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PubMed2004

The structure of the high-energy spin excitations in a high-transition-temperature superconductor.

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity. The high transition temperatures (high-T(c)) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials. The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T(c). The most obvious feature in the magnetic excitations of high-T(c) superconductors such as YBa2Cu3O6+x is the so-called 'resonance'. Although the resonance may be strongly coupled to the superconductivity, it is unlikely to be the main cause, because it has not been found in the La2-x(Ba,Sr)(x)CuO4 family and is not universally present in Bi2Sr2CaCu2O8+delta (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa2Cu3O6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T(c), and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La2-xBa(x)CuO4 (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

Journal Article↗
PubMed2003

A coherent three-dimensional Fermi surface in a high-transition-temperature superconductor.

All conventional metals are known to possess a three-dimensional Fermi surface, which is the locus in reciprocal space of the long-lived electronic excitations that govern their electronic properties at low temperatures. These excitations should have well-defined momenta with components in all three dimensions. The high-transition-temperature (high-T(c)) copper oxide superconductors have unusual, highly two-dimensional properties above the superconducting transition. This, coupled with a lack of unambiguous evidence for a three-dimensional Fermi surface, has led to many new and exotic models for the underlying electronic ground state. Here we report the observation of polar angular magnetoresistance oscillations in the overdoped superconductor Tl2Ba2CuO6+delta in high magnetic fields, which firmly establishes the existence of a coherent three-dimensional Fermi surface. Analysis of the oscillations reveals that at certain symmetry points, however, this surface is strictly two-dimensional. This striking form of the Fermi surface topography, long-predicted by electronic band structure calculations, provides a natural explanation for a wide range of anisotropic properties both in the normal and superconducting states. Our data reveal that, despite their extreme electrical anisotropy, the high-T(c) materials at high doping levels can be understood within a framework of conventional three-dimensional metal physics.

Journal Article↗
PubMed1997

Solute-induced shift of phase transition temperature in Di-saturated PC liposomes: adoption of ripple phase creates osmotic stress.

We have examined the calorimetric behavior of large liposomes consisting of symmetric saturated chain phosphatidylcholines. Most notably, for systems made in solutions containing solute (e.g., NaCl, glucose, etc.) there was an additional major endotherm just below the main phase transition temperature. The new endotherm was found to represent a population of lipid whose main phase transition was shifted to lower temperature due to an induced osmotic stress across the membrane. Absent for isoosmotic systems, the osmotic stress was created when the liposome internal volume decreased, a consequence of the Lbeta' (gel) to Pbeta' (rippled) phase transition. That is, rippling of the membrane caused vesicle volume to decrease (> or = 28%) and because the free flow of water outward was restricted by solute, an osmotic gradient was created where none had existed before. The distribution of enthalpy between the new shifted Tm and the expected Tm correlated with the percent of lipid in the outer bilayer and it was concluded that only the outer bilayer sensed the induced stress. Internalized liposome structures were shielded, thus explaining the persistence of the expected Tm in preparations made in solute. The shift in Tm (deltaTm) was discrete and linearly dependent upon lipid chain length for the PC series di-17:0 (deltaTm approximately 1.4 degrees C) through di-20:0 (deltaTm approximately 0.6 degrees C), suggesting a structural change (i.e., lipid packing/orientation) was involved. Although freeze-fracture electron microscopy of stressed and unstressed bilayers revealed no differences in ripple periodicity there were differences in surface features and in vesicle shape. The fact that this phenomenon has gone unnoticed for MLVs is probably due to the fact that these systems are known to exclude solute and thus exist under osmotic compression.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2005

Photoinduced intramolecular charge separation in a polymer solid below the glass transition temperature.

