氯乙烯SET-DT悬浮聚合动力学和成粒过程的相互关系

以碘仿为引发剂、连二亚硫酸钠/碳酸氢钠为催化体系、聚乙烯醇(PVA)和/或纤维素衍生物(MC)为分散体系,进行氯乙烯单电子转移-蜕化链转移(SET-DT)活性自由基悬浮聚合,采用在线示踪气相色谱法和激光粒度分析系统研究分散剂种类和浓度、搅拌转速等对聚合动力学和单体液滴/聚合物颗粒粒径分布的影响。发现在相同搅拌转速下,以MC为分散剂的氯乙烯聚合速率最大,以PVA为分散剂时反应速率最小;分散剂种类固定时,聚合速率随分散剂浓度增大而增大。SET-DT悬浮聚合过程中,水相连二亚硫酸钠分解产生的自由基向单体液滴的扩散速率与液滴粒径分布和皮膜结构有关,因此聚合成粒过程影响聚合动力学。尽管不同条件下的聚合均经历液-液分散、液滴黏并、树脂颗粒稳定(转化率>30%)等成粒阶段,但各阶段的液滴/颗粒平均尺寸随分散体系和搅拌转速的变化而变化,引起聚合速率变化;采用MC为分散剂得到的PVC树脂皮膜少,有利于水相产生的自由基向单体相的扩散,聚合速率大。     

Modeling of imidization kinetics

We investigated the applicability of a new kinetic model to the polyamic acid imidization process, which has been generally represented as involving two steps, fast and slow, with specific rate constants due to the existence of kinetically nonequivalent amic acid states. A time dependent nonlinear function of the form sech(-at) for the rate expression allowed us to simulate the general features of the imidization process. In the early stage, it is almost a constant, in accordance with the process of imidization of highly active amic acid groups, which is eventually limited by the chemical conversion of amic acid groups into imide rings. In the final stage, the reaction rate decreases exponentially because it is controlled by the conversion rate of amic acid groups from inactive to active states. Overall reaction rate is determined by the change in the rate-determining stage, and thus eventually exhibits a sharp drop in reaction rate. Comparison showed good agreement with experimental data. Activation energies calculated from the new model were in good agreement with those obtained from the first order model. The applicability of the model was further assessed by modeling a process involving two separate imidization processes. Simulation results showed good quantitative and qualitative correlations with experimental data. The model is also in good agreement with another complicated model.     

Emulsion polymerization: particle growth kinetics

An attempt to develop a predictive and manageable mathematical model for particle growth in emulsion homopolymerization was carried out by fitting the time evolution of the conversion in the chemically initiated seeded emulsion polymerization of styrene carried out under a wide range of experimental conditions with models of different complexity. Model discrimination based on the best fitting of the experimental data was carried out. No advantage was gained by increasing the complexity of the mathematical model. The dependence of the radical entry and exit rate parameters on the particle size was used to elucidate between the different mechanisms proposed for these processes.     

Modelling particle size distributions and secondary particle formation in emulsion polymerisation

An extensive model is given for the particle size distribution (PSD), particle number, particle size and amount of secondary nucleation in emulsion polymerisations. This incorporates what are thought to be all of the complex competing processes: aqueous phase kinetics for all radical species arising from both initiator and from exit (desorption), radical balance inside the particles, particle formation by both micellar and homogeneous nucleation mechanisms, and coagulation (the rate of which is obtained using the Healy–Hogg extension of DLVO theory). The predictions of the model are compared to extensive experimental results on rates, time evolution of the particle size distribution, and relative amounts of secondary nucleation, for styrene initiated by persulfate with sodium dodecyl sulfate and with sodium dihexyl sulfosuccinate as surfactants. For this system values of almost all of the many parameters needed for the model are available from independent measurements, and thus no significant parameter adjustment is plausible. Accord with experiment is imperfect but quite acceptable, supporting the validity of the various mechanisms in the model. Effects such as the experimental variation of particle number with ionic strength, as well as calculated coagulation rate coefficients as functions of particle size, suggest that coagulation of precursor (i.e., newly-formed) particles is a significant effect, even above the cmc. The modelling also suggests why secondary nucleation occurs readily in systems stabilised with polymeric surfactant.     

