Preparation and characterization of silica—polyimide composite membranes coated on porous tubes for CO2 separation

Silica-polyimide microcomposite membranes were prepared on γ-alumina-coated α-alumina support tubes, and their gas permeation properties were evaluated with He, N₂ and CO₂. Smoothing of the substrate surface and hybridization of silica and polyamic acid were both effective to form defect-free thin composite membranes. The CO₂ permeance of a membrane with a silica content of 68 wt% was one order of magnitude higher than that of a polyimide membrane having the same thickness. The permselectivity of CO₂ to N₂ was 30 at 30°C and 13 at 100°C. Contributions of the silica and polyimide phases to permeance of the composite membrane were analyzed with a two-phase permeation model. The effective thickness of the rate-controlling polyimide phase was less than one-tenth of the total thickness of the silica-polyimide membrane.     

Synthesis and characterization of amino functionalized ladder-like polysilsesquioxanes and their hybrid films with polyimide

Ladder-like polysilsesquioxanes (LPSQs) with different amino contents have been synthesized by controlling of the dosage of Pd/C catalyst. The concentration and activity of amino groups were investigated by Fourier transform infrared spectroscopy. Polyimide (PI)/LPSQ hybrid films have been prepared by incorporating of the obtained LPSQs with different amino contents into PI matrix, respectively. The interfacial interactions between PI matrix and LPSQ were studied with scanning electron microscopy and X-ray photoelectron spectroscopy, meanwhile the thermal and mechanical properties of the hybrid films were studied using dynamic mechanical analysis and tensile tests. The results indicate that the functionality of LPSQ has great effects on the interfacial interactions and the properties of hybrid films. With the increase of amino content, both the interfacial interactions and the cross-linking density of hybrids enhanced, which results in the decline of surface silicon concentration, increase of Young’s modulus and drop of elongation at break. Excessive amino content makes the hybrid films brittle and leads to incomplete imidization.     

Preparation and characterization of a polyimide membrane

Two different polyimide semipermeable membranes have been prepared from two base polyimides of PMDA/ODA and BTDA/ODA by incorporation of lithium chloride with the respective poly(amic acids) and subsequently leaching out the maximum possible lithium chloride by water at slightly elevated temperature. The water and various organic vapor permeability of the lithium chloride modified films has been found to be better compared to the respective control films. BTDA/ODA based polyimide films show overall lower permeability.     

Preparation and characterization of a novel polyimide liquid crystal vertical alignment layer

A series of diamines with a side chain containing rigid biphenyl unit and nonpolar alkoxy side end group [4-alkoxy-biphenol-3′,5′-diaminobenzoate] (Cm-BBDA, m = 4, 6, 12) were synthesized and their chemical structures were confirmed by Fourier Transform Infrared Spectroscopy (FT-IR) and nuclear magetic resonance spectroscopy (¹H NMR). Then three polyimides (PIs) were prepared by copolymerization of pyromellitic dianhydride (PMDA), 4,4′-methylenedianiline (MDA) and Cm-BBDA in N-methyl-2-pyrrolodone (NMP), and chemical structures of all PIs were confirmed by FT-IR. Structural identification of all poly(amic acid)s (PAA) was performed by ¹H NMR. Liquid crystal (LC) cells were fabricated using these PIs as the alignment layer for characterization of the alignment properties of LCs. It was found that the planar alignment was obtained when PI with side chain containing alkoxy side end group of 4 carbon atoms was employed and the vertical alignment was observed when alkoxy side end groups of 6 or 12 carbon atoms were included. A uniform vertical alignment was validated by polarizing microscopy. It was testified that LC vertical alignment possessed high thermal stability.     

Electrocatalytic activity of starch/Fe3O4/zeolite bionanocomposite for oxygen reduction reaction

The present work demonstrated an eco-friendly and facile method for the preparation of starch/Fe₃O₄/zeolite-bionanocomposite (BNC) at moderate temperature. Zeolite and starch were used as solid support and stabilizer, respectively. The analysis of UV–vis showed the appearance of surface plasmon resonance. From PXRD analysis, the incorporation of magnetite nanoparticles (NPs) in zeolite substrate results in reducing of intensities and broadening of the zeolite peaks of BNC. The TEM analysis showed the formation of highly distributed Fe₃O₄-NPs with an average diameter and standard deviation of 9.24 ± 3.57 nm. The FESEM and EDX analyses imply that Fe₃O₄-NPs were homogeneously formed on the surface of the zeolite substrate. VSM analysis illustrated the as prepared BNC possessed magnetic behaviour with a saturation magnetization and coercivity of 1.84 emu g⁻¹ and 17.76 G, respectively. The prepared BNC showed potential applicability in energy as low-cost electrode material. The BNC was used as a non-precious catalyst for oxygen reduction reaction (ORR) in the alkaline medium. The presence of starch and zeolite promoted long term stability up to 1000 cycles and avoid the dissolution and agglomeration of iron oxide. The ORR commences at the onset potential of 0 V follows by the two successive reduction peaks at −0.48 V and −1.00 V.     

