Pressure Scanning Volumetry of Physically Aged Polymer Glasses, Fictive Pressure, and Memory Effect.

Physical aging of a glass decreases its volume, V, entropy, and enthalpy, H, toward the equilibrium state values. For glasses usually formed by cooling a melt, the effect is modeled in terms of non-exponential, nonlinear structural relaxation by using a plot of the heat capacity, Cp = (dH/dT)p, against T obtained from differential scanning calorimetry (DSC) cooling and heating scans. A melt becomes glass also on isothermal pressurizing and the glass formed becomes liquid on depressurizing, showing a hysteresis of the sigmoid-shape plot of -(dV/dp)T against p, which resembles the thermal hysteresis observed in the Cp against T plots. By analogy with DSC, it was named pressure scanning volumetry (PSV). Here, we use the known values of non-exponential and nonlinearity parameters ß and x and volume of activation for structural relaxation time, ΔV*, of atactic poly(propylene) to investigate the effect of aging pressure, page, of aging time, tage, and of the pressurizing rate on aging features in PSV scans. The scans show a post-pg→l feature on depressurizing before the -(dV/dp)T overshoot peak appears. We provide quantitative plots (i) of the monotonic decrease of V and increase of fictive pressure, pf, with tage and (ii) of the memory (Kovacs) effect in V and pf of the polymer and (iii) provide generic plots of -(dV/dp)T against p for different combinations of ß, x, and ΔV*. The study is of academic significance because PSV scans show a change in the density fluctuation response. It is of technological significance in polymer-extrusion processing and it may stimulate the commercial development of computer-controlled, high-pressure equipment.

Dipolar relaxation, conductivity, and polar order in AgCN.

By using dielectric spectroscopy in a broad range of temperatures and frequencies, we have investigated dipolar relaxations, the dc conductivity, and the possible occurrence of polar order in AgCN. The conductivity contributions dominate the dielectric response at elevated temperatures and low frequencies, most likely arising from the mobility of the small silver ions. In addition, we observe the dipolar relaxation dynamics of the dumbbell-shaped CN- ions, whose temperature dependence follows the Arrhenius behavior with a hindering barrier of 0.59 eV (57 kJ/mol). It correlates well with a systematic development of the relaxation dynamics with the cation radius, previously observed in various alkali cyanides. By comparison with the latter, we conclude that AgCN does not exhibit a plastic high-temperature phase with free rotation of the cyanide ions. Instead, our results indicate that a phase with quadrupolar order, revealing dipolar head-to-tail disorder of the CN- ions, exists at elevated temperatures up to the decomposition temperature, which crosses over to long-range polar order of the CN dipole moments below about 475 K. Dipole ordering was also reported for NaCN and KCN, and a comparison with these systems suggests a critical relaxation rate of 105-107 Hz, marking the onset of dipolar order in the cyanides. The detected relaxation dynamics in this order-disorder type polar state points to glasslike freezing below about 195 K of a fraction of non-ordered CN dipoles.

Endothermic Effects on Heating Physically Aged Sucrose Glasses and the Clausius Theorem Violation in Glass Thermodynamics.

Experimental heat capacity, Cp,app, of some physically aged polymers had shown an endothermic peak or a shoulder on heating when the material was still in the glassy state. In the first part of this paper, we report observation of a similar feature in a molecular glass, sucrose, indicating increase in the enthalpy and entropy from kinetic unfreezing of molecular motions in the solid state. Aging decreases Cp,app of glass. Increase in the aging time, tage, or aging temperature, Tage, interferes with the onset temperature of Cp,app increase toward the liquid state value. When the endothermic feature is not obvious in the Cp,app-T plots of the glassy state, its presence may be discerned in a plot of dCp,app/dT against T. Molecular motions producing this feature have implications for the state point in a potential energy landscape of an aging glass. In the second part of the paper, we use the Cp,app data to examine how much our violation of the Clausius theorem affects the entropy determined from the Cp,app d ln T integral. In addition to calculating this integral for a closed cycle of (irreversible) cooling and heating paths, we suggest an analysis which uses the δCp,app d ln T integrals (δCp,app is the difference between the Cp,app of the aged and the unaged glass) measured only on the heating paths. The closed cycle Cp,app d ln T integral value is negligibly small. The δCp,app d ln T integral value increases with tage. It is equal to the enthalpy lost on aging divided by Tage. Clausius theorem violation has no significant effect on determination of the entropy from Cp,app d ln T integral of an aged glass.

