X‐ray studies on the double melting behavior of poly(butylene terephthalate)

The double melting behavior of poly(butylene terephthalate) (PBT) was studied with differential scanning calorimetry (DSC) and wide‐angle X‐ray analysis. DSC melting curves of melt‐crystallized PBT samples, which we prepared by cooling from the melt (250 °C) at various cooling rates, showed two endothermic peaks and an exothermic peak located between these melting peaks. The cooling rate effect on these peaks was investigated. The melt‐crystallized PBT sample cooled at 24 K min^?1 was heated at a rate of 1 K min^?1, and its diffraction patterns were obtained successively at a rate of one pattern per minute with an X‐ray measurement system equipped with a position‐sensitive proportional counter. The diffraction pattern did not change in the melting process, except for the change in its peak height. This suggests that the double melting behavior does not originate from a change in the crystal structure. The temperature dependence of the diffraction intensity was obtained from the diffraction patterns. With increasing temperature, the intensity decreased gradually in the low‐temperature region and then increased distinctly before a steep decrease due to the final melting. In other words, the temperature‐dependence curve of the diffraction intensity showed a peak that is interpreted as proof of the recrystallization in the melting process. The peak temperature was 216 °C. The temperature‐dependence curve of the enthalpy change obtained by the integration of the DSC curve almost coincided with that of the diffraction intensity. The double melting behavior in the heating process of PBT is concluded to originate from the increase of crystallinity, that is, recrystallization. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2005–2015, 2001     

Polymorphic effects at the eutectic melting in the H2O–glycine system

Solid mixtures of ice with three glycine polymorphs were heated up to the eutectic melting and the DSC has detected the eutectic temperatures of ?2.8 °C for α-, ?3.6 °C for β-, and ?2.8 °C for γ-glycine. DSC peaks of the eutectic melting are rather strange in shape, indicating unclear processes in the solutions. Accurate DSC measurements of extremely small samples can probably provide us with the physicochemical tool for the investigation of polymorphic differences among different solutions. This may be important in relation to different bioavailability of solutions prepared from different polymorphs. Eutectic temperature of ?4.7 °C in water–glycine system allows us to suggest that the unknown polymorph of glycine exists.     

Thermal and stability study of tetracene using differential scanning calorimetry

A thermal analysis study was made of tetracene using differential scanning calorimetry (DSC). The effect of different scan speeds was investigated. At scan speeds of 0.625 to 10°C min^?1 two large rounded exothermic peaks were produced. The peaks occurred at an increasingly high temperature as the scan speed increased (for example, the peaks occurred at 128 and 130°C at a scan speed of 0.625°C min^?1 and at 148 and 150°C at a scan speed of 10°C min^?1. When tetracene was heated at a scan speed of 80°C min^?1 only one large sharp exothermic peak was produced. It is believed that the two peaks obtained at scan speeds of 0.625 to 10°C min^?1 represent decomposition of the tetracene in two successive stages, while the one peak obtained at 80°C min^?1 represents an explosion. A stability test for tetracene is proposed that involves heating of the tetracene in aluminum pans from the DSC apparatus in ovens at 100, 75, and 60°C, removing the pans and samples at intervals of 30 min, 24 h, and 7 days, respectively, subjecting the samples to DSC at 1.25°C min^?1, and noting the time interval in the oven that produces a DSC curve that shows obliteration of the second peak. Two lots of tetracene made by different processes showed marked differences in stability characteristics.     

