微反应器内聚合物合成研究进展

微反应器作为化学工程学科的前沿和热点方向,逐渐成为聚合物合成的新装备、新工艺与新产品开发的重要平台,得到学术界和产业界的广泛关注。微反应器可实现可控的多相微尺度流动,能够强化聚合反应中的混合、传质和传热过程,严格控制反应时间,实现反应单元的模块化组合。与传统搅拌反应器相比,这些特点使得微反应器在控制聚合物分子量分布,简化反应环境,提高反应选择性,调节聚合物分子结构和宏观形貌等方面展现出了一定优势。本文全面综述了聚合物合成微反应器理论和技术的研究进展,并在新过程和新产品开发、反应动力学测量、微尺度基础研究和反应器放大等方面进行了展望。     

A study on the micromixing performance in microreactors for polymer solutions

Microreactors are widely applied for various polymerization processes to produce polymers with controlled molecular weight and structures. In this work, the mixing of polymer solutions in capillary microreactors was investigated with the aid of the biazo‐coupling reaction system and high‐speed camera. A smaller inner diameter, a higher Dean number and a lower Torsion number could promote the micromixing performance in microreactors. Unlike the mixing of Newtonian fluids in microreactors or micromixers, the mixing of polymer solutions follows different mechanisms and it is more difficult to reach homogeneous condition. However, the good micromixing performance in capillary microreactor systems for polymer solutions still could be obtained with high shear rates and long enough capillary length at relatively high correlated Reynolds numbers (N_Re > 29). Furthermore, the micromixing time in the capillary microreactors was evaluated based on a modified stretching efficiency model and its value was in the range of 0.1–8.0 ms. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3479–3490, 2018     

Deposition of polymer complex layers onto nonwoven textiles

Polymeric complex layers are deposited onto polypropylene and polyester nonwoven fabrics via the layer by layer method. It is found that grafting after previous surface activation with an aqueous ammonium persulfate solution is a beneficial method to form the first acidic layer. In order to form polymer complexes, poly (acrylic acid), poly(itaconic acid), poly(allylamine hydrochloride), and poly(N‐dimethylaminoethyl methacrylate) are used. The deposition of subsequent layers to form polymer complexes modifies the properties of nonwovens. Moreover, it is found that the deposited layers can be microreactors for chemical processes such as polymerization of aniline and formation of polymers and copolymers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1700–1705, 2007     

Highly loaded UV curable nanosilica dispersions for rapid prototyping applications

Silica microcomponents are attractive for applications requiring high temperature stability and chemical durability such as in microreactors. Rapid prototyping methods of forming ceramic dispersions hold great potential for manufacturing of microreactors. Stereolithography for example can benefit from highly solid loaded, low viscosity and high transparency dispersions. This article presents the development of >40 vol.%, UV curable transparent silica nanodispersions having viscosities suitable for stereolithography applications. The main parameters enabling these high loading dispersions are minimization of the Van der Waals attractive forces and affinity of monomer end group for the silica surface (e.g. hydrogen bonding). The minimization of the Van der Waals attraction is achieved by refractive index matching of the nanosilica and UV curable acrylate monomers. This results also in highly transparent dispersions having curing depths in the order of 10 mm in the UVA range. The transformation of selected dispersions to self-supporting silica glass sheets through UV curing, debinding and sintering was investigated by TGA/DTA, dilatometry and XRD.     

Devolatilization: A critical sequential operation for in situ compatibilization of immiscible polymer blends by one-step reactive extrusion

The main objective of this study was to show that an efficient removal of residual monomers is very important for in situ compatibilization of immiscible polymer blends by one step reactive extrusion. One-step reactive extrusion means that when compatibilizing two immiscible polymers with one polymer containing potential functional groups and the other being chemically inert with respect to these functional groups, functionalization of the chemically inert polymer and its subsequent interfacial reaction with the functional polymer are accomplished in a single extrusion process. Two model blend systems were chosen: polypropylene/poly(butylene terephthalate) (PP/PBT) and high density polyethylene/polyamide 6 (HDPE/PA6). A co-rotating intermeshing twin screw extruder was used to process these blends. Glycidyl methacrylate (GMA) and maleic anhydride (MA) were used to functionalize PP and HDPE, respectively. Results showed that the mechanical properties of both blends in terms of elongation at break and impact strength were improved to a much greater extent with up-stream devolatilization compared with down-stream devolatilization.     

