Thermophysics of a crack propagating in poly(methyl methacrylate)

Temperature kinetics along the crack propagation path in the bulk of PMMA at 20°C and under impact loading is explained. The initial experimental data are taken from [1, 2]. The kinetic temperature-time curve includes six different regions. In this study, the sequence of regions in the temperature kinetics is interpreted as a sequence of stages corresponding to the extension and breakdown of polymer chains in a microregion with dimensions of ≈130 μm located on the trajectory of crack propagation. At the first stage with a duration of ≈8 μs, the temperature increases by ≈10°C. This temperature rise that is related to the arrival of a loading wave front. At the second stage with a duration of ≈5 μs, temperature decreases by ≈8°C. This process is controlled by the thermal elasticity of polymer chains. At the third stage, temperature increases up to ≈90°C owing to the plastic flow of the material. At this stage, ahead of a crack, a craze is formed. Rearrangement of the isotropic polymer structure into an anisotropic polymer structure proceeds within ≈7 μs. At the fourth stage, temperature strongly decreases (nearly down to −150°C). This temperature reduction is provided by thermoelastic deformation of valence angles and chemical C-C bonds of the polymer backbone. At the fifth stage, the material heats up owing to the dissipation of the stored elastic energy due to the breakdown of extended polymer chains. This stage of the temperature rise on polymer chains spans over ≈18 μs. This phenomenon indicates the marked spatial continuity (diffuse character) of a crack tip and is related to a broad length distribution of ruptured chains. During the sixth stage, temperature remains at a constant level of ≈100°C for more than 100 μs. This stage reflects the emergence of exothermic processes on the newly formed fractured surface.     

Synthesis and thermal behaviour of silicon containing poly(ester imide)s

A series of silicon containing poly(ester imide)s [PEIs] were synthesized using novel vinyl silane diester anhydride (VSEA) and various aromatic and aliphatic dimines by two-step process includes ring-opening polyaddition reaction to form poly(amic acid) and thermal cyclo-dehydration process to obtain poly(ester imide)s. VSEA was synthesized by using dichloro methylvinylsilane and trimellitic anhydride in the presence of K₂CO₃ by nucleophilic substitution reaction. The PEIs were characterized by FTIR spectroscopy. The thermal properties of PEIs were investigated by using differential scanning calorimetry (DSC) and thermogravimetric analysis (TG) methods. The prepared PEIs showed glass transition temperatures in the range of 320–350°C and their 5% mass loss was recorded in the temperature range of 500–520°C in nitrogen atmosphere. These had char yield in the range of 45–55% at 800°C.     

Controlled rate thermal analysis of sepiolite

Controlled rate thermal analysis (CRTA) technology offers better resolution and a more detailed interpretation of the decomposition processes of a clay mineral such as sepiolite via approaching equilibrium conditions of decomposition through the elimination of the slow transfer of heat to the sample as a controlling parameter on the process of decomposition. Constant-rate decomposition processes of non-isothermal nature reveal changes in the sepiolite as the sepiolite is converted to an anhydride. In the dynamic experiment two dehydration steps are observed over the ~20–170 and 170–350 °C temperature range. In the dynamic experiment three dehydroxylation steps are observed over the temperature ranges 201–337, 337–638 and 638–982 °C. The CRTA technology enables the separation of the thermal decomposition steps.     

Polymorphism and stability of norfloxacin, (1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7-(1-piperazinil)-3-quinolinocarboxylic acid

Norfloxacin was studied by thermal methods (TG and DSC), X-ray powder diffraction, and by FT-IR, UV-VIS and NMR spectroscopy. The drug substance can be prepared in two different crystalline forms and in amorphous state, depending on the experimental conditions of preparation. DSC examinations were carried out at various heating rates and by cycling the samples in the temperature range 50°–250°C. The unstable crystalline form undergoes two irreversible solid-solid phase transitions at 176.5° and 197.6°C. The polymorph melts in the temperature range 218.5°–220.0°C.

