The effects of heating and cooling rate on the characteristics of the calorimetric glass transition for glassy polymers

DSC was used to study the characteristic endotherm which occurs when reheating a glass back to the liquid state. Anionic polystyrene, PMMA, polycarbonate, and polysulfone were tested, varying both the cooling and reheating rates over two decades. In this initial paper on these results, an experimental technique optimized for glassy polymers is explained, and the proper method for correcting thermal lag is outlined. The variation of the onset, peak and fictive temperatures is shown for polystyrene and polysulfone. Results are also reported for the first time on the shift of the endothermal glass transition, measured while cooling.     

Influence of heating and cooling rates on the glass transition temperature and the fragility parameter of sorbitol and fructose as measured by DSC

The glass transition temperatures of sorbitol and fructose were characterized by four points determined on DSC heating thermograms (onset, mid-point, peak and end-point), plus the limit fictive temperature. The variations of these temperature values, observed as functions of cooling and heating rates, were used to determine the fragility parameter, as defined by Angell [1] to characterize the temperature dependence of the dynamic behavior of glass-forming liquids in the temperature range above the glass transition. The apparent activation energy values, determined for the different temperatures studied, were similar for fructose and sorbitol. These values were compared to data obtained from other techniques, such as mechanical spectroscopy. The variations of the apparent activation values, observed in experiments involving cooling and heating at the same rate, slow cooling followed by rate-heating, or rate-cooling followed by fast heating, were explained by aging effects occurring during the heating step.     

Towards a solution for viscous heating in ultra-high pressure liquid chromatography using intermediate cooling

A generic solution is proposed for the deleterious viscous heating effects in adiabatic or near-adiabatic systems that can be expected when trying to push the column operating pressures above the currently available range of ultra-high pressures (i.e., 1200 bar). A set of proof-of-principle experiments, mainly using existing commercial equipment, is presented. The solution is based on splitting up a column with given length L into n segments with length L/n, and providing an active cooling to the capillaries connecting the segments. In this way, the viscous heat is removed at a location where the radial heat removal does not lead to an efficiency loss (i.e., in the thin connection capillaries), while the column segments can be operated under near-adiabatic conditions without suffering from an unacceptable rise of the mobile phase temperature. Experimental results indicate that the column segmentation does not lead to a significant efficiency loss (comparing the performance of a 10 cm column with a 2 cm × 5 cm column system), whereas, as expected, the system displays a much improved temperature stability, both in time (because of the shortened temperature transient times) and in space (reduction of the average axial temperature rise by a factor n). The method also prevents a large backflow of heat along the column wall that would lead to large efficiency losses if one would attempt to operate columns at pressures of 1500 bar or more. A real-world pharmaceutical example is given where this improved temperature robustness could help in moderating the changes in selectivity during method transfer from a low to a high pressure operation, although the complex non-linear behavior of the viscous heating and high pressure effects result in lower than expected improvement.     

Correlation between quasielastic Raman scattering and configurational entropy in an ionic liquid

Low-frequency (5-200 cm(-1)) Raman spectra are reported for the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, [bmim]PF(6), in glassy, supercooled liquid, and normal liquid phases (77-330 K). Raman spectra of [bmim]PF(6) agree with previous results obtained by optical Kerr effect spectroscopy and molecular dynamics simulation. Both the superposition model and the coupling model give reasonable fit to low-frequency Raman spectra of [bmim]PF(6). The configurational entropy of [bmim]PF(6) has been evaluated as a function of temperature using recently reported data of heat capacity. The calculated configurational entropy is inserted in the Adam-Gibbs theory for supercooled liquids, giving a good fit to non-Arrhenius behavior of viscosity and diffusive process, with the latter revealed by a recent neutron scattering investigation of [bmim]PF(6). There is a remarkable linear dependence between intensity of quasielastic Raman scattering and configurational entropy from 77 K up to the melting point of [bmim]PF(6). This correlation offers insight into the nature of dynamical processes probed by low-frequency Raman spectra of ionic liquids.     

Freezing on heating of liquid solutions

We report a reversible liquid-solid transition upon heating of a simple solution composed of a-cyclodextrine (alpha CD), water, and 4-methylpyridine. These solutions are homogeneous and transparent at ambient temperature and solidify when heated to temperatures between 45 degrees and 75 degrees. Quasielastic and elastic neutron scattering show that molecular motions are slowed down in the solid and that crystalline order is established. The solution "freezes on heating." This process is fully reversible, on cooling the solid melts. A rearrangement of hydrogen bonds is postulated to be responsible for the observed phenomenon.     