Cure modelling of polyester thermosets in a glass mould

The curing of unsaturated polyester was studied experimentally and using a model of the process. The kinetic parameters were calculated from the heat flow–time curves obtained by differential scanning calorimetry (Mettler Toledo DSC 823), operating in a non-isothermal mode. The temperature–time histories were studied in a cylindrical glass mould. A potential use of glass as a mould for polymer curing is found in the production of optical sensors. Here, glass was selected as a mould material because it is UV transparent, chemically inert and easy to clean. The thermal properties of glass moulds coupled with the intrinsic curing kinetics are of a significant interest in such investigations. Taking into account the heat transferred by the convection from the air to the mould surface and the conduction through the mould wall and resin, as well as the kinetics of the heat generated in the cure reaction, a numerical model has been constructed. The contributions to the rise in temperature from the heat conduction and chemical reaction are different in different parts of the composite, which can explain the temperature–time histories. The introduction of a carbonate based filler reduced the amount of heat released in the composite and, as a result, lowered the temperatures through the resin. A good agreement between experimental data and the predicted mathematical model of the curing process in the mould has been observed.     

γ-radiation grafting: A novel approach for the preparation of robust and highly selective supported metal complex catalysts

The technique of γ radiation grafting is shown to be of value in functionalising mechanically strong, chemically simple and chemically inert polymers such as polypropylene. The technique leads to the incorporation of several functional groups at a single site on the polymer which is of value in creating a three-dimensional catalytically active site. Such sites enable the local environment to further enhance the natural selectivity arising from the molecular nature of a metal complex catalyst. Accordingly, supported metal complex catalysts prepared in this way have higher selectivities than their homogeneous analogues. The approach is illustrated by reference to cobalt and rhodium(I) olefin hydroformylation catalysts; in both cases the supported catalysts show greater selectivity towards the formation of normal as opposed to branched aldehydes than their homogeneous analogues.     

Current Strategies to Improve the Bioactivity of PEEK

The synthetic thermoplastic polymer polyetheretherketone (PEEK) is becoming a popular component of clinical orthopedic and spinal applications, but its practical use suffers from several limitations. Although PEEK is biocompatible, chemically stable, radiolucent and has an elastic modulus similar to that of normal human bone, it is biologically inert, preventing good integration with adjacent bone tissues upon implantation. Recent efforts have focused on increasing the bioactivity of PEEK to improve the bone-implant interface. Two main strategies have been used to overcome the inert character of PEEK. One approach is surface modification to activate PEEK through surface treatment alone or in combination with a surface coating. Another strategy is to prepare bioactive PEEK composites by impregnating bioactive materials into PEEK substrate. Researchers believe that modified bioactive PEEK will have a wide range of orthopedic applications.     

An overview of ceramic molds for investment casting of nickel superalloys

Accelerating advancements in technological systems have demonstrated a need for alloys with drastically improved thermomechanical and chemical properties, called superalloys. Ceramic molds are typically used in near-net shape investment casting processes of superalloy components due to their chemical inertness and high-temperature capabilities. Ceramic molds, however, often suffer from shortcomings in vital properties including flexural strength, thermal shock resistance, permeability, dimensional stability, corrosion resistance, and leachability, which have restricted their ability to adequately process modern alloy castings. This study analyses these limitations and illustrates how to address them, particularly regarding ceramic mold and slurry design, processing of shells and cores, material selection, and testing and characterization. By utilizing advanced processing methods including additive manufacturing and gel-casting, more dimensionally accurate and preferentially built molds can be formed. Additionally, by varying the mold composition to achieve more chemically inert structures, reactions with the mold can be mitigated to reduce chemically induced defects.     

A pore-scale model for microfibrous ammonia cracking microreactors via lattice Boltzmann method

Microfibrous microreactors with high reactive surface-to-volume ratio are good choices for ammonia cracking, which is one of the main strategies for CO-free hydrogen production. In the current study, a numerical model based on the lattice Boltzmann method (LBM) is presented to investigate ammonia cracking microreactors with coupled physiochemical thermal processes at the pore scale. Several sets of transport phenomena such as fluid flow, species transport, heat transfer and chemical reaction are taken into account. Moreover, to model the species transport in the ammonia cracking microreactor an active approach is applied for the first time. The model is validated and then employed to simulate the reactive transport in five different microreactors with dissimilar structural parameters. Comparison of the results shows that the fibers orientation is an effective geometric parameter that can greatly influence the hydrogen production efficiency.     

