Resistive‐type humidity sensors based on PVP–Co and PVP–I2 complexes

Poly(vinylpyrrolidone) films containing cobalt chloride or iodine were investigated to obtain information on their possible use as a humidity sensor element. FTIR and UV‐VIS spectroscopies were used to characterize the PVP–I₂ and PVP–Co complexes. Infrared spectroscopy revealed a structural change of both shape and intensity of the carbonyl and lactam bands, indicating the formation of an ion‐coordination polymer. The J–E curves for pure PVP, PVP–I₂, and PVP–Co films obey ohm's law at low voltages, deviate from the linear response at higher voltages, and finally display breakdown behavior. An increase in current density of the PVP matrix with iodine or cobalt doping is attributed to the formation of charge transfer complexes. The observed hysteresis of the I–V characteristics implies that there was some standing voltage in the film, which could be attributed to a disorientation of polar side groups of PVP. The electrical conductivities of the polymeric complexes were very sensitive to environmental humidity. An explanation of the humidity‐sensing behavior of the PVP–I₂ and PVP–Co complexes is presented. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 459–469, 2001     

Effect of Prepolymerization on the Kinetics of Ethylene Polymerization and Ethylene/1‐Hexene Copolymerization with a Ziegler–Natta Catalyst in Slurry Reactors

The effect of prepolymerization on ethylene homopolymerization and ethylene/1‐hexene copolymerization with a commercial TiCl₄/MgCl₂ catalyst was investigated and the apparent homo‐ and copolymerization rate constants were estimated by varying polymerization temperature, pressure, time, and 1‐hexene/ethylene molar ratio during the prepolymerization. The apparent rate constants for activation, propagation, and deactivation depend on the prepolymerization conditions, showing that the prepolymerization stage strongly regulates the behavior of the catalyst in the main polymerization. Interestingly, the surface morphology of the prepolymer particles correlates to and explains these changes in polymerization kinetics behavior.

     

X-ray scattering and multivariate analysis for classification of organic samples: a comparative study using Rh tube and synchrotron radiation

This work compares the performance of X-ray tube induced and synchrotron induced energy dispersive X-ray fluorescence in generating scattering profiles when organic samples are irradiated. In the first case, this effect produces sharper peaks, well defined in the spectra, whereas synchrotron-induced scatter are seen as broad bands. These effects may be used for classifying simple materials like graphite, coke, activated carbon and carbon nanotubes, all having the same composition but different structures, using multivariate data analysis. In a second sample set, the method was applied to liquid samples of different alcohols (methanol, ethanol, 1-propanol and 2-propanol). Classifications were obtained in both cases independent of the X-ray source (synchrotron radiation or X-ray tube radiation), showing that the use of chemometric tools combined with X-ray spectrometry can efficiently distinguish organic samples by using scattering effects.     

‘Higher-order’ azomethine ylides in the synthesis of functionalized pyrroles and 5-oxo-5H-pyrrolizines

Azafulvenium methides generated by the thermal extrusion of SO₂ from 1-methyl- and 1,1-dimethyl-1H,3H-pyrrolo[1,2-c]thiazole-2,2-dioxides undergo [1,8]H sigmatropic shifts to give vinylpyrroles. Flash vacuum pyrolysis of the C-vinylpyrroles affords 5-oxo-5H-pyrrolizines or C-allyl-1H-pyrroles.     

Montmorillonite incorporated with magnesium oxide

MgO was incorporated into montmorillonite (MMT) though the controlled formation of Mg(OH)₂ followed by calcination. The evidence for Mg²⁺ in MMT before calcination was concluded from TG curves where two different temperature ranges for Mg(OH)2 dehydroxylation suggested that the location of Mg²⁺ at its internal and external surfaces. FTIR by diffuse reflectance technique of calcined sample showed a typical MgO band. XRD indicated a decrease in structural order and no collapse of clay structure. The organic compounds adsorption to MMT modified with MgO as suggested by increased degradation temperatures occurred at internal and external clay surfaces besides in an area formed probably by MgO and MMT layers.     

Semi-empirical topological method for the prediction of the chromatographic retention of cis- and trans-alkene isomers and alkanes.

A new index is proposed for the prediction of the chromatographic retention of the cis- and trans-n-alkene isomers and alkanes. This index is based on the hypothesis that the chromatographic retention of the molecule is due to the interaction of each carbon atom with the stationary phase, and consequently the index is reduced by its neighbours' steric effects. The topological values are obtained by a numerical approximation considering the general behaviour of the chromatographic retention of the compounds. The simple linear regressions between the chromatographic retention and the index proposed for all branched alkanes and also isomers of the studied straight-chain C5 and C14 alkenes (1-ene, cis- and trans-2-, 3-, 4-, 5-, 6- and 7-enes) is very good (the correlation coefficient is r = 0.9999), and the elution sequence is correct for most of them. The models have a high predictive ability, as established by cross-validation values (r2cv). Thus, this new method, different from those already existent, can be used as complementary tool for the elucidation of the molecular structure, or prediction of the chromatographic retention of the cis- and trans-alkene isomers and branched alkanes. It could be extended with success, in the future, to the other types of compounds.     

