Matrix-filler interactions in a co-ground ecocomposite: Surface properties and behaviour in water

Ecocomposites made up of polystyrene and starch were produced by co-grinding. The mechanism by which the composite is formed was identified by following the particle size and morphology. The size reduction of the matrix particles is favoured by the presence of starch which adheres on polystyrene surface, playing the role of agglomeration inhibitor between matrix particles. Thus, the filler is well dispersed in the matrix, permitting a good homogeneity of the composite properties. The hydrophilic behaviour of starch is reduced by co-grinding, resulting of a decrease of the acid and non dispersive components of the surface energies. Consequently the interactions between the initially hydrophobic matrix and hydrophilic filler are enhanced without using a compatibilizer. Thus, the water-resistance of the co-ground composite materials is better compared to blends since blends pellets introduced in water are rapidly disintegrated while an adapted co-grinding time permits to avoid this problem. It was seen that the diffusion coefficient of water in the composite pellets decreases with an increase of the co-grinding time for the lower filler rates, while it is the opposite for high filler rates. Moreover, the diffusion coefficient increases with the filler ratio.     

以纳米球形聚电解质刷为载体原位还原制备贵金属纳米粒子的研究

以具有核壳结构的纳米球形聚电解质刷为载体,通过吸附不同种类的贵金属离子并将其原位还原,得到分布和粒径不同的贵金属纳米粒子。采用动态光散射(DLS)研究了金属离子浓度和pH对壳层聚电解质链长的影响,使用透射电镜(TEM)观察了贵金属粒子在纳米球形聚电解质刷中的分布,并测量了金属粒子的尺寸。结果表明:金属离子浓度增大,聚电解质链长减小;pH>5时,负载金属粒子之后的聚电解质链长度均大于负载前的长度。不同金属粒子的负载情况相差较大:相同温度下,纳米金属粒子的尺寸符合Ni>Ag>Co的顺序;较高温下制得的金属粒子尺寸要高于低温下制得的金属粒子尺寸;三种纳米金属粒子中,Co纳米粒子具有最好的分布和最小的尺寸。     

影响塑件尺寸精度的原因分析

注塑成型件的尺寸精度一直是业界关心的话题,通过对影响塑件尺寸精度概念的描述,讲述了影响塑件尺寸精度的三大主要原因,并对其原因进行了详细的分析,构成了直接影响塑件尺寸精度要素,这对塑件产品设计、模具设计,材料选择,工艺制定和控制塑件尺寸精度都有一定的借鉴作用。     

Termini and main-chain composition of monomethoxy-terminated poly(ethylene glycol) studied by two-dimensional column chromatography

Samples of poly(ethylene glycol) especially those with functionalized ends have dimeric, trimeric, and tetrameric components at twice, three times, and four times as high a molecular weight (MW) as that of the main component. We used two-dimensional column chromatography to evaluate the MW and identify the terminal groups in each component. The first step is high osmotic pressure chromatography to prepare a sufficient amount of fractions that contain different percentages of the multimeric components. In the second step, each of the fractions was characterized by using regular size exclusion chromatography and high-resolution NMR. Decomposition of the chromatogram combined with isolation of the terminal methoxy peak in the NMR spectrum led to unique identification of the terminal groups. We thus found that the monomeric component is monomethoxy-terminated whereas the dimeric and other multimeric components are mostly dihydroxy-terminated.     

微米级单分散聚苯乙烯微球的研究进展

综述了近年微米级单分散聚苯乙烯(PS)微球的研究状况和技术进展,重点讨论了反应单体、引发剂、分散剂、反应介质等对微球粒径及粒径分布的影响,并对PS微球的制备方法进行了介绍。最后对今后PS微球的发展趋势及应用前景进行了分析和展望。     

Influence of Atomizing Parameters on Droplet Properties in a Pulse Combustion Spray Dryer

A pulse combustor employed in a spray-drying system offers a new approach for liquid atomization that yields high-quality powders at low cost. Using a pulse combustion atomizer, there is no need for any form of nozzle dispersion and its atomization mechanism differs from those of conventional atomizers, such as rotary atomizers and pressure and pneumatic nozzles. In this work, based on the analysis of atomization mechanism, experiments of unsteady pulsating atomization were carried out in an experimental system of a Helmholtz-type pulse combustor. An optical analyzer was used for measuring the mean diameter of atomized droplet and droplet distribution. The effects of liquid feed rate, air flow oscillatory frequency, and liquid viscosity on atomized droplet size and size distribution were investigated and analyzed. The results indicate that the uniform droplet size distribution can be obtained under the conditions of a low feed rate, high-frequency pulsating flow, and moderate viscosity. The range of the droplets' Sauter mean diameter (SMD) is between 50 and 80 µm. The pulsating air flow from the pulse combustor can be used to atomize liquid or slurry without a nozzle and the atomizing quality can meet the requirements of spray drying.     