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Zusammenfassung Norfloxacin wurde mittels thermischer Methoden (TG und DSC), weiterhin mittels der Debye-Scherrer-Methode und FTIR-, UV-VIS-und NMR-Spektroskopie untersucht. Je nach den experimentellen Bedingungen bei der Herstellung kann die Wirkstoffsubstanz in zwei verschiedenen kristallinen und in einer amorphen Form hergestellt werden. Die DSC-Untersuchungen wurden bei zahlreichen Aufheizgeschwindigkeiten und durch abwechselnden Temperaturwechsel zwischen Raum- und Schmelztemperatur durchgeführt. Die unstabile kristalline Form unterliegt zwei irreversiblen Feststoff-Feststoff-Umwandlungen bei 176.5° und bei 195.6°C. Das polymorphe Material schmilzt im Temperaturbereich 218.5°–220.0°C.

     

Influence of Doping of NaNO3 on the Solid Phase Thermal Decomposition of Bitumen and Cement

The thermal stability of solidified NaNO₃ salts in bitumen and cement has been investigated for safety considerations in the field of solidification of radioactive waste. The thermal decomposition of bitumen and cement in presence of NaNO₃ in a temperature range 22–650°C has been studied. The fraction decomposed of the pure samples and mixtures showed slow linear reactions followed by acceleratory and decay stages. Data are analyzed according to both Freeman-Carroll and Coats-Redfern kinetics to evaluate the activation energy and the order of reactions of all mixtures. It is found that the activation energies of bitumen and cement were 594 and 203 kJ mol^-1, respectively. The order of reactions of bitumen and cement was 2 and 4, respectively. The addition of NaNO₃ shortens the duration of the induction period in all mixtures. It was concluded that solidification of radioactive waste containing NaNO3 in bitumen and cement should be applied in the temperature range 22–300°C. At temperature higher than300°C solidification should be in cement. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal stability of the ‘cave’ mineral ardealite Ca<Subscript>2</Subscript>(HPO<Subscript>4</Subscript>)(SO<Subscript>4</Subscript>)·4H<Subscript>2</Subscript>O

Thermogravimetry combined with evolved gas mass spectrometry has been used to characterise the mineral ardealite and to ascertain the thermal stability of this ‘cave’ mineral. The mineral ardealite Ca₂(HPO₄)(SO₄)·4H₂O is formed through the reaction of calcite with bat guano. The mineral shows disorder, and the composition varies depending on the origin of the mineral. Thermal analysis shows that the mineral starts to decompose over the temperature range of 100–150 °C with some loss of water. The critical temperature for water loss is around 215 °C, and above this temperature, the mineral structure is altered. It is concluded that the mineral starts to decompose at 125 °C, with all waters of hydration being lost after 226 °C. Some loss of sulphate occurs over a broad temperature range centred upon 565 °C. The final decomposition temperature is 823 °C with loss of the sulphate and phosphate anions.     

Kinetics of the complex process of thermo-oxidative degradation of poly(vinyl alcohol)

The thermo-oxidative degradation of poly(vinyl alcohol) (PVA) has been investigated by TG+DTG+DTA simultaneous analysis performed in static air atmosphere, at four heating rates, namely 3, 5, 10 and 15 K min⁻¹. TG, DTG and DTA curves showed that, in the temperature range 25–700°C, four successive processes occur. The first process consisting in the loss of physical adsorbed water is followed by three processes of thermal and/or thermo-oxidative degradations. The processing of the non-isothermal data corresponding to the second process (the first process of thermo-oxidation) was performed by using Netzsch Thermokinetics — A Software Module for Kinetic Analysis. The dependence of the activation energy evaluated by Friedman’s isoconversional method on the conversion degree shows that the investigated process is complex one. The mechanism of this process and the corresponding kinetic parameters were determined by Multivariate Non-linear Regression Program and checked for quasi-isothermal experimental data. It was pointed out that the first process of thermo-oxidation of PVA consists in three consecutive steps having Avrami-Erofeev kinetic model. The obtained results can be used for prediction of the thermal lifetime of PVA corresponding to a certain temperature of use and an endpoint criterion.     