ASA/SEBS/paraffin composites as phase change material for potential cooling and heating applications in building

Phase change material (PCM) is able to melt and crystalize with a high heat of phase change at constant temperature, which provides new and green cooling and heating strategies for buildings. In this work, PCMs for buildings composed of acrylonitrile‐styrene‐acrylate copolymer (ASA), polystyrene‐b‐poly(ethylene/butylene)‐b‐polystyrene triblock copolymer (SEBS) and paraffin were fabricated by melt blending. The results of the accelerated leakage test indicated an excellent ability of PCMs to keep paraffin from leakage. Thermal properties suggested that the phase change enthalpy of PCMs increased with the increasing content of paraffin and their phase change temperature was close to the comfortable sensible temperature of human body, which made it quite suitable for building cooling and heating. Besides, PCMs presented excellent stability and reusability after several thermal cycling tests. The temperature test conducted with self‐designed cylindrical devices gave a more sufficient and direct demonstration of the cooling and heating effect. Remarkably, excellent cooling and heating performance (both as high as 15°C) of the composites could be obtained with the addition of paraffin. And the time span of the cooling and heating process was as long as 5 and 7.5 hours, respectively. Owing to its excellent cooling and heating capabilities, the ASA/SEBS/paraffin composites are of great potential to be applied in building temperature control.     

Entropy and enthalpy change in hydrated ion clusters

A new electrothermodynamic theory was recently developed by (Suck) Salk to treat the hydration of ions suspended in a gas containing water vapor. This theory was successfully applied to a study of the free energy of formation of both positive and negative ions and to an analysis of ion mobility data. Here the new theory is used to study entropy and enthalpy for a variety of hydrated ions. The results compare favorably to experiment and are significantly superior to Thomson theory results.     

Two-particle entropy and structural ordering in liquid water

Entropies of simple point charge (SPC) water were calculated over the temperature range 278-363 K using the two-particle correlation function approximation. Then, the total two-particle contribution to the entropy of the system was divided into three parts, which we call translational, configurational, and orientational. The configurational term describes the contribution to entropy, which originates from spatial distribution of surrounding water molecules (treated as points, represented by the center of mass) around the central one. It has been shown that this term can serve as the metric of the overall orientational ordering in liquid water. Analyzing each of these three terms as a function of intermolecular distance, r, we also find a rational definition of the hydration shell around the water molecule; the estimated radii of the first and second hydration shells are 0.35 nm and 0.58 nm, respectively. We find, moreover, that the first hydration shell around the water molecule participates roughly in 70% of the total orientational entropy of water, and this rate is roughly temperature independent.     

Gas chromatography with ballistic heating and ultrafast cooling of column

An overview of the existing methods for minimization of the analysis time in gas chromatography (GC) is presented and a new system for fast temperature programming and very fast cooling down is evaluated. In this study, a system of coaxial tubes, a heating/cooling module (HC-M), was developed and studied with a capillary column placed inside the HC-M. The module itself was heated by a GC oven and cooled down by an external cooling medium. The HC-M was heated at rates of up to 330 °C min⁻¹ and cooled at the rate of 6000 °C min⁻¹. The GC system was prepared for the next run within a few seconds. The HC-M permits good separation reproducibility, comparable with that of a conventional GC, expressed in terms of relative retention times and peak areas of analytes reproducibilities. The HC-M can be used within any commercial gas chromatograph.     

Analysis of the residual entropy of amorphous polyethylene at 0 K*

The residual entropy of amorphous polyethylene (PE) at 0 K is discussed within the framework of the heat capacity (C_p). The measured C_p of the liquid was extended from the glass transition to low temperature by separately finding its three parts—the vibrational, conformational, and external contributions—and extrapolating each to low temperature. The vibrational C_p was calculated from the frequency distributions of the group vibrations on the basis of force constants obtained from experimental infrared and Raman spectra as well as the skeletal vibrations in the amorphous solid (glass) obtained from fitting of the appropriate experimental C_p to Debye functions in the form suggested by Tarasov. The conformational part of C_p was evaluated from a fit of the heat capacity of the liquid, decreased by the contributions of the vibrational and external parts, to a one‐dimensional Ising model that can be extrapolated to 0 K and requires two discrete states described by stiffness, cooperativity, and a degeneracy parameter. The external part was computed from the experimental data for expansivity and compressibility, fitted to an empirical equation of state, and modified at low temperatures in accordance with the Nernst–Lindemann approximation. The computed C_p of the liquid PE agreed with the experiment from 600 K to the beginning of the glass transition at about 260 K. Extending the heat capacity to 0 K, bypassing the freezing of the large‐amplitude conformational motion in the glass transition, led to a positive residual entropy and enthalpy and avoided the so‐called Kauzmann paradox. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1245–1253, 2002     