Integration of LEFM and Strength Criteria for the Prediction of Notched Strength of Polymeric Composites

The prediction of the notched strength of polymeric composites has been the object of much interest in recent years. Theories based on both linear elastic fracture mechanics (LEFM) and strength criteria have been proposed by numerous investigators. In the present paper a model that integrates the concepts of a general strength criterion with the energy concept of LEFM is presented. This enables the transition between the extremes of both theories. Predictions made by the model are shown to compare well with experimental data for center-slit specimens of a variety of material systems. The use of the proposed model in ranking materials on the basis of notch sensitivity is also shown.     

Preparation of high loading Pt nanoparticles on ordered mesoporous carbon with a controlled Pt size and its effects on oxygen reduction and methanol oxidation reactions

A series of ordered mesoporous carbon (OMC) supported Pt (Pt/OMC) catalysts with a controlled Pt size from 2.7 to 6.7 nm at high Pt loading around 60 wt.% have been prepared and their electrocatalytic activities for the electrode reactions relevant to the direct methanol fuel cells have been investigated. The Pt/OMC catalysts with a high dispersion (Pt size around 3 nm) could be prepared by the use of a modified, sequential impregnation–reduction method. The Pt/OMC catalysts containing larger Pt particles were obtained by increasing reduction temperature under hydrogen flow and Pt loading, and by performing impregnation–reduction in a single cycle. The oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities of Pt/OMC catalysts as a function of Pt size were investigated at room temperature in 0.1 M HClO₄ and (0.1 M HClO₄ + 0.5 M methanol), respectively. The specific activity of Pt/OMC for ORR steeply increased up to 3.3 nm and became independent of Pt size from 3.3 to 6.7 nm, and the mass activity curve exhibited maximum activity at 3.3 nm. The MOR activity of Pt/OMC also exhibited the similar trend with the ORR activity, as the maximum of mass activity was also found at 3.3 nm. The results of the present work indicate that the Pt catalysts of ca. 3 nm is an optimum particle size for both ORR and MOR, and this information may be translated into design of high performance membrane electrode assembly.     

Optimization of cushion conditions in micro multi-point sheet forming

Improvement of product quality is required more than ever for mass production of micro formed parts. The process of micro multi-point forming with cushion has been simulated in this paper by finite element model which incorporates a constitutive relation with consideration of size effect. The effect of the cushion material, cushion thickness and coefficient of friction on the surface quality of the deformed sheet has been investigated. It has been found that a proper selection of the cushion material and optimum cushion thickness can significantly improve the surface quality. In addition, this paper also discusses the influence of the cushion material and coefficient of friction on the sheet thickness distribution. Better surface quality and thickness distribution can be obtained with good lubricating condition.     

Size segregation of spherical nickel pellets in the surface flow of a packed bed: Experiments and Discrete Element Method simulations

The size segregation of binary mixtures of spherical nickel pellets flowing into a packed bed was investigated with Discrete Element Method (DEM) simulations and physical experiments in 30 cm and 60 cm wide rectangular test cells. Each test cell approximates a vertical slice of a cylindrical packed bed, with a rising feed tube on one side of the cell representing the stationary frame of reference in the packed bed. As the feed tube is raised, the pellets flow laterally into the test cell to form a sloping surface inclined to the horizontal by the angle of repose. The lateral flow of pellets is confined near the surface of the packed bed, and was intermittent in character (i.e. surging). Velocity vectors show the detailed flow field in the simulated test cells. The smaller pellets were found to be concentrated near the core of the granular assembly, and the larger pellets segregate to the outer wall farthest from the feed tube. The degree of segregation, or coefficient of variation (variance/mean), is proportional to the diameter ratio α of the pellets and the length of the surface, and inversely proportional to the mass fraction of the smaller pellets within the range of parameters studied. The DEM simulations had an average deviation in mass fraction of 0.07 and maximum deviation of 0.22 from the experimental data.