Hydration Heats of Zeolites For Evaluation of Heat Exchangers

The enthalpy of hydration of purified clinoptilolite from Beli Plast, Bulgaria, and various cation-exchanged types such as Na-, K-, Ca- and Mg-clinoptilolite was determined by the adiabatic water-vapor absorption calorimeter. The hydration enthalpy becomes more exothermic in the sequence K, Na, Ca, Mg depending on hydration energy values of exchanged cations. Na-clinoptilolite would be an efficient heat exchanger in a wide temperature range of dehydration, 180–300°C, while Mg-clinoptilolite in higher temperatures, 300–350°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of water on the dielectric behaviour of chitosan films

 The dielectric properties of chitosan films with a degree of deacetylation of 86% have been investigated in the frequency range of 10^3–10⁶ Hz covering a broad range of temperatures from −150 to 150 °C. The variation of the dielectric pro-perties with temperature has been associated with two dielectric processes: (a) a local relaxation attributed to the presence of hydrogen-bonded water appearing at low temperatures (b) a conduction process related to water molecules which becomes desorbed upon heating at T>80 °C. Isothermal dielectric experiments have been performed in order to follow, in real time, the occurrence of both, the water sorption and desorption processes. Received: 20 June 1996 Accepted: 19 November 1996     

A TG analysis of the effect of calcination conditions on the properties of reactive magnesia

The reactivity of MgO obtained from calcination of magnesium carbonate at different temperatures has been investigated by means of hydration in a constant relative humidity environment at 40°C for periods up to 24 days. Natural magnesite and AR grade basic MgCO₃ calcined in the range of 500–1000°C was characterised in terms of surface area, crystallite size, morphology, and hydration rate. It was found that the hydration rate is dependent on the surface area and crystallite size where temperature was the main variable affecting them. The most reactive MgO was produced at the lowest calcination temperature with the highest surface area and the smallest crystallite size. The basic MgO specimens showed higher degree of hydration compared to the natural MgO specimens due to the smaller surface area and larger crystallite size. The low MgO content of the starting natural magnesite is also attributable to the lower reactivity. This preliminary study serves as a mean to investigate potential utilisation of reactive MgO as a supplementary cementitious material in eco-friendly cements.     

Thermogravimetry as a possible tool for determining modification degree of thermally treated Norway spruce wood

Thermal treatment is one of environmental friendly wood modification processes, developed in order to improve wood’s natural durability and dimensional stability. Beside wood species, mainly isothermal temperature of heat-treatment and process duration affect these properties, which also correlate with the mass losses caused by the treatment. However, there is a lack of suitable external quality control methods. In this work thermogravimetry as a potential method for determining the degree of thermal modification is presented. Several calibration curves, representing the mass losses in a certain temperature range (the values obtained from the TG curves) compared to weight losses caused by previous heat-treatment (known values), were established for spruce wood samples modified at different isothermal temperatures (from 170 to 220 °C). Linear plot and good correlation factors (R ² = 0.95 and 0.96) were obtained for the TG mass losses from 130 to 280 °C and from 130 to 300 °C, both under nitrogen atmosphere. The predominant cause of mass loss in this temperature region was depolymerisation and thermal decomposition of hemicelluloses residues. Lower correlation factors were obtained under the air atmosphere and in the wider temperature range, respectively.     

Mechanism and Kinetics of Thermal Dissociation of Inclusion Complex of Benzaldehyde with β-Cyclodextrin

The inclusion complex of benzaldehyde (BA) with β-cyclodextrin (β-CD) was prepared and was studied by thermal analysis and X-ray diffractometry. The composition of the complex was identified by TG and elemental analysis as β-CD·BA·9H₂O. TG and DSC studies showed that the thermal dissociation of β-CD·BA·9H₂O took place in three stages: dehydration in the range 70-120°C; dissociation of β-CD·BA in the range 235-270°C; and decomposition of β-CD above 280°C. The kinetics of dissociation of β-CD·BA in flowing dry nitrogen was studied by means of TG both at constant temperature and at linearly increasing temperature. The results showed that the dissociation of β-CD·BA was dominated by a one-dimensional random nucleation and subsequent growth process (A₂). The activation energy E was 124. 8 kJ mol^-1, and the pre-exponential factor A 5.04·10¹¹ min^-1. This revised version was published online in July 2006 with corrections to the Cover Date.     