Thermal analysis of sulfurization of polyacrylonitrile with elemental sulfur

Thermal analysis of sulfurization of polyacrylonitrile (PAN) with elemental sulfur was investigated by thermogravimetry and differential thermal analysis of the mixture of polyacrylonitrile and elemental sulfur up to 600°C. Due to the volatilization of sulfur, the different heating rate (10 and 20 K min⁻¹) and different mixture proportion of polyacrylonitrile and elemental sulfur were adopted to run the analysis. The different heating rates make the DSC curves of sulfur different, but make the DSC curves of PAN similar. In the DSC curve of sulfur for the heating rate of 20 K min⁻¹ around 400°C, a small exothermic peak occurs at 400°C in the wide endothermic peak around 380∼420°C, indicative of that there is an exothermic reaction around 400°C. In the DSC curves of the mixture, the peaks around 320°C are exothermic as the content of sulfur is below 3.5:1 and endothermic as the content of sulfur is over 4:1, indicating that one of the reactions between PAN and sulfur takes place around 320°C. In the TG curves of the mixture, the mass losses begin at 220°C, and sharply drop down from 280°C. The curves for the low sulfur content obviously show two steps of mass loss, and curves for the high sulfur content show only one step of mass loss, indicative of more sulfur is benefit for the complete sulfurization of PAN. This study demonstrates that the TG/DSC analysis can give the parameter for the sulfurization, even if the starting mixture contains the volatile sulfur.     

Studies on ignition and afterburning processes of KClO4/Mg pyrotechnics heated in air

Thermal behavior of KClO₄/Mg pyrotechnic mixtures heated in air was investigated by thermal analysis. Effects of oxygen balance and heating rates on the TG?CDSC curves of mixtures were examined. Results showed that DSC curves of the mixtures had two exothermic processes when heated from room temperature to 700?°C, and TG curve exhibited a slight mass gain followed by a two-stage mass fall and then a significant mass increase. The exothermic peak at lower temperature and higher temperature corresponded to the ignition process and afterburning process, respectively. Under the heating rate of 10?°C?min^?1, the peak temperatures for ignition and afterburning process of stoichiometric KClO₄/Mg (58.8/41.2) was 543 and 615?°C, respectively. When Mg content increased to 50%, the peak ignition temperature decreased to 530?°C, but the second exothermic peak changed little. Reaction kinetics of the two exothermic processes for the stoichiometric mixture was calculated using Kissinger method. Apparent activation energies for ignition and afterburning process were 153.6 and 289.5?kJ?mol^?1, respectively. A five-step reaction pathway was proposed for the ignition process in air, and activation energies for each step were also calculated. These results should provide reference for formula design and safety storage of KClO₄/Mg-containing pyrotechnics.     

Origin of double melting peaks in drawn nylon 6 yarns

The differential scanning calorimetry (DSC) melting curves of drawn nylon 6 were studied from the standpoint of reorganization of the crystals during the heating process. A new method was presented to obtain the DSC curve associated with the growth and melting of the original crystals, and that with the recrystallization and final melting process, separately. The results obtained show that, in the case of a heating rate of 10°C/min, the original crystals in the sample start perfecting themselves at temperatures far below their initial melting temperature and melt out below 222°C, recrystallization starts at about 210°C, and the newly emerged crystals melt out at 228°C. The superposition of two such constructed DSC curves reproduces the observed DSC curve well. Therefore, the double melting peaks of the sample are considered to be the result of superposition of three processes which occur successively during heating; perfection of the original crystals, melting of the perfected crystals concurrently with recrystallization, and melting of the recrystallized crystals.     

Temperature‐modulated differential scanning calorimetry studies on the origin of double melting peaks in isothermally melt‐crystallized poly(L‐lactic acid)

In this work, the melting behaviors of nonisothermally and isothermally melt‐crystallized poly(L ‐lactic acid) (PLLA) from the melt were investigated with differential scanning calorimetry (DSC) and temperature‐modulated differential scanning calorimetry (TMDSC). The isothermal melt crystallizations of PLLA at a temperature in the range of 100–110 °C for 120 min or at 110 °C for a time in the range of 10–180 min appeared to exhibit double melting peaks in the DSC heating curves of 10 °C/min. TMDSC analysis revealed that the melting–recrystallization mechanism dominated the formation of the double melting peaks in PLLA samples following melt crystallizations at 110 °C for a shorter time (≤30 min) or at a lower temperature (100, 103, or 105 °C) for 120 min, whereas the double lamellar thickness model dominated the formation of the double melting peaks in those PLLA samples crystallized at a higher temperature (108 or 110 °C) for 120 min or at 110 °C for a longer time (≥45 min). © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 466–474, 2007     

