Sharp Endothermic Peak Research Articles

A study on thermal properties of β-Ca(PO 3) 2 whiskers and on devitrification mechanism of calcium phosphate glass system

β-Ca(PO 3) 2 whiskers were successfully extracted from crystallized calcium phosphate glasses by leaching with distilled water or aqueous solution of 0.1–0.5 N NaOH. β-Ca(PO 3) 2whiskers of aspect ratios (16–400) were obtained from the crystallized glasses with Ca/P molar ratio smaller than 0.4. It is expected that the β-Ca(PO 3) 2 whiskers are available for new applications to biomedical fields. In this study, the morphology and the melting point of β-Ca(PO 3) 2 whiskers that obtain from the crystallized glasses with different particle size will be investigated. β-Ca(PO 3) 2 whiskers having aspect ratios of (16–400) with diameter of 3–30 μm was found from a crystallized glass with Ca/P molar ratio of 0.25, but the morphology of β-Ca(PO 3) 2 whiskers of crystallized glass before leaching are not clear because of the existence of glass phase, so the extraction step is needed for preparation of the β-Ca(PO 3) 2 whiskers. The curves of TG and DTA for the resultant powder before extraction step gave a high mass loss percentage (approximation 14%) and melt completely at about 900°C with a broad endothermic peak in DTA curve. But the resultant whiskers that after extraction step gave only 1% mass loss in TG curve and melt completely at 980°C with a sharp endothermic peak.

Characterization of crosslinked guar by thermal analysis

Guar gum (GG) was crosslinked with increasing amounts of glutaraldehyde (GA). The resulting crosslinked products were analyzed by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and wide-angle X-ray diffraction (WAXD) to determine the influence of crosslinking on the polysaccharide structure. Each crosslinked product thermogram was characterized by two or three sharp endothermic peaks and a wide exothermic peak, with the latter indicating that a decomposition chain reaction occurred. It was observed that: (a) crosslinking altered the structure of GG; (b) the thermal stability of crosslinked GG depended on the amount of GA used with excess amount of the crosslinker reducing the stability of crosslinked GG; (c) the reaction of GG with GA occurred either by inter- or intra-crosslinking, depending on the amount of GA used; (d) crosslinking of GG with GA resulted in the formation of new regions in the polysaccharide, characterized by modified structures, whose abundance and density were crosslinker concentration dependent.

Superplastic behavior and cavitation in high-strain-rate superplastic Si3N4p /Al-Mg-Si composites

High-strain-rate superplastic behavior has been investigated for Si3N4p /Al-Mg-Si (6061) composites with a V f =20 and 30 pct, respectively, where V f is the volume fraction of reinforcements. A maximum elongation was attained at a temperature close to the onset temperature for melting for both composites. The maximum elongation for the 30 vol pct composite was larger than that for the 20 vol pct composite. Development of cavities transverse to the tensile direction is responsible for the lower maximum elongation of the 20 vol pct composite. However, development of the transverse cavities was limited to the optimum superplastic temperature for the 30 vol pct composite. The differential scanning calorimetry (DSC) investigation showed that a sharp endothermic peak appeared for the 30 vol pct composite, indicating that sufficient partial melting occurs. It is, therefore, likely that the stress concentrations are sufficiently relaxed by a liquid phase and that the development of transverse cavities is limited for the 30 vol pct composite.

Amphotropic behaviour of a series of single-chain monoaza crown ethers

A series of single-chain monoaza crown ethers was synthesized to study their amphotropic behaviour. Differential scanning calorimetry (DSC) and texture determination show that these compounds exhibit a smectic phase. Surface pressure-area isotherms indicate that all the compounds form stable monolayers at the air-water interface. Transmission electron microscopy (TEM) shows that these crown ethers form vesicles when dispersed in water. DSC of aqueous dispersions yields sharp endothermic peaks which may be attributed to the gel-to-liquid crystalline phase transition. The absorption spectra of aqueous dispersions reflect the formation of J-like aggregates in bilayer assemblies.

