Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Summary High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na⁺. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.     

A thermogravimetric study of the alunites of sodium, potassium and ammonium

Thermogravimetry in tandem with mass spectrometry has been used to characterise the thermal decomposition of synthetic alunites of potassium, sodium and ammonium. Three mechanisms of decomposition are observed (a) dehydration, (b) dehydroxylation and (c) desulphation. The thermal decomposition of the three alunites is different. For NH₄-alunite, an additional process of de-ammoniation is observed which occurs simultaneously with dehydration. Dehydroxylation takes place in a series of four steps. De-sulphation occurs for K-alunite at 680 °C in a single step in comparison with Na and NH₄ alunites where de-sulphation is observed in a series of four steps. The temperature of desulphation is cation dependent. The thermal decomposition is not completed until around 800 °C.     

Thermal treatment of whewellite—a thermal analysis and Raman spectroscopic study

Thermal transformations of natural calcium oxalate monohydrate known in mineralogy as whewellite have been undertaken using a combination of thermal analysis and Raman microscopy with the use of a thermal stage. High resolution thermogravimetry shows that three mass loss steps occur at 162, 479 and 684 °C.Evolved gas mass spectrometry shows that water is evolved in the first step and carbon dioxide in the second and third mass loss steps. The changes in the molecular structure of whewellite can be followed by the use of the in situ Raman spectroscopy of whewellite at the elevated temperatures. The whewellite is stable up to around 161 °C, above which temperature the anhydrous calcium oxalate is formed. At 479 °C, the oxalate transforms to calcium carbonate with loss of carbon dioxide. Above 684 °C, calcium oxide is formed.     

Thermal decomposition of natural and synthetic plumbojarosites: Importance in ‘archeochemistry’

Plumbojarosite and argentoplumbojarosite were sources of lead and silver in ancient and medieval times. The understanding of the chemistry of the thermal decomposition of these minerals is of vital importance in ‘archeochemistry’. The thermal decomposition of plumbojarosite was studied using a combination of thermogravimetric analysis coupled to a mass spectrometer. Three mass loss steps are observed at 376, 420 and 502 °C. These are attributed to dehydroxylation, loss of sulphate occurs at 599 °C, and loss of oxygen and formation of lead occurs at 844 and 953 °C. The temperatures of the thermal decomposition of the natural jarosite were found to be less than that for the synthetic jarosite. This is attributed to a depression of freezing point effect induced by impurities in the natural jarosite. Raman spectroscopy was used to study the structure of plumbojarosite. Plumbojarosites are characterised by stretching bands at 1176, 1108, 1019 and 1003 cm⁻¹ and bending modes at 623 and 582 cm⁻¹. Changes in the molecular structure during thermal decomposition were followed by infrared emission spectroscopy. The technique shows the loss of intensity in the hydroxyl stretching region attributed to dehydroxylation. Loss of sulphate only occurs after dehydroxylation. Lead is formed at higher temperatures through oxygen evolution.     

Thermal decomposition of syngenite, K2Ca(SO4)2·H2O

The thermal decomposition of syngenite, K₂Ca(SO₄)₂·H₂O, formed during the treatment of liquid manure has been studied by thermal gravimetric analysis, differential scanning calorimetry, high temperature X-ray diffraction (XRD) and infrared emission spectroscopy (IES). Gypsum was found as a minor impurity resulting in a minor weight loss due to dehydration around 100 °C. The main endothermic dehydration and decomposition stage of syngenite to crystalline K₂Ca₂(SO₄)₃ and amorphous K₂SO₄ is observed around 200 °C. The reaction involves a solid-state re-crystallisation, while water and the K₂SO₄ diffuse out of the existing lattice. The additional weight loss steps around 250 and 350 °C are probably due to presence of larger syngenite particles, which exhibit slower decomposition due to the slower diffusion of water and K₂SO₄ out of the crystal lattice. A minor endothermic sulphate loss around 450 °C is not due to the decomposition of syngenite or its products or of the gypsum impurity. The origin of this sulphate is not clear.     