The preparation of novel silica modified polyimide membranes: synthesis,characterization, and gas separation properties

In this study, new monomers having siloxane groups were synthesized as an intermediate for preparation of siloxane modified polyimide polymers. Then with these monomers, the synthesis of uncrosslinked and crosslinked polyimide–siloxane hybrid polymer membranes were achieved. The purposes of the preparation of modified polyimides were to modify the thermal and chemical stability, and mechanical strength of polyimides, and to improve the gas separation properties of polymers. The new diamine monomer having siloxane groups was prepared from 3,5‐diaminobenzoic acid (3,5‐DABA) and 3‐aminopropyltrimethoxysilane (3‐APTMS) in N‐methyl‐2‐pyrollidone (NMP) at 180°C. The modified polyimide membranes having different amount of siloxane groups were synthesized from pyromellitic dianhydride (PMDA), 4,4‐oxydianiline (ODA), and 3,5‐diaminobenzamido‐N‐propyltrimethoxy silane (DABA/PTMS) in NMP using a two‐step thermal imidization process. The synthesis of modified polyimide membranes were characterized by Fourier transform infrared spectroscopy (FTIR). The thermal analysis of the polyimides were carried out by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Water absorption and swelling experiments were also carried out for the investigation of structural properties of polymers. FTIR observations confirmed that the polyimide membranes with new diamine intermediate were successfully obtained. Thermal analysis showed that the uncrosslinked copolyimides exhibited two glass transition temperatures, indicating that they were separated microphases and it was found that all the modified copolyimides had showed higher glass transition temperature (T_g) than unmodified polyimides. The separation properties of the prepared polyimide membranes were also characterized by permeability for O₂ and N₂ gases and ideal selectivity values were calculated. Copyright © 2009 John Wiley & Sons, Ltd.     

Surface orientation effect of asymmetric polyimide hollow fibers on their gas transport properties

In this study, we focused on the shear stress effects within a spinneret during hollow fiber spinning on the formation of the hollow fibers and their gas transport properties. We fabricated asymmetric polyimide hollow fibers with a completely defect-free thin skin layer using a dry/wet phase inversion process. The apparent calculated skin layer thickness of the hollow fiber was 280 nm and the O₂ permeance was 2.9×10⁻⁵ cm³ (STP)/(cm² s cmHg). Interestingly, the skin layer thickness was reduced at the high shear rate. In addition, the gas permeances and selectivities of the hollow fibers increased with the increasing shear rate. We concluded that the oriented skin layer of the hollow fiber induced by shear stress had a significant influence on the formation of the skin layer and its gas transport properties. From the ATR-IR spectra results, it was clear that the surface skin layer of the hollow fiber was parallel oriented.     

Preparation of polyimide/BaTiO3/Ag nanocomposite films via in situ technique and study of their dielectric behavior

Triphase polyimide nanocomposite films were fabricated using barium titanate （BaTiO3） with high dielectric constant and silver （Ag） with high conductivity as fillers. In situ method was utilized to obtain the homogeneous dispersion of nanoparticles. The in situ polymerization of polyimide precursor-poly（amic acid） was performed in the presence of BaTiO3 particles. Silver compound 1,1,1-trifluoro-2,4-pentadionato silver（I） was added into the BaTiO3 containing poly（amic acid） solution to achieve silver nanoparticles via in situ self metallization technique. The thermally induced reduction converted silver （I） to metallic silver with concomitant imidization of poly（amic acid） to polyimide. Both BaTiO3 and silver nanoparticles were uniformly dispersed in the polyimide substrate. The dependence of dielectric behavior on the BaTiO3 and Ag contents was studied. The incorporation of small amount of silver nanoparticles greatly increased dielectric constant of composite films.     

ZnO/Ag复合纳米粒子的制备与表征

以乙酸锌为前驱物,乙醇为溶剂,油酸钠为表面修饰剂,采用溶液化学法,在乙醇体系中制得纳米Zn O。然后缓慢加入一定量的硝酸银乙醇溶液,在乙醇的还原作用下将Ag+还原为Ag纳米粒子,制得Zn O/Ag复合纳米粒子。通过紫外-可见吸收光谱(UV-Vis)、荧光光谱(FL)、透射电子显微镜(TEM)和X射线衍射(XRD)等方法对所制备的氧化锌-银复合纳米粒子样品进行表征。结果表明,所合成的Zn O/Ag复合纳米粒子为球形,尺寸为20-30nm且粒径分布较窄。Ag纳米粒子附着于Zn O纳米粒子表面,并起到良好的表面修饰作用。对制备Zn O/Ag复合纳米粒子的机理进行了初步探究。     

