Conventional Differential Scanning Calorimetry Research Articles

The glass transition temperature of anhydrous amorphous calcium carbonate

Abstract Amorphous calcium carbonate (ACC) is the least stable polymorph of calcium carbonates. It has been identified to play an important role in nature (e.g., biomineralization and speleothem formation), where it acts as a precursor for the transformation to more stable polymorphs such as calcite. Furthermore, the use of ACC in technical applications requires a robust understanding of the material’s properties. We present the first study that reveals the existence of a glass transition for synthetic and anhydrous ACC. The glass transition occurs at 339 °C. Such measurements are impossible with conventional differential scanning calorimetry (DSC) due to the high tendency of ACC to crystallize. Fast scanning DSC with heating rates of 500 °C/s and higher, however, can be used to separate the endothermic glass transition signature from the exothermic crystallization event since crystallization is shifted to higher temperatures. This allows the detection and quantification of the glass transition for ACC. These observations indicate that ACC is a structural glass and are especially significant because the synthesis of ACC, precipitation from a solution followed by lyophilization, contrasts with the more conventional and well-known route of glass formation–the rapid cooling of a melt. Moreover, we prove that a structural glass can be produced from a simple single-component carbonate system.

Thermal analysis of crystallization reveals subtle interactions in pristine graphene-polycaprolactone nanocomposites

In this study, we investigate the melting behavior and crystallization of nanocomposites of poly(ε-caprolactone) (PCL), a biodegradable polymer, with pristine graphene, an economically feasible filler widely available from natural sources. Nanocomposites with pristine graphene loads between 0.01 and 5 wt% were prepared via solvent casting and primarily probed by Differential Scanning Calorimetry (DSC). Conventional DSC shows that the presence of graphene increases PCL crystallinity. Non-isothermal crystallization was studied using Mo's model, whereas other parameters as Activation Energy and Nucleation Activity were obtained. It is shown that graphene increases crystallization rates acting as nucleant, with no signatures of retardant effects. Compared with other classic nano-loads, like ad-hoc modified bentonite also analyzed for comparison, pristine graphene is more effective as nucleant, which indicates that it is better dispersed in PCL. Analysis by Self Successive Annealing (SSA), also carried out by DSC, reveals that graphene hinders the formation of crystals with lamellar thickness above 7.3 nm, as found in regular PCL. It may indicate that molecular interactions between PCL and pristine graphene disrupts the movement of polymeric chains, consequently limiting lamellar growth. Evidence of such interaction is found by Infrared and Raman spectroscopies that reveal broadening of the carbonyl peak of PCL and alteration of G and D' bands of graphene.

Fragility and Tendency to Crystallization for Structurally Related Compounds.

The present study was designed to investigate the physical stability of three organic materials with similar chemical structures. The examined compounds revealed completely different crystallization tendencies in their supercooled liquid states and were classified into three distinct classes based on their tendency to crystallize. (S)-4-Benzyl-2-oxazolidinone easily crystallizes during cooling from the melt; (S)-4-Benzylthiazolidine-2-thione does not crystallize during cooling from the melt, but crystallizes easily during subsequent reheating above Tg; and (S)-4-Benzyloxazolidine-2-thione does not crystallize either during cooling from the melt or during reheating. Such different tendencies to crystallize are observed despite the very similar chemical structures of the compounds, which only differ in oxide or sulfur atoms in one of their rings. We also studied the isothermal crystallization kinetics of the materials that were shown to transform into a crystalline state. Molecular dynamics and thermal properties were thoroughly investigated using broadband dielectric spectroscopy, as well as conventional and temperature-modulated differential scanning calorimetry in the wide temperature range. It was found that all three glass formers have the same dynamic fragility (m = 93), calculated directly from dielectric structural relaxation times. This result verifies that dynamic fragility is not related to the tendency to crystallize. In addition, thermodynamic fragility predictions were also made using calorimetric data. It was found that the thermodynamic fragility evaluated based on the width of the glass transition, observed in the temperature dependence of heat capacity, correlates best with the tendency to crystallize.

