Metallocene-methylaluminoxane catalysts for olefin polymerization. V. Comparison of Cp2ZrCl2 and CpZrCl3

The ethylene polymerization by Cp₂ZrCl₂/MAO (Cp = η⁵: cyclopentadienyl; MAO = methyl aluminoxane) and CpZrCl₃/MAO have been studied. The MW and PD (= M _w/M _w) of polymers obtained after 2.5-60 min are the same, which indicate short chain lifetime. The values of rate constants for Cp₂ZrCl₂ at 70°C are: k_p = 168?1670 (M s)^?1 and k_tr^A1 = 0.012-0.81 s^?1 depending upon [Zr] and [MAO,] k_tr^β = 0.28 s^?1, and k_tr^H = 0.2 M^?1 torr^?1/2 s^?1. These chain transfer rate constant values are two to three orders of magnitude greater than the corresponding values found for MgCl₂ supported titanium catalysts. One significant difference between the heterogeneous and homogeneous catalysts is that the former decays according to an apparent second order kinetics, whereas the latter decay is simple first order at 0°C and biphasic first order at higher temperatures. The productivity of the catalysts depends weekly on temperature while the MW decreases strongly with increase of temperature above 30°C. All the active species were formed upon mixing Cp₂ZrCl₂ with MAO while it took up to 20 min for the CpZnCl₃/MAO system. The productivity of the former increase more strongly with the decrease of [Zr] than the latter. Otherwise, the two catalyst systems have all their kinetic parameters differing less than a factor of two.     

Synthesis and characterization of polyhedral Pt nanoparticles: their catalytic property, surface attachment, self-aggregation and assembly

In this paper, we presented the preparation procedure of Pt nanoparticles with the well-controlled polyhedral morphology and size by a modified polyol method using AgNO(3) in accordance with the reduction of H(2)PtCl(6) in EG at high temperature around 160°C. The methods of UV-vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution (HR) TEM measurements were used to characterize their surface morphology, size, and crystal structure. We have observed that the polyhedral Pt nanoparticles of sharp edges and corners were produced in the preferential homogenous growth as well as the formation of porous and large Pt particles by self-aggregation and assembly originating from as-prepared polyhedral Pt nanoparticles. It is most impressive to find that the arrangement of Pt nanoparticles was observed in their surface attachments, self-aggregation, random and directed surface self-assembly by the bottom-up approach. Their high electrocatalytic activity for methanol oxidation was predicted. The findings and results showed that the polyhedral Pt nanoparticle-based catalysts exhibited the high electrocatalytic activity for their potential applications in developing the efficient Pt-based catalysts for direct methanol fuel cells.     

Electroosmotic pumping in microchips with nonhomogeneous distribution of electrolytes

A general equation to calculate the node pressure at a junction in a microfluidic network is presented. The node pressure is generated from both the hydrodynamic flow due to the external applied hydraulic pressures and the electrokinetic flow resulted from the applied electric field. Pure electroosmotic flow has a plug-flow profile and pressure flow has a parabolic flow profile. In a first order approximation, these two flows can be treated separately, and the total flow is the sum of the two. An externally applied pressure simply creates a constant offset in the node pressure as long as the flow resistances remain the same. In a nonhomogeneous microfluidic network, where the electrical resistivity or the electroosmotic mobility is not constant everywhere, the differences in electroosmotic flow in various sections of the network will create an electroosmotically induced pressure at the internal nodes. Our theoretical approach can easily be extended to networks with more than one internal node. One prediction of this theory is that any variation in electroosmotic mobility or solution resistivity in different network branches will generate a pressure, and can thus be used as a pump. As an example, we demonstrate electroosmotic pumping in a high-low buffer system.     

Development of tyrosinase biosensor based on pH-sensitive field-effect transistors for phenols determination in water solutions

An enzyme biosensor for the determination of 4-chlorophenol in water solutions based on potentiometric pH-sensitive field-effect transistors as semiconductor transducer and tyrosinase immobilised in saturated glutaraldehyde vapours as biorecognition element has been described for the first time. The main analytical characteristics were studied under different conditions, as well as the possibility to optimise these working parameters. Different factors, such as pH of immobilisation, the enzyme loading and time of immobilisation in glutaralaldehyde vapours were investigated with regard to the influence on sensitivity, limit of detection, dynamic range, and operational and storage stability. The best result gives a limit of detection close to 20 ppm and a dynamic range from 25 to 1000 ppm with sensitivity 2 mV mM(-1). The operational stability was not less-than15 h and the R.S.D. were approximately 3% for intra-sensors responses and approximately 7% for inter-sensors responses. The storage stability was >15 days.     

