Effect of latex concentration on epoxidation of natural rubber (NR) latex

Effect of latex concentration on the extent of epoxidation and physical properties of epoxidized natural rubber (ENR) was evaluated as a function of latex concentration in the range of 20–60% by weight. The epoxidation rate increased with increasing latex concentration. Physical properties and sequence distribution of an epoxy group of ENR with 30 mol % epoxy content was invariant with the latex concentration employed for epoxidation.     

An \mathcal O (N \log N) Fast Direct Solver for Partial Hierarchically Semi-Separable Matrices

This article describes a fast direct solver (i.e., not iterative) for partial hierarchically semi-separable systems. This solver requires a storage of $\mathcal O (N \log N)$ and has a computational complexity of $\mathcal O (N \log N)$ arithmetic operations. The numerical benchmarks presented illustrate the method in the context of interpolation using radial basis functions. The key ingredients behind this fast solver are recursion, efficient low rank factorization using Chebyshev interpolation, and the Sherman–Morrison–Woodbury formula. The algorithm and the analysis are worked out in detail. The performance of the algorithm is illustrated for a variety of radial basis functions and target accuracies.     

Efficient broadcast and multicast on multistage interconnectionnetworks using multiport encoding

This paper proposes anew approach for implementing fast multicast and broadcast in unidirectional and bidirectional multistage interconnection networks (MINs) with multiport encoded multidestination worms. For a MIN with n stages, such worms use n header flits each. One flit is used for each stage of the network and it indicates the output ports to which a multicast message needs to be replicated. A multiport encoded worm with (d₁, d₂..., d_n, 1⩽d_i⩽k) degrees of replication for the respective stages is capable of covering (d₁×d_x×...×d_n) destinations with a single communication start-up. In this paper, a switch architecture is proposed for implementing multidestination worms without deadlock. Three grouping algorithms of varying complexity are presented to derive the associated multiport encoded worms for a multicast to an arbitrary set of destinations. Using these worms, a multinomial tree-based scheme is proposed to implement the multicast. This scheme significantly reduces broadcast/multicast latency compared to schemes using unicast messages. Simulation studies for both unidirectional and bidirectional MIN systems indicate that improvement in broadcast/multicast latency up to a factor of four is feasible using the new approach. Interestingly, this approach is able to implement multicast with reduced latency as the number of destinations increases beyond a certain number. Compared to implementing unicast messages, this approach requires little additional logic at the switches. Thus, the scheme demonstrates significant potential for implementing efficient collective communication operations on current and future MIN-based systems     

Effect of the laser heating of nanotube nuclei on the nanotube type population

Many potential applications of carbon nanotubes are expected to benefit from the availability of single-walled carbon nanotube materials enriched in metallic species, and specifically armchair nanotubes. The present work focuses on the modification of the pulsed laser vaporization (PLV) technique to selectively produce certain carbon nanotube structures. Nanotube nuclei were “warmed-up” with an additional laser pulse, timed to coincide approximately with the nucleation event. The effect of the second laser on the carbon vapor temperature was studied by emission spectroscopy. Nanotube type populations with and without warm-up were compared by means of absorption, photoluminescence, and Raman spectroscopy. It was found that the warm-up of nanotube nuclei with a laser pulse has a noticeable, albeit small, effect on the nanotube population. The intensity of spectral features associated with (9,7) nanotube and its large chiral angle neighbors increased, while small chiral angle nanotubes decreased, with exception of the (15,0) tube. This experiment demonstrates that nanotube population during PLV synthesis can be manipulated in a controlled fashion.      

Silica‐supported bis(imino)pyridyl iron(II) catalyst: nature of the support–catalyst interactions

Ethylene polymerizations were performed using silica‐supported 2,6‐bis[1‐(2,6‐diisopropylphenylimino) ethyl] pyridine iron(II) dichloride with methylaluminoxane (MAO) as co‐catalyst. Silica was calcined at 600, 400 and 200 °C under vacuum for 8 h. The effect of calcination temperature of silica on the polymerization activity and the properties of the polymers obtained were examined. Catalyst–support interactions were examined by both a chemical method and XPS. It was observed that upon supporting the catalyst on the surface of silica, there is an increase in the binding energy of the metal center. However, no change in the metal binding energy was observed on supporting the catalyst to silica calcined at different temperatures. Ethylene polymerizations were performed using MAO as co‐catalyst. Catalysts were also prepared by first pretreating silica with MAO, followed by addition of the Fe(II) catalyst and contacting a complex of Fe(II) catalyst–MAO with silica previously calcined at 400 °C for 8 h. The results indicate that there is no chemical bonding between the support and the catalyst. Copyright © 2006 Society of Chemical Industry     

Methylaluminoxane: synthesis,characterization and catalysis of ethylene polymerization

Summary Three types of methylaluminoxanes (MAO-1, 2 and 3) were synthesized with different average molecular weights. In each MAO synthesis, two types of MAOs were isolated (Fractions A and B). Fractions A and B differ by the presence of different methyl/aluminum molar ratio and mol% trimethylaluminum. Kinetics of ethylene polymerization were studied using both fractions A and B of MAOs 1,2 and 3 as cocatalysts and Cp₂ZrCl₂ as catalyst at atmospheric pressure of ethylene. The poly(ethylene)s were analyzed for their molecular weight.     

Laser ablation process for single-walled carbon nanotube production

Different types of lasers are now routinely used to prepare single-walled carbon nanotubes. The original method developed by researchers at Rice University used a "double-pulse laser oven" process. Several researchers have used variations of the lasers to include one-laser pulse (green or infrared), different pulse widths (ns to micros as well as continuous wave), and different laser wavelengths (e.g., CO2, or free electron lasers in the near to far infrared). Some of these variations are tried with different combinations and concentrations of metal catalysts, buffer gases (e.g., helium), oven temperatures, flow conditions, and even different porosities of the graphite targets. This article is an attempt to cover all these variations and their relative merits. Possible growth mechanisms under these different conditions will also be discussed.     

Modeling studies on simultaneous adsorption of phenol and resorcinol onto granular activated carbon from simulated aqueous solution

The modelling study on simultaneous adsorption of phenol and resorcinol onto granular activated carbon (GAC) in multicomponent solution was carried out at 303K by conducting batch experiments at initial concentration range of 100-1000 mg/l. Three equilibrium isotherm models for multicomponent adsorption studies were considered. In order to determine the parameters of multicomponent adsorption isotherms, individual adsorption studies of phenol and resorcinol on GAC were also carried out. The experimental data of single and multicomponent adsorption were fitted to these models. The parameters of multicomponent models were estimated using error minimization technique on MATLAB R2007a. It has been observed that for low initial concentration of adsorbate (100-200mg/l), modified Langmuir model represents the data very well with the adsorption constant (Q(0)), 216.1, 0.032 and average relative error (ARE) of 8.34, 8.31 for phenol and resorcinol respectively. Whereas, for high initial concentration of adsorbate (400-1000 mg/l), extended Freundlich model represents the data very well with adsorption constant (K(F)) of 25.41, 24.25 and ARE of 7.0, 6.46 for phenol and resorcinol respectively. The effect of pH of solution, adsorbent dose and initial concentrations of phenol and resorcinol on adsorption behaviour was also investigated.