Influence of cross-link density on the non-isothermal crystallization kinetics of polybutylene terephthalate

Kinetics of crystallization of the samples of cross-linked polybutylene terephthalate with different cross-link densities was studied experimentally using conventional differential scanning calorimetry (cooling rates 5 to 40 K·min–1) and fast scanning calorimetry (cooling rates 0.5 to 5000 K·s–1). The samples were obtained by copolycondensation of terephthaloyl chloride, 1,4-butanediol, and glycerol. The cross-linking degree was characterized using 1H NMR spectroscopy. An increase of the number of glycerol cross-links in polymer leads to a decrease in the crystallization and melting temperatures and suppression of crystallization process. The critical rate of crystallization decreases monotonically with increasing cross-linking degree from 2700 K·s–1 for linear polybutylene terephthalate to 110 K·s–1 for the sample with 270 mol·m–3 cross-link density. At the cross-link densities higher than 270 mol·m–3, the polymer does not crystallize even at 5 K·min–1 cooling rate. The observed dependence of the cold crystallization enthalpy on the previous cooling rate suggests that the homogeneous nuclei formation process also slows down with an increase in the cross-linking degree.

An innovative and low-cost system for in situ and real-time cure monitoring using electrical impedancemetry for thermoset and CFRP laminate

This paper is divided into five sections. The first one introduces the context of our study dealing with the monitoring of the cure of structural composites made of carbon fibers and thermoset matrix (CFRPs). The second one presents a brief state of the art. The third one deals with the materials characterized in this study, and the design and the production of a new bench dedicated to electrical impedance (EI) and thermal measurements. The fourth one compares the results obtained on CFRP and unreinforced matrix samples using conventional differential scanning calorimetry and EI measurements. A conclusion and some prospectives close the paper. In a recent previous work, we studied phases transitions in CFRP during their curing by EI showing that changes in the electrical complex impedance Z can easily be related to those of conventional parameters such as the thermoset matrix degree of cure (DOC) α. This work was carried out using a commercial single-channel impedance analyzer: HIOKI. In addition to its high acquisition cost, this device presented certain limitations in terms of measurement channels (only one) and data acquisition rate (60 s for a frequency sweep). These facts led us to develop a new EI measurement bench for monitoring the impedance changes in CFRP part for aeronautical applications (airframe structures). The innovative new multi-channel (8) bench we designed and manufactured is based on the Digilent PmodIA module (never used previously for this purpose) and an analog-front-end developed for EI measurements. It costs only 15% of the HIOKI analyzer and has a data acquisition rate 48 times higher (1,23 s versus 60 s). All this while maintaining an equivalent impedance measurement range: 100 mΩ to 1 MΩ. The proposed approach makes it possible not only to monitor the DOC α, but also to detect potential cure cycle issues. This latter demonstration was carried out on a new composite material (vacuum-bag oven-cured), NC66/1808NA (supplied by CTMI), which, to our knowledge, had never been characterized in the literature. Compared with preliminary results, similar behavior is obtained on two different CFRPs using two different benches. This clearly underlines the value and quality of the study. Experimental validation of our approach will contribute to CFRP structural health monitoring.

TATB thermal decomposition: An improved kinetic model for explosive safety analysis

AbstractWe investigate and model the cook‐off behavior of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) to understand the response of explosive systems in abnormal thermal environments. Decomposition has been explored via conventional ODTX (one‐dimensional time‐to‐explosion), PODTX (ODTX with pressure‐measurement), PyGC‐MS (pyrolysis gas chromatography mass spectrometry), TGA (thermo‐gravimetric analysis), DSC (differential scanning calorimetry), and IR (infrared spectroscopy) experiments under isothermal and ramped temperature profiles. The data were used to fit rate parameters for proposed reaction schemes in a MATLAB thermo‐chemical computational model. These parameterizations were carried out utilizing a genetic algorithm optimization method on LLNL's high‐performance computing clusters, which enabled significant parallelization. These results include a multi‐step reaction decomposition model, identification of likely autocatalytic gas‐phase species, accurate high‐temperature sensitization, and prediction of confined system pressurization. This model will be scalable to several applications involving TATB‐based explosives, like LX‐17, including thermal safety models of full‐scale systems.