Photoinduced intramolecular charge separation (CS) in a polar polymer glass, cyanoethylated pullulan (CN-PUL), was studied below the glass transition temperature (Tg=395 K). A series of three carbazole (Cz: donor)-cyclohexane (S: spacer)-acceptor (A: acceptor) molecules (Cz-S-A) was used as intramolecular donor-acceptor dyads. The photoinduced CS rate was evaluated by the fluorescence decay measurement at temperatures from 100 to 400 K. The CS rate (kCS) increased above 200 K even far below Tg where micro-Brownian motions of the whole polymer chain are frozen. Below 200 K, on the other hand, kCS showed weak dependence on temperature. The temperature dependence of kCS is discussed in terms of the dielectric relaxation time of the polymer matrix. Consequently, CS below Tg was well explained by a thermally nonequilibrium electron transfer (ET) formula above 200 K and by a two-mode quantum-mechanical ET formula below 200 K. The increase in kCS above 200 K is mainly caused by a thermally activated low-frequency matrix mode originating from the side-chain relaxation of polar cyano groups. The weak temperature dependence of kCS can be explained by a nuclear-tunneling effect caused by a high-frequency matrix mode (variant Planck's over 2piomegH=250 cm-1) and an intramolecular vibrational mode (variant Planck's over 2piomegaQ=1300 cm-1). The high-frequency mode of the polymer matrix was attributed to a vibrational or librational motion of polar groups in the CN-PUL glassy solid.

Journal Article↗
PubMed2004

Enhancement of the superconducting transition temperature of MgB2 by a strain-induced bond-stretching mode softening.

We report a systematic increase of the superconducting transition temperature T(c) with a biaxial tensile strain in MgB2 films to well beyond the bulk value. The tensile strain increases with the MgB2 film thickness, caused primarily by the coalescence of initially nucleated discrete islands (the Volmer-Weber growth mode.) The T(c) increase was observed in epitaxial films on SiC and sapphire substrates, although the T(c) values were different for the two substrates due to different lattice parameters and thermal expansion coefficients. We identified, by first-principles calculations, the underlying mechanism for the T(c) increase to be the softening of the bond-stretching E(2g) phonon mode, and we confirmed this conclusion by Raman scattering measurements. The result suggests that the E(2g) phonon softening is a possible avenue to achieve even higher T(c) in MgB2-related material systems.

Journal Article↗
PubMed2004

The properties of free polymer surfaces and their influence on the glass transition temperature of thin polystyrene films.

We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (T(g)) of thin polystyrene (PS) films. Direct measurements of the near-surface properties of PS films are made by monitoring the embedding of 10 and 20 nm diameter gold spheres into the surface of spin-cast PS films. At a temperature T = 378 K( > T(g)), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below T(g) (363 K < T < 370 K), both sets of spheres embed 3-4 nm into the PS films and stop. These studies suggest that a liquid-like surface layer exists in glassy PS films and also provide an estimate for the lower bound of the thickness of this layer of 3-4 nm. This qualitative idea is supported by a series of calculations based upon a previously developed theoretical model for the indentation of nanoscale spheres into linear viscoelastic materials. Comparing data with simulations shows that this surface layer has properties similar to those of a bulk sample of PS having a temperature of 374 K. Ellipsometric measurements of the T(g) are also performed on thin spin-cast PS films with thicknesses in the range 8 nm < h < 290 nm. Measurements are performed on thin PS films that have been capped by thermally evaporating 5 nm thick metal (Au and Al) capping layers on top of the polymer. The measured T(g) values (as well as polymer metal interface structure) in such samples depend on the metal used as the capping layer, and cast doubt on the general validity of using evaporative deposition to cover the free surface. We also prepared films that were capped by a new non-evaporative procedure. These films were shown to have a T(g) that is the same as that of bulk PS (370+/-1 K) for all film thicknesses measured (> 7 nm). The subsequent removal of the metal layer from these films was shown to restore a thickness-dependent T(g) in these samples that was essentially the same as that observed for uncapped PS films. An estimate of the thickness of the liquid-like surface layer was also extracted from the ellipsometry measurements and was found to be 5+/-1 nm. The combined ellipsometry and embedding studies provide strong evidence for the existence of a liquid-like surface layer in thin glassy PS films. They show that the presence of the free surface is an important parameter in determining the existence of T(g) reductions in thin PS films.

Journal Article↗
PubMed1986

An analytical model to predict the effects of heating rate and applied load on glass transition temperatures of dental porcelain.