Effects of Microstructure and Sample's Surface to Volume Ratio on Pressure-Induced Nucleation and Transformation to Crystalline and Apparently Amorphous Solids.

Heterogeneous nucleation in a polycrystalline solid occurs (i) at the surface of its container, (ii) at its microstructural sites, namely, grain boundaries, grain junctions, and intergrain-strain regions, and (iii) at the defect sites, namely, dislocations, vacancies, and stacking faults (planar defects) in its single crystal grains. We analyze their thermodynamic and kinetic effects in terms of classical nucleation theory by taking into account (a) the increase in Gibbs free energy, G, due to the lattice misfit of the nuclei forming in the parent phase and (b) the decrease in G due to the angle subtended by the nucleus on the external surface, grain boundaries, and grain junctions. Hence we deduce that several combinations of nucleation sites in different materials and also in different polycrystalline samples of the same material may produce the same energy barrier against nucleation and overall growth. The overall nucleation and growth rates are dependent upon the surface to volume ratio, χ, of a sample in a vessel and the vessel's material. Pressurizing a crystalline solid is known to produce either its polymorphic crystal form or a solid that shows no Bragg peaks and appears amorphous. We argue that when self-diffusion rate becomes slower than the pressurizing rate, (dP/dt)T, a multiplicity of states nucleating at different sites become kinetically frozen on their path to crystal growth. In such a case, the transformed solid would appear amorphous. A solid of high χ would transform to a polymorph when (dP/dt)T is low and to a state that appears amorphous when (dP/dt)T is high. Known studies of 0.08-0.1 cm3 volume samples in diamond-anvil high pressure cells provide qualitative evidence of formation of both crystal polymorphs and apparently amorphous solids. Methods are suggested for observing such an occurrence in large polycrystalline samples.

Source of JG-Relaxation in the Entropy of Glass.

Intrinsic mobility of the glassy state is known as Johari-Goldstein (JG)-relaxation. Its source is either rotational and translational motions of a small number of molecules or small-angle orientational motion of all molecules, which may include restricted translational motion of part of their flexible segments. We examine the merits of the two views by using the excess entropy, Sexc, the difference between the entropy of a glass and its crystal state. Its value at the glass-forming temperature, Sexc( Tg), is the sum of (1) Sexc from phonons and anharmonic forces and (2) the configurational entropy from (a) the JG-process and, (b) the unfrozen modes of the α-process. If all molecules participated in the JG-process, the minimum configurational entropy from this process would be R ln(2) [= 5.76 J/(mol K)], and [ Sexc( Tg) - Sexc(0 K)] cannot be less than that. Analysis of data for 33 liquid and 3 liquid crystal glasses and 7 orientational glasses shows that its value is less than or comparable to R ln(2) for 10 liquids and 2 orientational glasses. If contributions from (1) and (2b) were known and subtracted from Sexc( Tg), the [ Sexc( Tg) - Sexc(0 K)] value would be less than R ln(2) for more glasses. Cooperative motions to explain our finding and heterogeneous dynamics to explain the broad JG-relaxation spectra would require sufficiently large local mobility regions. We also argue that all molecules cannot participate in small-angle orientational fluctuations, cite evidence against such fluctuations for the JG-process, and note that nucleation and crystallization in a glass structure indicate such regions. After discussing the consequences for the entropy theory, the available NMR studies, the Eshelby inclusions, the potential energy landscape, and the Gardner transition, we conclude that JG-process likely occurs in local regions in internal equilibrium in a glass structure.