Determination of amphetamine and its metabolite p-hydroxyamphetamine in rat urine by reversed-phase high-performance liquid chromatography after dabsyl derivatization.

The consumption of amphetamine is illicit and controlled due to both the elicited behavioural deviations and the toxicity effects reported in abusers. Thus, amphetamine levels in biological samples must be monitored in several clinical and forensic circumstances. In spite of the interspecies differences in the preferred route of biotransformation, benzylmethylketone, benzoic acid and 4-hydroxyamphetamine are the principal metabolites of amphetamine. However, the clinical and forensic studies are focused in the parent compound and in 4-hydroxyamphetamine since benzylmethylketone is a minor metabolite in human and benzoic acid is also an endogenous compound. In the present study amphetamine and its metabolite, 4-hydroxyamphetamine, are quantified in urine by HPLC after derivatization with 4-dimethylaminoazobenzene-4'-sulfonyl chloride (dabsyl chloride). This derivatization procedure transforms amphetamine and its hydroxylated metabolite in compounds with similar lipofilicity, enabling their quantitative and simultaneous extraction with an organic solvent. The precision of the HPLC technique was 7.3 and 10.0% for amphetamine and 4-hydroxyamphetamine derivatives, respectively. For the overall procedure, including enzymatic hydrolysis, derivatization and extraction of the derivatives, the obtained values were 9.3 and 6.2%. Recoveries obtained from spiked urines for amphetamine and 4-hydroxyamphetamine were better than 97% and 94% (mean value), respectively. The detection limits of the method was 10 ng for both compounds. The principal advantages of the present proposed method are the stability of the dabsyl derivatives at room temperature and the detection carried out in the visible region, reducing the interferences detected.     

Mathematical modeling of crystallization analysis fractionation of ethylene/1‐hexene copolymers

Crystallization analysis fractionation (Crystaf) is a polymer characterization technique for estimating the chemical composition distributions of semicrystalline copolymers. Although Crystaf has been widely used during the recent years, it is still a relatively new polymer characterization technique. More quantitative understanding of its fractionation mechanism is essential for further developments. In this work, three ethylene/1‐hexene copolymers with different 1‐hexene fractions, but similar number‐average molecular weights, were analyzed by Crystaf at several cooling rates. A mathematical model was proposed to describe the effect of comonomer fraction and cooling rate on Crystaf fractionation from a fundamental point of view. The model describes the experimental Crystaf profiles of ethylene/1‐hexene copolymers with different 1‐hexene fractions measured at distinct cooling rates very well. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1010–1017, 2007     

An improved flow-based procedure for microdetermination of total tannins in beverages with minimized reagent consumption

A flow system designed with solenoid micro-pumps is introduced for spectrophotometric determination of total tannins based on the Folin– Denis reaction. The procedure minimizes the main drawbacks related to the AOAC batch procedure, i.e. interferences from reducing species in the samples, high reagent consumption and waste generation, and low sampling rate. Linear response was observed for tannic acid concentrations in the range 2–100 mg L⁻¹, with a detection limit (99.7% confidence level) of 0.3 mg L⁻¹. The sampling rate and coefficient of variation (n = 10) were estimated as 75 measurements per hour and 1.1%, respectively. Results of determination of total tannin in tea, beer and wine samples were in agreement with those achieved by the batch reference procedure at the 95% confidence level. In comparison to the batch procedure, the reagent consumption and effluent generation were 83 and 60-fold lower, respectively. Correspondence: Fábio R. P. Rocha, Instituto de Química, Universidade de S?o Paulo, PO Box 26077, S?o Paulo, 05513-970 SP, Brazil     

SBS/Pani · DBSA mixture plasticized with DOP and NCLS - Effect of the plasticizers on the probability density of volume resistivity measurements

Plasticized mixtures of styrene-butadiene-styrene block copolymer (SBS) and Polyaniline (Pani) were prepared in a Haake internal mixer. Two different plasticizers were used: dioctyl phthalate (DOP) and cashew nut shell liquid (CNSL). Pani and plasticizers were characterized by FTIR and the resistive behavior of plasticized mixtures was investigated along the electrification time. It is shown that obtained experimental data are subject to deterministic dynamic fluctuations that cannot be described by single normal probability distribution functions (PDF) along the time. The PDF analysis shows that obtained PDFs must be described as sums of at least three distinct Gaussian distributions with different areas. It is also shown that the Gaussian component with larger area may provide better representation of the measured volume resistivity values.