Synthesis and thermal decomposition of GAP-Poly(BAMO) copolymer

An energetic copolymer of glycidyl azide polymer (GAP) and poly(bis(azidomethyl)oxetane (Poly(BAMO)) was synthesized using the Borontrifluoride-dimethyl ether complex/diol initiator system. The synthesized copolymer exhibited the characteristics of an energetic thermoplastic elastomer (ETPE). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the thermal decomposition behavior and the results were compared with that of the constituent homopolymers. The main weight loss step in all the polymers coincides with the exothermic dissociation of the azido groups in the side chain. In contrast with the behavior of the homopolymers, the copolymer shows a broad exothermic shoulder peak at 298 °C after the main exothermic decomposition peak at 228 °C. Kinetic analysis was performed by Vyazovkin's model-free method, which suggests that the activation energy of the main decomposition step is around 145 kJ/mol and for the second shoulder it is around 220 kJ/mol. Fourier transform infra red (FTIR) spectra of the degradation residues show that the azido groups in the copolymer decompose in two stages at different temperatures which is responsible for the double decomposition behavior.     

Synthesis and characterization of novel polyamides from new aromatic phosphonate diamine monomer

New aliphatic-aromatic and fully aromatic phosphonate polyamides were prepared by polycondensation reaction of our synthesized aromatic diamine: tetraethyl[(2,5-diamino-3,6-dimethylbenzene-1,4-diyl)dimethanediyl]bis(phosphonate) with the specific di-acylchloride (adipoyl chloride, isophthaloyl chloride and terephthaloyl chloride). The chemical structure of all samples were characterized by (¹H and ³¹P) NMR, MALDI-TOF MS, FT-IR tools, whereas their thermal properties were determined by DSC and TGA techniques. The phosponate polyadipamide (referred as PAP) is a semi-crystalline sample with a melting temperature at about 261 °C and glass transition (T_g) of 71 °C. All polymers show two thermal degradation steps in the temperature range 270-550 °C. Each polymer, independently its structure, shows the first maximum rate of thermal decomposition temperature (PDT) around 300-310 °C, which may be due to thermal degradation of phoshonate groups. MALDI-TOF spectra, beside the linear oligomers terminated with the specific groups expected in accord to the synthesis procedure, reveals the presence of cyclic oligomers in the polyadipamide and polyisophthalamide samples.     

Study of the decomposition of aqueous citratoperoxo-Ti(IV)-gel precursors for titania by means of TGA-MS and FTIR

An aqueous solution-gel route is developed for the preparation of TiO₂. In this report, we study an aqueous citratoperoxo-Ti(IV)-precursor at pH 2.0, which is compatible with polyvinyl alcohol (PVA) and therefore can be applied for the preparation of a thick mesoporous TiO₂ film.With regard to deposition of films, it is important to gain insight in the behaviour of the precursor during thermal treatment. Therefore, the thermal decomposition mechanism of a citratoperoxo-Ti(IV)-gel and a PVA modified citratoperoxo-Ti(IV)-gel is studied. Weight losses and evolved gasses are characterized by TGA-MS (5 °C min⁻¹), while gel structure and changes in the solid upon heating are studied by means of FTIR. For both gels, decomposition in dry air can be divided into five regions. After drying of the samples in the first region (∼100 °C), decomposition of the organic matter not coordinated to the metal ions occurs (∼200 °C). The third region (∼310 °C) involves the decomposition of citrato ligands. Finally, the residual organic matter is combusted in the last two regions. Only in dry air it is possible to fully remove the organic matrix in both gels at temperatures below 600 °C.It is also proven that the citratoperoxo-Ti(IV)-complexes, seen at pH 7.0, exist in the precursor gel already at pH 2.0.     

Thermal behavior of aluminum powder and potassium perchlorate mixtures by DTA and TG

In this work the thermal decomposition characteristics of micron sized aluminum powder + potassium perchlorate pyrotechnic systems were studied with thermal analytical techniques. The results show that the reactivity of aluminum powder in air increases as the particle size decreases. Pure aluminum with 5 μm particle size has a fusion temperature about 647 °C, but this temperature for 18 μm powder is 660 °C. Pure potassium perchlorate has an endothermic peak at 300 °C corresponding to a rhombic-cubic transition, a fusion temperature around 590 °C and decomposes at 592 °C. DTA curves for Al₅/KClO₄ (30:70) mixture show a maximum peak temperature for thermal decomposition at 400 °C. Increasing the particle size of aluminum powder increases the ignition temperature of the mixture. The oxidation temperature increased by enhance in the aluminum content of the mixture.     

