Analysis of aromatic sulfur compounds in gas oils using GC with sulfur chemiluminescence detection and high-resolution MS

The analysis of alkylbenzothiophenes (alkyl-BT) and alkyl-dibenzothiophenes (alkyl-DBT) in light cycle oil (LCO) and straight run (SR) gas oils is described. A detailed identification and quantitative analysis of alkyl-BT and alkyl-DBT present in LCO gas oils was carried out using GC-SCD. For the SR gas oils, the simultaneous presence of thiophenic and nonthiophenic compounds does not allow for a selective analysis of thiophenic compounds by GC-SCD. A new method using gas chromatography coupled with high-resolution mass spectrometry (GC-HRMS) is proposed to selectively detect and quantify the alkyl-BT and alkyl-DBT in SR gas oils. The development of the method and comparison of results between GC-SCD and GC-HRMS are presented.     

Reaction mechanism,kinetics and network structure of the DGEBA-DDS system

Different techniques were used to analyse the reaction mechanisms and kinetics of the system 2,2-[4-(2,3-epoxypropyl)phenyl]propane polycondensed with 4,4′-sulphonyl-dianiline in the temperature range 100?185°C. The results show the role of catalysts, of stoichiometry and of curing cycles in the reaction mechanisms and paths. Based on chemical balances of residual epoxy groups, and of hydroxy and ether groups formed, we have shown the effect of these functions on the thermal properties of the material.     

Structure–property relationships for styrene crosslinked polyesters. I. Network structure and rubbery elastic modulus

Ten unsaturated polyester prepolymers differing by the nature of the diol, by the maleate/phthalate molar ratio, or by the chain length, were crosslinked by about 40% by weight of styrene and then analyzed by IR spectrophotometry, solid state NMR, and rubbery elasticity measurements. The spectrometric measurements revealed a practically complete conversion of the styrene/fumarate copolymerization. The rubbery elastic modulus varies in a complex way with the presumed network structure. A non-ideality factor f was defined from the current theories; f is a decreasing function of the average number of flexible bonds between crosslinks. The networks based on dimeric diols display a practically ideal behavior. The influence of dangling chains, corresponding essentially to polyester chain ends, is taken into account by the Graessley's formula in which trifunctional and tetrafunctional crosslinks are distinguished. A method for the estimation of the nature and weight fraction of dangling chains was proposed.     

The role of impurities in the mechanisms and kinetics of reactions of epoxy resins and their effects on final resin properties

A series of commercial tetrafunctional epoxy prepolymers were step-growth polymerised with aromatic amines. The roles of non-epoxy chain ends and of impurities in the polymer were shown in terms of the mechanisms, reaction kinetics and final thermal properties of the material. The pure prepolymer was studied after preparative chromatographic separation. In the temperature range 100–180°C, epoxy-primary amine reactions predominate at the beginning, followed by epoxy-hydroxyl and epoxy-secondary amine reactions at the end of the reaction sequence. These reactions may be either intermolecular forming cross-linked networks, or intramolecular forming rings, the latter occurring primarily at the end of heat treatment. At temperatures higher than 180°C, there are breakdown reactions and homopolymerisation of epoxy groups. Based on data obtained with ¹³C solid phase cross polarization magic angle spinning nuclear magnetic resonance, inverse phase gas chromatography and Calvet microcalorimetry, the progress of the reaction could be determined up to the gel point and in the vitrification zone. Based on structural results (assays of residual functions and those formed in the cross-linked networks), it was possible to establish relationships between structures and thermal properties.     

Inverse gas chromatography used to follow kinetics of epoxy-amine reactions in the molten state

Polycondensation kinetics of epoxy-amine model systems were investigated with inverse gas chromatography. The results obtained were applied to the study of cross-linking kinetics of two epoxy prepolymers, tetraglycidyl diamino diphenyl methane and diglycidyl ether of bisphenol A, with diamino diphenyl sulphone. Data furnished by inverse gas chromatography showed zones of gelation and of vitrification. Based on kinetics established by isothermal microcalorimetry, it was possible to determine reaction advancement in these transition zones. The results show that advancement of both reaction systems to the transition points T₁ and T₂ is very sensitive to curing temperature and to the stoichiometry of the mixture. Finally, the vitreous transition temperatures of the cross-linked systems were determined and compared to those determined with differential scanning calorimetry.     