Crystallization Behavior of Semicrystalline Polymers Characterized by an In Situ Fluorescence Technique and its Sensing Mechanism

AbstractIn this study, fluorescence spectroscopy is used to study the bulk nonisothermal and isothermal crystallization of poly(L‐lactic acid) (PLLA) by monitoring in situ, the temperature (T)‐ and time (t)‐dependent fluorescence intensity of the fluorophores incorporated into the PLLA matrix, respectively. The crystallization behavior characterized by fluorescence resembles those measured by conventional differential scanning calorimetry, confirming the validity of this fluorescence technique for sensing crystallization. Combined in situ fluorescence results and ex situ X‐ray diffraction characterizations demonstrate that this fluorescence technique shows great sensitivity not only to the degree of crystallinity but also to the crystalline microstructures formed during crystallization (e.g., α vs α' form PLLA crystals). Moreover, complementary fluorescence microscopy helps reveal the fluorescence's intrinsic crystallization sensing mechanism: As crystals form during crystallization, the bulky fluorescent probes excluded from the crystalline regions reside in the rigid/mobile amorphous (i.e., non‐crystalline) regions of the resulting semicrystalline matrix, and their intramolecular motions are restricted by the neighboring crystalline domains; and thus, their fluorescence intensity is greatly enhanced upon crystallization. Owing to its high sensitivity, fluorescence is powerful for studying the crystallization behavior of ultrathin polymer films (e.g., 75 nm) and elucidating confinement and interfacial effects through future studies.

Nano-Scanning Calorimetry Applied to Phase Change Processes in GeTe Thin Films

The parallel nano-scanning calorimeter (PnSC) is a device for calorimetric measurement of nanoscale materials. Compared with traditional scanning calorimetry of bulk materials, PnSC has a faster heating rate and higher sensitivity when testing small masses of samples. Here, nano-scanning calorimetry is used to investigate the phase transition behavior of germanium telluride (GeTe). Amorphous GeTe is deposited on the heating element of PnSC, and the sample is analyzed in the process of heating crystallization. At a heating rate of approximately 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\text{3}}$</tex-math> </inline-formula> K/S, the amorphous deposited GeTe changes to rhombic GeTe at 185 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C and to cubic GeTe as the temperature continues to rise to 412 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> C. The phase change process of GeTe was also determined using conventional differential scanning calorimetry (DSC). The obtained phase change enthalpies and phase change temperatures were compared with those determined by nano-scanning calorimetry. The composition of the low and high-temperature phases was determined by X-ray diffraction and Raman spectroscopy.

Fast heating inhibits endothermic solid-solid polymorphic transition giving a melting of low temperature polymorph with the next cold crystallization

Heating at the rate of several hundred Kelvin per second in fast scanning calorimetry (FSC) experiments was found to change the route of polymorphic transformation. Upon the increase of the heating rate first the endothermic solid-state polymorphic transition is hindered to give a consecutive melting of low-temperature form and crystallization of high temperature polymorph. At higher heating rates, only melting of low-temperature polymorph is observed much below an equilibrium melting point. This thermal behavior with depression of final melting point by 30 K was observed for tert‑butylthiacalix[4]arene tetrasubstituted with (ethoxycarbonyl)methoxy groups (1), which does not sublimate at heating. A similar shift of the endothermic solid-state polymorphic transition by temperature at fast heating was found for caffeine (2), however the melting of low-temperature polymorph is not observed due to sublimation of the compound at elevated temperatures. The heating rates, at which a significant depression of the melting points of 1 and 2 may be expected in absence of sublimation, were calculated using equations of thermal kinetics derived from the data of conventional differential scanning calorimetry. The results give a practical guide for the preparation of glassy forms of the substances capable of endothermic solid-solid transition below melting point of high-temperature form.

Heat capacity of microgram oxide samples by fast scanning calorimetry.