Secondary particle formation in seeded suspension polymerization

Seeded suspension polymerization can be applied to obtain core-shell particles with particle diameters larger than 1000 μm, which finds application in the rigid foam industry, for instance. However, depending on the operation conditions, the formation of secondary particles decreases drastically the efficiency of monomer incorporation in the seed particles. In the present work, the mechanism of secondary particles formation during seeded suspension polymerization was investigated using monomers (styrene, methyl acrylate and methyl methacrylate) and initiators (benzoyl peroxide and lauroyl peroxide) with different water solubilities and, in the case of the initiators, also different decomposition rates. Results showed that there was no seed polymer in the composition of the secondary particles but only pure polymer from the monomer feed, meaning that they were not formed by erosive breakage of the swelled seeds. The fraction of secondary particles increased when monomers with higher water solubility and initiators with decreasing water solubility were used. These results were consistent with the formation of secondary particles by homogeneous nucleation and monomer droplet nucleation.     

The kinetics of ettringite formation

Conversion of hemihydrate to gypsum, and ettringite formation in the system C₃A---CaSO₄·1/2H₂O---H₂O three hydration temperatures using x-ray diffraction. Analysis of the kinetics of gypsum formation suggests that the rate limiting step changes from an initial dependence on the hemihydrate surface area to a dependence on the gypsum growth rate. The formation of ettringite was found to be controlled by diffusion. In accord with the needle-like morphology that ettringite exhibits, a nucleation and growth model predicted one-dimensional growth. An apparent activation energy calculated for ettringite formation is consistent with a diffusionally controlled process.     

Modeling kinetics of distortion in porous bi-layered structures

Shape distortions during constrained sintering experiment of bi-layer porous and dense cerium gadolinium oxide (CGO) structures have been modeled. Technologies like solid oxide fuel cells require co-firing thin layers with different green densities, which often exhibit differential shrinkage because of different sintering rates of the materials resulting in undesired distortions of the component. An analytical model based on the continuum theory of sintering has been developed to describe the kinetics of densification and distortion in the sintering processes. A new approach is used to extract the material parameters controlling shape distortion through optimizing the model to experimental data of free shrinkage strains. The significant influence of weight of the sample (gravity) on the kinetics of distortion is taken in to consideration. The modeling predictions indicate good agreement with the results of sintering of a bi-layered CGO system in terms of evolutions of bow, porosities and also layer thickness.     

Mixing effects on particle formation in supercritical fluids

The process termed solution enhanced dispersion by supercritical fluids (SEDS™) is investigated. In the process particles are created in the rapid antisolvent process using a twin-fluid nozzle to co-introduce the SCF antisolvent and solution. Results of experimental and numerical studies are presented for two regions of pressure: above the mixture critical pressure where a single-phase exists for all solvent–antisolvent compositions, and below the mixture critical pressure where the two-phase region is observed. In experimental studies paracetamol (in the single-phase system) and nicotinic acid (in the two-phase system) were precipitated from ethanol solution using supercritical CO₂ as an antisolvent. To interpret the phenomena affecting creation of the supersaturation and to predict suprsaturation distribution, balances of momentum (flow), species (mixing), energy (heating and cooling) and population (droplet and crystal size distributions) are applied. The Favre averaged k–? model of the CFD code FLUENT is applied together with specific models for precipitation subprocesses and Peng–Robinson equation of state. This includes application of the PDF closure procedure for precipitation and the drop breakage kernel that is based on multifractal theory of turbulence for modelling drop dispersion. Thermodynamic effects of mixing and decompression are included as well. Predicted values not always agree with experimental data but anyhow simulations predict all trends observed in experiments.     

Modeling process kinetics in Cu/C nanocomposite synthesis

We have studied the kinetics of chemical transformations in polyacrylonitrile (PAN) and composite Cu/PAN (C _Cu = 10% (mass)) at P = 10^?5 Pa in the range of temperatures of 20–700°C during heating according to the linear law with a velocity of 10°C/min with the method of thermogravimetry. From the conducted kinetic studies of pyrolysis, it can be inferred that kinetic data of the destruction reaction of PAN and a composite of Cu/PAN are described with differential dependences on the basis of heterogeneous chemical reaction equations. Experimental data prove the heterogeneous reactions of the first order. The limiting stages of the process depend on the kinetic processes of chemical bonds opening in the polymer and composite. For the reaction of composite Cu/PAN in a vacuum, the determinant is the decomposition of copper salt and a PAN complex with the formation of copper nanoparticles at 470–570 K.     

