Numerical simulation of substrate effects on spinodal decomposition in polymer binary mixture: morphology and dynamics

The characteristic features of the morphology and dynamics of binary mixture for substrate-directed spinodal decomposition (SDSD) in three dimensions have been studied using numerical simulations. The simulation results show that the formation of the wetting layer on the substrate interface follows the power-law growth. It is found that the continuous influence of anisotropic diffusive behavior arose by wetting of substrate induces the wetting component spreading onto the interface of the substrate randomly. The phase morphology and averaged size in the vicinity of the substrate fluctuate greatly due to the wetting of the substrate. The self-similar evolution of the cross-sections parallel and perpendicular to the substrate interface is discussed by relevant scale law functions, respectively. And the mechanisms of the growth in both the parallel and perpendicular directions are also investigated.     

Photocrosslinking kinetics of polymer blends undergoing spinodal decomposition process

Photocrosslinking reaction kinetics of poly(2-chlorostyrene) performed inside the spinodal region of poly(2-chlorostyrene)/poly(vinyl methyl ether) (P2CS/PVME) blends was investigated by means of ultraviolet (UV)-visible absorption spectroscopy. The reaction was performed via photodimerization of anthracene moieties chemically labeled on the P2CS chains. The crosslinking kinetics of (P2CS/PVME) blends submitted to a temperature jump from the one-phase into the spinodal regions was observed by monitoring the irradiation time dependence of the absorbances of anthracene as well as of the blend in two regions of wavelengths. One is inside and the other is outside the absorption range of anthracene. The contribution of the sample cloudiness to the absorbance of anthracene was subtracted from the absorption data by using an empirical power law experimentally established between the incident wavelengths and the absorption of the blends. It was found that the reaction kinetics approximately follows the mean-field kinetics inside the spinodal region, resembling the behavior of the crosslinking reaction performed in the miscible region at relatively low crosslinking densities. On the other hand, the method described here fails to estimate the crosslinking densities when the phase separation proceeds rapidly, overcoming the reaction. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:885–893, 1998     

Morphology control in symmetric polymer blends using spinodal decomposition

This paper studied, through modeling and computer simulation, the thermal-induced phase separation phenomenon in a symmetric polymer blend via spinodal decomposition. The one-dimensional model consisted of the Cahn–Hilliard theory for spinodal decomposition, and incorporated the Flory–Huggins–deGennes free energy equation, the slow mode mobility theory and reptation model for polymer diffusion. The numerical results replicated frequently reported experimental observations published in the literature for the early and intermediate stages of spinodal decomposition for symmetric polymer blends. Furthermore, the numerical results indicate that a dimensionless diffusion coefficient may be used as a parameter to control the formation and evolution of the phase-separated regions during spinodal decomposition as a means to customize functional polymeric materials with predefined material properties.     

Later-stage spinodal decomposition in polymer solution under high pressure—analyses of qm and Im

Later-stage spinodal decomposition (SD) of polymer solutions (polypropylene/trichlorofluoromethane) induced by pressure-jump was examined in situ as a function of pressure P by using time-resolved light scattering method with the cell designed for high pressure and high temperature. The time-evolution of the magnitude of scattering vector q_m(t,P) at maximum scattered intensity and the maximum scattered intensity I_m(t,P) were analyzed in order to characterize the coarsening processes of the later-stage SD, where t refers to time after the onset of pressure-jump. The changes in q_m(t,P) and I_m(t,P) with t at different P's were found to fall onto the respective master curves on the reduced plots, indicating that the scaling postulate is valid not only for the coarsening behaviors at different temperatures but for those at different P's.     