A nearly pure monoclinic nanocrystalline zirconia

Generally, monoclinic zirconia is considered to be much more difficult to prepare at low temperatures and particularly in a pure state. The present work is the first example that shows that the hydrous zirconia formed by precipitation can yield a nearly pure nanocrystalline monoclinic zirconia at a temperature as low as 320 °C. The crystallite size of the monoclinic zirconia produced in the present work is around 15 nm, and it does not change appreciably as calcination temperature is increased from 320 to or above 400 °C. Such a small monoclinic crystallite arises from some of the chemical and physical factors built into the solution-gelation-xerogel process such as acidic preparation-pH, rapid precipitation, and moderate aging time and drying temperature, which result in a structure different from those of the existing zirconium hydroxides. In addition, the hydrous zirconia exhibits a unique thermal behavior in two respects: first, a sudden weight drop in the region of exothermic peak of the thermogravimetric curve is seen, suggesting that the main decomposition of the hydrous zirconia occurs in this region; second, there is an endothermic peak at high temperature in the differential thermal analysis curve, indicating the presence of coordinated water in the hydrous zirconia.     

Preparation, thermal decomposition and lifetime of Eu(III)-phenanthroline complex doped xerogel

The main purpose of this work is proposing a new method of using non-isothermal formal kinetics analysis to predict the lifetime of luminescent complex materials. The Eu(III)-phenanthroline complex doped xerogel has been in situ synthesized by a catalyst-free sol-gel method. The photoluminescence spectra and TG curves of the xerogel verify the formation and decomposition of Eu(III)-phenanthroline complex in xerogel. The decomposition of the xerogel formally occurs in three steps. The Friedman and FWO isoconversional methods and multivariate non-linear regression method are used for formal kinetic analysis. The overall decomposition process below 800 °C is fitted by three-step consecutive reaction. The best fitted model for each step is F_n (n order reaction, the corresponding function f(α) is (1 − α)ⁿ). Correlation coefficient is 0.99956. The lifetime values of xerogel at different temperatures are predicted based on non-isothermal kinetic models by the 5% decomposition of europium organic complex.     

Thermal stability of solid and aqueous solutions of humic acid

The effects of temperature on the stability of a soil humic acid were studied in the present work. Solid samples of Gohy-573 humic acid (HA) and dissolved ones in aqueous solution (pH 6.0, 0.1 mol L⁻¹ NaClO₄) were investigated in order to understand the impact of temperature on the chemical properties of the material. The methods applied to solid samples in the present investigation were thermogravimetric analysis (TGA), temperature-programmed desorption coupled with mass spectrometry (TPD-MS), and in situ diffuse reflectance infrared Fourier transformed spectroscopy (in situ DRIFTS). Humic acid samples were studied in the 25-800 °C range, with focus on thermal/chemical processes up to 250 °C. The reversibility of the changes observed was investigated by cyclic changes to specified temperature ranges (40-110 °C). All measurements were conducted under inert-gas atmosphere in order to avoid samples combustion at increased temperatures. Aqueous solutions were analyzed by UV-vis absorption spectroscopy after storage at temperatures up to 95 °C, and storage times up to 1 week. For temperatures below 100 °C experiments on solid and aqueous samples have shown results which were consistent to each other. The amount of water desorbed is temperature dependent and up to 70 °C this process was totally reversible. Above 70 °C an irreversible loss of water was also observed, which according to UV-vis spectroscopy corresponds to water produced by condensation leading to more condensed polyaromatic structures. The water released up to 110 °C was about 7 wt% of the total mass of the dried humic acid, where less than 50% corresponded to reversibly adsorbed water. At higher temperatures (>110 °C), gradual decomposition resulting in the formation of carbon dioxide (110-240 °C), and carbon monoxide (140-240 °C) takes place. Hence, thermal treatment of Gohy-573 humic acid above 70 °C results in irreversible structural changes, that could affect chemical properties (e.g., complex formation) of the material.     