Tailoring the physical properties of some high birefringence isothiocyanato-based liquid crystals

This paper describes the synthesis and characterization of several liquid crystal compounds having a tolane or terphenyl core structure and a high polarizability, isothiocyanato (NCS), terminal group. The synthesized compounds have high optical birefringerce (0.35 and 0.52), the highest being exhibited by the olefin-tolanes. Several eutectic mixtures are formulated and show improved properties over the single compounds, such as a broad nematic range, low melting temperature and relatively high clearing point. The birefringence of these mixtures is in the range 0.35–0.37. UV, viscosity and electro-optical measurements of the synthesized compounds are also reported. Dipole moment and polarizability calculations were made using CS-MOPAC and Hyperchem software computational programmes.     

A New Sterically Highly Hindered 7‐Membered Cyclic Nitroxide for the Controlled Living Radical Polymerization

The synthesis of a new sterically highly hindered 7‐membered alkoxyamine, 2,2,7,7‐tetraethyl‐1‐(1‐phenylethoxy)‐1,4‐diazepan‐5‐one ( 4 ), starting from known 2,2,6,6‐tetraethyl‐1‐(1‐phenylethoxy)piperidin‐4‐one ( 3 ) via a Beckmann‐type rearrangement is presented. It is shown that ring‐enlargement by insertion of an NH moiety in going from 3 to 4 leads to a more efficient regulator for nitroxide‐mediated controlled living radical styrene (= ethenylbenzene) and butyl acrylate (= butyl prop‐2‐enoate) polymerization. In addition to the polymerization experiments, kinetic data on the reversible C? O bond homolysis of alkoxyamines 3 and 4 are presented.     

Syntheses and structures of new luminescent B(III) complexes: BPh2(2-(2-pyridyl)naphtho[b]imidazole) and BF2(2-(2-pyridyl)naphtho[b]imidazole)

A novel chelating ligand, 2-(2-pyridyl)naphtho[b]imidazole (PNI) has been synthesized and structurally characterized. The PNI molecule reacts readily with BPh₃ and BF₃ to form BPh₂(2-(2-pyridyl)naphtho[b]imidazole) (1) and BF₂(2-(2-pyridyl)naphtho[b]imidazole) (2), respectively. The structures of PNI and 1 were determined by single crystal X-ray diffraction, while that 2 by spectroscopic methods. The structure of 1 reveals that the boron center is four-coordinated. The electroluminescent device using 1 as the emitting layer shows bright yellow luminescence with a maximum emission peak at 540 nm. Complexes 1 and 2 form a new family of organometallic emitting materials which could be of interest for practical applications.     

Fabrication and Visualization of Metal‐Ion Patterns on Glass by Dip‐Pen Nanolithography

Fluorescent self‐assembled monolayers (SAMs) are used as dip‐pen nanolithography (DPN) substrates for the fabrication of patterns of Ca²⁺ and Cu²⁺ ions. The driving force for the transfer of these ions from an atomic force microscopy (AFM) tip to the surface is their complexation to organic ligands on the monolayer. By means of fluorescent surfaces, the patterns can be visualized under a fluorescence microscope. We use a custom‐built atomic force fluorescence microscope (AFFM), a combination of atomic force and confocal fluorescence microscopes, to deposit the metal ions onto the sensing SAMs by DPN and to subsequently visualize modulations of fluorescence intensity in a sequential write–read mode.     

Utilization of balanced detection with optical hard-limiters in T/W OCDMA system

In this paper, we utilize optical hard-limiters to further enhance the system performance in balanced detection T/W optical CDMA system. In T/W optical CDMA systems, it had been investigated that the balanced detection scheme is not superior to the OOK scheme because of the adverse impact of beat noise. The employment of optical hard-limiter is an effective way to alleviate the deteriorating effect of beat noise. The carrier-hopping prime code is utilized as signature sequences whose out of phase autocorrelation is zero and the cross-correlation is at most one. Due to the orthogonality of signature sequence, the correlator of shifted version of carrier-hopping prime code is utilized in the receiver. The numerical results reveal that our proposed scheme can greatly reduce the error floor; also it is superior to that using OOK scheme with optical hard-limiter. Most important of all, our proposed scheme has fixed and optimum thresholds which do not vary with system parameters; and this will lead to dynamic estimation of optimal thresholds unnecessary in the receiver.     

Experimental evaluation of a non-linear coded excitation method for contrast imaging

Previously, we have shown that for a single bubble, using chirps as the excitation signal improves both the linear and the non-linear response. Computer simulations of randomly distributed contrast agent bubbles show an increase of 10-13 dB in response when comparing pulse excitations with chirp excitations that have equal bandwidths and peak amplitudes. Second harmonic compression of simulated bubble echoes with chirp excitation shows low side-lobe levels and limited loss of axial resolution when compared to pulse excitation. Experimental results from water tank measurements with SonoVue contrast agent are in agreement with computer simulations showing increased signal-to-noise ratio and an increase of approximately 12 dB at the second harmonic when comparing pulse and chirp excitation.