Fast-scan vs conventional differential scanning calorimetry (DSC) techniques in detection of crystallization events of tolbutamide–polyethylene glycol composite

This study investigated the capacity of fast-scan (400 °C min^?1) against conventional (10 °C min^?1) differential scanning calorimetry (DSC) techniques to track crystallization phenomenon in tolbutamide–polyethylene glycol 3000 composites prepared by hot melt method (mass ratios 1:1, 1:5, and 1:9) and stored at 25 and 75 % relative humidities. Drug crystallization in composites was indicated by X-ray diffractometry (XRD) and scanning electron microscopy characterization over 40 days storage. With reference to XRD as gold measurement standard, fast-scan DSC could not map the crystallization events of composites (Pearson correlation: fast-scan DSC peak temperature and enthalpy versus XRD peak intensity and area, p > 0.05). Conventional DSC was able to indicate marked drug crystallization through an increase in endothermic enthalpy value of peaks at high temperature regimes between 250 and 360 °C due to formation of high melting point crystal form.     

Polymorphic transformation of indomethacin in precirol solid dispersion system

The polymorphic transformation of indomethacin (IMC) in the presence of Precirol during heating was investigated by differential scanning calorimetry (DSC), infrared (IR) spectroscopy, microscopic Fourier transform infrared (FT-IR)/DSC system, and powder X-ray diffractometry with heating. The results indicate that in the presence of Precirol the original γ form of IMC was first transformed to a transition state, and then to a new polymorph by heating or exposure to IR radiation. The transition state of the melted sample gave three endothermic peaks, at 34, 48 and 127°C, and one exothermic peak, at 54°C. The stable melted sample exhibited two endothermic peaks, at 58 and 127°C, which were due to the fusion of Precirol and the new polymorph of IMC, respectively. This new polymorph of IMC also exhibited two specific IR absorption peaks, at 1693 and 1675 cm^?1. Microscopic FT-IR/DSC was used to examine the correlation between the structural transformation and its thermal response, and demonstrated the existence of the transition state of the melted sample. X-ray diffractometry with heating confirmed the appearance of the new polymorph of IMC in the presence of Precirol after heating.     

Study of thermal stabilization for polystyrene/carbon nanocomposites via TG/DSC techniques

The degradability and durability for polymer–nanocomposites under various environmental conditions are from the essential fields of research. This study was carried out to examine the thermal stability of polystyrene loaded by carbon (C) nanoparticles up to 20 wt% content. The thermal degradation of PS/C nanocomposites were studied by thermogravimetry analysis and differential scanning calorimetry (DSC) under non-isothermal condition and inert gas atmosphere at constant heating rate 10 °C min^?1. The variation of degradation characteristic temperatures as a function of C content has been a non-monotonic behavior. The obtained results suggested that the C nanoparticles act as a promoter slowing down the degradation and providing a protective barrier to the nanocomposite, except 5 wt% C content. The latter exception was confirmed by DSC curve through the emergence of a small endothermic peak before the fundamental endothermic, melting, one.     

Calorimetric analysis of the multiple melting behavior of melt-crystallized poly(l-lactic acid) with a low optical purity

The polymorphous crystallization and multiple melting behavior of poly(l-lactic acid) (PLLA) with an optical purity of 92 % were investigated after isothermally crystallized from the melt state by wide-angle X-ray diffraction and differential scanning calorimetry. Owing to the low optical purity, it was found that the disordered (α′) and ordered (α) crystalline phases of PLLA were formed in the samples crystallized at lower (<95 °C) and higher (≥95 °C) temperatures, respectively. The melting behavior of PLLA is different in three regions of crystallization temperature (T _c) divided into Region I (T _c < 95 °C), Region II (95 °C ≤ T _c < 120 °C), and Region III (T _c ≥ 120 °C). In Region I, an exothermic peak was observed between the low-temperature and high-temperature endothermic peaks, which results from the solid–solid phase transition of α′-form crystal to α one. In Region II, the double-melting peaks can be mainly ascribed to the melting–recrystallization–remelting of less stable α crystals. In Region III, the single endotherm shows that the α crystals formed at higher temperatures are stable enough and melt directly without the recrystallization process during heating.     