Kinetic studies on the thermal dehydration of 4-aminopyridinium (1,2-dithiooxalato-S,S′)metalate(II) dihydrate complexes (M = Ni, Pd and Pt)

Abstract The thermal dehydration processes of 4-aminopyridinium (1,2-dithiooxalato-S,S′)metalate(II) dihydrate (M = Ni, Pd and Pt) have been studied under inert (Ar) and oxidative (Ar + O2) atmospheres by thermogravimetric (TG,DTG and DTA) and DSC techniques and using non-isothermal conditions. The kinetic analysis of the dehydration curves has been performed applying 21 different rate law expressions for theoretical solid-state models (nucleation, growth, nucleation-growth and diffusion). In both atmospheres, the best fits correspond to random nucleation rate laws. The activation energies obtained are 89.6 kJ mol−1 for the Ni compound, 94.1 kJ mol−1 for the Pd compound and 85.1 kJ mol−1 for the Pt compound in the oxidative atmosphere. In the inert atmosphere, the activation energy values are 101.5, 101.5 and 96.1 kJ mol−1 for the Ni, Pd and Pt compounds, respectively. These values are in accord with the fact that the three compounds are isostructural. The DSC curves show sharp endothermic peaks for the dehydration processes of the three compounds in both atmospheres and the dehydration enthalpies were computed from them.

Synthesis and characterization of tough polycyclic polyamides containing 4,9‐Diamantyl moieties in the main chain

A series of new polyamides were synthesized by direct polycondensation of the 4,9-diamantane dicarboxylic acid (I) with various aromatic diamines in N-methyl-2-pyrrolidone (NMP) containing lithium chloride. The polyamides had inherent viscosities of 0.56–1.85 dL/g. Dynamic mechanical analysis revealed the polymers IIIa–IIId to have main melting transitions at 403, 431, 423, and 452°C, respectively. Moreover, these polymers were quite stable at high temperatures and maintained good mechanical properties (G′ = ca. 108 Pa) up to temperatures close to the main transition well above 400°C. Although the polyamides contained rigid 4,9-diamantyl moieties in the main chain, the tensile properties of the polyamides showed toughness. Elongations of polyamides IIIa and IIIb reached 38.3 and 31.7%, respectively, before breaking. A glass transition was not observed. However, polyamide IIIc shows a melting transition with a sharp endothermic peak at 423°C by DSC measurement. Additionally, the introduction of 4,9-diamantyl units into the polyamide backbone resulted in polyamides with high thermal stability and good mechanical properties. © 1996 John Wiley & Sons, Inc.

Synthesis and characterization of tough polycyclic polyamides containing 4,9-Diamantyl moieties in the main chain

A series of new polyamides were synthesized by direct polycondensation of the 4,9-diamantane dicarboxylic acid (I) with various aromatic diamines in N-methyl-2-pyrrolidone (NMP) containing lithium chloride. The polyamides had inherent viscosities of 0.56-1.85 dL/g. Dynamic mechanical analysis revealed the polymers IIIa-IIId to have main melting transitions at 403, 431, 423, and 452°C, respectively. Moreover, these polymers were quite stable at high temperatures and maintained good mechanical properties (G' = ca. 10 8 Pa) up to temperatures close to the main transition well above 400°C. Although the polyamides contained rigid 4,9-diamantyl moieties in the main chain, the tensile properties of the polyamides showed toughness. Elongations of polyamides IIIa and IIIb reached 38.3 and 31.7%, respectively, before breaking. A glass transition was not observed. However, polyamide IIIc shows a melting transition with a sharp endothermic peak at 423°C by DSC measurement. Additionally, the introduction of 4,9-diamantyl units into the polyamide backbone resulted in polyamides with high thermal stability and good mechanical properties.

Thermal analysis of synthesis of wulfenite

A derivatograph was used in a thermal analysis study of the synthesis of wulfenite (lead molybdate) by the sintering of cerussite or lead oxide with molybdite. The reaction products were identified microscopically and by using a Siemens crystalloflex diffractometer. The DTA curves of mixtures of cerussite with molybdite show first the characteristic peaks of cerussite. The sharp endothermic peak at 300°C reflects the dehydration of hydrocerussite associated with cerussite. The endothermic peak at 350°C indicates the first step of cerussite decomposition, into PbO·PbCO3, and that at 400°C indicates the second step of its decomposition, into lead oxide. The formation of wulfenite takes place at 520°C in an exothermic reaction. The medium endothermic peaks at 880 and 955°C reflect the melting and volatilization of unreacted lead and molybdenum oxides. The DTA curve of sintering of molybdite with lead oxide reveals the formation of wulfenite at 500°C. The melting and volatilization of unreacted lead and molybdenum oxides appear in only one large and sharp endothermic peak at 980°C. The resulting wulfenite is pale-yellow in thin section, and crystallizes in the tetragonal system, in the form of square tabular crystals, with distinct (011) cleavage.