聚酰亚胺碳分子筛气体分离膜的研究进展

聚酰亚胺碳分子筛膜由于具有较高的热稳定性、耐化学性、气体渗透性和选择性,而受到广泛关注。根据近年来聚酰亚胺碳分子筛膜在气体分离方面的研究现状,详细介绍了填充改性、对前驱体进行预处理和聚酰亚胺单体改性的研究成果,并展望了聚酰亚胺碳分子筛分离膜的发展趋势,以期为未来高效分离膜的研发提供参考。     

Preparation, characterization of the Ta-doped ZnO nanoparticles and their photocatalytic activity under visible-light illumination

This paper describes a novel catalyst of the Ta-doped ZnO nanocrystals prepared by a modified polymerizable complex method using the water-soluble tantalum precursor as the sources of Ta. The catalysts were characterized by means of various analytical techniques as a function of Ta content (x=0–4 mol%) systematically. A remarkable advantage of the results was confirmed that dopant Ta enhanced the visible-light absorption of ZnO and the low-solubility tantalum doping could restrain the growth of crystal and minish the particle size. The relationship between the physicochemical property and the photocatalytic performance was discussed, and it was found that the photocatalytic activity in the photochemical degradation of methylene blue under visible-light irradiation (λ420 nm) was dependent on the contents of the dopant, which could affect the particle size, concentration of surface hydroxyl groups and active hydrogen-related defect sites, and the visible-light absorption. The highest photocatalytic activity was obtained for the 1.0 mol% Ta-doped ZnO sample.     

Preparation and properties of soluble polyamideimides containing oligo(tetrafluoroethene) segments and application in gas separation

A series of PMDA-, BTDA- and 6F-polyamideimides containing well-defined oligo(tetrafluoroethene) segments were derived from N,N′-bis(aminophenyl)perfluoroalkane-α, ω-dicarboxamides with n =1,2,3 tetrafluoroethene units. The influence of the fluorine content, microstructures and the oligo(tetrafluoroethene) segment lengths on phase transition temperatures, thermal degradation, contact angles, pure gas permeabilities and permselectivities was studied.     

Fabrication and gas sensing property of honeycomb-like ZnO

We report the structural characterization and proposed formation mechanism of honeycomb-like ZnO conglomerations fabricated by direct precipitation method. X-ray diffraction （XRD）, energy-disperse X-ray spectrometry （EDS）, scanning electron microscopy （SEM） showed that the as-prepared ZnO calcined at 700 ℃ were micron sphere particles with honeycomb-like structure. In the UV-vis absorbing spectrum, it was observed that there is a new additional absorption band at 260 nm, and it was speculated that the absorption may be caused by defects on the surface and interface of honeycomb-like ZnO. The as-products showed high sensitivity and short response time to sulfured hydrogen gas. These results demonstrate that honeycomb-like ZnO conglomerations are very promising materials for fabricating H2S gas sensors.     

Synthesis,gas barrier and thermal properties of polyimide containing rigid planar fluorene moieties

In order to obtain pristine polyimides with high barrier properties, pyromellitic dianhydride (PMDA) and 9H-fluorene-2,7-diamine (FDA) containing rigid planar fluorene moieties were used to prepare polyimide (FPI) via a conventional two-step polymerization process in this paper. The synthesized polyimide shows good barrier properties, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 1.01 cm³ m^?2 day^?1 and 2.35 g m^?2 day^?1, respectively. The effect of rigid planar structure in main chain on the barrier properties of polyimide was studied by means of wide angle X-ray diffractograms (WAXD), molecular dynamics simulations and positron annihilation lifetime spectroscopy (PALS), which was rarely reported before. The results reveal that the good barrier properties of FPI are mainly due to the high crystallinity, high chain rigidity and low free volume, which are resulted from the rigid planar structure. Additionally, the polyimide exhibits excellent thermal and dimensional stability with 5 wt% loss temperature of 519°C, glass transition temperature of 370°C and coefficient of thermal expansion (CTE) of 5.72 ppm/K. The good gas barrier and thermostability endow the polyimide with promising potential in flexible electronics encapsulation applications.     