Effects of molecular weight distribution on the spinodal temperature of polymer mixtures

The effects of molecular weight distribution on the phase stability of polymer mixtures were explored theoretically and experimentally. Based on the lattice‐fluid theory and volume‐fluctuation thermodynamics, the spinodal conditions for a lattice‐fluid mixture of two polymers with molecular weight distribution were derived. The results indicated that the phase stability of a polymer mixture decreases by increasing the molecular weight distribution of polymers in the blend. To confirm the theoretical results with experiments, the changes in the spinodal temperatures of poly(ethyl methacrylate)/polystyrene (PEMA/PS) blends and tetramethyl polycarbonate/polystyrene (TMPC/PS) blends were examined when each component has a different molecular weight distribution. When the weight‐average molecular weight of each component is the same, a blend composed of polymers having broad molecular weight distribution always exhibited lower phase separation than that composed of polymers having narrow molecular weight distribution at the same blend composition. Copyright © 2004 Society of Chemical Industry     

Effect of aliovalent dopants on the kinetics of spinodal decomposition in rutile-type TiO2-VO2

The effects of aliovalent dopants on the kinetics of spinodal decomposition in a rutile-type TiO₂-VO₂ system were studied by means of x-ray diffraction and transmission electron microscopy. In this study, as-prepared polycrystalline samples were annealed at 673 k inside the spinodal. For undoped samples, phase decomposition occurred along the [001] direction, and a lamellar structure formed on the nanometer length scale. The phase decomposition kinetics in this system were sensitive to the type of aliovalent dopant; the decomposition rate in a sample doped with Al₂O₃ was more than one order of magnitude higher than a sample doped with Nb₂O₅. These observations could be explained on the basis of the premise that cation interstitial mobility is greater than cation vacancy mobility. The obtained results are consistent with those for a TiO₂-SnO₂ system, indicating that phase separation in both rutile-type oxide systems can be interpreted through a common framework.     

Effects of the electrical conductivity and orientation of silicon substrate on the synthesis of multi-walled carbon nanotubes by thermal chemical vapor deposition

We studied the effects of the electrical conductivity and orientation of silicon substrate on both catalytic Fe thin film and the structure and morphology of multi-walled carbon nanotube (MWNT) grown by low-pressure chemical vapor deposition. Both p-type Si(100) and Si(111) substrates with three different doping concentrations (high, low, undoped) were used to evaluate the formation of catalytic nanoparticles and the growth of MWNTs. The morphology of catalytic nanoparticles such as size and density was characterized by field-emission scanning electron microscopy, Cs-corrected energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural characteristics of MWNTs grown on different combinations of silicon substrate orientation and electrical conductivities (σ) were also systematically analyzed. Based on the experimental results, growth modes of MWNTs could be controlled by choosing an appropriate combination of σ and orientation of Si substrates.     

Numerical investigation on the number of active surface sites of carbon catalysts in the decomposition of methane

The number of active sites on the surface of carbon catalysts is an important factor in determining their activity in the decomposition of methane. Although several studies have been performed to identify the nature of these sites, no method has been established to estimate their number. A method is presented to estimate this value, and its effect on hydrogen production is evaluated, along with that of temperature and residence time. For this purpose, the thermocatalytic decomposition of methane is modeled with the inclusion of the number of active sites of the catalyst in the kinetics. The results of the model indicate the high influence of variations of small residence times in this process, and the reduction of this effect at high temperatures. Also, the effect of the number of surface sites is shown to be more prominent at low residence times and temperatures. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2228–2234, 2014     

A multiscale mass transfer model for gas-solid riser flows: Part II—Sub-grid simulation of ozone decomposition

This article is to test the EMMS-based multiscale mass transfer model through computational fluid dynamics (CFD) simulation of ozone decomposition in a circulating fluidized bed (CFB) reactor. Three modeling approaches, namely types A, B and C, are classified according to their drag coefficient closure and mass transfer equations. Simulation results show that the routine approach (type C) with assumption of homogeneous flow and concentration overestimates the ozone conversion rate, introduction of structure-dependent drag force will improve the model prediction (type B), while the best fit to experimental data is obtained by the multiscale mass transfer approach (type A), which takes into account the sub-grid heterogeneity of both flow and concentration. In general, multiscale behavior of mass transfer is more distinct especially for the dense riser flow. The fair agreement between our new model with literature data suggests a fresh paradigm for the CFB related reaction simulation.     