微反应器研究及展望

综合概括了微反应器(微通道反应器)的基本概念,把微反应器与其他微通道设备相区别;从化学反应工程的角度按气固相催化微反应器、液液相微反应器、气液相微反应器和气液固三相催化微反应器等类型对各种新型微反应器予以简略而突出的介绍;从其几何特性出发系统而深入地阐述了微反应器具有的一系列超越传统反应器的独特优越性;简略介绍了微反应器的制作、研究现状和展望。     

Study of polyaniline films degradation by thin-layer bidimensional spectroelectrochemistry

Degradation of polyaniline films electrodeposited on optically transparent gold electrodes has been followed by thin-layer bidimensional spectroelectrochemistry. The influence of the anodic potential reached during multiple potential scans has been assessed. Correlation of changes in the spectral signals in normal and parallel arrangement has enabled identifying species of different nature, both soluble and insoluble, electroactive and inert, during the degradation of the polymer film. The three bidimensional spectroelectrochemistry signals have demonstrated the occurrence of different types of degradation processes depending on the applied potential, ranging from the simple release of oligomers and hydrolysis products retained in the film during electropolymerization to more dramatic modifications of the film.     

XPS studies of in situ plasma-modified polymer surfaces

X-ray photoelectron spectroscopy (XPS) has been used to study the chemical effects of both inert (argon) and reactive (oxygen, nitrogen, and mixed gas) plasma treatments done in situ on a variety of polymer surfaces. Inert gas plasma treatments introduce no new detectable chemical species onto the polymer surface but can induce degradation and rearrangement of the polymer surface. However, plasma treatments with reactive gases create new chemical species which drastically alter the chemical reactivity of the polymer surface. These studies have also shown that the surface population of chemical species formed after plasma treatment is dependent on both the chemical structure of the polymer and the plasma gas. The effects of direct and radiative energy-transfer processes in a plasma have also been studied. Polymers containing certain functional groups were found to be more susceptible to damage via radiative energy transfer. Ageing studies of plasma-modified polymer surfaces exposed to the atmosphere have shown that the ageing process consists of two distinct phases. The initial phase, which occurs rapidly, involves adsorption of atmospheric contaminants and, in some cases, specific chemical reactions. The second phase, which occurs slowly, is due to surface reorganization.     

Heat treatment of 6H-SiC under different gaseous environments

Silicon carbide is a useful material for the reactors in chemical processes. In recent years, microreactors have gained significant attentions due to the high demand for process miniaturization. As heat and mass-transfer are highly improved inside the gas flow channels in microreactors, any change on the surface of inner channels under heating becomes critical to the performance of microreactors. To investigate the surface changes of silicon carbide during the heat treatment, 6H-SiC coupons were processed in five different gases—Ar, N₂, air, 0.9% O₂ in Ar and 50% H₂O in air—that are commonly encountered in high temperature chemical processes. While the formation of oxide film was found to be dependent on the partial pressure of oxidizing gas, surface decomposition was found from the treatment in nitrogen environment. Characterization of the SiC surface by Raman spectroscopy and SEM–EDX revealed that a graphitic layer has formed at the oxide film/SiC interface. Crystallinity of graphitic layer at the interface seemed to be dependent on the partial pressure of oxidizing gas, which was revealed by the relationship between G peak position and R(I_D/I_G). The intensity ratio of FTO(0)/FTO(2/6) bands showed that stacking faults on the surface of SiC coupons were reduced after heat treatment.     

Role of Hydrogen Peroxide in Selective OH Group Functionalization of Polypropylene Surfaces using Underwater Capillary Discharge