An analytical model was developed to predict the effects of heating rate and load on the glass transition temperature (Tg) of dental porcelain. An equation similar to the Moynihan equation relating heating rate to Tg was found for midspan load. The validity of the analytical model was demonstrated by using viscosity data from one published study to predict Tg behavior in an independent study. Values of Tg calculated with the analytical model for a dilatometer specimen were found to be higher than Tg values calculated for a bending beam viscometer.

Chemical Phenomena↗
PubMed2003

Effect of water potential on sol-gel transition and intermolecular interaction of gelatin near the transition temperature.

The sol-gel transition of gelatin, measured by thermal analysis and viscosity measurement, was analyzed in terms of the change in hydration state of polymer molecules. A new thermodynamic model was proposed in which the effect of water potential is explicitly taken into account for the evaluation of the free energy change in the sol-gel transition process. Because of the large number of water molecules involved and the small free energy change in the transition process, the contribution of water activity, a(W), was proved to be not negligible in the sol-gel transition process in solutions containing such low-molecular cosolutes as sugars, glycerol, urea, and formamide. The gel-stabilization effect of sugars and glycerol was linear with a(W), which seemed consistent with the contribution of water potential in the proposed model. The different stabilization effect among sugars and glycerol was explained by the difference in solvent ordering, which affects hydrophobic interaction among protein molecules. The gel-destabilization effect of urea and formamide could be explained only by the direct binding of them to protein molecules through hydrogen bonding. On the contrary, the polymer-polymer interaction, measured by the viscosity analysis, in polyethyleneglycol and dextran solutions was not sensitive to the change in a(W), suggesting that no substantial change in hydration state with a(W) occurred in these polymer solutions.

Animals↗
PubMed1992

The influence of polymer glass transition temperature and molecular weight on drug release from tablets containing poly(DL-lactic acid).

Five molecular weight grades of poly(DL-lactic acid) were characterized using gel permeation chromatography, differential scanning calorimetry, and viscometry to determine the effect of molecular weight on the glass transition temperature and the intrinsic viscosity. In addition, dynamic mechanical thermal analysis was used to assess the dynamic storage modulus and the damping factor of the polymer samples by detecting motional and structural transitions over a wide temperature range. Significant relationships were found between the molecular weight and these polymer properties. The five grades of poly(DL-lactic acid) were also incorporated as binders into matrix tablet formulations containing the model drug theophylline and microcrystalline cellulose. Dissolution studies showed significant correlations between the properties of the polymer and the matrix release profiles of the tablets. The release of theophylline slowed down progressively as the polymer molecular weight increased. The differences in release became less significant and reached a limiting asymptotic value as the molecular weight increased to 138,000. Further, tablet index testing was utilized to determine the compaction properties of the polymer granulations. Although there was no correlation with the molecular weight of PLA, brittle fracture index testing indicated very low brittleness for all granulations tested. However, bonding index determinations correlated very well with both the physical-mechanical properties of the polymer and drug release profiles.

Biomechanical Phenomena↗
PubMed2001

Apparent nonergodic behavior of supercooled liquids above the glass transition temperature.

A speckle pattern can be observed in the polarized component of light scattered from glass forming liquids far above their glass transition temperature. This speckle pattern fluctuates with characteristic time that corresponds to the relaxation time of the additional ultraslow component in the correlation function and is about seven orders of magnitude longer than the relaxation time of the alpha-process. This slow process is out of the experimental time window when the alpha-process is measured by means of the photon correlation spectroscopy and results in an apparent nonergodicity which can be seen as a baseline offset in the ensemble-averaged correlation function. In contrast, the time-averaged field correlation functions which have been measured in practically all light scattering studies always decay to zero. The slow process contributes a q-dependent excess intensity to the polarized component of scattered light. The values of the nonergodicity parameters obtained from the static and dynamic light scattering experiments are equal. Both the slow component and the excess intensity result from denser regions of fractal character which develop in glass-forming liquids on approaching the glass transition.

Journal Article↗
PubMedDate not supplied

Prediction of glass transition temperatures from monomer and repeat unit structure using computational neural networks.

