Critical Conversion Research Articles

Calculation of the effect of substitution on the yield and topological parameters of hyperbranched polymers synthesized by the cyclotrimerization of mono- and bifunctional monomers

A kinetic model allowing for the effect of substitution has been developed for the synthesis of hyperbranched polymers through the cocyclotrimerization of mono- and bifunctional monomers. The calculation has been performed by means of a mathematical apparatus for generating functions. New theoretical relationships have been obtained that enable one to predict critical conversion and variations in the structural and molecular-mass parameters of hyperbranched polymers during reaction in relation to the initial monomer ratio, relative reactivities of functional groups, and variations in the reactivities during the process.

A new solution of transient confined–unconfined flow driven by a pumping well

A new solution of transient confined–unconfined flow driven by a pumping well is developed and compared to previous approximate solutions of Moench and Prickett [Moench AF, Prickett TA. Radial flow in an infinite aquifer undergoing conversion from artesian to water table conditions. Water Resour Res 1972;8:494–9] and Hu and Chen [Hu L, Chen C. Analytical methods for transient flow to a well in a confined–unconfined aquifer. Ground Water 2008;46(4):642–6]. The problem is rewritten in dimensionless form with the Boltzmann transform. The nonlinear equation for flow in the unconfined zone is solved with the Runge–Kutta method. Position of the conversion interface is determined with an iteration scheme. This study shows that the confined–unconfined flow depends on three dimensionless parameters that represent the confined–unconfined storativity ratio ( a D ), the ratio of the initial hydraulic head over the aquifer thickness ( f i ), and the dimensionless pumping rate ( q D ). The rate of expansion of the unconfined zone increases with q D , but decreases with a D and f i . Differences between the two previous approximate solutions and the new solution of this study are observable in the estimated position of the conversion interface and the drawdown–time curves. The new solution can be applied to estimate the time for confined–unconfined conversion to occur (critical conversion time), and the time when the pumping well becomes dry (critical drying time). The critical conversion time is found to be very sensitive to the initial hydraulic head. The critical drying time is often much larger than the critical conversion time and may never be observed during a finite pumping period.

Investigation of Coal Char−Slag Transition during Oxidation: Effect of Temperature and Residual Carbon

The transition of coal char to molten slag at high conversion was studied for a bituminous coal using a laminar entrained-flow reactor under oxidizing conditions. Post-oxidized char particles were analyzed by various techniques including loss-on-ignition, gas adsorption analysis, and scanning electron microscopy to determine carbon content, internal surface area and pore size distribution, and char morphology, respectively. These analyses provide information concerning the effect of temperature and residual carbon on the transition from porous char to molten slag. Results showed that, at temperatures above the ash flow temperature, the transition from porous char to molten slag occurred at about 90% conversion for the coal used in this study. No transition occurred at temperatures below the ash flow temperature. This finding explains previous observations that there is a coal-dependent critical carbon conversion at which the ash stickiness increases dramatically. This result also indicates that surface area can be used as a criterion for determining the critical conversion of the transition. In addition, it was found that the randomly overlapping pore model cannot be directly applied to predict the surface area evolution of char particles during the transition without considering the reopening of closed micropores during the initial reaction and the ash fusion effect.

The critical conversion efficiency of light energy to hydrogen from photocatalytic water decomposition

The hydrogen production from photocatalytic water decomposition is one of the best approaches that convert solar energy to hydrogen energy. At present, the hydrogen production from photocatalytic water decomposition has a low efficiency. In order to determine a valuable hydrogen production process from photocatalytic water decomposition in the aspect of energy conversion, the concept of critical conversion efficiency of light energy is proposed in this paper. Critical conversion efficiency of light energy is defined as the conversion efficiency of light energy when the hydrogen energy output is equal to an equivalent energy input of the system. The criterion introduced is that only if the conversion efficiency of light energy is greater than the critical one, an energy profit can be achieved in a system. A hypothetical system with 20,000 tons annual hydrogen production by using Na2S/Na2SO3 as the sacrificial reagent is analyzed and calculated in this paper to determine the critical conversion efficiency of light energy by means of energy balance, cumulative exergy balance, and emergy balance, respectively. The results show that the critical conversion efficiency of light energy determined by emergy balance is the most reasonable, which gives the figure around 15%.

