How much time is needed to form a kinetically stable glass? AC calorimetric study of vapor-deposited glasses of ethylcyclohexane

Glasses of ethylcyclohexane produced by physical vapor deposition have been characterized by in situ alternating current chip nanocalorimetry. Consistent with previous work on other organic molecules, we observe that glasses of high kinetic stability are formed at substrate temperatures around 0.85 Tg, where Tg is the conventional glass transition temperature. Ethylcyclohexane is the least fragile organic glass-former for which stable glass formation has been established. The isothermal transformation of the vapor-deposited glasses into the supercooled liquid state was also measured. At seven substrate temperatures, the transformation time was measured for glasses prepared with deposition rates across a range of four orders of magnitude. At low substrate temperatures, the transformation time is strongly dependent upon deposition rate, while the dependence weakens as Tg is approached from below. These data provide an estimate for the surface equilibration time required to maximize kinetic stability at each substrate temperature. This surface equilibration time is much smaller than the bulk α-relaxation time and within two orders of magnitude of the β-relaxation time of the ordinary glass. Kinetically stable glasses are formed even for substrate temperatures below the Vogel and the Kauzmann temperatures. Surprisingly, glasses formed in the limit of slow deposition at the lowest substrate temperatures are not as kinetically stable as those formed near 0.85 Tg.

Molecular origin of enhanced proton conductivity in anhydrous ionic systems

Ionic systems with enhanced proton conductivity are widely viewed as promising electrolytes in fuel cells and batteries. Nevertheless, a major challenge toward their commercial applications is determination of the factors controlling the fast proton hopping in anhydrous conditions. To address this issue, we have studied novel proton-conducting materials formed via a chemical reaction of lidocaine base with a series of acids characterized by a various number of proton-active sites. From ambient and high pressure experimental data, we have found that there are fundamental differences in the conducting properties of the examined salts. On the other hand, DFT calculations revealed that the internal proton hopping within the cation structure strongly affects the pathways of mobility of the charge carrier. These findings offer a fresh look on the Grotthuss-type mechanism in protic ionic glasses as well as provide new ideas for the design of anhydrous materials with exceptionally high proton conductivity.

New menthol polymorphs identified by flash scanning calorimetry

Menthol polymorphism screening by thermal microscopy in order to corroborate flash DSC results.

Determination of volatility of ionic liquids at the nanoscale by means of ultra-fast scanning calorimetry

The determination of vaporization enthalpies of extremely low volatility ionic liquids is challenging and time consuming due to the low values of vapor pressure. In addition, these liquids tend to decompose even at temperatures where the vapor pressure is still low. Conventional methods for determination of vaporization enthalpies are thus limited to temperatures below the decomposition temperature. Here we present a new method for the determination of vaporization enthalpies of such liquids using differential fast scanning calorimetry. We have developed and proven this method using [EMIm][NTf2] at temperatures of up to 750 K and in different atmospheres. It was demonstrated that evaporation is still the dominating process of mass loss even at such highly elevated temperatures. In addition, since the method allows very high heating rates (up to 10(5) K s(-1)), much higher temperatures can be reached in the measurement of the mass loss rate as compared to common devices without significant decomposition of the ionic liquid. We discuss the advantages and limits of this new method of vaporization enthalpy determination and compare the results with data obtained from established methods.

Isothermal Time-Temperature-Precipitation Diagram for an Aluminum Alloy 6005A by In Situ DSC Experiments