Morphology and electric conductivity of cross-linked polyethylene–carbon black blends prepared by gelation/ crystallization from solutions

 Ultra-high-molecular-weight polyethylene (UHMWPE) – carbon black (CB) blends were prepared by gelation/ crystallization from PE dilute solutions containing CB particles. The UHMWPE/CB composition chosen were 1/0.15, 1/0.25, 1/0.5, 1/0.75, 1/1, 1/3, 1/5, and 1/9, etc. The cross-linking of PE chains was performed by chemical reaction of dicumyl-peroxide at 160 ^°C. X-ray diffraction patterns indicate that the crystallinity of PE within the blends decreased drastically through the chemical reaction at high temperature. The sample preparation method by gelation/crystallization provided the UHMWPE–CB system with various CB contents up to 90% and the conductivities for the resultant specimens were in the range from 10^-9 to 1 Ω^-1 cm^-1 corresponding to the electric conductivity range of semiconductors. The blends assured thermal stability of electric conductivity by cross-linking of PE chains, although the mechanical property such as the storage and loss moduli were very sensitive to temperature. The conductivity of the blends with CB content ≥20% were almost independent of temperature up to 220 ^°C and the values in the heating and cooling processes were almost the same. On the other hand, for the UHMWPE–CB blends with 13% CB content corresponding to the critical one, temperature dependence of electric resistivity showed positive temperature coefficient (PTC) effect. The PTC intensities for non-cross-linked and cross-linked materials were lower than that of the corresponding low-molecular-weight-polyethylene (LMWPE)–CB blend but the maximum peak appeared at 160 ^°C which is higher than the peak temperature of LMWPE–CB blend. Received: 10 December 1997 Accepted: 9 April 1998     

Organosoluble and thermally stable polyimides derived from a new diamine containing bulky-flexible triaryl pyridine pendent group

A novel kind of aromatic diamine,N-(4-(4-(2,6-diphenyl pyridine-4-yl)phenoxy)phenyl)-3,5-diaminobezamide (DPDAB),was synthesized via aromatic nucleophilic substitution of 3,5-dinitrobenzoylchloride with 4-(4-(2,6- diphenylpyridine-4-yl)phenoxy)aniline(DPPA),followed by palladium-catalyzed hydrazine reduction.This monomer was used to prepare polyimides(PIs)based on reaction with several commerically avaiable tetracarboxylic dianhydrides such as pyromellatic dianhydride(PMDA),benzophenone tetracarboxylic acide dianhydride(BTDA)and bicycle[2.2.2]oct-7-enc- 2,3,5,6-tetracarboxylic dianhydride(BCDA).These PIs had inherent viscosity in the range of 0.34-0.76 dL/g and showed good solubility in various aprotic polar solvents.The glass-transition tempratures(T_gs)of the PIs were in the range of 184-302℃,and showed high thermal stability with 10%weight loss in the temperature range of 360-500℃under nitrogen atmosphere.     

On the nature of decrease in the particles size of nanostructured nickel oxide obtained by the sol-gel method in the presence of surfactants

A processes of formation of nanostructured powders of nickel oxide by annealing in the temperature range of 200–700°C of the nickel hydroxide obtained by the sol-gel method at 80°C from solutions of nickel nitrate by precipitation with alkali in the presence of surfactant AF-12 (polyethylene oxide alkylphenyl ether) was investigated. The formation of nanostructured powders of nickel oxide in the presence of a surfactant reduces the size of nanoparticles to 20–25 nm, which is 1.5 times smaller than the particles obtained without a surfactant. The effective influence of surfactant on the particle size begins in the temperature range of its decomposition and evaporation equal 350–400°C.     

Synthesis and characterization of some Schiff-base-containing polyimides

The diamine, 4-aminophenyloxy-N-4-[(4-amiophenyloxy)benzylidene]aniline, was prepared via the nucleophilic substitution reaction and was polymerized with different dianhydrides either by one-step solution polymerization reaction or two-step procedure. The latter includes ring-opening polyaddition to give poly(amic acid), followed by cyclodehydration to polyimides. The inherent viscosity ranges from 0.61–0.79 dl/g. Some of the polymers were soluble in most of the organic solvents such as DMSO, DMF, DMAc, NMP, and m-cresol even at room temperature. The degradation temperature of the resultant polymers falls in the ranges from 240–500 °C in nitrogen with only 10% weight loss. Specific heat capacity at 200 °C ranges from 1.0929–2.6275 J g⁻¹ k⁻¹. The temperature at which the maximum degradation of the polymer occurs ranges from 600–630 °C. The glass transition temperature (Tg) values of the polyimides ranged from 185 to 272 °C. The activation energy and enthalpy of the polyimides ranged from 47.5–55.0 kJ/mole and 45.7–53.0 kJ/mole and the moisture absorption in the range of 0.23–0.72%.     

