Design and performance of improved lumped-distributed Wilkinsondivider topology

An improved lumped-distributed planar 3 dB Wilkinson divider is presented. The technique reduces the number of components and via-grounds. An analysis based on ideal transmission lines provides design equations for the proposed Wilkinson divider. The topology gives good results at L-band and uses less than half of the substrate area of the standard topology     

Silica‐supported (nBuCp)2ZrCl2: effect of catalyst active center distribution on ethylene–1‐hexene copolymerization

Metallocenes are a modern innovation in polyolefin catalysis research. Therefore, two supported metallocene catalysts—silica/MAO/(ⁿBuCp)₂ZrCl₂ (Catalyst 1) and silica/ⁿBuSnCl₃/MAO/(ⁿBuCp)₂ZrCl₂ (Catalyst 2), where MAO is methylaluminoxane—were synthesized, and subsequently used to prepare, without separate feeding of MAO, ethylene–1‐hexene Copolymer 1 and Copolymer 2, respectively. Fouling‐free copolymerization, catalyst kinetic stability and production of free‐flowing polymer particles (replicating the catalyst particle size distribution) confirmed the occurrence of heterogeneous catalysis. The catalyst active center distribution was modeled by deconvoluting the measured molecular weight distribution and copolymer composition distribution. Five different active center types were predicted for each catalyst, which was corroborated by successive self‐nucleation and annealing experiments, as well as by an extended X‐ray absorption fine structure spectroscopy report published in the literature. Hence, metallocenes impregnated particularly on an MAO‐pretreated support may be rightly envisioned to comprise an ensemble of isolated single sites that have varying coordination environments. This study shows how the active center distribution and the design of supported MAO anions affect copolymerization activity, polymerization mechanism and the resulting polymer microstructures. Catalyst 2 showed less copolymerization activity than Catalyst 1. Strong chain transfer and positive co‐monomer effect—both by 1‐hexene—were common. Each copolymer demonstrated vinyl, vinylidene and trans‐vinylene end groups, and compositional heterogeneity. All these findings were explained, as appropriate, considering the modeled active center distribution, MAO cage structure repeat units, proposed catalyst surface chemistry, segregation effects and the literature that concerns and supports this study. While doing so, new insights were obtained. Additionally, future research, along the direction of the present work, is recommended. © 2013 Society of Chemical Industry     

Injection-locked balanced oscillator-doubler

A sub-harmonic injection-locked balanced microwave oscillator-doubler configuration is presented. A highly stabilised signal with low harmonic distortion and high fundamental signal rejection is produced without using any balun. The integrated approach results in a small circuit size and low power consumption requirement     

Effect of molecular weight and average comonomer content on the crystallization analysis fractionation (Crystaf) of ethylene α-olefin copolymers

The effect of molecular weight and average comonomer content on the crystallization analysis fractionation (Crystaf) of ethylene/1-hexene copolymers was investigated experimentally and modeled via stochastic simulations. Molecular weight and comonomer content are the main structural parameters that affect the crystallizability of polymer molecules from dilute solutions. Sets of samples with varying molecular weight and comonomer content were prepared to study each effect separately. Although both structural parameters significantly affect the shape of Crystaf profiles, comonomer content is the main determining factor for Crystaf peak location for most molecular weights of interest. The results of the stochastic simulations show good qualitative agreement with the experimental data, but also indicate some clear limitations that might be related to the kinetics of crystallization effects during Crystaf analysis.     

Stochastic simulation for morphological development during the isothermal crystallization of semicrystalline polymers: A case study of syndiotactic polypropylene

A stochastic simulation scheme for predicting morphological development during nucleation and subsequent crystal growth based on predetermined crystallization kinetic data of a semicrystalline polymer under quiescent isothermal conditions is proposed. Based on previously obtained crystallization kinetic data for syndiotactic polypropylene (s‐PP) used as the input information, the simulation scheme was successful in predicting the morphological development of s‐PP during isothermal crystallization from the melt state. The predicted development of crystallinity during crystallization was reanalyzed with the Avrami macrokinetic model, and good agreement between the predicted and theoretical values for s‐PP was observed. On the basis of this simulation scheme, both the spherulite size and its distribution during the course of crystallization could also be predicted. Although the spherulitic growth rate influenced both the spherulite size and its distribution during the course of crystallization, it had no effect on the final spherulitic morphology or the resulting average spherulitic size. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Injection-locked balanced oscillator

A novel injection-locked balanced oscillator (ILBO) topology is proposed. The circuit is simple to design, small and has low power consumption. The external injection signal is applied at the centre point of the balanced oscillator with a minor increase in complexity for the matching circuit. The technique is demonstrated practically in the 1.8 GHz band. An excellent amplitude and phase balance of the ILBO is achieved     

Influence of the longest ethylene and isotactic propylene sequences on crystallization elution fractionation of ethylene/propylene copolymers

Crystallization elution fractionation (CEF) is the newest crystallization-based technique for estimating the chemical composition distribution of ethylene/1-olefin copolymers. Understanding the separation mechanism of CEF for ethylene/propylene copolymers over their full compositional range is challenging because the crystallizabilities of the copolymer chains depend on the longest ethylene sequence and on longest isotactic propylene sequence. We developed a mathematical model to describe the CEF mechanism for ethylene/propylene copolymers over the entire compositional range using population balances for the crystallization and dissolution stages. The joint distribution of longest ethylene and isotactic propylene sequences determines how the copolymer populations crystallize and dissolve. The model was validated with experimental CEF profiles of ethylene/propylene copolymers varying from pure ethylene to propylene homopolymers.