Calculation of the critical conversion and topological parameters for hyperbranched polyisocyanurate polymers prepared via cocyclotrimerization of mono- and diisocyanates

A new approach to the design of hyperbranched polymers, the cocyclotrimerization of mono-and diisocyanates is proposed. The kinetic scheme for the synthesis of hyperbranched polyisocyanurates was considered and calculated in the generating-function formalism. New theoretical relationships were obtained that make it possible to predict the critical conversion and a change in the structural and molecular-mass characteristics of hyperbranched polyisocyanurates in the course of the reaction, depending on the initial ratio of monoand diisocyanates and on the relative reactivity of isocyanate groups.

The theory of nonrandom polycondensation for the description of sol–gel polymerization of alkoxisilanes

AbstractThe earlier developed theory of nonrandom polycondensation (Kuchanov, et al. J Stat Phys 122, 875, 2006) is applied for the description of sol–gel polymerization of alkoxisilanes. The accuracy of simple approximate approaches used in literature to calculate critical conversion at the gel‐point is evaluated. The results of calculation of the characteristics of gel's polymer network governing its elastic properties are presented for the first time. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3093–3116, 2007

Structure development in polyurethane networks based on star-like precursors

A network formation from an OH-functional star oligomer and two isocyanates (HDI and HDI-trimer) was investigated by changes in the critical molar ratio of OH groups $$(r_{\rm OH})_{\rm c}$$ as a function of dilution. The critical ratio $$(r_{\rm OH})_{\rm c}$$ is such critical excess of OH groups at which the critical conversion of NCO at the gel point groups reaches 100%. The dilution effect was interpreted by pronounced cyclization, which was higher in the case of the HDI-trimer compared with the HDI.

UO 2 oxidation site densities determined by one- and two-electron oxidants

The oxidation site density (number of oxidation sites per m 2) for UO 2 powder was determined by measuring the amount of oxidant needed to fully oxidize the surface (denoted the critical oxidant conversion). The point where the surface becomes fully oxidized is identified by a change in reaction order from first to zeroth order in HC O 3 - free systems. At the critical oxidant conversion the kinetics of the reaction becomes completely governed by dissolution of oxidized UO 2. The oxidants used in this study are H 2O 2 (two-electron oxidant) and IrC l 6 2 - (one-electron oxidant). The oxidation site densities determined using the two different oxidants are (2.1 ± 0.1) × 10 −4 and (2.7 ± 0.5) × 10 −4 mol m −2, respectively, expressed in two electron equivalents. The fairly good agreement between the two oxidants implies that the methodology used indeed gives a reasonable measure of the oxidation site density. In addition, oxidation site densities for different size fractions of UO 2 powder were determined. The results are discussed in terms of surface roughness.

On the prediction of internal particle morphology in suspension polymerization of vinyl chloride. Part I:: The effect of primary particle size distribution

The present study provides a comprehensive investigation on the determination of the primary particle size distribution in the suspension “powder” polymerization of vinyl chloride. The primary particle size distribution inside the polymerizing monomer droplets is determined by the solution of a population balance equation governing the nucleation, growth, and aggregation of the primary particles. The stability of the colloidal primary particles is expressed in terms of the electrostatic and steric stabilization forces. The primary particle stability model includes the effects of agitation, temperature, electrolyte as well as primary and secondary stabilizer concentrations. It also includes both diffusive and shear-induced particle destabilization mechanisms. The proposed stability model is shown to accurately describe existing experimental data on particle number, mean particle size and particle size distribution for both bulk and suspension vinyl chloride polymerizations. The primary particle population balance model can predict the critical monomer conversion at which massive particle aggregation occurs leading to the formation of a continuous network of primary polymer particles inside the polymerizing monomer droplets. A detailed investigation on the predicted critical monomer conversion is carried out including its dependence on the rate of agitation, temperature, electrolyte concentration, as well as the concentrations of the primary and secondary stabilizers.