Time-temperature-precipitation (TTP) diagrams deliver important material data, such as temperature and time ranges critical for precipitation during the quenching step of the age hardening procedure. Although the quenching step is continuous, isothermal TTP diagrams are often applied. Together with a so-called Quench Factor Analysis, they can be used to describe very different cooling paths. Typically, these diagrams are constructed based on mechanical properties or microstructures after an interrupted quenching, i.e., ex situ analyses. In recent years, an in situ calorimetric method to record continuous cooling precipitation diagrams of aluminum alloys has been developed to the application level by our group. This method has now been transferred to isothermal experiments, in which the whole heat treatment cycle was performed in a differential scanning calorimeter. The Al-Mg-Si-wrought alloy 6005A was investigated. Solution annealing at 540 °C and overcritical quenching to several temperatures between 450 °C and 250 °C were followed by isothermal soaking. Based on the heat flow curves during isothermal soaking, TTP diagrams were determined. An appropriate evaluation method has been developed. It was found that three different precipitation reactions in characteristic temperature intervals exist. Some of the low temperature reactions are not accessible in continuous cooling experiments and require isothermal studies.

Effect of aging the glass of isotactic polybutene-1 on form II nucleation and cold crystallization

The effect of aging initially fully amorphous isotactic polybutene-1 (iPB-1) at temperatures between 243 and 283 K on form II nucleation and cold crystallization has been quantified by fast scanning chip calorimetry. Aging of amorphous iPB-1 at temperatures close to the glass transition temperature leads to formation of nuclei which accelerate subsequent cold crystallization. Analysis of the enthalpy of cold crystallization on heating differently aged samples revealed a maximum rate of nucleation at around 265 K. In contrast, the maximum rate of form II crystallization is observed at distinctly higher temperature of 330-340 K. It is suggested that formation of form II crystal nuclei in the glassy state requires prior densification of the glass since acceleration of cold crystallization on heating the aged glass is detected only after completion of the enthalpy relaxation. The analysis of the rates of nucleation and cold crystallization of iPB-1 at low temperatures is a necessary completion of prior work on the phase transition behavior, and contributes to further understanding of mechanisms of crystal nucleation in polymers.

High frequency alternating current chip nano calorimeter with laser heating

Heat capacity spectroscopy at frequencies up to 100 kHz is commonly performed by thermal effusivity measurements applying the 3ω-technique. Here we show that AC-calorimetry using a thin film chip sensor allows for the measurement of frequency dependent heat capacity in the thin film limit up to about 1 MHz. Using films thinner than the thermal length of the thermal wave (~1 μm) at such frequencies is advantageous because it provides heat capacity alone and not in combination with other quantities like thermal conductivity, at least on a qualitative basis. The used calorimetric sensor and the sample are each less than 1 μm thick. For high frequency AC-calorimetry, high cooling rates at very small temperature differences are required. This is realized by minimizing the heated spot to the size of the on chip thermocouple (3 × 6 μm(2)). A modulated laser beam shaped and positioned by a glass fiber is used as the heat source. The device was used to measure the complex heat capacity in the vicinity of the dynamic glass transition (structural relaxation) of poly(methyl methacrylate). Combining different calorimeters finally provides data between 10(-3) Hz and 10(6) Hz. In this frequency range the dynamic glass transition shifts about 120 K.

Das kontinuierliche Zeit-Temperatur-Ausscheidungs-Diagramm einer Aluminiumlegierung EN AW-6005A*

Der Einfluss der Abschreckgeschwindigkeit auf das Ausscheidungsverhalten von Aluminiumlegierungen wird, in Anlehnung an ZTU-Diagramme von Stählen, in kontinuierlichen Zeit-Temperatur-Ausscheidungs-Diagrammen dargestellt. Solche Diagramme sind bisher kaum verfügbar, können jetzt aber mit Hilfe der Differential Scanning Calorimetry (DSC) erfasst werden. Wird eine Charge der Aluminiumlegierung EN AW-6005A in einem Kühlgeschwindigkeitsbereich von 0,1–30 K/min von Lösungsglühbedingungen in einem DSC abgekühlt, sind in den Abkühlkurven zwei exotherme Ausscheidungsreaktionen zu erkennen, eine Hoch- sowie eine Niedertemperaturreaktion. Um Aufschluss darüber zu erhalten, welche Phasen dabei ausgeschieden werden, wurden umfangreiche Gefügeanalysen mittels Licht- und Rasterelektronenmikroskopie, energiedispersiver Röntgen-Mikroanalyse (EDX), Röntgendiffraktometrie (XRD) und Elektronen-Rückstreubeugung (EBSD) sowie Härteprüfungen an unterschiedlich abgekühlten Proben durchgeführt. Es konnte gezeigt werden, dass bei der Hochtemperaturreaktion Mg2Si ausgeschieden wird. Die Keimbildung der Mg2Si-Partikel erfolgt überwiegend heterogen an Primärausscheidungen.