Compatibility study of quetiapine fumarate with widely used sustained release excipients

The drug-excipient compatibility study of quetiapine fumarate, with widely used sustained release excipients, was carried out employing differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The selected excipients were HPMC K100M, sodium alginate, xanthan gum, Eudragit RSPO, hydrogenated castor oil, carnauba wax, and PEO WSR 303. Equal proportion of drug and excipients was utilized in the interaction study. FT-IR spectra indicated the absence of interaction between drug and excipients. The DSC curve showed a sharp endothermic melting peak at 173.26 °C for quetiapine fumarate. Post melting interaction was observed for carnauba wax, Eudragit RSPO, and hydrogenated castor oil probably due to solubilization of drug in the melted excipient. No interaction was observed for other excipients. The physical mixtures stored at 30 ± 2 °C/65 ± 5% RH did not show any significant degradation of the drug. The concept of systemically conducted preformulation studies will facilitate dossier submission to the drug control authority.     

The effect of oxygen content on the melting of the eutectic of physiological salinity solution

The effect of oxygen concentration on the melting of this eutectic was investigated by DSC. In the deoxygenized solution, an endothermic peak attributed to the eutectic transition was observed in the course of heating, and its peak temperature is around ?21.5°C. Another endothermic peak appeared at lower temperature in the presence of oxygen. As the oxygen content in the solution increases, the temperature of this peak is shifted to lower temperature. The transition at the lower temperature are associated with the melting of eutectic carrying oxygen. The same results are given in the NMR data.     

Thermal analysis of the double-melting behavior of poly(L-lactic acid)

The melting and crystallization behavior of poly(L -lactic acid) (PLLA; weight-average molecular weight = 3 × 10⁵) was studied with differential scanning calorimetry (DSC). DSC curves for PLLA samples were obtained at various cooling rates (CRs) from the melt (210 °C). The peak crystallization temperature and the exothermic heat of crystallization determined from the DSC curve decreased almost linearly with increasing log(CR). DSC melting curves for the melt-crystallized samples were obtained at various heating rates (HRs). The double-melting behavior was confirmed by the double endothermic peaks, a high-temperature peak (H) and a low-temperature peak (L), that appeared in the DSC curves at slow HRs for the samples prepared with a slow CR. Peak L increased with increasing HR, whereas peak H decreased. The peak melting temperatures of L and H [T_m(L) and T_m(H)] decreased linearly with log(HR). The appearance region of the double-melting peaks (L and H) was illustrated in a CR–HR map. Peak L decreased with increasing CR, whereas peak H increased. T_m(L) and T_m(H) decreased almost linearly with log(CR). The characteristics of the crystallization and double-melting behavior were explained by the slow rates of crystallization and recrystallization, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 25–32, 2004     

Thermal and spectral behavior of (Y,Eu)VO4 powder

Thermal analysis of Y_xEu_1?xVO₄ powder (used as “phosphor” coating for a high pressure mercury lamp) was done under a non-isothermal linear regime, both in a dynamic air regime and in a nitrogen atmosphere. The heating in air atmosphere gave on TG curve small rate of mass increase due to oxygenation and two endothermic effects are observed on DTA and DSC curves. By contrary, in nitrogen atmosphere a continuous stepped mass loss of powder (around 0.65 %), is recorded in the range of temperatures from room temperatures to 1,200 °C, and only one endothermic effect, to eliminate the gases accumulated on the crystallite surface. The powder was heated for 3 h in a Nabertherm furnace at 350, 800, and 1,100 °C using quite similar rate for heating program followed by a furnace cooling to room temperature. XRD and FTIR analyses showed the sample purification by thermal treatment and a very small increase of nanocrystallite sizes. The time evolution of the optical emission spectra in the range from 186.2 to 877.47 nm were recorded for different lamp powers in two different situations: with the outer bulb coated with Y_xEu_1?xVO₄ type “phosphor”, and without it. We observed that UV-Hg lines are absorbed by Y_xEu_1?xVO₄ type “phosphor” with different percents (100 % for 253.73 nm, 95 % for 312.65 nm, and 33 % for 365.12 nm) but the heating of the powder do not influence the UV-absorption properties of the powder.     