Ideal Chemical Conversion Concept for the Industrial Production of Ethene from Hydrocarbons

This review considers and evaluates alternative process concepts for the production of ethene on an industrial scale. A fundamental perspective is chosen, focusing on the ability to create a near-optimum conversion path from hydrocarbon (C2+) feed to ethene. The critical conversion aspects are quickly achieving a high temperature at a low hydrocarbon pressure and, after a short reaction time, arresting the composition with a high ethene content by rapid cooling. The features of an ideal process involve a maximum olefin yield, no remains of energy carriers or auxiliary chemicals in the product, minimal ecological impact, minimum energy input per unit product, high availability, and a low degree of complexity of the reaction section. The majority of the current ethene production processes are an evolutionary redesign from earlier existing processes that is able to add a large amount of thermal energy in a short period of time and at elevated temperature levels in the range of 600−1200 °C, enabled by better high-temperature-resistant materials. The process concepts reviewed in this paper are examined from a different point of viewnamely, how well they meet the criteria of the ideal process. The specific energy requirements of the processes are limited in the available literature; therefore, these requirements are systematically determined with Aspen Plus software and the aid of SPYRO for an ethene plant with a fixed ethane feed. A wide variety of process concepts is covered: dehydrogenation, direct heating, and indirect heating. Although none of the reported processes fulfils all ideals, the new combination of the adapted firing furnace with ceramic reactors internals and the shock wave reactor come close to it.

Microfabrication by X‐ray‐induced polymerization above the lower critical solution temperature

AbstractA polymer synthesis method is presented in which chain growth driven by exothermic reaction stimulates a gradual chain collapse. The globular precipitates in such systems can be restrained from coalescing by polymerizing in a quiescent environment. Time‐resolved small‐angle scattering study of the methacrylic acid polymerization kinetics in a quiescent system above its lower critical solution temperature (LCST) in water reveals the following features of this method: (a) growing oligomers remain as rigid chains until a critical chain length is reached, at which they undergo chain collapse, (b) radius of gyration increases linearly with time until a critical conversion is reached, and (c) radius of gyration remains constant after the critical conversion, even while conversion is gradually increasing. Following this self‐stabilizing growth mechanism, we show that nanoparticles can be directly synthesized by polymerizing N‐isopropylacrylamide above its LCST in water. The average size of nanoparticles obtained from a polymer–solvent system is expected to be the maximum extent of reaction spread at that monomer concentration. This hypothesis was then verified by polymerizing N‐isopropylacrylamide above their LCST in water, but by initiating the reaction with X‐rays shielded by a mask. The microfabricated patterns conform well to the size and shape of the mask used confirming that the growing chains do not propagate beyond the exposed regions as long as the reaction temperature is maintained above the LCST. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 429–425, 2006

Comprehensive Study of Free Radical Copolymerization Using a Monte Carlo Simulation Method, 1

AbstractSummary: In order to investigate the influence of reactivity ratios and initial feed composition on the microstructure of macromolecules in free radical copolymerization, a comprehensive study was carried out using a Monte Carlo simulation method. As a result, a new procedure was introduced to modify the works of others on the initiation step. The variation of the copolymer composition and the fashion of the arrangement of monomers in simulated chains were evaluated as a function of copolymerization parameters. The model was capable of monitoring any change in azeotropy as well as the magnitude and direction of composition drift from the azeotrope point. The maximum reachable conversion (MRC) was predicted for different combinations of initial feed compositions and reactivity ratios. According to the simulation results, a critical conversion where the macromolecules produced inherited the maximum allowed alternation was obtained for the reactivity ratios given.Change of sequence distribution of simulated copolymer chains with conversion for various initial feed compositions on a triangular graph (rA = 0.5, rB = 0.9).magnified imageChange of sequence distribution of simulated copolymer chains with conversion for various initial feed compositions on a triangular graph (rA = 0.5, rB = 0.9).