Quantitative structure-property relationships (QSPR) are developed to correlate glass transition temperatures and chemical structure. Both monomer and repeat unit structures are used to build several QSPR models for Parts 1 and 2 of this study, respectively. Models are developed using numerical descriptors, which encode important information about chemical structure (topological, electronic, and geometric). Multiple linear regression analysis (MLRA) and computational neural networks (CNNs) are used to generate the models after descriptor generation. Optimization routines (simulated annealing and genetic algorithm) are utilized to find information-rich subsets of descriptors for prediction. A 10-descriptor CNN model was found to be optimal in predicting T(g) values using the monomer structure (Part 1) for 165 polymers. A committee of 10 CNNs produced a training set rms error of 10.1K (r2 = 0.98) and a prediction set rms error of 21.7 K (r2 = 0.92). An 11-descriptor CNN model was developed for 251 polymers using the repeat unit structure (Part 2). A committee of CNNs produced a training set rms error of 21.1K (r2 = 0.96) and a prediction set rms error of 21.9 K (r2 = 0.96).

Journal Article↗
PubMed2003

Self-diffusion of tris-naphthylbenzene near the glass transition temperature.

We present a direct measurement of self-diffusion of a single-component glass-forming liquid at the glass transition temperature. Forward recoil spectrometry is used to measure the concentration profiles of deuterio and protio 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB) following annealing-induced diffusion in a vapor-deposited bilayer. These experiments extend the range of measured diffusion coefficients in TNB by 6 orders of magnitude. The results indicate a decoupling of translational diffusion coefficients from viscosity or rotation. At T(g), D(T) is 400 times larger than expected from the Stokes-Einstein equation.

Journal Article↗
PubMed2001

Pressure dependence of the superconducting transition temperature of magnesium diboride.

We studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB(2)). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB(2).

Journal Article↗
PubMed2001

Influence of various drugs on the glass transition temperature of poly(vinylpyrrolidone): a thermodynamic and spectroscopic investigation.

PURPOSE: To determine the influence of hydrogen bonding and solubility parameter on the glass transition temperature (T(g)) of various drug-poly(vinylpyrrolidone) blends. METHODS: The T(g) of PVP films containing either acetaminophen, naproxen, salicylamide, carbamazepine, griseofulvin or propranolol hydrochloride were measured using differential scanning calorimetry. Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction was used to characterize the specific interactions between the drug-PVP blends and the physical state of the films, respectively. The total solubility parameter and its individual components were calculated using the method of Van Krevelen. RESULTS: Salicylamide displayed the greatest plasticizing effect, depressing the T(g) to the minimum. This was consistent with the FTIR data, which indicated the presence of hydrogen bonding with PVP. Griseofulvin showed the least plasticizing effect due to lack of interaction with PVP. All the drugs except griseofulvin were amorphous within the film up to 30% (w/w) drug composition. The correlation between the various components of the solubility parameters and the plasticizing effect of drugs was very poor. CONCLUSIONS: Spectroscopic investigation for the presence of interaction between the drugs and PVP proved to be extremely predictive of the plasticizing effect of various drugs. In contrast, solubility parameters appeared to be far less sensitive indicators of drug-PVP miscibility.

Chemistry, Pharmaceutical↗
PubMed2003

Evaluation of different calorimetric methods to determine the glass transition temperature and molecular mobility below T(g) for amorphous drugs.

The purpose of the present study was to compare different calorimetric methods used to determine the glass transition temperature (T(g)) and to evaluate the relaxation behaviour and hence the stability of amorphous drugs below their T(g). Data showed that the values of the activation energy for the transition of a glass to its super-cooled liquid state qualitatively correlate with the values of the mean molecular relaxation time constant of ketoconazole, itraconazole and miconazole, three structurally related drugs. Estimation of the molecular mobility by activation energy calculation indicated that loperamide was more stable than its two building blocks T263 and R731. It was further shown that the most commonly used approach to determine T(g) (T(g (1/2 c(p))) leads to erroneous values when enthalpy recovery is significant. In this case, an alternative method based on enthalpic considerations leads to results in accordance to basic thermodynamics. Estimation of molecular mobility based on activation energy calculations is therefore considered to be a valuable alternative for the method based on measurement of the extent of relaxation. When enthalpy relaxation is important, the use of T(g 1/2c(p)) leads to an overestimation of the T(g).

