High char‐yielding poly[acrylonitrile‐co‐(itaconic acid)‐co‐(methyl acrylate)]: synthesis and properties

Polyacrylonitrile terpolymers of various compositions consisting of acrylonitrile (AN), itaconic acid (IA) and methyl acrylate (MA) were synthesized by solution polymerization in dimethylsulfoxide. Increase in concentration of either IA or MA retarded the overall polymerization rate and the polymer molecular weight. The system consisting of AN + MA and varying IA concentration was more prone to retardation in comparison with the system composed of AN + IA with variable MA concentration. The retardation factors were quantified. Minor quantities of MA boost the reactivity of IA in the terpolymer system. The terpolymer was richer in MA vis‐à‐vis the feed. The thermal characteristics of the terpolymer were examined as a function of its composition. In contrast to the copolymer of AN and IA requiring 1–1.5 mol% IA, the terpolymer required an IA content of approximately 2.5 mol% for optimum thermal stability. The polymer with 90 mol% AN, 2.5 mol% IA and 7.5 mol% MA exhibited reasonably good char‐forming characteristics and thermal stability. The overall crystallinity and crystallite size of the polymers were found to decrease on incorporation of the comonomers. The ‘aromatization index’ of the copolymer increased with the temperature of pyrolysis through re‐organization of the tetrahydropyridine ladder structure. Copyright © 2005 Society of Chemical Industry     

Ethylene Oligomerizations by Diazene Bridged Ni(II) Catalysts Derived from Pyrazole-Scaffold-Based Binucleating Ligands with Alkyl and Aryl Pendant Arms

Abstract  

The coordination and organometallic chemistry of a series of diazene (N₂)-bridged Ni(II) catalysts derived from pyrazole-scaffold-based ligands bearing alkyl and aryl pendent arms was investigated. Binucleating ligands were obtained as products of the condensation reaction between 3,5-dichloroformyl-1H-pyrazole and aliphatic/aromatic primary/secondary amines under anhydrous conditions. The Ni(II) catalysts were activated with ethyl aluminum sesquichloride (EASC) to oligomerize the ethylene mainly into C4, C6, C8, and C10 fractions with activities up to 1.2 and 0.5 × 10⁶ g (mol-Ni)⁻¹ bar⁻¹ h⁻¹ at 30 and 50 °C, respectively. All catalysts were found to be electrochemically active in the working potential range of −2 to +2 V. A change in the potential of Ni(II) was provoked by the N₄ donor bridging ligands, increasing the ethylene oligomerization activity.     

“String and bead” model of copper modified polycarbosilane: synthesis and applications

Journal of Materials Science - The synthesis of metal modified polycarbosilanes is currently an area of significant activity. These polymers can be processed to advanced materials such as ceramic...     

Simple and low-cost synthetic route for SiBCN ceramic powder from a boron-modified cyclotrisilazane

This study reports a simple and low-cost synthetic route for preparing SiBCN ceramic powder via pyrolysis of boron-modified cyclotrisilazane (BCTS). BCTS resins were synthesized by reacting boric acid with 1, 3, 5-trimethyl-1′, 3′, 5′-trivinylcyclotrisilazane (CTS) in the molar ratio of 1:1, 1:3, and 1:5. The boron modification in CTS resin resulted in optimum properties for preceramic polymers such as solubility in common solvents, processable viscosity (<20 cps) and high ceramic yield (>80 wt. %). The polymer to ceramic conversion was carried out at 1450 and 1650°C under a nitrogen atmosphere. The study demonstrated that the changes in CTS concentration and pyrolysis temperature significantly affected the evolution of ceramic phases, morphology, and elemental composition which were thoroughly investigated through XRD, SEM, and HRTEM techniques. The results revealed the formation of β-SiC, β-Si₃N₄, and oxide ceramic phases with BCTS in the molar ratio of 1:1 and 1:3; whereas, β-SiC, β-Si₃N₄, and turbostratic BN(C) ceramic phases were obtained with BCTS in the molar ratio of 1:5.     

A comparative study on Cf/PyC/SiC minicomposites prepared via CVI process for hypersonic engine application

Carbon fiber reinforced silicon carbide (SiC) minicomposites were prepared from three variants of commercially available carbon fibers, viz., T300‐3K, T300J‐3K, and T300‐6K. The SiC matrix infiltration was done via chemical vapor infiltration process using methyltrichlorosilane as the precursor. Minicomposites were characterized for the composition and morphology of the matrix material deposited. It was found that the matrix contains 2H‐SiC along with the major phase 3C‐SiC. Cyclic tensile tests were carried out on the composites to understand the damage mechanism and load bearing characteristics under cyclic loading conditions. The dependence of peak and residual strains on the fiber volume fraction was studied. Oxidation of the CMCs in air at 1500°C was studied and the result was explained based on a five part process.     

Microfluidics assisted fabrication of microspheres by poly(2–hydroxyethyl methacrylate)-block-poly(l-histidine) hybrid materials and their utilization as potential drug encapsulants

A new series of synthetic polymer bioconjugate hybrid materials consisting of poly(2-hydroxyethyl methacrylate)[p(HEMA)]as a synthetic polymer block and poly(l-histidine) [p(His)] as a polypeptide block were synthesized by combining atom transfer radical polymerization of HEMA with ring opening polymerization of benzyl-N-carboxy-l-histidine anhydride. The resulting biocompatible p(HEMA)₂₅-b-p(His)_n (n = 15, 25, 35, and 45) hybrid polymers were investigated for their use as pH-sensitive drug delivery system. Highly uniform microspheres were fabricated by using the microfluidics assisted self-assembly of these block polymers and utilized them as carriers for a chemotherapeutic agent doxorubicin and a beta-lactam antibiotic ampicillin for the evaluation of the drug-loading and release behaviors according to pH, demonstrating the release is sensitive to pH. The capability of the new p(HEMA)-b-p(His) hybrid materials to self-assemble in microenvironments and to effectively encapsulate drug molecules serves as suitable carriers for the delivery of drug molecules with varying physiochemical properties.     