Instability and thermal conductivity of pressure-densified and elastically altered orientational glass of Buckminsterfullerene.

We report on the temperature, pressure, and time (T, p, and t)-dependent features of thermal conductivity, κ, of partially ordered, non-equilibrium state of C60-OG, the orientational glass of Buckminsterfullerene (at T below the orientational freezing temperature Tog) made more unstable (i) by partially depressurizing its high-p formed state to elastically expand it and (ii) by further pressurizing that state to elastically contract it. The sub-Tog effects observed on heating of C60-OG differ from those of glasses because phonon propagation depends on the ratio of two well-defined orientational states of C60 molecules and the density of the solid. A broad peak-like feature appears at T near Tog in the κ-T plots of C60-OG formed at 0.7 GPa, depressurized to 0.2 GPa and heated at 0.2 GPa, which we attribute to partial overlap of the sub-Tog and Tog features. A sub-Tog local minimum appears in the κ-T plots at T well below Tog of C60-OG formed at 0.1 GPa, pressurized to 0.5 GPa and heated at 0.5 GPa and it corresponds to the state of maximum disorder. Although Buckminsterfullerene is regarded as an orientationally disordered crystal, variation of its properties with T and p is qualitatively different from other such crystals. We discuss the findings in terms of the nature of its disorder, sensitivity of its rotational dynamics to temperature, and the absence of the Johari-Goldstein relaxation. All seem to affect the phenomenology of its glass-like transition.

Decrease in electrical resistivity on depletion of islands of mobility during aging of a bulk metal glass.

The effect of structural relaxation on electrical resistivity, ρglass, of strain-free Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk metallic glass was studied during isothermal aging at several temperatures, Tas. Since cooling of a liquid metal increases its resistivity ρliq, one expects ρglass to increase on aging toward ρliq at T = Ta. Instead, ρglass decreased non-exponentially with the aging time. The activation energy of aging kinetics is 189 kJ mol-1, which is higher than the activation energy of the Johari-Goldstein (JG) relaxation. After considering the sample's contraction, phase separation, and crystallization as possible causes of the decrease in ρglass, we attribute the decrease to depletion of islands of atomic mobility, soft spots, or static heterogeneity. Vibrations of the atoms in these local (loosely packed) regions and in the region's interfacial area contribute to electron scattering. As these deplete on aging, the contribution decreases and ρglass decreases, with a concomitant decrease in macroscopic volume, enthalpy, and entropy (V, H, and S). Local regions of faster mobility also decrease on cooling as V, H, and S of a liquid decrease, but structure fluctuations dominate electron scattering of a liquid metal and ρliq increases effectively according to the Ziman-Nagel theory for a homogenously disordered structure. Whether depletion of such local regions initiates the structural relaxation of a glass, or vice versa, may be resolved by finding a glass that physically ages but shows no JG relaxation.

Increasing the Ambient Pressure Solubility by Forming a Glass at High Pressure and Its Thermodynamics, a Much Sought-After Pharmaceutical Advantage.

With the objective of increasing the bioavailability of poorly soluble curative compounds, we describe a thermodynamics-based method for increasing their solubility, σ. It requires forming their pressure-densified glassy (PDG) state by supercooling the melt under a high pressure to form glass, depressurizing, and recovering the glass at a low temperature. First, we formally show that the excess free energy of PDG is higher at ambient pressure than that of a glass (normally) formed by supercooling the melt at ambient pressure (NG), and therefore its σ will be higher. For a given compound, σ would increase with the pressure under which the liquid is cooled to form PDG and also with increase in the cooling rate. Second, we propose that this increase may be determined by using differential scanning calorimetry heating scans and, more accurately and directly, by vapor pressure measurement. Analysis of calorimetry data of poly(styrene) shows that the magnitude of increase in σ is considerable and is expected to be much higher for curative compounds. Since σ is related to vapor pressure through the free energy change, and to the dissolution rate, Γ, the PDG state a compound would not only sublimate rapidly but also dissolve rapidly. We discuss the stability of PDG relative to NG against structural relaxation and crystallization and, hence, of their bioavailability with time.