Quantitative scanning calorimetry on microgram-sized samples opens a broad, new range of opportunities for studying the thermodynamic properties of quantity-limited materials, including those produced under extreme conditions or found as rare accessory minerals in nature. We calibrated the Mettler Toledo Flash DSC 2+ calorimeter to obtain quantitative heat capacities in the range 200-350 °C, using samples weighing between 2 and 11.5 μg. Our technique is applied to a new set of oxide materials to which it has never been used before, without the need for melting, glass transitions, or phase transformations. Heat capacity data were obtained for silica in the high pressure stishovite (rutile) structure, dense post-stishovite glass, standard fused quartz, and for TiO2 rutile. These heat capacities agree within 5%-15% with the literature values reported for rutile, stishovite, and fused SiO2 glass. The heat capacity of post-stishovite glass, made by heating stishovite to 1000 °C, is a newly reported value. After accurate calibrations, measured heat capacities were then used to calculate masses for samples in the microgram range, a substantial improvement over measurement in conventional microbalances, which have uncertainties approaching 50%-100% for such small samples. Since the typical uncertainty of heat capacities measured on 10-100 mg samples in conventional differential scanning calorimetry is typically 7% (1%-5% with careful work), flash differential scanning calorimetry, using samples a factor of 1000 smaller, increases the uncertainty of heat capacity measurements by a factor of <3, opening the door for meaningful measurements on ultra-small, high-pressure samples and other quantity-limited materials.

Novel approach to characterize the cross‐linking effect of liquid silicone rubber via cavity pressure analysis during injection molding

AbstractThe processing of liquid silicone rubber (LSR) in the injection molding process to molded parts offers high economic potential due to its great scalability. Unlike the well‐researched and understood injection molding process of thermoplastics, LSR still lacks basic process knowledge. The cavity pressure curve shows a completely different pattern, as the cold LSR expands strongly once placed in the hot mold due to volume dilation. During processing of LSR exothermic and irreversible cross‐linking, occurs. In this work, a method is introduced to display the cross‐linking reaction in the cavity pressure curve. Using a self‐developed testing device and conventional differential scanning calorimetry measurements, a correlation between cross‐linking and the pressure signal can be demonstrated. A typical deflection is evaluated in the pressure curve by differentiating, which is attributed to the cross‐linking. The relationships established are confirmed by pressure measurements in the mold during the processing of LSR in the injection molding process. The knowledge gained contributes to a better understanding of the process, helps to optimize existing processes and save resources. Existing cavity‐pressure‐measuring‐systems are to be supplemented by an evaluation logic and thus, in addition to cycle time optimization, process variations are to be detected, energy consumption optimized and quality ensured and verified.

Determination of Melting Parameters of Cyclodextrins Using Fast Scanning Calorimetry.

The first evidence of native cyclodextrins fusion was registered using fast scanning calorimetry (FSC) with heating rates up to 40,000 K s-1. The endothermal effects, detected at low heating rates, correspond to the decomposition processes. Upon the increase of the heating rate the onset of these effects shifts to higher temperatures, reaching a limiting value at high heating rates. The limiting temperatures were identified as the melting points of α-, β- and γ-cyclodextrins, as the decomposition processes are suppressed at high heating rates. For γ-cyclodextrin the fusion enthalpy was measured. The activation energies of thermal decomposition of cyclodextrins were determined by dependence of the observed thermal effects on heating rates from 4 K min-1 in conventional differential scanning calorimetry to 40,000 K s-1 in FSC. The lower thermal stability and activation energy of decomposition of β-cyclodextrin than for the other two cyclodextrins were found, which may be explained by preliminary phase transition and chemical reaction without mass loss. The obtained values of fusion parameters of cyclodextrins are needed in theoretical models widely used for prediction of solubility and solution rates and in preparation of cyclodextrin inclusion compounds involving heating.

Phase-formation maps of CuZrAlCo metallic glass explored by in situ ultrafast techniques

Crystallization of Cu47.5Zr48Al4Co0.5 metallic glass upon isokinetic and isothermal annealing is studied by conventional and ultrafast differential scanning calorimetry, resistive flash-annealing, and in situ high-energy X-ray diffraction. The fast-heating experiments of Cu47.5Zr48Al4Co0.5 metallic glass are complemented by in situ high-energy XRD studies of the undercooled liquid by using electromagnetic levitation. The combination of complementary techniques reveals a competition between the Cu10Zr7, B2 CuZr and τ4 Cu2ZrAl phases crystallization depending on heating rate and shows the specific role of Co alloying. A continuous-heating-transformation diagram for a heating rate exceeding six orders of magnitude, Φ = 0.0833 − 90 000 K s−1, and a time-temperature-transformation diagram are constructed.