Synthesis and Properties of New Polyamides Based on Diamantane

A series of new polyamides was synthesized by direct polycondensation of 1,6-diamantanedicarboxylic acid (I) with various aromatic diamines in N-methyl-2-pyrrolidone (NMP) containing lithium chloride. In this process, triphenyl phosphite and pyridine were used as condensing agents. High performance of direct polycondensation with triphenyl phosphite as a catalyst to I was subsequently obtained. The polyamides had inherent viscosities of 0.43-1.48 dL/g. Dynamic mechanical analysis revealed polymers IIIc and IIId to have main melting transitions at 400 and 374 °C, respectively. Moreover, these polymers were quite temperature stable and maintained good mechanical properties (G' ∼10 8 Pa) up to temperatures close to the main transition well above 350 °C. A glass transition phenomenon, in the conventional sense, was not observed. Polyamide IIIc showed a melting transition with a sharp endothermic peak appearing at 412 °C. The 5% weight loss temperature was in the range 405-420 °C in a nitrogen atmosphere. Additionally, polyamides IIIc and IIId had tensile strengths of 82.9 and 62.3 MPa, elongations to break of 13.7 and 6%, and initial moduli of 1.4 and 1.6 GPa, respectively. Interestingly, compounds I and DMF formed a single crystal bonded with a hydrogen bond. The structure of I exhibits molecular symmetry.

Thermal Behavior of Methyl p-hydroxybenzoate in Controlled-Pore Glass Solid Dispersion

Physicochemical properties of methyl p-hydroxybenzoate (MPHB) in a mixture with controlled-pore glass of 120 Å (CPG120) were investigated by differential scanning calorimetry (DSC), powder X-ray diffraction measurement, and IR spectroscopy. In the DSC thermogram of a mixture of 40% MPHB and 60% CPG120, a broad endothermic peak (108-112°C) and a sharp endothermic peak due to the fusion of MPHB were observed. In the following DSC run, the temperature of the endothermic peak (108-112°C) was shifted to a lower temperature (80-100°C). The effects of grinding and atmospheric gas pressure on the DSC thermograms were also studied. Increase of the grinding time caused a decrease in the DSC peak area of fusion, and the adsorption of MPHB into the CPG pores was enhanced with grinding. The peak at 108-112°C moved to higher temperatures in the condition of increased gas pressure. In the second DSC run, the temperature of the endothermic peak depended upon the pore size of the mixed CPG. These results indicated that MPHB in the mixture adsorbed into the pores of the CPG during the first DSC run and that the adsorbed and crystallized MPHB fused in the pores in the second run.

Thermal change of SnI 2 thin films. Part 4. TG-DTA and DSC

A thermal change of a tin iodide (SnI 2) evaporated film as deposited on a glass substrate has been successfully detected by TG-DTA and evaluated semiquantitatively by DSC in nitrogen and air (heating rate 10°C min −1); the film area was ≈0.5 and ≈ 0.25 cm 2 for TG-DTA and DSC, respectively. In nitrogen, a sharp endothermic peak at 317.9°C was interpreted as denoting the melting of SnI 2; an endothermic weight loss at 260–420°C was interpreted as the sublimation and evaporation of SnI 2. An exothermic weight loss at 180–290°C in air was interpreted as the oxidation reaction proposed in our previous papers [Thermochim. Acta, 232 (1994) 29–36; 232 (1994) 37–45; 243 (1994) 95–100], namely 2SnI 2(s) + O 2(g)→SnO 2(s) + SnI 4(g).