Structural relaxation and thermal conductivity of high-pressure formed, high-density di-n-butyl phthalate glass and pressure induced departures from equilibrium state.

We report a study of structural relaxation of high-density glasses of di-n-butyl phthalate (DBP) by measuring thermal conductivity, κ, under conditions of pressure and temperature (p,T) designed to modify both the vibrational and configurational states of a glass. Various high-density glassy states of DBP were formed by (i) cooling the liquid under a fixed high p and partially depressurizing the glass, (ii) isothermal annealing of the depressurized glass, and (iii) pressurizing the glass formed by cooling the liquid under low p. At a given low p, κ of the glass formed by cooling under high p is higher than that of the glass formed by cooling under low p, and the difference increases as glass formation p is increased. κ of the glass formed under 1 GPa is ∼20% higher at ambient p than κ of the glass formed at ambient p. On heating at low p, κ decreases until the glass to liquid transition range is reached. This is the opposite of the increase in κ observed when a glass formed under a certain p is heated under the same p. At a given high p, κ of the low-density glass formed by cooling at low p is lower than that of the high-density glass formed by cooling at that high p. On heating at high p, κ increases until the glass to liquid transition range is reached. The effects observed are due to a thermally assisted approach toward equilibrium at p different from the glass formation p. In all cases, the density, enthalpy, and entropy would change until the glasses become metastable liquids at a fixed p, thus qualitatively relating κ to variation in these properties.

Sub-Tg features of glasses formed by cooling glycerol under pressure - Additional incompatibility of vibrational with configurational states in the depressurized, high density glass.

The vibrational state of a glass is naturally incompatible with its configurational state, which makes the glass structurally unstable. When a glass is kept at constant temperature, both the vibrational and configurational states of a glass change with time until it becomes metastable (equilibrium) liquid and the two states become compatible. The process, known as structural relaxation, occurs at a progressively higher rate during heating, and the properties of a glass change accordingly. We add to this incompatibility by depressurizing a glass that had been formed by cooling a liquid under a high pressure, p, and then investigate the effects of the added incompatibility by studying thermal conductivity, κ, and the heat capacity per unit volume ρCp of the depressurized glass. We use glycerol for the purpose and study first the changes in the features of κ and of ρCp during glass formation on cooling under a set of different p. We then partially depressurize the glass and study the effect of the p-induced instability on the features of κ and ρCp as the glass is isobarically heated to the liquid state. At a given low p, the glass configuration that was formed by cooling at high-p had a higher κ than the glass configuration that was formed by cooling at a low p. The difference is more when the glass is formed at a higher p and/or is depressurized to a lower p. On heating at a low p, its κ decreases before its glass-liquid transition range is reached. The effect is the opposite of the increase in κ observed on heating a glass at the same p under which it was formed. It is caused by thermally assisted loss of the added incompatibility of configurational and vibrational states of a high-p formed glass kept at low p. If a glass formed under a low-p is pressurized and then heated under high p, it would show the opposite effect, i.e., its κ would first increase to its high p value before its glass-to-liquid transition range.

Effects of electric field on thermodynamics and ordering of a dipolar liquid.

