Etude de la pyrolyse du polystyrene par thermogravimetrie en montee lineaire de temperature

After corroboration of the kinetic law obtained isothermally for polystyrene pyrolysis, experiments were carried out at constant heating rates. It was found that one mechanism governs the overall reaction, and that utilization of the characteristic parameters of the system for the maximum rate (Kissinger's method) gives the best results in the determination of the apparent activation energy and the pre-exponential factor.     

Etude vibrationnelle des oxydes PbO α et PbO β: Interpretation en relation avec la dilatation thermique

A normal coordinate analysis of the Raman and ir vibrational spectra of PbO α and PbO β allowed us to calculate the force field for both phases. The cohesion between the structural layers is realized by means of attractive interactions involving the lone pair of the Pb^II atoms. The analysis of the potential energy distribution shows that these interactions are much weaker than the PbO bonds in the layers. The calculated force constants for these bonds vary inversely as a function of their distance and allow one to explain the value of the thermal expansion coefficients in the layers. On the other hand, the abnormally small value of the thermal expansion along the normal direction to the layers, particularly for PbO β, may be explained only in assuming the existence of a negative contribution due to a modification of the relative orientation of the dipoles (Pb^II-lone pair) situated on two adjacent layers.     

Etude du changement de phase solide-liquide en relation avec la stabilite thermique des formes polymorphes et pseudopolymorphes du lorazepam et identification spectrale

Thermal decomposition of lorazepam begins by lose of a water mole between two groups NH-OH and occurs at different temperatures depending on the formes of polymorphs. Melting temperature of a form can be determined only if it is below that of decomposition.The thermoanalytical and spectral study of lorazepam commercial drugs (a) and (b) recrystallisation product at 20°C in benzine (c), in carbon tetrachlorid (d), in acetonitrile (e) were carried out.Thermal behaviour, RX diffraction, IR spectroscopy allow us to identify polymorph (a, b, c) and two solvates (d and e). The solvates stoechiometry (d and e) has been determined by thermogravimetry (3-2) d, (2-1) e, and confirmed by magnetic nuclear resonance for (e).

Nous exprimons nos vifs remerciements à Madame Eliane Toreilles, Directeur de recherche au CNRS (Laboratoire de Chimie Organiques UA 458 CNRS, Ecole Nationale Supérieure de Chimie de Montpellier) pour sa collaboration personnelle en spectroscopie.     

Etude des transitions de phases du Stearate de lithium: Influence de l'histoire thermique

The phase transitions of lithium stearate have been studied by differential scanning calorimetry and the observation of thin foils under polarized light. A new phase has been demonstrated. The phase transition temperatures and enthalpies have been measured and the dependence of some of them on the earlier cycle characteristics (number, maximum temperature and temperature rate) is demonstrated.     

Influence de la temperature et du gradient de vitesse sur la viscosite des solutions diluees de xanthane

The dependence of the relative viscosity of a dilute xanthan solution (0.4 g·l^?1) has been studied as a function of temperature (20–70°) and of the ionic strength of the solvent. When the rate of shear exceeds 100 sec^?1, the viscosity can be expressed as γ̊ the exponent (n ? 1) varies with the polymer conformation. In aqueous solution in the absence or in the presence of added salt at a temperature above the melting temperature T_M (depending on the ionic strength of the solvent), the exponent (n ? 1) is ?0.285 and corresponds to the unordered conformation; at temperatures below T_M, the local helical conformation is rigid and (n ? 1) is ～ ?0.44 almost independent of the temperature.     

Etude de la degradation thermique des oligomeres du polyacrylonitrile

To obtain more information on the reactions during the pyrolysis of polyacrylonitrile at 290° in air, we studied the thermal degradation of its trimers, tetramers and pentamers having the general formula: R  CH(CN)  CH₂  CH(CN)_n?1  R where R = H or CH₃ and n = 3,4,5. Study by gas chromatography of the volatile compounds produced during pyrolysis at 290° and 770° showed that the first step of the degradation is homolytic scission of the chain. There are then intra and intermolecular polymerisations of the nitrile groups and formation of a ladder polymer. This reaction is clearly exothermic only for pentamers. This increase of the reactivity with DP is borne out by a kinetic study by EPR of the pyrolysis in air at 290°, oxidation of the ladder polymer giving a paramagnetic solid. Its EPR characteristics (band width and number of spins per g) are very dependent of the DP and nature of the end-groups, H or CH₃, of the pyrolysed oligomer.     