Kinetics of enthalpy recovery studied by temperature-modulated differential scanning calorimetry

An analyzing method using periodic temperature modulation is applied to the kinetics of enthalpy recovery that occurs when an amorphous material in the glassy state devitrifies during heating. In particular, this study demonstrates that an analytical expression of the kinetic response can be formulated by assuming temperature modulation with a periodic temperature jump and isothermal holding. This analysis provides a reasonable interpretation for the well-known fact that the magnitude of the dynamic heat capacity of complex quantity obtained by temperature modulation does not exhibit a large peak corresponding to the enthalpy recovery peak. Furthermore, it is demonstrated that the phase angle peak of the dynamic heat capacity was affected by the kinetics. Its quantitative evaluation provides useful information on the relaxation time during glass transition. Numerical simulations and experimental results of atactic polystyrene examined by conventional differential scanning calorimetry and fast scanning calorimetry confirm these findings.

Amorphization and Crystallization of Hexanitroazobenzene (HNAB) Using Conventional DSC and Flash DSC

AbstractThe present work presents results from an investigation of the glass transition and crystallization behaviors of HNAB tested over more than five orders of magnitude of cooling rate from 0.005 °C/s to 600 °C/s (0.3 to 36000 °C/min) by a combination of conventional and Flash differential scanning calorimetry (DSC). The work quantifies the influence of the thermal amorphization route on the properties of this high explosive. Cooling rates faster than 100 °C/s (6000 °C/min) result in amorphous HNAB as expected from prior work, but we also find that amorphization of the HNAB occurs at cooling rates slower than 0.008 °C/s (0.5 °C/min). The behavior of the amorphous HNAB made by slow cooling is compared with that of amorphous HNAB made by fast cooling, as well as with that made by solvent casting in terms of glass transition temperature, apparent activation energy of glass transition, and dynamic fragility parameter m. Besides, the non‐isothermal crystallization response as a function of cooling rate is also reported. The thermal stability and decomposition energy of amorphous HNAB are compared with those of the crystalline counterpart, being similar heats of decomposition of 3295 and 3392 J/g, respectively; suggesting that the amorphous HNAB will have similar thermal stability and chemical energy to the crystalline form.

Merging of relaxations and step-like increase of accompanying supercooled liquid region in metallic glasses via ultrafast nanocalorimetry

Glassy materials under external stimuli usually display multiple and complex relaxations. The relaxations and the evolution paths of glassy materials significantly affect their properties and are closely related to many key issues in glass physics, such as glass transition and thermoplastic forming. However, until now, the relaxation dynamics in the presence of external stimuli and the microscopic atomic motion of glassy materials have been unclear due to the lack of structural information. By combining Flash and conventional differential scanning calorimetry (DSC), we applied a very large range of heating rate of six orders of magnitude and investigated the relaxation dynamics of three typical metallic glasses. We discovered the merging of distinct relaxation events with increasing rate of heating. Most interestingly, the experiments revealed new behaviors with step-like increases in the supercooled liquid region and excess heat capacity during the merging of multiple relaxations. A comprehensive scheme was proposed for the evolution of the thermal relaxation spectrum, the heterogeneity of the corresponding atomic motion and the potential energy landscape with rate of heating. These experimental results shed light on the mechanism of atomic rearrangement during heating and provided a new approach to regulate the physical properties of amorphous materials by controlling their intrinsic relaxation dynamics.

Differential scanning calorimetry for authentication of edible fats and oils - What can we learn from the past to face the current challenges?