Morphology of homogeneous copolymers of ethylene and 1‐octene. III. Structural changes during heating as revealed by time‐resolved SAXS and WAXD

The structural changes of two linear polyethylenes, LPEs, with different molar mass and of two homogeneous copolymers of ethylene and 1‐octene with comparable comonomer content but different molar mass were monitored during heating at 10 °C per minute using synchrotron radiation SAXS. Two sets of samples, cooled at 0.1 °C per minute and quenched in liquid nitrogen, respectively, were studied. All LPEs display surface melting between room temperature and the end melting temperature, whereas complete melting, according to lamellar thickness, only occurs at the highest temperatures where DSC displays a pronounced melting peak. There is recrystallization followed by isothermal lamellar thickening if annealing steps are inserted. The lamellar crystals of slowly cooled homogeneous copolymers melt in the reverse order of their formation, that is, crystals melt according to their thickness. Quenching creates unstable crystals through the cocrystallization of ethylene sequences with different length. These crystals repeatedly melt and co‐recrystallize during heating. The exothermic heat due to recrystallization partially compensates the endothermic heat due to melting resulting in a narrow overall DSC melting peak with its maximum at a higher temperature than the melting peak of slowly cooled copolymers. With increasing temperature, the crystallinity of quenched copolymers overtakes the one of slowly cooled samples due to co‐recrystallization by which an overcrowding of leaving chains at the crystal surfaces is avoided. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1975–1991, 2000     

Differential thermal analysis of low-rank coals

Differential thermal analysis (DTA) of low-rank coals of high lignite to subbituminous rank from coal mines of Pakistan is reported. The studies carried out in dynamic oxygen atmosphere indicate that the exothermic reactions occur between 300 and 650°C and that the samples undergo stepwise oxidation of the organic matter rather than a continuous process as indicated by the pattern of shoulders from 250 to 350°C accompanying the main peak around 450°C. The effect of heating rate, particle size and volatile content was also studied in relation to oxidation. The results show that the increase in heating rate from 10 to 80 deg min⁻¹ results in a marked shift in all the events in the DTA curve towards higher temperatures. As for the effect of particle size, the DTA records of 100–75, 150–100, 250–150 μm and greater than 250 μm fractions show that the magnitude and position of shoulder peaks are more sensitive to changes in particle sizes compared to the main peak. The curves recorded to study the effect of changing volatile content of samples between 30–40% indicate a complex pattern of shoulders accompanying the main peak. In general, the number of shoulder peaks increases with increasing volatile content of samples but their positions do not follow any trend. The DTA curves recorded in nitrogen contain ill-de-fined oxothermic effects over the 300–750°C temperature range. These curves consist of an endothermic peak around 150°C, two exothermic shoulders in the temperature region 300–400°C and a large broad exothermic whip between 500 and 700°C. The heating rates have similar effects as in oxygen while the particle size do not influence the results. It has been concluded that the organic matter in the coals studied here is extremely heterogeneous with different burning characteristics; as a result it is very difficult to quantify energy changes associated with poorly resolved exothermic events along the DTA curve. The effects also dominate in N₂ atmosphere thus making identification of mineral matter difficult. The overall pattern of DTA events in oxygen can be correlated with the heating rate, particle size and volatile content of samples.     

