Thermal analysis of polymer layered silicate nanocomposites

It has been shown, for three different polymer layered silicate (PLS) nanocomposite systems, how differential scanning calorimetry (DSC) can identify the different reactions of homopolymerisation and of crosslinking that occur in the intra- and extra-gallery regions of these nanocomposites, respectively, and hence how DSC can be used to assess the cure conditions for optimising their nanostructure. The PLS nanocomposites are based upon: (i) diglycidyl ether of bisphenol-A (DGEBA) cured with a polyoxypropylene diamine; (ii) DGEBA cured with an –NH₂ terminated hyperbranched polymer (HBP); and (iii) tri-glycidyl p-amino phenol (TGAP) cured with a diamine. In each case, the existence of both intra- and extra-gallery reactions in the DSC cure curves, and whether they occur simultaneously or sequentially, and in what order, are identified and correlated with the nanostructure as observed by small angle X-ray scattering and transmission electron microscopy. In particular, it is shown that the intra-gallery reaction must precede the extra-gallery for significant exfoliation to occur. In accordance with this scenario, the TGAP/diamine system displays the greatest degree of exfoliation, the DGEBA/diamine system the least, with the DGEBA/HBP system intermediate. For those systems in which significant exfoliation occurs, the DSC cure curves also allow the optimum cure conditions, such as the isothermal cure temperature, to be determined.     

Natural sesquiterpenoids

This review covers the isolation, structural determination, synthesis and chemical and microbiological transformations of natural sesquiterpenoids. The literature from January to December 2005 is reviewed,and 386 references are cited.     

Simultaneous spectrophotometric determination of diuretics by using multivariate calibration methods

The wavelength range and number of factors used in partial least-squares (PLS) calibration for the resolution of the dihydralazine (DHZ)-hydrochlorothiazide (HCT) binary mixture and the dihydralazine-hydrochlorothiazide-reserpine ternary mixture were optimized in terms of the relative standard error (R.S.E.) and relative mean standard error (R.M.S.E.). Under the optimum conditions thus established, synthetic mixtures of the analytes can be resolved with errors and relative standard deviations (R.S.D) less than 4.5 and 1.0%, respectively. The ensuing method, which was validated by comparison with high performance liquid chromatography (HPLC), also gives good results with real samples (pharmaceutical preparations).     

Innovative detection methods for aquatic algal toxins and their presence in the food chain

Detection of aquatic algal toxins has become critical for the protection of human health. During the last 5 years, techniques such as optical, electrochemical, and piezoelectric biosensors or fluorescent-microsphere-based assays have been developed for the detection of aquatic algal toxins, in addition to optimization of existing techniques, to achieve higher sensitivities, specificity, and speed or multidetection. New toxins have also been incorporated in the array of analytical and biological methods. The impact of the former innovation on this field is highlighted by recent changes in legal regulations, with liquid chromatography–mass spectrometry becoming the official reference method for marine lipophilic toxins and replacing the mouse bioassay in many countries. This review summarizes the large international effort to provide routine testing laboratories with fast, sensitive, high-throughput, multitoxin, validated methods for the screening of seafood, algae, and water samples.     

Determination of the content of fatty acid methyl esters (FAME) in biodiesel samples obtained by esterification using 1H-NMR spectroscopy

Three different calibration curves based on (1)H-NMR spectroscopy (300 MHz) were used for quantifying the reaction yield during biodiesel synthesis by esterification of fatty acids mixtures and methanol. For this purpose, the integrated intensities of the hydrogens of the ester methoxy group (3.67 ppm) were correlated with the areas related to the various protons of the alkyl chain (olefinic hydrogens: 5.30-5.46 ppm; aliphatic: 2.67-2.78 ppm, 2.30 ppm, 1.96-2.12 ppm, 1.56-1.68 ppm, 1.22-1.42 ppm, 0.98 ppm, and 0.84-0.92 ppm). The first curve was obtained using the peaks relating the olefinic hydrogens, a second with the parafinic protons and the third curve using the integrated intensities of all the hydrogens. A total of 35 samples were examined: 25 samples to build the three different calibration curves and ten samples to serve as external validation samples. The results showed no statistical differences among the three methods, and all presented prediction errors less than 2.45% with a co-efficient of variation (CV) of 4.66%.     

Curing kinetics of the epoxy system diglycidyl ether of bisphenol A/isophoronediamine by Fourier transform infrared spectroscopy

The curing reaction of an epoxy system consisting of a diglycidyl ether of bisphenol A (BADGE, n = 0) and isophoronediamine (IPD), was studied by Fourier transform infrared spectroscopy (FTIR). The degree of conversions and the reaction rates at different isothermal curing temperatures were calculated from the infrared spectra using a method derived from Beer's law. This method is based on the ratio of the height of the characteristic absorbance peak to reference absorbance peak. A kinetic model proposed by Sourour and Kamal has been used to fit experimental data. Copyright © 2008 John Wiley & Sons, Ltd.     

Study of the influence of a reactive diluent on the rheological properties of an epoxy-diamine system

The generalized temperature integral I(m, x) appears in non-isothermal kinetic analysis when the frequency factor depends on the temperature. A procedure based on Gaussian quadrature to obtain analytical approximations for the integral I(m, x) was proposed. The results showed good agreement between the obtained approximation values and those obtained by numerical integration. Unless other approximations found in literature, the methodology presented in this paper can be easily generalized in order to obtain approximations with the maximum of accurate.     

Synthesis of Dibenzocyclooctadienes by Anodic Oxidation

the use of electrochemical oxidation was applied to the synthesis of dibenzocyclooctadiene lactones related to steganacin skeleton. This reaction, performed with compounds 1 and 2, led to results comparable with the oxidation with RuTFA, TTFA and VOF₃.