Determination of the thermal modification degree of beech wood using thermogravimetry

Thermal modification is one of the environmental friendly wood preservation technologies. During this process, changes of the main woody cell wall components occur, which lead to improved dimensional stability, lower hygroscopicity and improvement in biological durability. Several chemical reactions which occur during thermal treatment of wood caused changes in wood properties. During TG measurements, thermal decomposition reactions, which was not completed during previous thermal modification process, continued in wood samples, meaning that more thermally treated samples exhibited lower mass losses in a certain or whole temperature range up to 600 °C. Therefore, mass loss, obtained within selected temperature range, could be used as a marker of previous thermal treatment. The aim of the present work is to evaluate suitability of a thermogravimetric method (TG) for determination of a degree of thermal treatment of beech wood. On the basis of thermally untreated sample and those which were thermally modified at 180, 190, 200, 210, 215 and 220 °C in the absence of oxygen, respectively, and with known values of mass loss during the modification processes, several calibration curves were constructed. They represent mass loss in a certain temperature range during TG measurement versus mass loss during previous thermal modification. In a temperature range from 130 to 300 °C and from 130 to 320 °C under nitrogen atmosphere, a linear dependence was observed; correlation coefficients R ² were 0.87 and 0.91, respectively. In wider temperature range and under air atmosphere, lower correlation coefficients were obtained. High correlation coefficient, higher than 0.95, was observed in a temperature range from 25 to 130 °C under both atmospheres. In this region, dehydration due to rehydration of thermally modified samples occurs. The results of this work were compared with those obtained for Norway spruce.     

Some Physico-chemical Properties of Doxazosin Mesylate Polymorphic Forms and its Amorphous State

Seven polymorphic modifications of doxazosin mesylate, designed as forms A, D, E, F, G, H, I, and the amorphous state were studied by thermal methods (TG and DSC), temperature resolved X-ray powder diffractometry, hot stage and scanning electron microscopy and by FT-IR spectroscopy. Amorphous form was obtained either by fast evaporation of the solvent or by fast cooling of the melt in the DSC. Polymorphs A and F were found to be stable in the temperature range from room temperature to their melting points at 277.9 and 276.5°C, respectively. Form G, which melts at 270.8°C, was found to be hygroscopic. Polymorph D undergoes irreversible solid–liquid–solid phase transition at 235.5°C to polymorph I which melts at 274.9°C. Form H, which melts at 258.0°C, was found to be unstable at high temperatures. DSC examinations revealed that form H is irreversibly transformed to polymorph F during heating above the temperature of about 240°C. The amorphous state was found to be stable at room temperature but when heating above the glass transition (T _g=144.1°C) it crystallizes at 221.6°C, what leads into a mixture of polymorphic forms. The new polymorphic form designed as E was identified in the mixture. The polymorph E is converted by heating to the more stable form F. The solubilities at 25°C for forms A, and F in methanol are 3.5 and 7.7 mg mL⁻¹and in water they are 3.8 and 6.2 mg mL⁻¹, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal behavior of loratadine

Simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) techniques were used for the characterization the thermal degradation of loratadine, ethyl-4-(8-chloro-5,6-dihydro-11H-benzo[5,6]cyclohepta[1,2-b]pyridin-11-ylidine)-1-piperidinecarboxylate. TG analysis revealed that the thermal decomposition occurs in one step in the 200–400°C range in nitrogen atmosphere. DTA and DSC curves showed that loratadine melts before the decomposition and the decomposition products are volatile in nitrogen. In air the decomposition follows very similar profile up to 300°C, but two exothermic events are observed in the 170–680°C temperature range. Flynn–Wall–Ozawa method was used for the solid-state kinetic analysis of loratadine thermal decomposition. The calculated activation energy (E _a) was 91±1 kJ mol^–1 for α between 0.02 and 0.2, where the mass loss is mainly due to the decomposition than to the evaporation of the decomposition products.     

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.