The effects of concentration and heating or cooling rate on the DTA curves of Al-Ce alloys

The crystallization of Al-Ce alloys was studied by DTA. The melting and freezing DTA curves of the investigated alloys exhibit two peaks, corresponding to the transformations of a high-purity phase and the eutectic phase, respectively. The peaks could be separated during both freezing and melting by changing the heating or cooling rate. The final temperature of the phase transformation is marked by the starting temperature of the second peak. A slight shoulder on the DTA peak, even on the opposite side to the maximum point, may correspond to the final temperature.The connection between the reaction time and the peak temperature for the sample was studied at different cooling and heating rates.It was concluded that a knowledge of the structure and composition of the studied system is necessary in the interpretation of the DTA curves.

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Zusammenfassung Die Kristallisation von Al-Ce-Legierungen wurde mittels DTA untersucht. Die Schmelz- und Erstarrungs-DTA-Kurven der geprüften Legierungen weisen zwei Peaks auf, welche der Umwandlung einer Phase hohen Reinheitsgrades, bzw. der eutektischen Phase entsprechen. Die Peaks konnten sowohl während des Erstarrens als auch während des Schmelzens durch Änderung der Aufheiz- oder Abkühlgeschwindigkeit getrennt werden. Die Endtemperatur der Phasenumwandlung wird durch die Anfangstemperatur des zweiten Peaks angedeutet. Eine kleine Schulter des DTA-Peaks, selbst an der entgegengesetzten Seite des Maximums, kann der Endtemperatur entsprechen.Der Zusammenhang zwischen der Reaktionszeit und der Peak-Temperatur der Probe wurde bei verschiedenen Aufheiz- und Abkühlgeschwindigkeiten untersucht.Es wurde gefolgert, daß bei der Deutung von DTA-Kurven die Kenntnis der Struktur und Zusammensetzung des untersuchten Systems nötig ist.

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Résumé La cristallisation d'alliages Al-Ce a été étudiée par ATD. Deux pics apparaissent sur les courbes ATD lors de la fusion et de la cristallisation des alliages étudiés. L'un est dû à la transformation de la phase de haute pureté et l'autre à la phase eutectiques. Les pics peuvent être séparés en changeant les vitesses de chauffage et de refroidissement, tant à la fusion qu'à la cristallisation. La température finale de la transformation de phase est marquée par la température initiale du second pic. Un faible épaulement du pic ATD, même du côté opposé au maximum, peut correspondre à la température finale.Le rapport entre la durée de la réaction et la température du pic a été étudié pour diverses vitesses de refroidissement et de chauffage.On en conclut que l'interprétation des courbes ATD exige de connaître la structure et la composition du système étudié.

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1-. , , , . , , . . , , . . , , .

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Paper presented at the Scientific Session on Thermal Analysis held at Balatonfüred, Hungary, on 14–16 October, 1976.

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The authors are grateful to Mrs. M. Laár for the excellent preparation of the alloys. Thanks are due to Miss K. Papp and Mr. J. Szeleczki for their help in the thermal analysis. We are also indebted to Mrs. M. Horváth for the metallographic work.     

Study and development of gel-like formulations based on liquid glass

Possibility of forming a gel with controlled strength and rheological properties from solutions of alkaline silicates and other chemical components at elevated temperatures was analyzed. The dependence of the strength of the gels on the concentration of acetic acid in solution and temperature was studied.     