We propose that an electric field's role in changing the structural disorder may be investigated by comparing the field-induced entropy decrease, ΔES, against the pressure-induced and cooling-induced entropy decreases, ΔpS and ΔTS, respectively, for the same increase in the dielectric α-relaxation time, Δτα, or in the viscosity. If these three quantities are found to be the same, the change in the number of microstates, Δln Ω = ΔS/R, would be the same whether there is an electric field-induced dipole vector alignment, or not. The available data [S. Samanta and R. Richert, J. Chem. Phys. 142, 044504 (2015)] show that ΔES ≅ ΔpS, and ΔES ≅ ΔTS. We further argue that in the case of conformational disorder without hydrodynamics, as for a flexible molecule's orientationally disordered or plastic crystal, ΔTS would be more negative than ΔES for the same increase in Δτα. For cyclo-octanol plastic crystal, whose octyl-ring would lose some of its dielectrically inactive conformational degrees of freedom on cooling, ΔTS is five-times ΔES. Hence the entropy of such crystals may not be related to their τα, an aspect relevant to certain biopolymer crystals. We also mention other effects of E. The findings are relevant to a number of recent studies on the analysis of the effect of electric field on a liquid's properties. The method can be used to study the role of other entropy-altering variables in liquid crystals and ferromagnetic liquids.

Aging kinetics of levoglucosan orientational glass as a rate dispersion process and consequences for the heterogeneous dynamics view.

Aging kinetics of a glass is currently modeled in terms of slowing of its α-relaxation dynamics, whose features are interpreted in terms of dynamic heterogeneity, i.e., formation and decay of spatially and temporally distinct nm-size regions. To test the merits of this view, we studied the calorimetric effects of aging an orientational glass of levoglucosan crystal in which such regions would not form in the same way as they form in liquids, and persist in structural glasses, because there is no liquid-like molecular diffusion in the crystal. By measuring the heat capacity, Cp, we determined the change in the enthalpy, H, and the entropy, S, during two aging-protocols: (a) keeping the samples isothermally at temperature, Ta, and measuring the changes after different aging times, ta, and (b) keeping the samples at different Tas and measuring the changes after the same ta. A model-free analysis of the data shows that as ta is increased (procedure (a)), H and S decrease according to a dispersive rate kinetics, and as Ta is increased (procedure (b)), H and S first increase, reach a local maximum at a certain Ta, and then decrease. Even though there is no translational diffusion to produce (liquid-like) free volume, and no translational-rotational decoupling, the aging features are indistinguishable from those of structural glasses. We also find that the Kohlrausch parameter, originally fitted to the glass-aging data, decreases with decrease in Ta, which is incompatible with the current use of the aging data for estimating the α-relaxation time. We argue that the vibrational state of a glass is naturally incompatible with its configurational state, and both change on aging until they are compatible, in the equilibrium liquid. So, dipolar fluctuations seen as the α-relaxation would not be the same motions that cause aging. We suggest that aging kinetics is intrinsically dispersive with its own characteristic rate constant and it does not yield the α-relaxation rate. In this view, thermodynamic and other properties define the fictive temperature; the real or imaginary components of a dynamic property do not define it. While particles' overall motions may still play a crucial role in (structural) glass physics, we conclude that translational diffusion alone is not a requirement for structure stabilization on aging of a kinetically frozen state.

Thermal conductivity of Glycerol's liquid, glass, and crystal states, glass-liquid-glass transition, and crystallization at high pressures.