Thermal decomposition of alkyl ammonium ions and its effects on surface polarity of organically treated nanoclay

The surface free energy and surface polarity of organically modified montmorillonite clay change when exposed to elevated temperatures. This was verified in this study using contact angle measurements and Wu's harmonic-mean equation. It was observed that the surface polarity of organically modified clay measured from the values of contact angle of water and diiodomethane reduced significantly owing to thermal degradation of the organic modifier of clay, the latter was confirmed by Fourier transform infra-red spectroscopy. As a consequence, nonpolar polymer, polypropylene (PP), was found to spread more on heat treated surface of organically modified clay than a polar polymer, polyamide 6 (PA6). The ramification of these phenomena on blending of PP, PA6, and organically modified clay is discussed.     

Numerical simulation of the effects of the design feature of a cyclone and the inlet flow velocity on the separation of CO2 particles from a CO2-COF2 mixture

In synthesizing COF₂ from CO, a considerable amount of CO₂ is produced. A method of solidifying CO₂ at low temperature and separating CO₂ particles from the COF₂ gas using a cyclone was designed and the separation efficiency according to the cyclone feature was studied. Optimal sizing and operation conditions of the cyclone were investigated by reviewing the flow velocity profile and the particle trajectory using a numerical analysis with computational fluid dynamics (CFD). The effects of the inlet flow velocity and the ratio of the cyclone diameter to the cone length (D/L) on the recovery efficiency were estimated. Results revealed that the separation efficiency increases with an increase in the ratio of D/L and a decrease in the cyclone size. The recovery efficiency of CO₂ increases with the increase in the inlet flow velocity. Based on these results, we could propose a concept and methodology to design the optimal features and sizing of a cyclone suitable for separating solid CO₂ from gaseous COF₂ at low temperature.     

DD分解炉燃烧与分解耦合过程的数值模拟

针对一实际尺寸的DD分解炉进行了煤燃烧与碳酸钙分解耦合过程的三维数值模拟研究,其中,对连续相采用Euler坐标系下的k-ε双方程湍流模型,采用离散相模型(discrete phase model)进行颗粒相的运动轨迹计算,采用组分运输模型(species transport model)结合涡耗散概念模型(EDC)模拟煤粉燃烧及生料分解过程,采用P-1辐射模型计算气体和颗粒之间的辐射换热。计算所得煤粉燃烬率为86%,碳酸钙分解率为92.9%,与工程实际数据吻合较好,表明模拟结果的可信性。研究结果表明:来自底部向上运动的高速烟气流与两股横向三次风相遇后,汇合成一股高速向上运动的主气流,携带着煤粉流在分解炉中心处向上运动,并偏向位于分解炉侧面的出口方向;煤粉的燃烧主要发生在分解炉下半柱体部分中心处,并形成了高温区;碳酸钙则围绕着高温区迅速分解,其分解过程主要发生在分解炉下半柱体部分。     

Substrate matters:The influences of substrate layers on the performances of thin-film composite reverse osmosis membranes

Thin-film composite (TFC) reverse osmosis (RO) membranes are playing the dominating role in desalination.Tremendous efforts have been put in the studies on the polyamide selective layers. However, the effect of the substrate layers is far less concerned. In this review, we summarize the works that consider the impacts of the substrates, including pore sizes, surface hydrophilicity, on the processes of interfacial polymerization and consequently on the morphologies of the active layers and on final RO performances of the compositemembranes. All the works indicate that the pore sizes and surface hydrophilicity of the substrate evidently influence the RO performances of the composite membranes. Unfortunately, we find that the observations and understandings on the substrate effect are frequently varied from case to case because of the lack of substrates with uniform pores and surface chemistries.We suggest using track-etchedmembranes or anodized aluminamembranes having relatively uniformpores and functionalizable porewalls asmodel substrates to elucidate the substrate effect.Moreover, we argue that homoporous membranes derived from block copolymers have the potential to be used as substrates for the large-scale production of high-performances TFC RO membranes.     

Effect of zeolite in surface discharge plasma on the decomposition of toluene

Toluene was decomposed in a surface discharge plasma reactor packed with various zeolites. The positioning effect of the zeolite bed was also investigated Reactor-B, in which the zeolite bed was located upstream, performed much better than Reactor-A, in which the zeolite bed was located downstream. Furthermore, the decomposition efficiency in Reactor-B increased with the capacity for toluene adsorption on zeolite, while that in Reactor-A did not. The toluene adsorbed in micropores was not decomposed effectively by direct electron impact, but was decomposed by active oxygen species generated in the plasma zone on the zeolite surface. A good correlation was also observed between toluene decomposition and ozone consumption in the downward-type reactor.     