Beating the heat--fast scanning melts silk beta sheet crystals

Beta-pleated-sheet crystals are among the most stable of protein secondary structures, and are responsible for the remarkable physical properties of many fibrous proteins, such as silk, or proteins forming plaques as in Alzheimer's disease. Previous thinking, and the accepted paradigm, was that beta-pleated-sheet crystals in the dry solid state were so stable they would not melt upon input of heat energy alone. Here we overturn that assumption and demonstrate that beta-pleated-sheet crystals melt directly from the solid state to become random coils, helices, and turns. We use fast scanning chip calorimetry at 2,000 K/s and report the first reversible thermal melting of protein beta-pleated-sheet crystals, exemplified by silk fibroin. The similarity between thermal melting behavior of lamellar crystals of synthetic polymers and beta-pleated-sheet crystals is confirmed. Significance for controlling beta-pleated-sheet content during thermal processing of biomaterials, as well as towards disease therapies, is envisioned based on these new findings.

Structure-Property Relations in Ionic Liquids: 1,2,3-Trimethyl-imidazolium and 1,2,3-Trimethyl-benzimidazolium bis-(trifluorsulfonyl)imide

Vaporization enthalpies for a pairs of the ionic liquids (ILs) 1,2,3-trimethyl-imidazolium and 1,2,3-trimethyl-benzimidazolium bistrifluorsulfonylimide and the corresponding molecular liquids 1,2-dimethyl-imidazole and 1,2-dimethyl-benzimidazole were determined using a quartz crystal microbalance method and TGA (for ILs) and the transpiration method (for molecular compounds). Differences in vaporization enthalpies in molecular and ionic liquids are discussed in context of structure-property relations.

On the theoretical determination of the Prigogine-Defay ratio in glass transition

In a recent analysis [J. W. P. Schmelzer and I. Gutzow, J. Chem. Phys. 125, 184511 (2006)] it was shown for the first time that--in contrast to earlier belief arising from the works of Prigogine and Defay [Chemical Thermodynamics (Longman, London, 1954), Chap. 19; The first French edition of this book was published in 1950] and Davies and Jones [Adv. Phys. 2, 370 (1953); and Proc. R. Soc. London, Ser. A 217, 26 (1953)]--a satisfactory theoretical interpretation of the experimentally observed values of the so-called Prigogine-Defay ratio Π, being a combination of jumps of thermodynamic coefficients at glass transition, can be given employing only one structural order parameter. According to this analysis, this ratio has to be, in full agreement with experimental findings, larger than one (Π > 1). Its particular value depends both on the thermodynamic properties of the system under consideration and on cooling and heating rates. Based on above-mentioned analysis, latter dependence on cooling rates has been studied in detail in another own preceding paper [T. V. Tropin, J. W. P. Schmelzer, and C. Schick, J. Non-Cryst. Solids 357, 1303 (2011)]. In the present analysis, an alternative general method of determination of the Prigogine-Defay ratio is outlined, allowing one to determine this ratio having at ones disposal the generalized equation of state of the glass-forming melts under consideration and, in particular, the knowledge of the equilibrium properties of the melts in the glass transformation range. Employing, as an illustration of the method, a particular model for the description of glass-forming melts, theoretical estimates are given for this ratio being, again, in good agreement with experimental data.