A molecular dynamics simulation study of surface effects on gas permeation in free-standing polyimide membranes

A 141100-atom model of a glassy ODPA–ODA polyimide free-standing membrane, corresponding to a thickness of two average radii of gyration for the 40-mers chains, has been studied using molecular dynamics (MD) simulations. Due to the large-scale of the fully atomistic model, a parallelized particle-mesh technique using an iterative solution of the Poisson equation had to be implemented for the efficient evaluation of the electrostatic interactions. With flattened-chain configurations, the density was found to adjust itself naturally in the middle of the membrane to 95% of the ODPA–ODA experimental value. At the free-standing surfaces, the density profile became sigmoïdal, indicating surface roughness. For comparison, two isotropic bulk models, one at the “normal” density as obtained for ODPA–ODA under ambient conditions and the other one at 95% of the normal-density, were built. Small gas probes were inserted into all three models in order to investigate whether the interfacial structure of the glassy free-standing membrane can influence penetrant transport. Gas diffusion in the low-density part of the interface was found to be very fast. The limiting value for the gas diffusion coefficient D_membrane is only attained when the probes enter more dense regions in the membrane. Indeed, D_membrane compares well with D_bulk obtained for the 95%-density bulk system, i.e. about twice that in the normal-density bulk. Solubility is larger in the membrane than in both bulk models, thus suggesting an effect of chain flattening in addition to the density. Adsorption is particularly high at the free-standing interfaces.     

Preparation and Ethanol Sensing Properties of ZnO Nanofibers

ZnO nanofibers with an average diameter of about 90 nm were prepared by an electrospinning method combined with a calcination process. The as-electrospun nanofibers before and after calcination were characterized by means of differential thermal analysis(DTA), thermal gravimetric analysis(TGA), X-ray diffraction(XRD) and scanning electron microscopy(SEM). The fibers after calcination at 600 °C belong to the hexagonal wurtzite structure. The sensor based on ZnO nanofibers exhibited excellent ethanol sensing properties at 206 °C such as good linear dependence in the low concentration(1―100 μL/L), high response, and good selectivity. Fast response(less than 2 s) and recovery(about 16 s) were also observed in our investigations.     

Preparation, characterization and dielectric properties of 4,4-diphenylmethane diisocyanate (MDI) based cross-linked polyimide films

Four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared by the reaction of different types of conventional poly(amic acid) intermediates with MDI as a cross-linking agent. Subsequently, they were thermally imidized in order to obtain corresponding cross-linked polyimide structure. The results of FTIR-ATR showed that MDI can effectively react with carboxylic acid groups of PAA to form cross-linked polyimide films. TGA, FTIR-ATR and SEM analyses were carried out for characterization of cross-linked polyimide (CPI) films. Moreover, the electrical properties such as dielectric breakdown strength, dielectric constant, I-V characteristics and loss factor of MDI based cross-linked polyimides have been checked. In addition, some physical properties such as water uptake, adhesion, hardness and solubility properties of the films were investigated.The results showed that all CPI films have good insulating properties such as high dielectric breakdown voltage, low loss factor (tan δ), leakage density and excellent physical properties.     

Zn基片上ZnO及ZnS/ZnO复合纳米阵列的液相合成及发光性质

采用温和的溶液路线在Zn基片上合成了单晶态的ZnO纳米棒阵列、 纳米片阵列和ZnS/ZnO复合双层纳米棒阵列. 使用X射线粉末衍射仪、 扫描电子显微镜、 高分辨透射电子显微镜、 X射线光电子能谱仪等对产物的组成、 结构及形貌进行了表征. 讨论了表面活性剂在液相合成中对产物形貌的调控作用. 通过室温发射光谱的测定, 研究了所得纳米阵列材料的发光性质.     

Preparation of mixed matrix membranes based on polyimide and aminated graphene oxide for CO2 separation

The aminated graphene oxide (GO) was prepared by the functionalization of pristine GO with ethylenediamine and then dispersed into the poly(amic acid) (the precursor of polyimide [PI]) solution followed by the chemical imidization to successfully fabricate the PI/amine‐functionalized GO mixed matrix membranes (MMMs) using in‐situ polymerization method. Chemical structure and morphology of the GO before and after amine modification were characterized by scanning electron microscopy, Raman spectrum, Fourier transform infrared, and X‐ray photoelectron spectroscopy. Scanning electron microscopy indicated that fine dispersion of GO throughout PI matrix was achieved, which indicates that the in‐situ polymerization approach can enhance the interfacial interaction between the GO and the PI matrix, and then improve the dispersion of carbon material in the polymer matrix. Compared with the conventional solution mixture method, the MMMs prepared with in‐situ polymerization method showed excellent CO₂ permeability and CO₂/N₂ selectivity. The MMMs doped with 3 wt.% aminated GO exhibited maximum gas separation performance with a CO₂ permeability of 12.34 Barrer and a CO₂/N₂ selectivity of 38.56. These results suggest that the amino groups on GO have strong interaction with the CO₂ molecules, which can significantly increase the solubility of polar gas. Our results provide an easy and efficient way to prepare MMMs with good mechanical behavior and excellent gas separation performance.