Polypentamethylnadimides obtained by Diels-Alder reaction

The reactions between benzylbromide or α,α′-dibromo-meta-xylene (DBMX) and sodium 1,2,3,4,5-pentamethylcycopenta-1,3-dienide (CpMe₅^?, Na^⊕) give only the expected pentamathylcyclopentadiene derivatives with a good yields. These products lead to endo isomer adducts via a Diels-Alder reaction with maleimide.New low molecular weight polypentamethylnadimides were obtained in this way from the Diels-Alder polymerization of bispentamethylcyclopentadiene monomers and commercial or synthetic bismaleimides. The polyadducts were characterised by size exclusion chromatography (SEC), matrix assisted laser desorption ionization-time of flight (MALDI-TOF), nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA).The low molecular weight was attributed to methyl steric hindrance. For the lowest molecular weight products, two different endcapped oligomers were observed.     

Occurrence of Ochratoxin A in Brazilian cocoa beans

In the present work studied the occurrence of Ochratoxin A (OTA) in dried beans from southern Bahia (Brazil) harvested in two periods of the year (early crop and harvest seasons) then fermented and dried under the sun. Nearly 92.5% of the cocoa samples presented OTA levels below the limits suggested by the European Community (2 μg/kg). The highest toxin content was found in fine and flavour cocoa beans harvested in the early crop. Ochratoxigenic species of the genera Aspergillus and Penicillium were isolated from the cocoa beans. Both harvesting time and production model influenced OTA content in cocoa. These results show that, according to the current world standards for Ochratoxin A levels, beans from southern Bahia give a good quality cocoa.     

Structure/properties relationship of nadimide-terminated fluorinated polyimides. Part II. New thermostable systems

We have established the chemical structure/mechanical properties relationships of new nadimide-terminated polyimides. Initially, the prepolymers were synthesized by polycondensation of nadic anhydride (NA), hexafluoroisopropylidene-2,2-bis(phthalic acid anhydride) (6HFDA) and two phenylene diamines, meta (mPDA) and para (pPDA) in N-methylpyrrolidone (NMP). After characterizing the microstructure and composition of these systems using chromatographic (HPLC, SEC) and spectroscopic techniques (¹H and ¹³C NMR), we determined their thermomechanical properties. The rheology curves obtained showed the presence of two transitions, α and β, associated with the glass transition and a second, sub-vitreous transition, because of local molecular movements. By studying the relationship between chemical structures and mechanical properties, we have shown the influence of endo/exo isomerization of nadimide extremities on the glass transition temperature T_g. In a given polymer, the value of T_g increases with increasing percentage of exo isomers. ©1997 SCI     

Light induced degradation of testosterone in waters

The degradation of testosterone under simulated irradiations was studied in phosphate buffers and in natural waters at various excitation wavelengths. The quantum yield of photolysis was significantly lower at 313 nm (2.4 × 10^− 3) than at 254 nm (0.225). The formation of several photoproducts was observed, some of them being rapidly transformed in turn while others show higher stability towards subsequent irradiations. The nature of the main products was tentatively identified, both deduced from their spectral and spectrometric data and by comparison with synthesised standard compounds. Among the obtained photoproducts, the main one is possibly a spiro-compound, hydroxylated derivative of testosterone originating from the photohydratation of the enone group. The photodegradation pathway includes also photorearrangements. One of them leads to (1,5,10)-cyclopropyl-17β-hydroxyandrostane-2-one. The pH of the water does not seem to affect the rate of phototransformation and the nature of the by-products.     

New thermostable systems: structure/property relationships of nadic terminated telechelic fluorine polyimides

Polyimides were synthesized by polycondensation of nadic anhydride monoester (NE), hexafluoroisopropylidene-2,2-bis (phthalic acid anhydride) (6HFDA) and Cardo diamine in aprotic/polar medium. After examining the microstructure and the composition of systems with a polymerization index n̄ = 2.075 and 5.18 by high performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) spectroscopy (¹H and ¹³C), their thermomechanical properties were investigated. The rheology curves obtained in the temperature range 50–400°C showed the presence of an α transition (glass transition temperature T_g) and a sub-glass β transition. The temperature of the maximum of the loss modulus G″ of these two transitions varied in the domain of molar mass M̄_n studied. The effect of endo/exo isomerization of the nadimide extremities on the values of the glass transition temperature has also been shown. When the percentage of exo isomer of a given polymer increased, the glass transition temperature T_g increased by 50°C.