpha relaxation process of the fragile glass former salol is investigated in the T-P domain by means of photon correlation spectroscopy. We find that a time-pressure superposition principle is obeyed for the relaxation function in addition to the time-temperature superposition. The behavior of the relaxation time is studied by using an extended version of the Adam-Gibbs model including the pressure dependence. The excellent conformity of the equation describing the bidimensional surface tau(T,P) to the experimental data provides a positive check for this model, here verified on photon correlation measurements. The same model gives a rationale of the phenomenological expressions recently introduced to describe the changes in the slow dynamics induced by varying both temperature and pressure. These findings suggest that the reduction of configurational entropy actually guides the liquid toward the glass transition.

Correlation between nonexponential relaxation and non-Arrhenius behavior under conditions of high compression

Photon correlation spectroscopy was used to investigate the behavior of the dynamical properties of 1,1'-di(4-methoxy-5-methyl-phenyl)cyclohexane (BMMPC) at elevated pressures. The fragility of BMMPC measured by the steepness index m(T) is decreasing and the nonexponentiality parameter beta(KWW) is increasing with increasing pressure. This result strongly suggests that the phenomenological correlation between the steepness index and nonexponentionality is also preserved under high compression. The pressure dependence of the structural relaxation times is well characterized by a simple activation volume form. The activation volume continuously increases with decreasing temperature, which is probably due to the increase of cooperativity of the structural relaxation process. Moreover, we found that the glass-transition temperature exhibits a significant dependence on pressure.

Low-frequency dielectric relaxation in rubber

A rubber sample is investigated by dielectric spectroscopy in the frequency range from 10(-2)<or=nu<or=10(7) Hz. Measurements are performed under both isothermal (at ambient pressure) and isobaric conditions. The spectral shape of the loss peaks is symmetric and temperature and pressure independent. Hence the temperature-pressure-frequency superposition principle is fulfilled in a cross-linked rubber sample. The Vogel-Fulcher-Tammann temperature law and its pressure counterpart well describe the respective temperature and pressure dependences of relaxation times. In addition we determined the activation volume from isothermal relaxation data.

Effect of pressure on the dynamics of glass formers

A description of the pressure dependence of the structural relaxation time has been derived from the Adam-Gibbs theory by writing the configurational entropy in terms of the excess heat capacity and the molar thermal expansion. This new equation was tested successfully on dielectric relaxation data for an epoxy compound over a wide range of temperature and pressure.

Pressure dependence of structural relaxation time in terms of the Adam-Gibbs model

A new equation describing the behavior of the structural relaxation time, tau(T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S(c), in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient alpha and the isothermal bulk modulus K0. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of tau versus temperature with the fragility parameter independent from pressure. The equation for tau(T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of tau(max) is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena.

Temperature and pressure scaling of the alpha relaxation process in fragile glass formers: A dynamic light scattering study

The dynamics of two fragile glass forming liquids was studied as a function of temperature and pressure using dynamic light scattering. On the basis of measured data we evaluated the pressure and temperature scaling of the alpha relaxation. All the isotherms can be superimposed and form a master curve when the reduced relaxation times are plotted versus reduced pressure. The Kohlrausch-Williams-Watts stretching parameter beta(KWW) is increasing with increasing temperature and decreasing pressure and plotted versus log<tau(KWW)> follows a master curve at all temperatures and pressures studied.

Scaling behavior of the alpha relaxation in fragile glass-forming liquids under conditions of high compression

The effect of pressure variation on dynamics of alpha relaxation process in poly[(phenyl glycidyl ether)-co-formaldehyde] has been investigated both under isothermal (T=293 K) and isobaric (P=0.1, 60, 120, 180, and 240 MPa) conditions using broad band dielectric spectroscopy (10(-2) to 10(6) Hz). The alpha relaxation is analyzed by means of the Havriliak-Negami relaxation function which has two shape parameters (alpha and gamma) to characterize non-Debye behavior. As a result we found that the shape parameters of the dielectric function collected for different pressures fall on master curves constructed by plotting alpha and alphagamma against the logarithm of relaxation time. The scaling of shape parameters for different pressure stems from pressure independence of fragility. This provides strong experimental evidence supporting correlation of non-Debye behavior with non-Arrhenius relaxation under high pressure. From an analysis of the shape parameters of relaxation function, in terms of the Schonhals and Schlosser model, we drew conclusions that the molecular mobility of PPG is controlled in the same way by temperature and pressure. The relaxation times exhibit a clear non-Arrhenius behavior under isothermal and isobaric condition.