Experimental estimates of compression heating and decompression cooling in ethylene glycol

The chemical shift difference, Δσ, between the methylene and hydroxyl protons in the high resolution ¹H nuclear magnetic resonance spectrum of ethylene glycol is shown to be pressure dependent. The equilibrium Δσ values for ethylene glycol are reported as a function of temperature and pressure between ambient conditions, 323 K and 2 kbar, respectively. This surface is used along with Δσ values measured in response to a rapid pressure increase to calculate a temperature rise that is used to infer a temperature change for water that is consistent with theoretical estimates. This work implies that compression heating and decompression cooling are not significant enough to interfere with pressure induced protein folding studies.     

In-situ heating study on the structural change of surfactant-templated germanium oxide mesostructure

Mesostructured germanium oxide has been well-synthesized by using a surfactant-templated approach under basic hydrothermal conditions. The cationic surfactant cetyltrimethylammonium bromide (CTAB) has formed nanotubes with uniform diameter of about 3.2 nm. Blanket-like morphology of the as-prepared sample has been observed with transmission electron microscopy (TEM). High-resolution TEM image reveals that the nanotubes are connected with inorganic germanium oxide and have self-assembled into periodic mesostructure. In-situ heating X-ray diffraction (XRD) patterns confirm that the germanium oxide is in amorphous phase in the temperature range from room temperature (RT) to 700 degrees C. In-situ heating small-angle X-ray scattering (SAXS) presents the mesostructural change with temperature. The local atomic structures around germanium atom have been obtained with in-situ heating X-ray adsorption fine structure (XAFS) techniques. The stability of this mesostructure has been determined to be correlated with the cationic surfactant CTAB. The structural evolution from the GeO 2/NaOH aqueous solution, the as-prepared sample to the sample heated at 700 degrees C, has been described, and the formation mechanism of mesostructured germanium oxide has been discussed.     

The use of phase-inverting emulsions to show the phenomenon of interfacial crystallization on both heating and cooling

Highly anomalous crystallization behavior has been achieved in phase-inverting emulsion systems by using nonionic surfactants that induce nucleation. In particular, nucleation can be inhibited at the phase inversion, allowing systems held at, or near, this temperature to undergo crystallization either on heating or cooling. This new phenomenon is demonstrated for 27.4 wt % aqueous glycine solutions emulsified in decane using Span 20 Tween 20 blends. The inhibitory effect on interfacial nucleation at/near the phase inversion is readily shown by the maximum in the induction time for crystallization found in systems at/near the phase-inversion temperature. These findings are unprecedented. An extremely rapid rise in nucleation rate is expected on cooling glycine solutions, owing to the associated increase in supersaturation, the driving force for crystallization. The origin of this highly anomalous behavior is thought to be the low droplet interfacial tension, gammaow, that occurs at the phase-inversion temperature, which results primarily in a substantially increased contact angle between the glycine critical nucleus and the droplet interface. This may present a paradigm shift in crystallization strategies through the use of tunable contact-angle nucleators.     

Reaction paths of tautomerization between hydroxypyridines and pyridones

Tautomerization paths of 2(and 4)-hydroxypyridine (called here HP) to 2(and 4)-pyridone (called here PY) with water molecules were investigated by the use of density functional theory calculations. Potential energies were compared for a number of water molecules. The 2-HP molecule was found to be isomerized most readily and concertedly to the 2-PY one via proton relays with two water molecules. The reaction pattern is invariant even when outer water molecules are added. The 4-HP(H(2)O)(n) --> 4-PY(H(2)O)(n) reaction model did not give small activation energies. However, a reaction of (4-HP)(2)(H(2)O)(2) --> (4-PY)(2)(H(2)O)(2) was found to occur readily through a transient ion-pair intermediate. The conversion processes of (2-PY)(2) to the tautomerization reacting system were discussed. The hydrogen-bond directionality regulates the tautomerization paths.     

Effect of residual water and microwave heating on the half-life of the reagents and reactive intermediates in peptide synthesis

Precise microwave heating has changed the way many small molecules are being synthesized and, currently, the field of solid-phase peptide synthesis is undergoing dramatic changes owing to the use of microwave heating. To fully reap the benefits of precise microwave heating for the formation of amide bonds in peptide synthesis, it is important to understand the kinetics of formation and break-down of activated esters and their N-acylation of the nascent peptide chain at elevated temperatures. Herein, we present systematic studies of, first, the rate of formation of activated esters by NMR spectroscopy and, second, their N-acylation during peptide synthesis. A study of the amount of residual water in the solvents revealed a significant effect on electrophilic reagents and intermediates. This observation was expanded into a general study of microwave heating in peptide synthesis.