Etude cinetique d'une decomposition thermique par couplage de la calorimetrie et de l'analyse thermique a vitesse de decomposition constante

A method is described where the simultaneous measurement of a thermal flow (by means of Tian-Calvet type calorimetry) and of a gas flow (by means of constant decomposition rate thermal analysis) allows the knowledge at any time of the instantaneous enthalpy of thermal dissociation. The method is used to study the thermal decomposition of an industrial Al(OH)₃ gibbsite sample.     

Etude thermodynamique de la decomposition thermique des hydrates du chlorure de magnesium

The thermodynamic study of the different compounds of the system MgCl₂, H₂O, HCl allows to draw the stability diagram of the 8 solid phases into a three-dimensional space (temperature, total pressure and ratio of HCl and H₂O partial pressures). The evolution of the composition of solid and gaseous phases during an isobaric heating is described. The conclusions of this study are in good agreement with the experimental results obtained in laboratory and with the industrial requirements set for the different steps of the dehydration of MgCl₂.     

Rearrangement thermique des α-ceto, α-cyano epoxydes en dioxoles : Etude du mecanisme de la reaction

The thermolysis of α-cyano α-keto epoxides to give dioxoles is a new rearrangement. The reactivity of the epoxides is dependent upon the nature of the substituents on the epoxide ring, and solvent polarity. When the reaction is carried in the presence of benzaldehyde, a 1,3 cycloadduct is obtained. These results are in good agreement with a carbonyl ylide intermediate.     

Etude de la chloration du PVC en masse—I: Evolution microstructurale de la chaine en fonction des conditions operatoires et consequences sur son comportement thermique

Bulk chlorination of polyvinylchloride has been studied between 40° and 140° with or without u.v. radiation. Microstructural analysis of the chlorinated polymer shows two domains of temperature for the chlorination. Below 80° products have two thermal transitions like a block copolymer; they consist of sequences of two types: vinyl and chlorinated vinyl. Above 80°, the spread of chlorine along the chain becomes more even and products have only one thermal transition at a temperature much higher than Tg of the original PVC.     

Etude de la stabilisation du polychlorure de vinyle avec des molecules modeles—I: Degradation thermique du chloro-4-hexene-2

This paper describes an extensive kinetic study of thermal dehydrochlorination for 4-chloro-2-hexene (model of an allylic unstable structure in polyvinylchloride) in solution in dichloroethane. di-ethyl-2-hexyl phthalate and other solvents. The reaction, which is reversible, has an ionic mechanism. It is catalysed by a charge transfer complex between hexadiene and hydrochloric acid. It is inhibited by complexing agents for hydrochloric acid, such as tetrahydrofuran. The addition of hydrochloric acid to conjugated dienes has been shown to occur also in the case of β-carotene.     

Oxydes de plomb: V. Etude de la texture des phases quadratique et orthorhombique de Pb3O4: Influence des defauts sur la transition de phase

Several samples of Pb₃O₄ have been prepared by oxidizing PbO in air at various temperatures in the range 705–815°K. A correlation is established between the nonstochiometry of the samples and their X-ray diffraction line profiles at 295°K which are characteristic of an orthorhombic distortion of the tetragonal lattice. In the high-temperature phase (T > 170°K), orthorhombic microdomains exist in the tetragonal matrix. The mean distortion increases with the nonstochiometry of the compound. Below 170°K Pb₃O₄ exhibits an orthorhombic phase with orthorhombic domains according to two orientation states, and para crystalline distortion. A model of texture is proposed and compared with the high-temperature one. The pretransitional effect which is observed between 250 and 170°K is correlated with the presence of orthorhombic microdomains in the high-temperature phase (tetragonal).     

Etude du fluorure de propargyle par RMN en phase nematique: Determination de la geometrie moleculaire et des signes des differentes constantes de couplage indirect

NMR spectra of proton, fluorine-19 and carbon-13 satellites of partially oriented propargyl fluoride have enabled the complete structure of this molecule to be determined. Agreement with microwave results is excellent, except for the carbon-fluorine bond length. Signs of all indirect coupling constants have been determined.     