Plasma chemical methods are well suited for introducing functional groups to the surfaces of chemically inert polymers such as polyolefins. However, a broad variety of functional groups are often formed. Unfortunately, for further chemical processing such as grafting of molecules for advanced applications a highly dense monotype functionalized polyolefin surface is needed. Therefore, the main task was to develop a selective surface functionalization process, which formed preferably only a single type of functional groups at the surface in high concentration. Amongst the novel plasma methods, the underwater plasma process (UWP) is one of most attractive options to solve the problem of monotype functionalization. Such plasma is an efficient source of ions, electrons, UV-radiation, high-frequency shock waves, radicals such as hydroxyl radical, and reactive neutral molecules such as hydrogen peroxide. In contrast to established gas phase glow discharge processes, the water phase limits the particle and radiation energies and thus the energy input into the polymer. By virtue of the liquid water environment, which moderates highly energetic plasma species, extensive oxidation, degradation, cross-linking and radical formation on the polymer are more limited as compared to gas plasma exposure. The variety of plasma produced species in the water phase is also much smaller because of the limited reaction possibilities of the plasma with water. The possibility to admix a broad variety of chemical additives makes underwater plasma even more attractive. Hydrogen peroxide and the catalyst (Fe-ZSM5) should influence and increase the equilibrium concentration of OH radicals in the underwater plasma process. It was found that these radicals played a very important role in OH functionalization of polyolefin surfaces. Hydrogen peroxide was identified to be the most prominent precursor for OH group formation in the UWP. The catalyst would affect the steady state of OH radical formation and its reaction with the substrate surface and thus accelerates the functionalization process.     

Ultrasonic investigations of linear and star shaped polybutadiene polymers in solutions of cyclohexane,hexane and ethylbenzene

Ultrasonic attenuation and velocity measurements are reported on solutions of linear and star shaped polybutadiene in cyclohexane, hexane and ethylbenzene. The linear polymers exhibit viscosity and ultrasonic attenuation characterics typical of linear polymers. In contrast, the star-shaped polymers exhibit viscosities which are considerably lower than those for the equivalent linear polymer. The ultrasonic attenuation is larger in the star shaped polymers than in the corresponding linear materials and this additional contribution to the attenuation is ascribed to the effects of relaxation of the chemical crosslink. In contrast, the adiabatic compressibilities of the linear and star shaped polymers are essentially independent of molecular weight and structure of the polymer and only a function of the solvent. Analysis of the data presented leads to the conclusion that in this system the excess attenuation is associated with the relaxation of the crosslinks and that the major contribution to such a relaxation comes from entropic rather than volume or enthalpic processes.     

Preparation of Spherical, Submicrometer Oxide Particles by Hydrolysis of Emulsified Alkoxide Droplets

A new method for forming spherical, submicrometer ceramic oxide particles by the hydrolysis of emulsified alkoxide droplets is reported. Emulsions are formed of alkoxide droplets dispersed in an inert, polar solvent. The alkoxide droplets are hydrolyzed to form oxide particles by adding water to the emulsion. It was shown that individual droplets acted as "microreactors" and controlled the powder size, shape, and composition. Both single-oxide and mixed-oxide powders were formed by this technique.     

PBT的固相缩聚

对固相缩聚的机理和影响因素进行了介绍。认为聚对苯二甲酸丁二酯 ( PBT)是一种重要的工程塑料 ,高的分子质量使其性能得到提高。作为生产 PBT高粘切片的有效方法 ,固相缩聚依赖于化学 (可逆化学反应 )和物理 (小分子的扩散 )两个过程     

Rayleigh–Benard Convection in a Chemically Equilibrium Gas Containing Chemically Inert Microparticles

A physicomathematical model of the Rayleigh–Benard convection in a chemically equilibrium gas containing chemically inert microparticles (Al₂O₃) is proposed. A linear analysis of convection in the Boussinesq approximation is performed. It is shown that addition of chemically inert microparticles increases the critical value of the Rayleigh number and stability of the convective process. The possibility of using chemically inert microparticles for controlling convection in a chemically reacting gas is demonstrated by an example of a chemically equilibrium gas.     

Electrochemical response of poly(o-ethoxyaniline) films produced by different techniques

Poly(o-ethoxyanilines) (POEA) were synthesized electrochemically and chemically in aqueous acid media. POEA films were readily obtained during the electrochemical deposition on platinum electrodes, while the chemically synthesized material was processed in the form of films either by drop coating or by Langmuir-Blodgett (LB) deposition. The open circuit potential values measured during the chemical synthesis, as well as the color changes observed, are in close agreement with those of the electro-polymerization indicating basically the same changes in oxidation states of the polymer structures. The electrochemical response of all films was investigated by cyclic voltammetry. Two redox processes were observed after stabilization in the electrolyte solution, which are associated with interconversion reactions involving different oxidation and protonation states. Ultra-thin LB films required the shortest time for stabilization. POEA films displayed electrochromism, with color changes from yellow to violet as a function of the potential.