Reaction kinetics of thermoplastic polyurethane polymerization in situ with poly(vinyl chloride)

In this study, the reaction kinetics of thermoplastic polyurethane (TPU) polymerization in situ with poly(vinyl chloride) (PVC) was characterized with differential scanning calorimetry. The presence of PVC appeared to enhance the thermodynamic drive for TPU hard segment phase separation and domain ordering from the PVC/TPU reactant mixture. Prior to hard segment phase separation, and thus some critical conversion, TPU polymerization in situ with PVC obeyed the same nth order, phenomenological kinetic rate law followed by neat TPU polymerization. In addition, the overall rate constant employed in the rate law increased in the presence of PVC. After hard segment phase separation, and the resulting physical cross-linking of the PVC/TPU reactant mixture, the reaction kinetics of TPU polymerization in situ with PVC became diffusion controlled.

X-ray Photoelectron Spectroscopy Study on E-beam Co-evaporated YBCO Precursors and their Post –deposition Conversion

AbstractElectron beam co-evaporation is one of the ex situ techniques being developed for cost-effective fabrication of YBa2Cu3O7-δ (YBCO) coated conductors. To achieve high critical current (Ic) and fast conversion, further understanding is needed of the chemical and microstructural characteristics of the precursors and the phase evolution process towards the epitaxial c-axis oriented YBCO film during the post-deposition conversion annealing. In this study, X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical and compositional profiles of the precursors. Samples quenched from the processing stage were also inspected for identifying the transient elemental chemical states during the YBCO phase formation. XPS survey and detailed spectra were recorded for the surfaces and through various depths. It is suggested that XPS spectra, especially the “Auger parameters” can be used for revealing the conversion reaction. Preliminary results indicate that the copper oxidation state might have important effect on precursor conversion behaviors.

Hydrogenated hydroxy-terminated polyisoprene (HHTPI) based urethane network: network properties

Polyurethane (PU) networks based on hydrogenated hydroxytelechelic polyisoprene (HHTPI) and isophorone diisocyanate isocyanurate (IIPDI) were prepared and studied. Determination of the critical conversion point ( p c=0.58) was obtained experimentally through rheological measurements. This value was similar to the value predicted using the Macosko Miller model. Differential scanning calorimetry (DSC) analyses show that PUs with a high degree of phase separation is obtained (81% for [NCO]/[OH]=1). This phase segregation decreased when hard segment content increased. Glass transition temperature of the soft segment, T gs=−66 °C, was little affected by hard segment content. Thus, the resulting polyurethanes exhibit elastomeric properties. For [NCO]/[OH]>1 ratios, a second transition at 70 °C was observed and attributed to hard segment interactions. Using swelling measurements, the solubility parameter δ, polyurethane–THF interaction parameter ( χ), crosslink densities ( ν e) and the molecular weight between crosslinks ( M ¯ c ) were determined.

Polymerization and Network Formation of UV-Curable Systems Monitored by Hyphenated Real-Time Dynamic Mechanical Analysis and Near-Infrared Spectroscopy

Hyphenated rapid real-time dynamic mechanical analysis (RT-DMA) and time-resolved near-infrared spectroscopy (RT-NIRS) have allowed simultaneous monitoring of acrylate photopolymerizations with regard to both their chemical conversion and mechanical properties. Up to 374 NIR spectra and up to 50 DMA data points can be accumulated within a second. We observe that modulus buildup does not linearly follow chemical conversion of acrylate bonds. The gel point is detected after passing a certain critical acrylate conversion. Beyond the gel point (determined when the phase angle drops below 45°) network formation proceeds exponentially with acrylate conversion. Experimental data reveal a critical dependence of the mechanical property development during the later stage of acrylate conversion. The experimental results across the modulus−conversion plot reveal the importance of final acrylate bond conversion as opposed to the early part of the conversion vs time (maximum rate of polymerization Rp) when considering ...

