Reaction epoxy-amine: Suivi du mecanisme reactionnel et de la cinetique par RMN 13C, 15N et HPLC

The curing reaction of diglycidyl ether of bisphenol A (DGEBA) with metaphenylene diamine (mPDA) was investigated with high performance liquid chromatography (HPLC) and Fourier transform nuclear magnetic resonance spectroscopy (FT NMR ¹³C and ¹⁵N). With the results obtained from a series of models a mechanism was proposed and the cure kinetics were obtained at 100°C. The major conclusion is that the cure proceeded mainly by chain extension, whereas crosslinking occurred in the reaction of hydroxyl groups with epoxides and resulted in the formation of ether linkages.     

Etude par RMN 13C à 62,89 MHz des branchements éthyles dans les copolymères ethylène-butène et polyéthylène basse densité

Carbon 13 nuclear magnetic resonance spectra (at 62,89 MHz) were obtained for a series of branched heavy alcanes (12-ethyl tricosane, 11,20-diethyl tricontane, 11,18-diethyl octacosane, 11,17-diethyl heptacosane, 11,16-diethyl hexacosane, 9,12-diethyl heneicosane, 5,7-diethyl docosane, 6,7-diethyl docosane, 2 éthyl-hexyl-12 tricosane), which provide a model set for describing the ethyl branched sequences in ethylene butene copolymers and low-density polyethylene (LDPE). For ethylene-butene copolymers we do not detect any head-to-head polymerization of butene as reported recently (the existence of a 1,2-ethyl pair has not been confirmed by the low-field signal at 41,3 ppm), but only isolated ethyl and 1-3-diethyl branches. The three peaks observed in the methyl region (broad signal) of the spectrum are assigned to butene centered triads, as opposed to branches in positions having different tacticities as reported earlier. Carbon 13 nuclear magnetic resonance spectra of high-pressure polymerized low-density polyethylene have been measured at 62,89 MHz. On the basis of Willbourn's double back biting mechanism, two kinds of complex branches, the 1,3-ethyl pair and 2-ethyl-hexyl, have been assigned. Finally, these results suggest that the ethyl branches in low-density polyethylene are not isolated branches.     

Analytical methods for the determination of selected steroid sex hormones and corticosteriods in wastewater

An analytical method for the determination of 12 selected estrogens, progestagens and corticosteroids is presented. The optimization of the method, including liquid chromatography separation, extraction on a solid phase, purification on a silica gel cartridge and detection by mass spectrometry, is described. Both the repeatability, with relative standard deviation ranging from 1.4 to 2.7%, and the accuracy, with recoveries ranging from 92.7 to 102.4%, were very satisfactory for ten of the target analytes. The limits of detection were lower than 1 ng/L for progestagens, androgens and corticosteroids, and ranged between 0.9 and 4.3 ng/L for estrogens. The results of the analysis of two sewage treatment plants in the area of Lyon (France) by this method reveal that all the compounds investigated are present in the effluents. The estrogen most frequently detected was estrone, with a median of 26.1 ng/L. The target progestagens were detected with concentrations ranging between 5 and 41 ng/L. Androgens were also present in most of the samples in the range 1–30 ng/L, while the corticosteroids were present only in one plant, with a median of 31.9 ng/L.     

Stereochemistry of rings. XI. Cyclohexane derivatives. Part 4. Structure of 1-oxocyclohexane-2,6-dione-4-spiro cyclopentane

The structure of the title compound was examined to elucidate the changes in the ring conformation of 1,3-cyclohexanedione derivatives when a heteroatom is introduced. The compound is orthorhombic, witha=17.540(3),b=7.457(2),c=6.661(2) Å and space groupP2₁2₁2₁. The structure was solved by direct methods and refined to a finalR of 0.072 for 527 observed reflections. The two mean ring planes form a dihedral angle of 88.9°.For Communication X see Bocelliet al. (1982a).For Part 3 see Bocelliet al. (1982b).     

Analysis of pesticide residues in essential oils of citrus fruit by GC–MS and HPLC–MS after solid-phase extraction

A robust reliable method for the analysis of residues of pesticides in citrus groves was developed. Residues of twelve pesticides were extracted from citrus essential oils by SPE, separated by liquid chromatography and analyzed by GC-MS. In addition, ten pesticides were extracted by SPE, separated and analyzed by electrospray HPLC-MS. In the case of lemon essential oils, all twenty residues were separated by liquid/solid extraction on a mixed Florisil-C(18) cartridge. The method enabled the analysis of the twenty pesticide residues at levels of 2 to 30 ppm with limits of detection ranging between 0.02 to 0.50 mg L(-1).     

Mechanism of thermal polymerization of cyanate ester systems: Chromatographic and spectroscopic studies

The mechanisms and kinetics of polymerization of mono- and difunctional cyanate esters are investigated using chromatographic (HPLC) and spectroscopic methods (UV, liquid and solid-state NMR, and FTIR). The results obtained after chromatographic separation and identification of the chemical species present in the reaction medium have enabled us to propose a reaction path and a kinetic model for these thermally polymerized systems. Finally, the polymerization of cyanate ester was studied in the presence of different catalysts (imidazole, AcAcCu and AcAcCr) added directly, without solvent, and showed their influence on mechanisms. © 1996 John Wiley & Sons, Inc.     

Reticulation de resines epoxydes sous microondes

A microwave treatment has been applied to the curing of a DGEBA epoxy resin with an aromatic diamine (DDS). The aim of study is to understand the energy transfer processes and the crosslinking mechanisms induced by pulsed microwave treatment. The “pulsed” technique induces an increase in the electrical field inside the polymer without changing the overall energy transfer, as compared with continuous microwave treatment of the same average power level. The cured samples were investigated by Fourier Transform Infra-red, ¹³C solid state NMR and dynamic mechanical measurements. Network structures were found to be affected by pulse repetition period and peak pulse power.     

Composites a matrices polyimides thermoplastiques mecanismes de polymerisation et proprietes

Model compounds have been used to determine the mechanisms of reaction of aromatic diamines with aromatic cyclic dianhydrides to form polyamic acids and polyimides by thermal or chemical dehydration. Substituted phthalic anhydride were used to model the anhydride functionality and either 4,4′-diaminodiphenyl (ether, sulfone, methane) or toluidines for the amine functionality. Chemical data obtained ‘in situ’ with Fourier transform infrared spectroscopy (FTIR), 13C nuclear magnetic resonance (liquid and solid states CP MAS) and X-rays diffractometry show that the degree of imidisation is temperature and solvent dependent. This analysis resulted in an analytical method of determining the anhydride - amic acid, and/or amic - imide ratios in the partly cured polyamic acid. Selected properties of films and composites were determined and compared with similar commercial compounds.