Azoles↗
PubMed1993

Dependence of the bilayer phase transition temperatures on the structural parameters of phosphatidylcholines.

Most saturated diacyl phosphatidylcholines C(X):C(Y)PC (saturated 1,2-diacyl-sn-glycero-3-phosphocholine with X carbons in the sn-1 acyl chain and Y carbons in the sn-2 acyl chain), in excess water, can self-assemble into lamellae which, upon heating, may undergo multiple thermotropic phase transitions at well-defined, discrete temperatures. The transition temperature corresponding to the main or the gel to liquid-crystalline phase transition (Tm) is known for many bilayers of fully hydrated phosphatidylcholines. In this study, we have analyzed the Tm values of 44 molecular species of phosphatidylcholines in terms of their structural and packing characteristics in the gel-state bilayer. Two general equations are thus derived: Tm = 162.26-3651.71 (1/N) - 88.42 (delta C/N) for C(X):C(Y)PC with X > or = Y, and Tm = 157.68-3525.44 (1/N) - 93.28 (delta C/N) for C(X):C(Y)PC with X < Y. Here, N is the minimal hydrophobic thickness of the dimeric C(X):C(Y)PC in the gel-state bilayer and delta C is the effective chain length difference between the sn-1 and sn-2 acyl chains for the monomeric C(X):C(Y)PC in the gel-state bilayer. The advantage of these two equations in predicting the Tm values for phosphatidylcholines with delta C/CL values in the range of 0.07 to 0.40 is their simplicity. A figure containing a total of 173 calculated Tm values is also presented.

Computer Simulation↗
PubMed1994

Crystallization of indomethacin from the amorphous state below and above its glass transition temperature.

The solid state crystallization of amorphous polymers, sugars, and inorganic glasses is often thought to be restricted to the region above the glass transition temperature, Tg, because insufficient molecular mobility (high viscosity) exists below Tg for nucleation and crystal growth. Here we report on the isothermal and nonisothermal crystallization of dry amorphous indomethacin in the temperature range of 20 degrees C above and below its Tg. These studies were carried out with two amorphous samples having different degrees of metastability relative to the crystalline state. It was shown that in both samples significant crystallization to the most stable polymorphic form occurred over several days when stored below Tg, and in some cases this process was preceded by the relaxation of one amorphous form to the other. At storage temperatures near to and above Tg the rates of crystallization increased as expected but a second less thermodynamically stable polymorph also appeared with the more stable crystal form. This behavior was explained by the possible relationship between the degree of metastability relative to the crystalline state of each amorphous form and the interfacial energy existing at the respective nucleation sites, in accord with the Ostwald step rule.

Calorimetry, Differential Scanning↗
PubMed1994

Polymerization kinetics, glass transition temperature and creep of acrylic bone cements.

Sulfix-6 and Zimmer LVC 60/30 bone cements were selected and the polymerization kinetics and resulting glass transition temperature Tg; creep behaviour in the dry or water-saturated state; and sorption and diffusion of water were studied. The calculation of conversion was based on a comparison of the residual polymerization heat measured by differential scanning calorimetry and the corresponding theoretical value. The conversion reached 99% after 90 min of quasi-adiabatic polymerization starting at 23 degrees C or after 10 min of isothermal polymerization at 37 degrees C. The Tgs of the cements prepared in the former way were about 82 and 100 degrees C, respectively. Creep rate of the bone cements at 37 degrees C decreased with the time of creeping. Sorbed water enhanced the compliance, but reduced the creep rate for long times so that water sorption during the service time may not have detrimental effects on the creep resistance of the cements. Both types of cements contained about 1% of low molar mass substances extractable by water. Measurements of the sorption kinetics of water showed that the diffusion coefficient is 0.14 x 10(-11) and 0.22 x 10(-11) m2/s and 1 yr sorption achieves 2.11% and 2.89% for Sulfix and Zimmer, respectively.

Adsorption↗