Synthesis and characterization of soluble polyimides derived from a novel unsymmetrical diamine monomer: 1,4-(2′,4″-diaminodiphenoxy)benzene

A new aromatic unsymmetrical diamine monomer, 1,4-(2′,4″-diaminodiphenoxy)benzene (OAPB), was successfully synthesized in three steps using hydroquinone as starting material and polymerized with various aromatic tetracarboxylic acid dianhydrides, including 4,4′-oxydiphthalic anhydride (ODPA), 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 2,2′-bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride (6FDA) and pyromellitic dianhydride (PMDA) via the conventional two-step thermal or chemical imidization method to produce a series of the unsymmetrical aromatic polyimides. The polyimides were characterized by solubility tests, viscosity measurements, IR, ¹H NMR, and ¹³C NMR spectroscopy, X-ray diffraction studies, and thermogravimetric analysis. The polyimides obtained had inherent viscosities ranged of 0.38-0.58 dL/g, and were easily dissolved in common organic solvents. The resulting strong and flexible PI films exhibited excellent thermal stability with the decomposition temperature (at 5% weight loss) of above 505 °C and the glass transition temperature in the range of 230-299 °C. Moreover, the polymer films showed outstanding mechanical properties with the tensile strengths of 41.4-108.5 MPa, elongation at breaks of 5-9% and initial moduli of 1.15-1.68 GPa.     

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)

The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, ¹H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44 °C for heating rate 20 °C/min and glass transition temperature at −36 °C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404 °C (heating rate 10 °C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E = 157 kJ/mol while the second mechanism is a first-order reaction with E = 221 kJ/mol, as calculated by the fitting of experimental measurements.     

Microwave-assisted digestion of organoarsenic compounds for the determination of total arsenic in aqueous, biological, and sediment samples using flow injection hydride generation electrothermal atomic absorption spectrometry

A microwave assisted wet digestion method for organoarsenic compounds and subsequent determination of total arsenic in aqueous, biological and sediment samples by means of flow injection hydride generation electrothermal atomic absorption spectrometry (FI-HG-ETAAS) is described. Sodium persulfate, sodium fluoride and nitric acid serve as digestion reagents, which allow a quantitative transformation of organoarsenic compounds to hydride forming species in a commercial microwave sample preparation system. The maximum operating pressures of the applied tetrafluorometoxil (TFM) liners are 75 bar (high pressure vessels) and 30 bar (medium pressure vessels), corresponding to maximum solution temperatures of 300 and 260 °C. For the investigated samples, digestion temperatures of 210-230 °C (medium pressure vessels) and 240-280 °C (high pressure vessels) were obtained.In medium pressure vessels, arsenic recovery from aqueous testing solutions of dimethylarsinic acid (DMA), phenylarsonic acid (PAA) and tetraphenylarsonium chloride (TPA) at initial concentrations of 100 and 10 μg l⁻¹ is complete, even in the presence of an excess of organic carbon (potassium hydrogen phthalate, 2000 mg l⁻¹) or fatty acids (linolenic acid 70%; linoleic acid ≈20-25%; Oleic acid ≈3%, 900-4500 mg l⁻¹).Arsenic recovery from aqueous arsenobetaine (ASB) solutions with the same initial concentrations is also complete if high pressure vessels and a higher concentration of fluoride ions are used, whereas the addition of organic carbon (potassium hydrogen phthalate, 2000 mg l⁻¹, fatty acids, 900-4500 mg l⁻¹) leads to a decrease in arsenic recovery of about 2-5%. In all cases, residual carbon contents are close to the limit of detection for the applied analytical method (15 mg l⁻¹).Results of arsenic analysis in reference standard materials revealed a significant dependence on the material’s nature (sediment samples, plant materials and seafood samples). Sediment samples and plant materials show recoveries for arsenic around 100% after a single-step digestion in medium pressure TFM liners. Seafood (fish/lobster/mussel samples) usually require either the use of high pressure vessels or a second digestion step, if medium pressure vessels are used.     

Thermal decomposition of potassium hexanitronickelate(II) hydrate

The thermal decomposition of the complex K₄[Ni(NO₂)₆]·H₂O has been investigated over the temperature range 25-600 °C by a combination of infrared spectroscopy, powder X-ray diffraction, FAB-mass spectrometry and elemental analysis. The first stage of reaction is loss of water and isomerisation of one of the coordinated nitro groups to form the complex K₄[Ni(NO₂)₄(ONO)]·NO₂. At temperatures around 200 °C the remaining nitro groups within the complex isomerise to the chelating nitrite form and this process acts as a precursor to the loss of NO₂ gas at temperatures above 270 °C. The product, which is stable up to 600 °C, is the complex K₄[Ni(ONO)₄]·NO₂, where the nickel atom is formally in the +1 oxidation state.     