Enhancement of solubility and dissolution rate of poorly water-soluble naproxen by complexation with 2-hydroxypropyl-β-cyclodextrin

The solubility and dissolution rate of naproxen (NPX) complexed with 2-hydroxypropyl-β-cyclodextrin (2-HPβCD) using coprecipitation, evaporation, freeze-drying and kneading method were investigated. Solubility of NPX linearly increased (correlation coefficient, 0.995) as 2-HP-βCD concentration increased, resulting in AL type phase solubility curve. Inclusion complexes prepared by four different methods were compared by differential scanning calorimetry (DSC). The NPX showed sharp endothermic peak around 156°C but inclusion complexes by evaporation, freeze-drying and kneading method showed very broad peak without distinct phase transition temperature. In contrast, inclusion complex prepared by coprecipitation method resulted in detectable peak around 156°C which is similar to NPX, suggesting incomplete formation of inclusion complex. Dissolution rate of inclusion complexes prepared by evaporation, freeze-drying and kneading except coprecipitation method was largely enhanced in the simulated gastric and intestinal fluid when compared to NPX powder and commercial NAXEN® tablet. However, about 65% of NPX in gastric fluid still remained unreleased but most of NPX dissolved within 5 min in intestinal fluid. In case of inclusion complex prepared by coprecipitation method, formation of inclusion complex appeared to be incomplete, resulting in no marked enhancement of dissolution rate. From these findings, inclusion complexes of poorly water-soluble NPX with 2-HPβCD were useful to increase solubility and dissolution rate, resulting in enhancement of bioavailability and minimization of gastrointestinal toxicity of drug upon oral administration of inclusion complex.

Structures of wholly aromatic copolyesters composed of 4-hydroxybenzoic acid, biphenol and terephthalic acid: 1. Effects of copolymer composition on thermal properties

The structures and phase transitions of thermotropic liquid-crystalline copolymers composed of 4- hydroxybenzoic acid (HBA), terephthalic acid (TPA) and biphenol (BP) (HBA/TPA/BP copolymer) with five different copolymer compositions were investigated using X-ray diffraction and thermal analyses. The HBA/TPA/BP copolymers exhibited nematic liquid-crystalline characteristics at elevated temperatures, and complicated fine structures formed in the solid state when cooled from the anisotropic nematic melt. The crystal to smectic E (SE) type and SE type to hexagonal phase transitions inevitably occurred at approximately 100°C and 300°C, respectively, in the samples of varied copolymer compositions. Particularly, the molecular chain of almost the same monomer fraction (HBA/TPA/BP = 35/32.5/32.5) showed sharp endothermic peaks on the differential scanning calorimetry curve; thereby, the crystallite size was considered to be comparatively uniform. The SE-type structure was frozen in almost all the quenched samples. Highly ordered crystallites were formed in the samples when they were annealed above 300°C where the hexagonal interchain packing order was formed.

Effect of Dehydration on the Formation of Levofloxacin Pseudopolymorphs.

Differential scanning calorimetry (DSC) curves of levofloxacin hemihydrate measured under various conditions showed different thermograms. These phenomena were attributed to be the dehydration. Dehydration caused a multiple-phase transition. Dehydration at a higher temperature (above 70°C) gave a sharp endothermic peak on the DSC curve due to the melting of the γ form, and at a lower temperature (below 50°C) gave a sharp endothermic peak due to the melting of the α form.In contrast, the thermal behavior of levofloxacin monohydrate was not affected by dehydration. The difference in the thermal behavior between the hemihydrate and the monohydrate might be attributed to a difference in the interaction between levofloxacin and crystal water. Observations by thermomicroscopy, the changes in powder X-ray diffraction patterns during heating, and single X-ray analysis all supported the above interpretation.

In situ observation of partial melting in superplastic aluminum alloy composites at high temperatures

The possibility of partial melting and its relations to the superplasticity at high strain rates were studied with transmission electron microscopy and differential scanning calorimetry in AlCuMg (2124), AlMg (5052), and AlMgSi (6061) alloys reinforced with Si 3N 4 particles. Calorimetry measurements of all three composites showed a sharp endothermic peak at an optimum superplastic temperature. At the same temperature, transmission electron microscopy showed the melting of grain boundaries and interfaces, suggesting direct correlations between partial melting and the superplasticity. Solute segregation was also observed at boundaries and interfaces, and was discussed as causes for partial melting.