Heat transfer and kinetics in the pyrolysis of shrinking biomass particle

The impact of shrinkage on pyrolysis of biomass particles is studied employing a kinetic model coupled with heat transfer model using a practically significant kinetic scheme consisting of physically measurable parameters. The numerical model is used to examine the impact of shrinkage on particle size, pyrolysis time, product yields, specific heat capacity and Biot number considering cylindrical geometry. Finite difference pure implicit scheme utilizing tri-diagonal matrix algorithm (TDMA) is employed for solving heat transfer model equation. Runge-Kutta fourth-order method is used for chemical kinetics model equations. Simulations are carried out for radius ranging from 0.0000125 to , temperature ranging from 303 to and shrinkage factors ranging from 0.0 to 1.0. The results obtained using the model used in the present study are in excellent agreement with many experimental studies, much better than the agreement with the earlier models reported in the literature. Shrinkage affects both the pyrolysis time and the product yield in thermally thick regime. However, it is found that shrinkage has negligible affect on pyrolysis in the thermally thin regime. The impact of shrinkage reflects on pyrolysis in several ways. It includes reduction of the residence time of gases within the particle, cooling of the char layer due to higher mass flux rates of pyrolysis products and thinning the pyrolysis reaction region.     

Correlation of the kinetics of aggregation and inactivation of L-glutamate dehyrogenase during drying and particle formation of a levitated microdroplet

Measurement of single droplet drying in the acoustic levitator has been used to detect a correlation between the kinetics of aggregation and inactivation of the model protein l-glutamate dehydrogenase (GDH) during drying and particle formation. The droplet drying was performed at one set of conditions: drying gas temperature?=?60°C; drying gas relative humidity?=?1%, and drying gas velocity?=?0.5?L/min. A centrifugation technique was used to remove insoluble aggregates from the rehydrated microparticles removed from the levitator chamber. Size exclusion chromatography could then be used to quantify the contents of nonaggregated native protein. During the constant-rate drying period up to the critical point of drying, there was retention of nonaggregated native structure. Only after the critical point of drying has been reached, a rapid decline in nonaggregated native content was observed owing to formation of insoluble aggregates. No sign of a soluble trimer of GDH was found. The kinetics of this behavior correlates closely with the kinetics of inactivation of the same enzyme in levitated microdroplets from the literature, i.e., retention of activity up to the critical point and rapid loss of activity after the critical point. The novelty of this work is that such a correlation between aggregation and inactivation can be detected during drying and particle formation on both sides of the critical point of drying.     

超临界流体技术制各类胡萝卜素纳米颗粒

Based on the solubility in supercritical CO2, two strategies in which CO2 plays different roles are used to make quercetine and astaxanthin particles by supercritical fluid technologies. The experimental results showed that micronized quercetine particles with mean particle size of 1.0-1.5 µm can be made via solution enhanced dis-persion by supercritical fluids (SEDS) process, in which CO2 worked as turbulent anti-solvent; while for astaxan-thin, micronized particles with mean particle size of 0.3-0.8 µm were also made successfully by rapid expansion supercritical solution (RESS) process.     

Modeling of nano-sized macromolecules in silane-based self-assembled nano-phase particle coatings

Molecular simulation approaches have been used to enhance the understanding of complex chemical interactions in coatings related processes. The Self-assembled NAno-phase Particle (SNAP) coating process relies on aqueous solution processes, similar to those used in conventional sol–gel synthesis, to form siloxane nano-sized structures, which are subsequently cross-linked upon film application. This process has been shown to produce a dense, protective thin film on metal substrates. The SNAP process involves design and selection of the coating constituents, based on the desired functionalities for network formation and cross-linking chemistry. In order to facilitate the design of coating components at the molecular level, it is imperative to gain a fundamental understanding of these complex phenomena.

Molecular simulations on several oligomers with different side chains have been performed to study components of the of Si–O networks during the SNAP particle formation process. Several ring structures of tetramethyl orthosilicate (TMOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS) have been considered. Geometry optimization of the cyclic Si–O structure formation has been performed, and ring strain parameters have been calculated.