To investigate the effects of local density fluctuations on phonon propagation in a hydrogen bonded structure, we studied the thermal conductivity κ of the crystal, liquid, and glassy states of pure glycerol as a function of the temperature, T, and the pressure, p. We find that the following: (i) κcrystal is 3.6-times the κliquid value at 140 K at 0.1 MPa and 2.2-times at 290 K, and it varies with T according to 138 × T(-0.95); (ii) the ratio κliquid (p)/κliquid (0.1 MPa) is 1.45 GPa(-1) at 280 K, which, unexpectedly, is about the same as κcrystal (p)/κcrystal (0.1 MPa) of 1.42 GPa(-1) at 298 K; (iii) κglass is relatively insensitive to T but sensitive to the applied p (1.38 GPa(-1) at 150 K); (iv) κglass-T plots show an enhanced, pressure-dependent peak-like feature, which is due to the glass to liquid transition on heating; (v) continuous heating cold-crystallizes ultraviscous glycerol under pressure, at a higher T when p is high; and (vi) glycerol formed by cooling at a high p and then measured at a low p has a significantly higher κ than the glass formed by cooling at a low p. On heating at a fixed low p, its κ decreases before its glass-liquid transition range at that p is reached. We attribute this effect to thermally assisted loss of the configurational and vibrational instabilities of a glass formed at high p and recovered at low p, which is different from the usual glass-aging effect. While the heat capacity, entropy, and volume of glycerol crystal are less than those for its glass and liquid, κcrystal of glycerol, like its elastic modulus and refractive index, is higher. We discuss these findings in terms of the role of fluctuations in local density and structure, and the relations between κ and the thermodynamic quantities.

Thermodynamic Analysis of the Two-Liquid Model for Anomalies of Water, HDL-LDL Fluctuations, and Liquid-Liquid Transition.

After reviewing the protocol-dependent properties of HDA, which thermally anneals to LDA, and the data gap over an unusually large T-range between HDA, LDA, and water, we investigate whether or not, despite HDA's ill-defined state and distinction from a glass, the HDL-LDL fluctuations view of the two-liquid model can explain water's anomalous behavior. An analysis of the density, ρ, compressibility, ß, heat capacity, Cp, and thermal conductivity, κ, of water over a monotonic (continuous) path bridging this data gap shows the following: (i) Such a path between ρwater at 320 K and ρHDA yields an untenable thermal expansion coefficient of water. (ii) There is neither a continuous path between ßwater at 353 K and ßHDA, nor between Cp,water at 363 K and Cp,HDA. (iii) The same value of ρwater, of ßwater, or of Cp,water at two temperatures separated by a maxima or a minima is incompatible with the HDL-LDL fluctuations view. (iv) κLDA at ∼140 K is about twice that of κ water at 253 K. (v) κHDA at 120 K is incompatible with κwater at T > 320 K. Thus, there is an internal inconsistency between the thermodynamics of HDA seen as a glass and that of water seen as an HDL-LDL mixture, which is incompatible with both the HDL-LDL fluctuations view and the liquid-liquid transition.

Effects of stacking disorder on thermal conductivity of cubic ice.

Cubic ice is said to have stacking disorder when the H2O sequences in its structure (space group Fd3̄m) are interlaced with hexagonal ice (space group P6(3)/mmc) sequences, known as stacking faults. Diffraction methods have shown that the extent of this disorder varies in samples made by different methods, thermal history, and the temperature T, but other physical properties of cubic and hexagonal ices barely differ. We had found that at 160 K, the thermal conductivity, κ, of cubic ice is ∼20% less than that of hexagonal ice, and this difference varies for cubic ice samples prepared by different methods and/or subjected to different thermal history. After reviewing the methods of forming cubic ice, we report an investigation of the effects of stacking disorder and other features by using new data, and by analyzing our previous data on the dependence of κ on T and on the pressure. We conclude that the lower κ of cubic ice and its weaker T-dependence is due mainly to stacking disorder and small crystal sizes. On in situ heating at 20-50 MPa pressure, κ increases and cubic ice irreversibly transforms more sharply to ice Ih, and at a higher T of ∼220 K, than it does in ex situ studies. Cooling and heating between 115 and 130 K at 0.1 K min(-1) rate yield the same κ value, indicating that the state of cubic ice in these conditions does not change with time and T. The increase in κ of cubic ice observed on heat-annealing before its conversion to hexagonal ice is attributed to the loss of stacking faults and other types of disorders, and to grain growth. After discussing the consequences of our findings on other properties, we suggest that detailed studies of variation of a given property of cubic ice with the fraction of stacking faults in its structure may reveal more about the effect of this disorder. A similar disorder may occur in the mono-layers of H2O adsorbed on a substrate, in bulk materials comprised of two dimensional layers, in diamond and in Zirconium and in numerous other crystals.