Degradation mechanism of diethylene glycol units in a terephthalate polymer

Diethylene glycol (DEG) is incorporated into poly(ethylene terephthalate) (PET) during industrial synthesis in order to control crystallisation kinetics. DEG is known to be a weak point in the thermal degradation of PET, which is problematic during the recycling of the polymer.Studies on the thermal decomposition of the model polymer poly(diethylene glycol terephthalate) (PDEGT) have been performed using TG, DSC, TVA and spectroscopic techniques. They revealed a degradation behaviour with two distinct steps, where the first step initiates some 100 K below the degradation temperature of PET. The second step is similar to the behaviour of PET.Based on our observations, a new degradation mechanism specific to DEG units is proposed, where random ether groups along the backbone can back-bite and form cyclic oligomers. These cyclic species, containing ether moieties, are evolved at 245 °C and constitute the first of the two steps of degradation observed for PDEGT.     

Effect of surfactant agent upon the structure of montmorillonite

The modification of sodium montmorillonite (MMT) through the incorporation of amphiphilic octadecylammonium cations in various concentrations (10–200% CEC) into the clay’s interlayer spaces has been studied. High resolution thermogravimetric analysis shows that the thermal decomposition of modified montmorillonite occurs in three steps. The first step of mass loss is related to dehydration of adsorbed water and water hydrating metal cations such as Na⁺. The second step of mass loss is attributed to the decomposition of surfactant. The third step is due to the loss of OH units during the dehydroxylation of the montmorillonite. The conformation of the surfactant cations in the confined space of the silicate galleries is investigated by X-ray diffraction analysis. These analyses are very important for any attempt to incorporate the organomodified MMT particles into different media for various applications such as polymer nanocomposite preparation.     

New hydrazinium salts of benzene tricarboxylic and tetracarboxylic acids—preparation and their thermal studies

Some new hydrazinium salts of benzene tricarboxylic and tetracarboxylic acids have been prepared by neutralisation of these acids with hydrazine hydrate in aqueous medium and characterised by conductance measurement, IR spectral and thermal analyses. Hemimellitic acid (H₃hml) forms monohydrazinium salt, trimellitic acid (H₃tml) and trimesic acid (H₃tms), mono and dihydrazinium salts, and pyromellitic acid (H₄pml) all the four salts with hydrazine hydrate. Conductance study indicates their electrolytic nature. IR spectra of all the salts show NN stretching frequencies of the N₂H₅⁺ in the region of 960-990 cm⁻¹. The hemimellitate salt undergoes endothermic dehydrazination at 154 °C, trimellitates and pyromellitates in the range of 191-271 °C, and trimesates in the range of 267-332 °C. Trimesates decompose to give CO₂ around 337 °C. All the salts then undergo strong exothermic decomposition in the range of 517-595 °C via the formation of respective acid intermediates first, then arenes, yielding carbon residue. A comparison of the thermal behaviour of pure acids with that of their salts reveals the fact that the acids do not withstand high temperature like salts. They show sharp endotherms at their melting points and then they decompose exothermally before 400 °C to give carbon residue.     

Determination of hydroxyls density in the silica-mesostructured cellular foams by thermogravimetry

Thermogravimetry was applied to determine the surface hydroxyls coverage in the mesostructured cellular foams (MCFs) calcined at different temperatures and then rehydroxylated by contacting with water vapor or liquid. The TG measurements were performed by heating MCFs in air stream using a three-step temperature program: (i) at rate of 5 °C min⁻¹ from 25 to 200 °C; (ii) held at 200 °C for 30 min; and (iii) heating at rate of 10 °C min⁻¹ up to 1100 °C. The hydroxyls content was calculated from weight loss during third step. The hydroxyls density appeared to depend strongly on the calcination temperature and the subsequent contact with water vapor. When MCFs were exposed for a short period (ca. 1 min) to moist air the hydroxyls content increased rapidly, more in the samples calcined at 300 °C than 500 °C, to attain surface densities of 4.75 and 1.6 OH nm⁻², respectively. The 2-h contact with water vapor resulted in slower further increase of hydroxyls densities, to values of 5.45 and 2.9 OH nm⁻², for samples calcined at 300 and 500 °C, but longer contacts had no significant effect. A similar trend was also observed when sample was treated with liquid water.