New method to determine the recombination rate constant between nitroxides and macroradicals

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Sequential catalytic growth of carbon nanotubes

Nanofilaments made of repeated units of multiwall carbon nanotubes (MWCNT) with a metal particle encapsulated at one end were synthesized by thermal chemical vapor deposition (CVD) of methane on Ni-Fe catalysts. The nanofilaments were characterized using scanning electron microscopy (SEM), low- and high-resolution transmission electron microscopy (TEM). A mechanism based on a sequential growth is here proposed to explain the structure. Using the structural information from the TEM micrographs, we demonstrate the existence of two different growth regimes depending on the size of the catalyst particle and estimate the critical carbon concentration in the catalyst particle to initiate the growth. (C) 2002 Elsevier Science B.V. All rights reserved

Sequentially grown carbon nanotubes

Nanofibers consisting of repeated units of multiwall carbon nanotubes (MWCNT) with a metal particle encapsulated at one end were synthesized by thermal chemical vapor deposition (CVD) of methane/hydrogen on Ni-Fe catalysts. The grown structures were studied by scanning electron microscopy (SEM), low and high-resolution transmission electron microscopy (TEM). Sequential growth is proposed to explain the periodicity of the nanofibers. The nanofibers are broken by sonication into single elementary units open at one end

Reducing the Degree of Branching in Polyacrylates via Midchain Radical Patching: A Quantitative Melt-State NMR Study

Degrees of branching were measured with 13C melt-state NMR in poly(n-butyl acrylates) synthesized in both the presence and absence of 1-octanethiol at various temperatures, and their precision calculated based on the signal-to-noise ratio of the quaternary carbon at the branching point. Nitrogen gas was used for all pneumatics. Samples synthesized with thiol were measured in 7 mm rotors at MAS rotational frequencies of 1.8 to 4.5 kHz, while samples synthesized without thiol were measured in 4 mm rotors at 9 kHz MAS. The relative standard deviation SD of DB was calculated from the signal-to-noise ratio of the quaternary carbon SNR. The accuracy of measured degrees of branching was ensured by full relaxation of the relevant signals between pulses. A significant decrease of branching was observed in the presence of a thiol during the polymerization process, experimentally confirming the patching effect of midchain tertiary radicals by the thiol

Determination of propagation rate coefficients for methyl and 2-ethylhexyl acrylate via high frequency PLP-SEC under consideration of the impact of chain branching

The radical propagation rate coefficients, kp, of methyl acrylate (MA) and 2-ethylhexyl acrylate (EHA) have been determined in bulk via high frequency (500 Hz) pulsed laser polymerization coupled to size exclusion chromatography (PLP-SEC) up to elevated temperatures (20 ≤ T/°C ≤ 80). Prior to the analysis of the generated polymeric material, an investigation into the branching behavior of the generated polymers has been undertaken, employing the concept of local dispersity, D(Ve). In addition, the Mark-Houwkink-Kuhn-Sakurada parameters for both poly(MA) and polyEHA were determined at each studied reaction temperature. The temperature averaged values read (K = 10.2 × 10-5 dL g-1; α = 0.741) and (K = 9.85 × 10-5 dL g-1; α = 0.719) for poly(MA) and polyEHA, respectively. The local dispersity data indicate that branching in polyEHA may be considerably more prevalent than in poly(MA), as with increasing temperature polymer microinhomogeneities are observed. Consequently, the Arrhenius parameters for kp of EHA are beset with a larger error than those of MA. The activation parameters in the temperature range between 20 and 80 °C read: EA MA = 18.5 (+0.8 to -0.9) kJ•mol-1 and AMA = 2.5 (+1.2 to -0.6) × 107 L•mol-1•s-1; EA EHA = 15.8 (+1.6 to -1.4) kJ•mol-1 and A EHA = 9.1 (+10.1 to -2.9) × 106 L•mol -1•s-1. © 2010 American Chemical Society

Toward a more general solution to the band-broadening problem in size separation of polymers

The molecular weight distributions (MWDs) and hydrodynamic volume distributions of polymers can reveal considerable mechanistic information on the polymerization process, and have significant effects on physical properties such as viscosity. While the broadening function for a particular SEC setup can be found using ultranarrow standards, these are extremely difficult to obtain. The present paper implements and tests a suggested technique (Aust. J. Chem. 2005, 58, 178) to enable the deconvolution of size distributions using broad standards, synthesized under conditions which are expected to produce a number MWD P(M) which is a single exponential. Broad standards with a wide range of (M) over bar (n) were synthesized for both styrene and methyl methacrylate (MMA), using low-conversion free-radical polymerization with appropriate choice of chain transfer agent (CTA) and initiator concentrations; standards with high (M) over bar (n)were synthesized at 25 degrees C without added initiator. The broadening function was obtained by assuming a flexible functional form (exponential Gaussian hybrid) and least-squares fitting its parameters so that the "theoretical" exponential P(M) curves for each sample, with exponents obtained experimentally, matched the experimental SEC distribution for styrene. The procedure was tested by using the same band-broadening function to deconvolute data for the original polystyrene "standards" and the polyMMA samples, using the Ishige deconvolution method. This method tends to amplify noise, and too tight a tolerance can lead to spurious structure in the deconvoluted distributions. Nevertheless, a tolerance range could be found which led to stable solutions, where the deconvoluted P(M) curves for both were indeed single exponential over the range of molecular weights where data with acceptable accuracy could be obtained. This suggests that this is a generally applicable method to correct for band broadening for a wide range of systems, although improved deconvolution methods are needed to obtain truly converged and stable solutions

High-resolution separation of oligo(acrylic acid) by capillary zone electrophoresis

Oligo(acrylic acid)s, produced by RAFT polymerization, have been separated and analyzed for the first time by capillary zone electrophoresis. The resolution obtained by capillary electrophoresis in borate buffers is far higher than that currently achieved using size exclusion chromatography. This work demonstrates that capillary electrophoresis is the technique of choice for the characterization of oligomers of acrylic acid and of other water-soluble monomers involved in emulsion polymerization processes