Kinetic-freezing and unfreezing of local-region fluctuations in a glass structure observed by heat capacity hysteresis.

Fluctuations confined to local regions in the structure of a glass are observed as the Johari-Goldstein (JG) relaxation. Properties of these regions and their atomic configuration are currently studied by relaxation techniques, by electron microscopy, and by high-energy X-ray scattering and extended x-ray absorption fine structure methods. One expects that these fluctuations (i) would kinetically freeze on cooling a glass, and the temperature coefficient of its enthalpy, dH/dT, would consequently show a gradual decrease with decrease in T, (ii) would kinetically unfreeze on heating the glass toward the glass-liquid transition temperature, Tg, and dH/dT would gradually increase, and (iii) there would be a thermal hysteresis indicating the time and temperature dependence of the enthalpy. Since no such features have been found, thermodynamic consequences of these fluctuations are debated. After searching for these features in glasses of different types, we found it in one of the most stable metal alloy glasses of composition Pd40Ni10Cu30P20. On cooling from its Tg, dH/dT decreased along a broad sigmoid-shape path as local-region fluctuations kinetically froze. On heating thereafter, dH/dT increased along a similar path as these fluctuations unfroze, and there is hysteresis in the cooling and heating paths, similar to that observed in the Tg-endotherm range. After eliminating other interpretations, we conclude that local-region fluctuations seen as the JG relaxation in the non-equilibrium state of a glass contribute to its entropy, and we suggest conditions under which such fluctuations may be observed.

Structural fluctuations and orientational glass of levoglucosan--High stability against ordering and absence of structural glass.

To investigate whether a non-exponential relaxation always indicates 2-4 nm-size regions of dynamic heterogeneity, we studied the kinetic freezing and unfreezing of structural fluctuations involving the rotational modes in orientationally disordered crystal (ODIC) of levoglucosan by calorimetry. The heat capacity, Cp, of levoglucosan measured over the 203 K-463 K range shows that its low-temperature, orientationally ordered crystal (ORC) transforms to ODIC phase on heating, which then melts to a low viscosity liquid. On cooling, the melt transforms to the ODIC which then does not transform to the ORC. Instead, the ODIC supercools. Fluctuations resulting from hindered (random) rotations of levoglucosan molecules confined to the lattice sites and from their conformational changes become progressively slower on cooling and an orientational glass (O-G) forms showing the sigmoid shape decrease in Cp characteristic of structural arrest like that of a glass. On heating the O-G state, rotational fluctuations begin to contribute to Cp at To-g of 247.8 K and there is an overshoot in Cp and thermal hysteresis (characteristic of physical ageing) in the temperature range of 230-260 K. The non-exponential relaxation parameter, ß(cal), determined by fitting the Cp data to a non-exponential, nonlinear model for relaxation of a glass is 0.60, which is similar to ß(cal) found for polymers, molecular liquids, and metal-alloy melts in which Brownian diffusion occurs. Such ß(cal) < 1 are seen to indicate 2-4 nm-size dynamically heterogeneous domains in an ultraviscous liquid near the glass formation, but its value of 0.60 for ODIC levoglucosan, in which Brownian diffusion does not occur, would not indicate such domains. Despite the lack of Brownian diffusion, we discuss these findings in the potential energy landscape paradigm. Levoglucosan melt, which is believed to vitrify and to stabilize a protein's disordered structure, did not supercool even at 200 K/min cooling rate. The findings have consequences for reports on the dielectric relaxation studies that indicated that levoglucosan melt supercools to form a structural glass of Tg of ∼245 K, and for computer simulation of its dynamics. Levoglucosan is the ninth ODIC that forms O-G. It does so more easily than the other eight.

Effects of configurational changes on electrical resistivity during glass-liquid transition of two bulk metal-alloy glasses.