Differential alternating current chip calorimeter for in situ investigation of vapor-deposited thin films

Physical vapor deposition can be used to produce thin films with interesting material properties including extraordinarily stable organic glasses. We describe an ac chip calorimeter for in situ heat capacity measurements of as-deposited nanometer thin films of organic glass formers. The calorimetric system is based on a differential ac chip calorimeter which is placed in the vacuum chamber for physical vapor deposition. The sample is directly deposited onto one calorimetric chip sensor while the other sensor is protected against deposition. The device and the temperature calibration procedure are described. The latter makes use of the phase transitions of cyclopentane and the frequency dependence of the dynamic glass transition of toluene and ethylbenzene. Sample thickness determination is based on a finite element modeling of the sensor sample arrangement. In the modeling, a layer of toluene was added to the sample sensor and its thickness was varied in an iterative way until the model fit the experimental data.

Benchmark Values: Thermochemistry of the Ionic Liquid [C4Py][Cl]

Differential scanning calorimetry (DSC) was used for the determination of the reaction enthalpy of the synthesis of the ionic liquid [C4Py][Cl] from pyridine and butyl chloride. A combination of DSC results with quantum chemical calculations presents an indirect technique to obtain enthalpy of vaporization of [C4Py][Cl]. In order to ascertain this indirect value, we used thermal gravimetric analysis (TGA) to derive enthalpy of vaporization directly from the isothermal mass-loss measurements. This new procedure was additionally validated with molecular dynamics (MD) simulations.

Crystallization kinetics and miscibility of blends of polyhydroxybutyrate (PHB) with ethylene vinyl acetate copolymers (EVA)

A series of ethylene vinyl acetate random copolymer EVA, with vinyl acetate (VA) varied from 9 to 91m%, was investigated by differential scanning calorimetry DSC and polarized optical microscopy. Biodegradable polymer blends of polyhydroxybutyrate PHB and EVA, having VA in EVA in range from 40m% till 91m%, were prepared by film casting from a chloroform solution. The miscibility and crystallization behavior of these blends were investigated. The isothermal crystallization behaviors of PHB and PHB/EVA blends are discussed in terms of the half time of crystallization t1/2. Experimental results indicated that blends of PHB/ EVA91 are completely miscible blend in the entire (0 to 100 m%) compositional ranges. Blends PHB/ EVA, for VA varied from 40 till 70 m% are immiscible as evidenced by the existence of unchanged composition independent glass transition temperatures (Tg), crystallization and melting behavior. The isothermal crystallization of PHB blends was investigated from room temperature till 130 °C. 80 °C was found to be the best temperature for comparison of different blends. At 80 °C t1/2 strongly depends on the content of VA in PHB/EVA blend, mainly due to differences in miscibility as well as due to differences in segmental mobility as identified by differences in glass transition temperature. Since both components in PHB/EVA80, pure PHB and pure EVA80, have glass transition temperatures close to 0 °C, it is difficult to decide its miscibility from Tg. However from the strong dependence of the value of crystallization half time t1/2 of PHB/EVA80 on blend composition, it was possible to reasonably infer that PHB/EVA80 is partially miscible.

SiO2 Nanofiller impact on crystallization kinetics during adiabatic anionic polymerization of ε-caprolactam

Anion activated adiabatic polymerization of ε-caprolactam was investigated in the presence of various quantities of nano-SiO2. The parallel processes of crystallization and polymerization are separated first. Based on the crystallization kinetics and electron microscopic investigations it was shown that nano-SiO2 particles are heterogeneous nuclei for the crystallization of poly-ε-caprolactam. Further the time dependence of mean length of amorphous polymer macromolecules is determined experimentally.

H2 storage and CO2 capture on a nanoscale metal organic framework with high thermal stability

A nanoscale aluminium-based metal organic framework (NMOF) with high thermal stability has been synthesized, which shows high H(2) and CO(2) uptake capacities and an excellent selectivity for CO(2) over N(2) and O(2).