1,721,047 research outputs found
Laser single pulse initiated RAFT polymerization for assessing chain-length dependent radical termination kinetics
No full textA novel method combining RAFT polymerization with pulsed-laser initiation for determining chain-length dependent termination rate coefficients, k(t), is presented. Degenerative chain-transfer in RAFT enables single-pulse pulsed-laser polymerization (SP-PLP) traces to be measured on systems with a narrow radical distribution that remains essentially unchanged during the experiment. SP-PLP-RAFT experiments at different polymerization times allow for determining kt as a function of chain length via classical kinetics assuming chain-length independent k(t)
Chain length dependent termination in butyl acrylate free-radical polymerization studied via stationary and pulsed laser initiated RAFT polymerization
No full textThe chain-length dependence of the termination rate coefficient, k t, in butyl acrylate free-radical polymerization has been determined by two independent methods, RAFT-SP-PLP and RAFT-CLD-T, both employing control of radical chain length by reversible addition fragmentation chain transfer (RAFT) polymerization. Within RAFT-SP-PLP, the polymerization induced by a laser single pulse is monitored via near-IR spectroscopy with a time resolution of microseconds. In RAFT-CLD-T, isothermal reaction rate measurements are carried out via DSC under stationary polymerization conditions. The resulting k t data refer to the situation of living/controlled radical polymerization, where both radical chain length and monomer conversion increase during the course of the reaction. The RAFT-SP-PLP measurements were carried out at 60°C and two pressures, 5 and 1000 bar. The RAFT-CLD-T experiments were run at ambient pressure and at two temperatures, 60 and 80°C, respectively. In absolute value, the termination rate coefficients for identical pressure and temperature deduced from the two methods differ by less than a factor of 2. For the dependence of kt on chain length, i, almost identical information is provided by the two techniques. The chain-length dependence of kt may be described by the power-law expression kt(i) = k t(1,1)i-α with, however, α being different for short-chain and long-chain radicals. RAFT-SP-PLP yields α1 = 1.25 for the short-chain regime from 1 < i < 30, and α2 = 0.22 for chain lengths above i = 50. RAFT-CLD-T results in α1 = 1.04 and α2 = 0.20 in identical chain length regimes. k t(1,1) values are found to be close to 1 × 109 L mol-1 s-1. © 2005 American Chemical Society
Determination of addition and fragmentation rate coefficients in RAFT polymerization via time-resolved ESR spectroscopy after laser pulse initiation
No full textA method for simultaneous determination of both the addition and fragmentation rate coefficients of the RAFT equilibrium reactions is presented, which is based on laser single pulse initiation in conjunction with microsecond time-resolved ESR spectroscopy. The build-up and subsequent decay in concentration of the intermediate radical are measured and k(ad) and k(beta) values are deduced from fitting the concentration versus time profiles to simple kinetic models
Termination kinetics of dibutyl itaconate free-radical polymerization studied via the SP-PLP-ESRTechnique
No full textThe termination kinetics of dibutyl itaconate(DBI) bulk polymerization was studied via SP-PLP-ESR single pulse-pulsed laser polymerization with time-resolved detection of free-radical concentration by electron-spin resonace, at temperature of PLP experiments,termination rate coefficients,k(1)(i,i), are measured for radicals of (almost) identical chain length (CL)i. CL-averaged , for chain lenghts up to 200 monomer units, and also k(t)(0) referring to termination of very small-size radicals are directly deduced from measured DBI radical concentration vs time traces. At 45degreesC, is (3.4 +/-0.6) (.) 10(5) L (.) mol(-1 .) s(-1) and k(t)(0) is (7.2 +/- 1.0) (.) 10(5) L (.) mol(-1) (.) s(-1). Both rate coefficients are independent of monomer conversion up to the highest experimental conversion of 18%. The associated activation energies are E-A() = 23.0 +/- 3.2 kJ (.) mol(-1) and E-A(k(0)(t)) = 27.6 +/- 2.8 kJ (.) mol(-1), respectively. "Model-dependent" and "model-free" analyses of radical concentration vs time profiles indicate a pronouced CL dependence of k(t)(i,i) for DBI radicals of moderate size, 5 and k(t)(0) from laser-induced photopolymerization of DBI
Free-radical polymerization kinetics of 2-acrylamido-2-methylpropanesulfonic acid in aqueous solution
No full textThe SP-PLP-NIR technique, which combines Pulsed laser polymerization (PLP) initiated by a single pulse (SP) with time-resolved monitoring of the resulting monomer conversion via near-infrared (NIR) spectroscopy, was used to investigate the kinetics in aqueous solution of 2-acrylamido-2-methylpropanesulfonic acid (AMPS). For initial AMPS concentrations of 2.79 mol(.)L(-1) (50 wt% AMPS) and 1.04 mol(.)L(-1) (20 wt% AMPS), the ratio of (chain length averaged) termination and propagation rate coefficients, /k(p), was measured up to almost complete monomer conversion at temperatures between 10 and 40 degrees C and ambient pressure. Up to 80% monomer conversion, /k(p) is only slightly lowered, whereas there is a clear decrease upon further increasing conversion. Variation of temperature and of pH does not significantly affect /k(p). For estimating individual rate coefficients, and k(p), in addition chemically initiated polymerizations have been carried out, in which AMPS conversion was monitored via in-line FT-NIR spectroscopy. The resulting and k(p) values, for 40 degrees C and an initial AMPS concentration of 2.79 mol(.)L(-1), are 2 x 10(7) L(.)mol(-1.)s(-1) and 1 x 10(5) L(.)mol(-1.)s(-1), respectively. Both rate coefficients are significantly higher at the lower AMPS content of 1.04 mol(.)L(-1). at this lower AMPS content may be understood in terms of termination Occurring under reaction diffusion control. The lowering in rate coefficients measured at the higher AMPS content is indicative of a reduced poly(AMPS) chain mobility
Free‐Radical Termination Kinetics Studied Using a Novel SP‐PLP‐ESR Technique
No full textA novel method for measuring termination rate coefficients, k(t), in free-radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator-derived radicals. During subsequent polmerization, radical concentration is monitored by time-resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain-length dependence of k(t), to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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