Thermodynamics of multiple stimuli-responsive polyelectrolytes with complexing ability towards the copper(II) ion

The thermodynamics of protonation of carboxylate groups in polymers containing amido and isopropyl moieties was studied in 0.1 M NaCl at different temperatures. Compared with polymers with the l-leucine moiety, poly(N-acryloyl- l-valine) showed a wider decreasing linearity of the basicity constant (log K) in relation to the degree of protonation (α), with a minimum at α = 0.65. Under these conditions, a sharp endothermic peak revealed the presence of hydrophobic interactions as the macromolecule assumed a compact coil conformation and water molecules were released with an increase in entropy. The graft polymers on the surface of a polyurethane film displayed larger potentiometric hysteresis loops with respect to the cellulose support, as the nature of the polymer was similar to that of the hydrophobic substrate. The reversible configuration of the graft polymer chains instantly responded to changes in pH and temperature, modifying the rate of water filtration through the membrane pores. As the temperature approached 31°C an abrupt increase in water permeation was observed even though the graft polymer was in the uncharged and coiled state. The polyelectrolyte formed a hydroxo-complex species, involving two monomer units (L −) of the polymer, with the Cu(II) ion. Electron paramagnetic resonance (EPR) spectra, supported by conductometric data, revealed Cu(OH) 2L 2 2− stoichiometry in solution. The potentiometric data showed a good fit when processed by the Superfit program and a decreasing pattern of log β in relation to pH.

Differential Thermal Analysis of Polyethylene Glycol and Glycerol Monostearate Suppository Bases Containing Theophylline

AbstractWater soluble polyethylene glycol 4000 (PEG4) suppositories were prepared containing 4% (w/w) theophylline. Various concentrations of polyethylene glycol 1000 (PEG1) and glycerol monostearate (GMS) were also added. Differential thermal analysis (DTA) of the PEG4 and PEG1 combination suppositories showed no melting endotherm for theophylline. But when theophylline concentration in the base was 12% (w/w) and above, sharp endothermic peak of theophylline was obtained. In contrast, when GMS was added as a base material above 50% (w/w) with PEG4, the melting endotherm of theophylline (4%, w/w) appeared at 273-274°C. The melting endotherm of the suppository bases increased up to 2 to 4°C due to storage at 4°C for six months.

Thermoanalytical Investigations of Fullerenes

Based on the results of decomposition studies by means of TG it was possible to show the differences of air oxidation of C60 and C70. Mixtures containing both fullerenes form solid solutions and cannot be distinguished by thermoanalytical methods. It was found that these mixtures adsorb oxygen before they decompose. The effect of the heating rate and atmosphere was investigated. DSC curves of pure C60-Fullerite show a sharp endothermic peak between 200 and 270 K which is due to a phase transition. The shape of the peak and the reversible character of this phase transition depends strongly on the sample preparation and purity. Vapor pressure measurements on C60 samples were carried out by the Knudsen method.

The Effect of the Particle Size on the Anomalous Peaks observed with the DSC Measurements on Henpentacontane and Dopentacontane

Abstract DSC curves for henpentacontane and dopentacontane, when crystallized from the melt, showed anomalous sharp endothermic peaks on the region of the high-temperature phases. Grounding the samples made the peaks disappear. The appearance of the peak may be ascribed to the kinetic factors, not to the first order phase transition.

Structural variations in antimony selenide

DTA graphs, X-ray diffraction patterns and electron micrographs showed that bulk Sb 2Se 3 obtained by furnace cooling or quenching the melt from 1123 K to room temperature has a crystalline nature of orthorhombic structure while the prepared thin films with any thickness showed amorphous structure. The sharp endothermic peak at 603 K showing phase transition extended as a plateau to about 623 K with the quenched material depending on the degree of crystallinity. Crystallization of thin films was attained by annealing at 423 K in air and at 448 K under vacuum. This amorphous into crystalline transformation showed itself as an additional exothermic peak at 453 K in the DTA graph. Increasing the temperature and time of annealing and also the film thickness enhanced crystallization which was found to start with the preferred orientation on the planes (020), (120) and (130). The observed structural changes were explained in view of the variations of atomic energy and internal stresses, the nature of boundary angles, the chain and layer structure of Sb 2Se 3 and the role played by surface defects during crystallization.