Thermal study of clay ceramic pastes containing sugarcane bagasse ash waste

This study focuses on the thermal and mineralogical transformations of clay ceramic pastes. The pastes contain different amounts of sugarcane bagasse ash waste. Thermal and mineralogical changes occurring during firing were characterized by differential thermal analysis, thermogravimetry analysis (TG), X-ray diffraction (XRD), and scanning electron microscopy. On heating three endothermic events within the 73.5–75.7, 276.9–283.5, and 567.1–573.5 °C temperature ranges were identified. The endothermic valleys could be mainly interpreted as the release of physically adsorbed water, dehydration of hydroxides, and dehydroxylation of kaolinite, respectively. Two exothermic events within the 618.9–690.1 and 948 °C temperature ranges were identified. The exothermic peaks are associated with the decomposition of organic compounds and crystallization of mullite from metakaolinite, respectively. TG results indicate that the clay ceramic pastes had a total mass loss in the 13.1–13.6 % range, and are dependent on the sugarcane bagasse ash waste amount added. It was found that the replacement of natural clay with sugarcane bagasse ash waste, in the range up to 20 wt%, influenced the thermal behavior and technological properties of the clay ceramic pastes. In addition, the thermal analysis results agree well with the XRD.     

Thermal behaviour of the system Fe(NO3)3?·?9H2O–Bi5O(OH)9(NO3)4?·?9H2O–glycine/urea and of their generated oxides (BiFeO3)

Activated carbons (AC), particularly those containing sulphur, are effective adsorbents for mercury (Hg) vapour at elevated temperatures. Activated carbon-based technologies are expected to become a major part of the strategy for controlling mercury emission from coal-fired power plants. Understanding the mechanism of mercury adsorption on sulphur impregnated activated carbons (SIAC) is essential to optimizing activated carbons for better mercury removal efficiency and to developing technologies for the handling of the spent AC. In this work thermal analysis before and after mercury uptake was carried out for the SIAC prepared under various conditions from oil-sand petroleum coke using a simultaneous differential thermal analyzer. Samples were heated at 20°C min⁻¹ under nitrogen in the temperature range from ambient to 1000°C. The DSC curves suggest both endothermic and exothermic changes during heating. The endothermic processes were attributed to evaporation of moisture and other volatile components. The exothermic processes existed in a wide temperature range of 150–850°C likely due to the oxidation reactions between carbon and adsorbed oxygen, oxygen-containing surface groups. The enthalpies of liquid mercury interaction with SIAC at different Hg/AC mass ratio were also measured at 30, 40 and 50°C using a differential scanning calorimeter. The combination of thermal analysis and calorimetry techniques enabled confirmation that the interaction of mercury with SIAC involves both physical and chemical processes.     

Effect of the Degree of Oil Biodegradation on the Crystallization of Methane Hydrate and Ice in Water-Oil Emulsions

To study the influence exerted by oxidized oil components on the nucleation and growth of gas hydrates the nucleation of methane hydrate and ice in 50 wt % emulsions of oil in native oil and two samples of the same oil subjected to biodegradation for 30 and 60 days (samples N, V30, and V60, respectively) were examined. In the course of measurements, the samples were cooled to–15°C at a constant rate of 0.14 deg min^–1 and then heated to the initial temperature. The initial methane pressure in the system was 15 MPa at 20°C. In the process, the temperatures were recorded at which heat effects corresponding to the formation of hydrate/ice and the melting of these. In the case of emulsion N, no exothermic effects were manifested in the cooling stage. In the heating stage, the endothermic effects of ice melting were found in half of the samples. No effects corresponding to the decomposition of the hydrate were observed. In experiment with V30 samples, the formation of the hydrate and ice was manifested as strong exothermic effects. Ice was formed in all the experiments, and the hydrate, only in 21% of the samples. Finally, in experiments with V60, ice and the hydrate were formed in 54 and 13% of cases, respectively. Their formation was manifested as weak exothermic effects in the cooling stage. Thus, it was demonstrated that the biodegradation level of oil samples affects the nucleation of methane hydrate and ice in emulsions formed on the basis of these samples.