Microwave-assisted synthesis of calix[4]resorcinarenes

Microwave-assisted synthesis of calix[4]resorcinarenes by cyclocondensation of various aldehydes and resorcinol catalysed by 12-tungstophosphoric acid type Keggin (H₃PW₁₂O₄₀·13H₂O) or concentrated HCl is described. Excellent isolated yields (up to 90%) were attained within short reaction times (typically, 3-5 min) when the reaction was performed under microwaves irradiation.     

High-performance thin-layer chromatography chemical fingerprinting: a modern technique for comparative assessment of a multivariate chromatogram analysis of Padina boergesenii

JPC – Journal of Planar Chromatography – Modern TLC - Gujarat’s coastline holds immense storage of valuable marine macroalgae. Padina boergesenii is normally familiar as leafy...     

The transducer vibratory profile effects on the detection of the transient ultrasonic field scattered by a rigid point reflector

In this study a generalized approach, based on the impulse response technique, is developed to evaluate the transient ultrasonic field scattered by a rigid point reflector and detected by a planar circular transducer characterized by a non-uniform vibratory profile. For this purpose, several analytical functions are used to represent the non-uniform vibration amplitude of the transducer. All these functions have maximum amplitude at the transducer’s center and decrease monotonically away from axis. The amplitude variation of these functions has a direct effect on the shape of the average pressure detected by the transducer. Some numerical results are presented to illustrate the effects of a non-uniformly vibrating source on the detected transient pressure. The results show the relative importance of the edge wave modifications in the case of source having non-uniform vibration amplitude distribution.     

Tailoring MWD of Bimodal Polyethylene in the Presence of Ziegler-Natta Catalyst

Summary: Linear poly (ethylene-co-1-butene) was produced through two-step polymerization in one reactor using a Ziegler-Natta catalyst, where in the first step, low molecular weight homopolymer of ethylene in the presence of hydrogen and in the next step, high molecular weight copolymer of ethylene with 1-butene in the absence of hydrogen were produced. Molecular weight distribution of bimodal polyethylene was tailored through adjustment of polymerization time of each stage and hydrogen concentration of the first stage. Increasing hydrogen concentration shifted the molecular weight distribution curve to the lower molecular weights and broadened molecular weight distribution while interestingly increased high molecular weight incorporation of copolymer produced in the second stage due to increasing of reaction rate in the second step. To achieve bimodal molecular weight distribution, the polymerization times of the first and the second steps, which are highly dependent on the amount of hydrogen, were adjusted properly. The effects of the mentioned parameters on the processability as well as rheological properties of some samples were investigated. The rheological results showed shear thinning behavior of all specimens and confirmed the changes in molecular weight and molecular weight distribution. It was also demonstrated that the melt miscibility between low molecular weight and high molecular weight fractions improved with increasing of chains having very low molecular weight.     

Nitroxide bound beta-cyclodextrin: is there an inclusion complex?

236CDTIPNO, a derivative of the persistent free radical TIPNO (1,1-dimethylethyl 2-methyl-1-phenylpropyl nitroxide) covalently bound to a permethylated-beta-cyclodextrin, was prepared. Self-association of 236CDTIPNO in water was proved by solvent- and competition-dependent EPR spectroscopy experiments with 2,6-di-O-Me-beta-cyclodextrin (DIMEB) and permethylated-beta-cyclodextrin (TRIMEB) as external hosts competing for accommodation of the TIPNO moiety. Temperature-dependent EPR spectra were simulated with a novel two-dimensional (field-temperature) EPR simulation program that afforded a full determination of the thermodynamic parameters characterizing the rate constants of the self-inclusion reaction derived from Arrhenius and Eyring models. This method allows separating the line broadening effects due to relaxation from a chemical exchange, even if only the fast exchange regime is accessible experimentally. The activation parameters for the forward and backward steps were consistent with an equilibrium between a nonassociated form and a weakly associated form, with activation free enthalpies for each reaction of around 34 kJ.mol(-)(1).     

MCNP5 evaluation of a response matrix of a Bonner Sphere Spectrometer with a high efficiency <Superscript>6</Superscript>LiI (Eu) detector from 0.01 eV to 20 MeV neutrons

A new approach has been suggested to establish reliable response matrix of a CRNA Bonner Sphere Spectrometer equipped with a high efficiency 10 mm diameter × 2 mm thick ⁶LiI (Eu) scintillation detector from thermal neutron energy up to 20 MeV. It combines the experimental response functions obtained using a continuous neutron spectrum emitted from an ²⁴¹Am-Be neutron source of known emission, with those obtained by Monte-Carlo (MC) calculations. Sensitivity analysis has been performed to check the influence of pertinent parameters on the MC computed detector response. The parameters of interest are the density of polyethylene moderators, the cross section library used in MCNP5 and two computing models depending on the interactions of the neutrons in the detector. The calculated responses of the spectrometer to the ²⁴¹Am-Be neutron source showed discrepancies depending on the sphere diameter when compared to our measured responses. However, this difference was smaller compared with the experiments available in the literature. The enhancement achieved in the present study is partly due to improved modeling in the MC calculations. To correct for the differences observed, a fitting factor has been deduced for each sphere diameter and used to adjust the MC responses to the experimental ones. The obtained results were subsequently compared to the experimental data of GSF-G spheres system obtained for a typical 4 mm diameter × 4 mm thick ⁶LiI scintillator for four ISO-8529 selected neutron energies (144.0 keV, 565 keV, 2.5 and 14.8 MeV). The present responses were similar in shape but higher, in absolute values, by about two and half times for all spheres owing essentially to the increase in active part of the detector. Finally, the CRNA-BSS response matrix containing 48 energy points from 0.01 eV to 20 MeV for 22 sphere diameters with a mean polyethylene density of 0.944 g/cm³ was constructed by applying a spline interpolation method. The released response matrix was then compared to data available in the literature evaluated by Monte-Carlo calculations for a 4 mm × 4 mm ⁶LiI crystal scintillator. The obtained response matrix is intended for radiation protection applications at CRNA.     

Characterizations of realized homogeneous Besov and Triebel-Lizorkin spaces via differences

Based on the role of the polynomial functions on the homogeneous Besov spaces, on the homogeneous Triebel-Lizorkin spaces and on their realized versions, we study and obtain characterizations of these spaces via difference operators in a certain sense.