Evolution des tenseurs de dilatation thermique en fonction de la temperature. II. Etude calorimetrique et cristallographique de la transition de phase ordre-desordre du thiophene chrome tricarbonyle C4H4SCr(CO)3

Benzenechromium tricarbonyl C₆H₆Cr(CO)₃ and thiophenechromium tricarbonyl C₄H₄SCr(CO)₃ are isomorphous at room temperature. We have measured, in the range 77–295°K, the variations of specific heat and of the tensor of thermal expansion for these two molecular compounds. The first one exhibits quite normal behavior; on the other hand, the second one exhibits a first order phase transition, monoclinic ? triclinic at T = 185°K, associated with an order-disorder phase transition. Structural evolution of thiophene-chromium tricarbonyl, during phase transition, has been explained from the variation of its tensor of thermal expansion.     

Etude de la Fragmentation en Spectrographie de Masse de Spiropyrannes Benzodithioliques. Influence de la Position et de la Nature des Substituants

The mass spectra of 12 benzodithiolic spiropyrans are reported. Their fragmentation pathways have been determined by exact mass and metastable transition measurements. The nature of the fragmentation is strongly dependent upon the position and nature of the substituents. A comparison is made with their photo and thermochemical decompositions.     

Mise en evidence et localisation des protons dans les bioxydes de plomb PbO2α et β chimiques et électrochimiques

Chemical and electrochemical samples of lead dioxides PbO₂α (orthorhombic) and PbO₂β (tetragonal) are studied by quasi-elastic neutron scattering and by X-ray diffraction between 77 and 550 K. The presence of OH^?-like proton species is established: the electrochemical species are characterized by two types of local proton motions ($\text{ΔE}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 300 μ}\text{eV}\text{, ΔE}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 40 μ}\text{eV}$). The thermal expansion of the chemical and electrochemical sample shows: (1) the loss of OH^?-like charged species after heating to 550 K, (2) an anisotropy of the OH ··· O bonds in the [PbO₆] octahedra chains which are oriented differently in the α and β structures. An interpretation of the role of protons is proposed.     

Etude du comportement thermique des n-alcanes dans des milieux hydrocarbones complexes par analyse calorimetrique differentielle. I. Etude du comportement thermique des n-alcanes en programmation lineaire de temperature

Middle distillates contain various amounts of n-alkanes which are responsible for low temperature properties. The thermal behavior of 14 alkanes ranging from n-C₁₂H₂₆ to C₂₅H₅₂ has been studied by means of a Mettler TA 2000 B heat flow DSC. Different hydrocarbon matrices such as gas oil (diesel) where n-alkanes have previously been removed, kerosene and gas oil containing a known distribution of n-alkanes have been used. A comparative study of the values of the enthalpy of dissolution obtained for each n-alkane in a gas oil with or without a distribution of n-alkanes has been carried out.     

Variations des proprietes d'adsorption d'eau de la montmorillonite-lithium en fonction du traitement thermique

Journal of Thermal Analysis and Calorimetry - L'adsorption de vapeur d'eau sur montmorillonite lithique en provenance de Marnia et Camp Berteaux a été analysée après...     

Etude de la stabilisation du polychlorure de vinyle avec des molecules modeles—II: Action des phosphites et des phosphines sur la degradation du Chloro-4-hexene-2

Phosphines are inhibitors of thermal degradation of 4-chloro-2-hexene because they form strong complexes with either HCl or ZnCl₂, which cause catalysis of the dehydrochlorination. Aliphatic phosphines are more efficient than aromatic phosphines. Aliphatic phosphites also act as stabilizers; they react with HCl, and may be substituted for allylic chlorine in the presence of ZnCl₂ as catalyst; the resulting phosphonate may be destroyed by HCl if phosphite is not in excess. Aromatic phosphites do not react at 60°; they only form a complex with HCl and, in the presence of ZnCl₂, they cause an increase of the dehydrochlorination rate. This effect is due to the reverse reaction, catalysed by a complex HCl-hexadiene, being slowed down because of the competing phosphite-HCl complex.     

Etude sur le comportement thermique de quelques polymeres ignifugeants contenant du phosphore et des halogenes

The thermal behaviour a series of polyphosphonates was studied by thermogravimetry (TG) and differential thermal analysis (DTA). The obtained data permitted the calculation of activation energies and reaction orders on decomposition zones by the Coats — Redfern method.