Morphology study of Nafion membranes prepared by solutions casting

The structures of Nafion membranes prepared by solutions casting from low aliphatic alcohols/water mixture solvents and N,N′‐dimethyl formamide (DMF) solvent were investigated using differential scanning calorimeter and small angle X‐ray scattering. The aggregation behavior of Nafion molecules in the casting solutions was also investigated using dynamic light scattering. We show that the morphology of membranes was strongly influenced by the conformations of Nafion molecules in the solutions. In aliphatic alcohol/water mixture solvents, which have a worse compatibility with Nafion backbones, the Nafion molecules aggregate and form fringed rod‐like structures. These primary rod‐like structures then aggregate again through fringed side chains to form secondary ionic aggregations. In DMF solvent, owing to its better compatibility with Nafion backbones, less Nafion molecules aggregate. The high degree of Nafion molecular aggregations in aliphatic alcohol/water mixture solvents leads to a high degree of hydrophobic and hydrophilic phase separation for membranes prepared by casting from Nafion/aliphatic alcohol/water solutions. However, the lower degree of molecular aggregations in DMF solvent results in a lower degree of hydrophobic and hydrophilic phase separation for membranes prepared by casting from Nafion/DMF solution. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3044–3057, 2005     

重铬酸铵与明胶光化学反应的光声光谱研究

重铬酸铵与明胶光化学反应的光声光谱研究陈钢进，杜浩，蔡铁权，方健文（浙江师范大学化学系金华３２１００４）（浙江师范大学物理系金华）关键词重铬酸盐明胶，光化学反应，光声光谱重铬酸盐明胶（ＤＣＧ）是全息照相的重要材料，全息图的形成过程是六价铬在光的激发下...     

Microviscosity in modified Nafion membranes measured by fluorescence probes

Sodium or quaternary ammonium modified Nafion membranes in forms of Nafion-Na+,Nafion-NMe4+,Nafion-NEt4+ and Nafion-NBu4+ have been prepared by neutralizing Nafion-H+ membrane with NaOH or tetraalkylammonium hydroxide aqueous solution.Intramolecular excimer formation and fluorescence polarization methods using l,3-di(1-pyrenyl)propane and acridine orange as probes respectively have been employed for measurements of the microviscosity within those modified Nafion membranes.Results show that the probes are located in the fluorocarbon/water interface in the cluster of the membranes and the microviscosities around the probe molecules are in the range of ca.120-1200 cp and increase in the order of Nafion-Na+相似文献   

Transport properties of composite membranes containing silicon dioxide and Nafion

Silicon dioxide (SiO₂) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO₂ membranes were fabricated with SiO₂ content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO₂ content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO₂ content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO₂ content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO₂ nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.     

Thermolysis of azobenzene

Thermolysis of azobenzene in a melt and in solution results in the formation of aniline. The possible directions of the reaction are considered. Comparison of the enthalpies of the reaction proves that the cleavage of a -bond in the N=N group to form a biradical intermediate is thermodynamically more favorable than that of a C-N -bond.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1562–1564, September, 1993.     

Pt/Nafion膜制备的研究

采用还原剂渗透还原法制备了不同Pt载量的Pt/Nafion膜,并利用扫描电镜(SEM),X射线衍射分析(XRD)对反应不同时间制得的Pt/Nafion膜中的Pt在膜内及膜表面的生长情况进行了研究。实验发现:Pt在膜内及膜表面均能发生生长,且主要沉积在膜内靠近膜表面处的一段区间。采用KBH~4还原H~2PtCl~6有利于Pt(111)晶面的生长。分析不同反应时间的横截面及正面照片,提出了Pt生长的可能机理。     

Pervaporation of water from aqueous sulfuric acid at elevated temperatures using Nafion membranes

The concentration of sulfuric acid by pervaporation has been studied using Nafion-112^® and Nafion-117^® membranes, which have been characterized in terms of flux, permeability, and separation factor at 100 and 120 °C. Feed acid concentrations investigated ranged from 40 to over 80 wt%. In general, water fluxes ranged from 100 to 8000 g/m² h, depending on feed acid concentration and separation factors as high as 10⁴ were observed. Membrane stability was probed using dynamic mechanical analysis that revealed an increase in the temperature at which the α transition is observed, which corresponds to the glass transition (T_g) of the hydrophilic domain, upon use, suggesting embrittlement of the polymer structure. Further studies showed that the embrittlement was due to an interaction with the acid and was not induced by the operating temperature.     