Fats and oils authentication has become an important issue recently, due to the growing interest in consumption of cold-pressed oils. Therefore, it is becoming more and more difficult to maintain official control over the growing assortment of new cold-pressed oils. Authenticity of plant oils is also an important issue for religious or cultural reasons.This review article focuses on the application of differential scanning calorimetry (DSC) in the field of assessing authenticity of various fats and oils (e.g. olive oil, palm oil, confectionery fats, butter). Extra virgin olive oil (EVOO) is the most comprehensively tested oil by means of the DSC technique in terms of the authenticity of origin as well as the adulteration with foreign oils. In most of the studies on DSC applicability for authentication, crystallization and melting curves were analyzed by the conventional DSC, although other modified DSC methods were also applied, such as isothermal freezing, modulated temperature DSC (MT-DSC) and fast DSC. However, the most promising are the melting profiles, which, due to the complexity of transitions, need advanced chemometric tools as well as tools for peaks deconvolution. The future prospect of using DSC in the authenticity assessment lies also in the use of DSC techniques along with other complementary chromatographic or spectroscopic techniques.

Characterization of wax appearance temperature of model oils using laser-induced voltage

As a component of crude oils, wax plays an important part in the flowability of waxy oils. The deposition of waxes poses a significant challenge in petroleum production. This paper proposes laser-induced voltage (LIV) to measure the wax appearance temperature (WAT). With a decreased temperature, the peak of the LIV signal (VP) decreases with a greater slope until the inflection point (TLIV) after that wax precipitates from the oil. After which, VP changes more slowly. Thus, the TLIV is confirmed as the WAT. Furthermore, the WAT was measured using conventional differential scanning calorimetry, and the results of the two methods are consistent. When a laser irradiates the oil sample, plasma is generated in the liquid due to cascade ionization and multiphoton absorption. The plasma moves based on the effects of an external electric field and generates the LIV. However, temperature changes influence variations in the LIV for waxy oils. In addition, when paraffin precipitates from the oil, it exits as sediment at the bottom of a cuvette, which can impede plasma movement. Therefore, there will be significant differences in the trends before and after WAT. This study demonstrates that LIV offers a way to measure WAT in waxy oils.

Determining and modelling a complete time-temperature-transformation diagram for a Pt-based metallic glass former through combination of conventional and fast scanning calorimetry

State of the art fast differential scanning calorimetry (FDSC) is used to complement conventional differential scanning calorimetry (DSC) studies about the isothermal time-temperature-transformation (TTT) diagram of the bulk metallic glass forming liquid Pt42.5Cu27Ni9.5P21 to allow a comprehensive study of the crystallization kinetics of this system over a broad temperature range. FDSC and DSC data align well in the low-temperature region of the crystallization nose but show distinct discrepancies in the high-temperature region as the FDSC studies reveal faster crystallization times. The results are mathematically described and discussed based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. Thereby, either homogeneous or heterogeneous nucleation is assumed, depending on the respective experimental conditions in FDSC and DSC studies. With this approach, the complete TTT diagram can be modelled as superposition of two sequential JMAK fits.

Estimation of a eutectic composition of imidazolium chloride ionic liquids using video imaging and impedance methods

The eutectic temperature Te and composition Xe of binary systems composed of three ionic liquids, 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl), 1-methyl-3-propylimidazolium chloride ([C3mim]Cl), and 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), were estimated using a video imaging method, in which the formation of a liquid phase on solidified ionic liquids at a given composition stored in a vial was detected from time-lapse images taken during temperature scanning. The advantage of this method is that multiple samples (five samples in this study) could be measured simultaneously, whereas its disadvantage is a less accurate determination of the melting point compared with the conventional differential scanning calorimetry (DSC) method. The Te and Xe of [C2mim]Cl–[C3mim]Cl, [C2mim]Cl–[C4mim]Cl, and [C3mim]Cl–[C4mim]Cl binary systems were roughly estimated. Furthermore, the Te of the ternary system of the [C2mim]Cl–[C3mim]Cl–[C4mim]Cl with limited compositions was estimated, which was lower than that of the binary systems. The transition of the electrochemical impedance of [C2mim]Cl ionic liquids was also measured using two Pt wire electrodes as a function of temperature. The differential inversed resistance curve (dR–1/dT)-T shows an obvious peak corresponding to the phase transition from a low conductive solid phase to a highly conductive liquid phase. This peak was comparable with the peak obtained in the DSC measurement.