巷道截面变窄瓦斯爆炸传播特性的数值模拟

为获得井下巷道截面变窄对瓦斯爆炸传播特性的影响,利用Fluent流体力学软件建立相应的数值模型,对井下截面变窄的巷道瓦斯爆炸进行数值模拟研究。探讨了巷道截面变窄的大小对瓦斯爆炸的影响程度以及瓦斯爆炸后压力和温度的变化。巷道截面变窄对瓦斯爆炸有明显的激励作用,且巷道越窄,激励作用越明显。在此基础上,分析了巷道壁面和三波结构对瓦斯爆炸的影响。结果表明,经壁面反射后的反射波和三波点的碰撞使爆炸的压力和温度显著升高且持续时间较长。研究结果对矿井建设的安全设计与安全设施设备的实施有重要意义。     

生物质三组分二元混合热解特性研究

采用TG-FTIR-MS研究了生物质三组分二元混合热解过程的失重特性和小分子气体逸出规律。结果表明二元混合组分热解过程降低了热解反应开始温度。纤维素与半纤维素混合热解过程热解反应受到抑制,热解失重率降低,H₂、CH₄和H₂O产量减小,CO和CO₂产量增加。木质素和半纤维素在混合热解过程中存在协同效应,促进热解反应进行,H₂产量增加,然而其他小分子气体产物的生成被抑制,协同效应的效果更有利于可冷凝挥发分产物生成,这种效应随着半纤维素比例增大而减弱。半纤维素和纤维素在整个热解过程表现出相互抑制的效果,其小分子气体产物产量减小,但随着纤维素比例增大,影响减弱。     

高分子表面活性剂自聚效应对溶液介观结构的影响

采用动态密度泛函理论分别以PEB［(PEO) ₁₁-(PBO)₁₁］和PL64［(PEO)₁₃-(PPO)₃₀-(PEO)₁₃］为研究对象,模拟了两嵌段和三嵌段共聚物高分子表面活性剂自聚效应对介观结构的影响.PEB在自聚效应的影响下可以形成多层囊泡结构,并且PEB聚集体的自聚程度越高,多层囊泡的层数越多,自聚程度越低,多层囊泡中包含的水分子越多.而PL64在自聚效应的影响下可以形成球形多孔结构以及球状和短棒状聚集结构.此外,通过比较序参数以及介观结构的演化过程,给出了PEB多层囊泡结构与PL64球形多孔结构和球状短棒状聚集结构的演化机理.     

Study of molecular shape and non-ideality effects on mixture adsorption isotherms of small molecules in carbon nanotubes: A grand canonical Monte Carlo simulation study

The sorption isotherms for binary mixtures of methane, ethane, propane and tetrafluoromethane have been determined in carbon nanotubes using configurational bias Monte Carlo simulation techniques. At high loadings, a curious maximum for equimolar gas-phase mixtures occurs with increasing pressure in the absolute adsorption isotherm of one or both adsorbing species. It was detected that there exist two fundamentally different reasons for this maximum. First, due to a higher packing efficiency, one component is able to displace the other component at high loadings. Here, it must be stressed that the displaced component is not necessarily the larger molecule. Second, non-ideality effects of the bulk gas phase can be made responsible for this maximum. The acceptance probability of a molecule insertion in a grand canonical Monte Carlo step is proportional to the component fugacity. If, owing to non-ideality effects of the gas phase, the fugacity of one component does not increase as steeply with pressure as the other component, a maximum can occur in the absolute adsorption isotherm of this component. These findings were demonstrated for various binary mixtures of CH₄, CF₄, C₂H₆ and C₃H₈.     

钝感剂对RDX热分解性能的影响研究

采用DSC对原料RDX和钝感剂与RDX的复合物进行了热分析测试,对样品的热分解动力学和热力学参数进行了计算和对比。结果表明,加入钝感剂(石墨或复合蜡)后,RDX在不同升温速率(2、5、10、20 K/min)下的放热峰峰温都得到了升高,热稳定性也有所提高,热力学参数发生明显的变化。