Consequences of increase in structural fluctuations on heating Pd40Ni10Cu30P20 and Zr46.75Ti8.25Cu7.5Ni10Be27.5 through their glass to liquid transition range were investigated by measuring the electrical resistivity, ρ, an electron scattering property. The temperature coefficient of resistivity (TCR = (1/ρ) dρ/dT) of the liquid and glassy states is negative. The plots of their ρ against T in the Tg (glass to liquid transition) range show a gradual change in the slope similar to the change observed generally for the plots of the density, elastic modulus, and refractive index. As fluctuations in the melt structure involve fewer configurations on cooling, ρ increases. In the energy landscape description, the melt's structure explores fewer minima with decrease in T, vibrational frequencies increase, and electron scattering and ρ increase. Plots of (-dρ/dT) against T resemble the plot of the specific heat of other glasses and show a sub-Tg feature and a rapid rise at T near Tg. Analysis shows that the magnitude of negative TCR is dominated by change in the phonon characteristics, and configurational fluctuations make it more negative. The TCR of the liquid and glassy states seems qualitatively consistent with the variation in the structure factor in Ziman's model for pure liquid metals as extended by Nagel to metal alloys and used to explain the negative TCR of a two-component metal glass.

Non-exponential relaxation, fictive temperatures, and dispersive kinetics in the liquid-glass-liquid transition range of acetaminophen, sulfathiazole, and their mixtures.

To investigate the effects of added molecular heterogeneity on the hysteretic features of liquid-glass-liquid transition, we studied acetaminophen, sulfathiazole, and three of their mixtures by calorimetry, and determined the T(g) and the fictive temperature, T(f), from changes in the enthalpy and entropy on the cooling and heating paths, as well as the non-exponential parameter, ß(cal). We find that, (i) T(f) for cooling is within 1-3 K of T(f) for heating and both are close to T(g), (ii) the closed loop entropy change in the liquid-glass-liquid range is negligibly small, (iii) T(g) and T(f) increase on increasing sulfathiazole in the mixture, (iv) ß(cal) first slightly increases when the second component is added and then decreases, and (v) ageing causes deviations from a non-exponential, nonlinear behavior of the glass. In terms of fluctuations in a potential energy landscape, adding a solute heterogeneity would shift the state point to another part of the landscape with a different distribution of barrier heights and a different number of minima accessible to the state point. Part of the change in ß(cal) is attributed to hydrogen-bond formation between the two components. Ageing changes the relaxation times distribution, more at short relaxation times than at long relaxation times, and multiplicity of relaxation modes implied by ß(cal) < 1 indicates that each mode contributing to the enthalpy has its own T(g) or T(f). ß(cal) differs from ß(age) determined from isothermal ageing, and the distribution parameter of α-relaxation times would differ from both ß(cal) and ß(age).

Change in entropy in thermal hysteresis of liquid-glass-liquid transition and consequences of violating the Clausius theorem.

Change in the entropy, dS, with change in the temperature T, with or without phase transformation, is determined only if the thermal path is reversible. For an irreversible thermal path, dS > dqirrev/T, an expression known as inequality in the Clausius theorem. In the glass formation range, Cp and enthalpy show a time-dependent hysteresis between the cooling and heating paths and the two Cp paths cross each other. We provide new data on Cp of poly(styrene) and use the previous Cp data [E. Tombari, C. Ferrari, G. Salvetti, and G. P. Johari, Phys. Rev. B 78, 144203 (2008)], both data obtained from measurements performed on cooling from melt to glass and heating from glass to melt at unusually slow rates, and show that violation of the Clausius theorem in such cases has insignificant consequences for determining the entropy of glass. We also report Cp of the samples annealed for different times at different temperatures in which the enthalpy spontaneously decreased. These measurements also show that violation of the Clausius theorem is relatively inconsequential for interpreting the entropy of the glassy state.