Synthesis,Ceramic Conversion and Microstructure Analysis of Zirconium Modified Polycarbosilane

Polymer derived ceramics have been widely being explored as high temperature structural components in aerospace as rocket nozzles, nose tip and leading edges of reusable launch vehicles. Polycarbosilane (PCS) was modified by a condensation reaction with zirconium acetylacetonate [Zr(acac)₄] to form polyzirconocarbosilane (PZrCS). A series of PZrCS were synthesized, which could be transformed into Si–Zr–C ceramic phases on pyrolysis. The ceramic yield of PCS was significantly improved by the introduction of zirconium into the system. The XRD patterns of the PZrCs show the characteristic peaks of ?SiC at 1300 °C and at 1500 °C the characteristic peaks of ZrC and ZrO₂ were observed. The carbothermal reaction in PZrCS was completed at 1650 °C and the resulting ceramic was non-oxide SiC/ZrC phase. The SEM images proved that the increase in concentration of zirconium in the final ceramic decreases the surface uniformity. HRTEM analysis of PZrCS heat treated at 1650 °C shows the evolution of oxide free ZrC/SiC phase with compatible grain boundaries without stacking fault. It could be concluded that the technique of introducing ultra-high temperature ceramic phases into the SiC matrix is an effective approach to improve the high-temperature performance of silicon based ceramics.     

Energy efficient bidirectional relay network with spatial modulation

S patial modulation is a potential candidate for 5G wireless communication systems that provides high spectral efficiency with high reliability and low complexity. Spatial modulation conveys information in the index of transmitting antenna along with conventional modulation scheme. Also, energy efficiency communication plays a vital role in 5G wireless communication. In this article, energy efficiency and spectral efficiency are focused on a bidirectional relay network. In the proposed bidirectional relay network, the energy consumption burden at the relay node is reduced by placing a power splitter that coordinates the energy harvesting and information processing at the relay node. Spatial modulation is employed at all nodes to reduce the effect of interchannel interference and synchronization problem in the receiver. The combined effect of spatial modulation in all nodes and energy harvest at the relay node are analyzed in the bidirectional relay network. The end‐to‐end outage probability expression for the bidirectional relay network is derived in terms of power splitting factor at relay node. Analytical simulation results have been verified by Monte‐Carlo simulations. The overall performance of the proposed system is compared with an existing literature and found that the proposed system is having better spectral efficiency and energy harvesting.     

Synthesis and stability of chitosan gold nanocomposites: Effect of time of heating and concentration of reactant

Gold nanoparticles (AuNPs) exhibit red colour and is suitable for development of indicators based on colour change. This work investigated the influence of concentration of gold precursor and time of thermal treatment on the properties of chitosan gold nanocomposites. Three concentrations of gold, 5 mM, 10 mM, and 15 mM were used and the heating time studied were 5 min and 15 min. The UV–vis spectra, transmission electron microscopic characteristics, zeta potential, and visible colour were inspected. All the samples have shown distinct colour change and the characteristic surface plasmonic resonance peak at ~530 nm. The spectral properties, zeta potential, and particle dispersity were significantly affected by the concentration and time. Furthermore, the nanoparticles were exposed to frozen condition at −18 °C ± 2 °C. The absorbance maxima of surface plasmonic resonance peak have changed significantly in all the samples. The visible colour of chitosan gold nanocomposites has changed on freezing and the microscopic analysis demonstrated the clustering and change in shape of nanoparticles on freezing. The results suggest that the chitosan gold nanocomposites synthesised with 10 mM gold chloride and 5 min of heating can be efficiently used as freeze indicator.     

Effect of oxalic acid on the rheological properties of dope solution of poly[acrylonitrile‐co‐(methyl acrylate)‐co‐(itaconic acid)]

The very high dope viscosity of concentrated dope of poly[acrylonitrile‐co‐(methyl acrylate)‐co‐(itaconic acid)] (with M?_v = 10.67 × 10⁵g mol^?1) in DMF could be diminished significantly by the addition of oxalic acid (OXA). The change in steady shear rheological behaviour caused by OXA has been analysed for the dope using a rheometer working in the viscosity mode. The temperature dependence of η₀ conformed to the Arrhenius‐Frenkel‐Eyring equation. ΔG_v decreased marginally with OXA concentration, and the least value was observed at an OXA concentration of 0.63 % by weight. Shear thinning behaviour was observed under higher shear rates for the terpolymer solutions in the presence and absence of OXA. The pseudoplasticity index (n) showed an abrupt initial increase on addition of OXA. The OXA concentration of 0.63 % by weight was advantageous for decreasing the viscosity of the polymer dope. The reduction in viscosity is attributed to the disturbed polymer‐polymer interactions by way of H‐bonding of OXA with the polymer. OXA‐containing dope at higher shear rate could achieve very low viscosities. Copyright © 2004 Society of Chemical Industry