Photochemical reaction kinetics in optically dense media: The influence of thermal reactions

The kinetics of photochemical reactions in optically dense media essentially free from diffusion was considered. The photochromic isomerization A ↔ B was studied as an example. If thermal isomerization is possible, a stationary state is achieved in time determined by rate constants for the thermal reactions. The concentration wave profile is changed during the photochemical reaction propagation. Low values of thermal reaction constants and decrease in sample optical density during photochemical isomerization were found to be essential for maximal wave penetration into the sample. Sharp concentration gradients of A and B can be observed when both the optical density is increased during photochemical isomerization and the quantum yield of the direct photochemical reaction A → B is higher than that of the reverse photochemical reaction B → A.     

Photochemical addition reaction of a polymer-bearing pendant vinyl ether with various thiol compounds

Photochemical addition reaction of the pendant vinyl ether group in the polymer ( P-1 ), which was synthesized by the alternate ring-opening copolymerization of glycidyl vinyl ether with phthalic anhydride, with various thiol compounds such as benzenethiol, phenylmethanethiol, 2-mercaptoacetic acid, ethyl 2-mercaptoacetate, N-acetyl-L -cysteine (AcCys), and 1,4-phenylenedi(methylthiol) was carried out using benzophenone (BP) as the photosensitizer in the THF solution. Each reaction proceeded very smoothly to give the corresponding polymers with high conversion, although the degree of reaction of the pendant vinyl ether group in P-1 was affected by the molar ratio between the thiol compounds and the vinyl ether group, and the amounts of photosensitizer BP added. Furthermore, it was also found that optically active polymer containing pendant N-acetyl-L -cysteine residue was synthesized by the photochemical addition reaction of P-1 with AcCys. The reactions of P-1 with dithiol or bisazide compounds occurred effectively to give gel products in the film state, and it was found that the polymer film containing P-1 and those compounds can be applied as negative-type photoresists with high practical photosensitivity. © 1993 John Wiley & Sons, Inc.     

Thermal stability of ion-exchange Nafion N117CS membranes

The effect of exchanged ions on the thermal stability of Nafion N117CS membranes was investigated by X-ray photoelectron spectra (XPS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and ion exchange capacity determinations. The ion exchange of alkaline metal ions was effective in improving the thermal stability of the Nafion N117CS membrane. Findings reveal that when Nafion was exchanged for cations with a larger ionic radius, the membrane attained superior thermal stability. On the other hand, we confirmed that the Na-exchange Nafion N117CS membrane possessed a distinctive degree of thermal stability among the alkaline ion-exchange Nafions, although the order of ionic radii is K > Na > Li. Thermal stability improved the most when the Nafion membrane was exchanged for alkaline ions, followed by divalent ions, then trivalent ions. As for the Nafion membrane when it was exchanged for divalent ions or trivalent ions, Nafion following the ion exchange had a thermal stability proportional to an increase in the ionic radius of the cation. This stability may be explained by the reduction of water content and a greater interaction between the sulfonate groups and the cations with larger ionic radii. Since the Al cations acted as a Lewis acid center, the decomposition of the ether bonds of the perfluoroalkylether pendant-chains of the Nafion membrane was observed for the Nafion N117CS membrane that had been exchanged for Al ions. The activation of molecular mobility in Nafion was observed between the decomposition stages of the loss of water and the loss of sulfonic groups. The temperature of activation of cation-exchange Nafion became much higher than that of Nafion in an acid form.     

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion‐based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long‐time creep of ～40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ～0.55 MPa in 125‐μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ～15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006     

Nafion/PTFE and zirconium phosphate modified Nafion/PTFE composite membranes for direct methanol fuel cells

We prepared Nafion/PTFE (NF) and zirconium phosphate (ZrP) hybridized Nafion/PTFE composite membranes (NF–ZrP). NF–ZrP composite membranes were prepared via two processes. One is impregnating sub-μm porous PTFE membrane directly in a Nafion/ZrOCl₂ solution (NF–Zr–d). The other is impregnating sub-μm porous PTFE membrane in a Nafion solution to prepare NF composite membrane, and then the NF membrane was impregnated in a ZrOCl₂ aqueous solution via in situ precipitation method (NF–Zr–I). The ZrOCl₂ inserted in NF composite membranes was then reacted with phosphoric acid to form ZrP and thus NF–ZrP–d and NF–ZrP–I composite membranes were obtained. The direct methanol fuel cell (DMFC) performances of membrane electrode assemblies prepared from Nafion-117, NF, NF–ZrP–d, and NF–ZrP–I composite membranes were investigated. The effects of introducing sub-μm porous PTFE film and ZrP particles into Nafion membranes on the DMFC performance were investigated. The influence of ZrP hybridizing process into NF membranes (the process of preparing NF–ZrP–I is inserting ZrOCl₂ into NF membranes after Nafion is annealed and the process of preparing NF–ZrP–d is mixing ZrOCl₂ into a Nafion solution before Nafion is annealed) on the morphology of NF–ZrP composite membranes and thus on the DMFC performance was also discussed.     