Methods of Calculation of the Critical Conversion in the Formation of Network Polymers

AbstractFor Abstract see ChemInform Abstract in Full Text.

Halloween genes encode P450 enzymes that mediate steroid hormone biosynthesis in Drosophila melanogaster

Mutation of members of the Halloween gene family results in embryonic lethality. We have shown that two of these genes code for enzymes responsible for specific steps in the synthesis of ecdysone, a polyhydroxylated sterol that is the precursor of the major molting hormone of all arthropods, 20-hydroxyecdysone. These two mitochondrial P450 enzymes, coded for by disembodied ( dib) (CYP302A1) and shadow ( sad) (CYP315A1), are the C22 and C2 hydroxylases, respectively, as shown by transfection of the gene into S2 cells and subsequent biochemical analysis. These are the last two enzymes in the ecdysone biosynthetic pathway. A third enzyme, necessary for the critical conversion of ecdysone to 20-hydroxyecdysone, the 20-monooxygenase, is encoded by shade ( shd) (CYP314A1). All three enzymes are mitochondrial although shade has motifs suggesting both mitochondrial and microsomal locations. By tagging these enzymes, their subcellular location has been confirmed by confocal microscopy. Shade is present in several tissues as expected while disembodied and shadow are restricted to the ring gland. The paradigm used should allow us to define the enzymes mediating the entire ecdysteroid biosynthetic pathway.

Estimating diffusion-controlled reaction parameters in photoinitiated polymerization of dimethacrylate macromonomers

The kinetics of photoinitiated polymerization of dimethacrylate macromonomers have been studied to determine the diffusion-controlled reaction parameters using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). A predicted kinetic rate expression with a diffusion control factor was employed to estimate an effective rate constant and to define the reaction-controlled and diffusion-controlled regimes in the photopolymerization. An effective rate constant,ke, can be obtained from the predicted kinetic rate expression. At the earlier stages of polymerization, the average values of kinetic rate constants do not vary during the reaction time. As the reaction conversion,α, reaches the critical conversion, αc, in the predicted kinetic expression, the reaction becomes to be controlled by diffusion due to the restricted mobility of dimethacrylate macromonomers. A drop in value of effective rate constant causes a drastic decrease of reaction rate at the later stages of polymerization. By determining the effective rate constants, the reaction-controlled and diffusion-controlled regimes were properly defined even in the photopolymerization reaction system.

Atom vacancy lines and surface patterning: The role of stress for Br-Si(100)-(2×1)at 700 K

Using variable-temperature scanning tunneling microscopy, we studied Br-induced roughening of Si(100)-$(2\ifmmode\times\else\texttimes\fi{}1)$ at 700 K. The roughening pathway requires Br-free dimers so that a saturated surface is inactive. Initial roughening involves the formation of atom vacancy lines and regrowth chains of Si dimers on the terraces. The atom vacancy lines grow longer through a stress-induced process that creates dimer vacancies. Br adatom repulsion then splits these dimer vacancies into pairs of single-atom vacancies. A $(3\ifmmode\times\else\texttimes\fi{}2)$ reconstruction derived from dimer rows and atom vacancy lines is energetically favored at high concentration due to Br-Br repulsive interactions and Br bond angle relaxation that is facilitated by the $(3\ifmmode\times\else\texttimes\fi{}2)$ structure. Though favored, the conversion is slow at high coverage. Continuous scanning over $\ensuremath{\sim}72$ h shows the correlation between Br concentration and surface morphology. Ultimately, a highly dynamic configuration is reached in which large regrowth islands and large terrace areas have $(3\ifmmode\times\else\texttimes\fi{}2)$ symmetry. These areas become unstable when the Br concentration drops below a critical value and conversion to $(2\ifmmode\times\else\texttimes\fi{}1)$ terraces and extended islands occurs.