双核有机金属化合物的光化学反应探析

有机金属化合物是指分子中至少含有一个金属-碳键（M-C键）的化合物,包括含M-C σ-键、M-Cnπ-键、ηn-M-Cn大π-键的化合物.双核金属-金属碳键的有机化合物的光化学反应包括光诱导金属-金属键的断裂、配体的离解与取代、光化学插入反应、光诱导异构化反应、歧化反应等.     

超分子凝胶中的光化学反应

超分子凝胶中的光化学反应是比较特殊的一类反应,通常是将具有光响应活性的基团或分子引入到超分子凝胶的自组装体系中,因此,能够将超分子凝胶独特的性质与光化学反应的优势有效地结合起来,构筑新型的光功能材料,这使得此类超分子凝胶在光信息存储、光开关及光转换器件等前沿领域具有广阔的应用前景.本文主要总结近年来国内外包括作者课题组对超分子凝胶中光化学反应方面的研究进展,以及其在多重响应凝胶、手性光学开关以及手性合成方面的应用.     

Preparation of Nafion Membranes for Reproducible Ammonia Quantification in Nitrogen Reduction Reaction Experiments

This study highlights the importance of following a strict protocol for Nafion membrane pretreatment for electrochemical nitrogen reduction reaction experiments. Atmospheric ammonia pollution can be introduced to the experimental setup through membranes and interpreted falsely as catalysis product from N₂. The sources of ammonia contamination vary drastically between locations worldwide and even within the same location between days depending on temperature, wind direction, fertilizer use, and manure accumulation in its vicinity. The study shows that significant amounts of ammonium is accumulated in the membranes after commonly practiced pretreatment methods, where the amount depends on the ammonia concentration in the surrounding of the experiment. Therefore, we introduce a new pretreatment method which removes all the ammonium in the membrane. The membranes can be stored for several days but a short final step in the method needs to be carried out right before NRR experiments.     

光异构化反应速率的量子理论

光异构化反应是在光场存在下，分子吸收光子引起的单分子化学反应，包括通常的异构化与环合、开环反应．一些作者用量子化学方法及分子轨道相关图和态相关图等方法对这类反应进行过研究［1］．本文用多声子光跃迁理论［2］研究光异构化反应，导出反应速率及其在低温条件下的解析表达式，对所得结果进行了讨论．1理论方法与结果在光异构化反应中，分子的电子状态与构型都发生变化，而且电子态的变化是与构型的改变紧密耦合的．分子的构型用分子的振动波函数来描写．由于电子与原子质量相差悬殊，可以采用绝热近似处理这一电子－振动相互作用…     

Molecular dynamics study of the cations,water molecules,and polymer chains in Nafion type membranes

The structure and dynamics of the hydrated cationic complexes in Nafion type membrane pores has been studied by the molecular dynamics approach. The mechanism of the cationic transport has been examined. The dependence of the cationic transport coefficients on temperature and the number of water molecules has been investigated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1233–1236, July, 1995.The present work was made possible in part by financial support of the International Science Foundation (Grant MPG000) and the Russian Foundation for Basic Research (Grant 93-03-4205).     

Determination of Biacetyl by Sensitized Photochemical Kinetic Fluorimetry

Abstract

A new in-situ photochemical kinetic fluorimetric method was proposed for the determination of biacetyl (BI). It is based on the sensitization of BI on the photochemical reaction of amaranth (AM). AM, a nonfluorescent compound was converted into an intensively fluorescent compound in a slightly alkaline medium by the sensitized photochemical reaction, and BI was indirectly determined by monitoring the change of the fluorescence intensity. The determination can be carried out by fixed-time method or tangent method. The kinetic behavior of the reaction and the effects of some experimental conditions were investigated and discussed. The calibration graph was rectilinear from 1.0 μg ml^?1 to 10.0 μg ml^?1 of BI (r = 0.999), the limit of detection was 1.0 ng ml^?1, and the coefficient of variation was 0.44% for 0.90 μg ml^?1 of BI (n = 6). The mechanism for the sensitization of BI was examined and the triplet-triplet energy transfer, in which BI acted as the energy donor and AM as the energy acceptor, was suggested to be the main cause. Its application to real samples has been tested.