Mass transport and electrode accessibility through periodic self-assembled nanoporous silica thin films

Ordered nanoporous silica films have attracted great interest for their potential use to template nanowires for photovoltaics and thermoelectrics. However, it is crucial to develop films such that an electrode under the nanoporous film is accessible to solution species via facile mass transport through well-defined pores. Here, we quantitatively measure the electrode accessibility and the effective species diffusivity for nearly all the known nanoporous silica film structures formed by evaporation-induced self-assembly upon dip-coating or spin-coating. Grazing-angle of incidence small-angle X-ray scattering was used to verify the nanoscale structure of the films and to ensure that all films were highly ordered and oriented. Electrochemical impedance spectroscopy (EIS) was then used to assess the transport properties. A model has been developed that separates the electrode/film kinetics and the film transport properties from the film/solution interface and bulk solution effects. Accounting for this, the accessible area of the nanoporous film coated FTO electrode (1-theta) is obtained from the high-frequency data, while the effective diffusivity of the ferrocene dimethanol (D(FDM)) redox couple is obtained from intermediate frequencies. It was found that the degree of order and orientation in the film, in addition to the symmetry/topology, is a dominant factor that determines these two key parameters. The EIS data show that the (211) oriented double gyroid, (110) oriented distorted body center cubic, and (211) distorted primitive cubic silica films have significant accessibility (larger than 26% of geometric area). However, the double-gyroid films showed the highest diffusivity by over an order of magnitude. Both the (10) oriented 2D hexagonal and (111) oriented rhombohedral films were found to be highly blocking with only small accessibility due to microporosity. The impedance data were also collected to study the stability of the nanoporous silica films in aqueous solutions as a function of pH. The distorted primitive silica film showed much faster degradation in pH 7 solution when compared to a blocking film such as the 2D hexagonal. However, silica films maintained their structure at pH 2 for at least 12 h.     

Catalysis in Organic Solvents with Lipase Immobilized by Sol-Gel Technique

A series of immobilized lipases were obtained by sol-gel process, using silica prepolymers prepared from tetramethoxysilane, methyltrimethoxysilane, propyltrimethoxysilane and (3-aminopropyl)triethoxysilane. The activities of these biocatalysts were compared with the lipase adsorbed on poly(methylhydroxysiloxane) and encapsulated into a silicone rubber, lipase entrapped in nanoporous silica matrix and commercial sol-gel lipase. Model reactions were the esterification of stearic acid and Corey lactone bisalcohol (an intermediate of prostaglandin synthesis). The positive effect of hydrophobic-hydrophilic interface, created by the addition of organosilanes, on the activity of biocatalysts was partially reduced by decreasing specific surface of mesopores. Hydrophobic solvents increased the activity of the lipase entrapped in tetramethoxysilane–methyltrimethoxysilane prepolymer in the sequence acetone < toluene < benzene < decane < hexane. The activity of silicone rubber-encapsulated biocatalysts was proportional to polymer swelling in organic solvents (hexane > toluene > acetone).     

Nanoporous thermosetting polymers

Potential applications of nanoporous thermosetting polymers include polyelectrolytes in fuel cells, separation membranes, adsorption media, and sensors. Design of nanoporous polymers for such applications entails controlling permeability by tailoring pore size, structure, and interface chemistry. Nanoporous thermosetting polymers are often synthesized via free radical mechanisms using solvents that phase separate during polymerization. In this work, a novel technique for the synthesis of nanoporous thermosets is presented that is based on the reactive encapsulation of an inert solvent using step-growth cross-linking polymerization without micro/macroscopic phase separation. The criteria for selecting such a monomer-polymer-solvent system are discussed based on FTIR analysis, observed micro/macroscopic phase separation, and thermodynamics of swelling. Investigation of resulting network pore structures by scanning electron microscopy (SEM) and small-angle X-ray scattering following extraction and supercritical drying using carbon dioxide showed that nanoporous polymeric materials with pore sizes ranging from 1 to 50 nm can be synthesized by varying the solvent content. The differences in the porous morphology of these materials compared to more common free radically polymerized analogues that exhibit phase separation were evident from SEM imaging. Furthermore, it was demonstrated that the chemical activity of the nanoporous materials obtained by our method could be tailored by grafting appropriate functional groups at the pore interface.     

Fabrication of an open Au/nanoporous film by water-in-oil emulsion-induced block copolymer micelles

Water-in-oil (W/O) emulsion-induced micelles with narrow size distributions of approximately 140 nm were prepared by sonicating the polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer in the toluene/water (50:1 vol %). The ordered nanoporous block copolymer films with the hydrophilic P2VP interior and the PS matrix were distinctly fabricated by casting the resultant solution on substrates, followed by evaporating the organic solvent and water. The porous diameter was estimated to be about 70 nm. Here, we successfully prepared the open nanoporous nanocomposites, the P2VP domain decorated by Au (5+/-0.4 nm) nanoparticles based on the methodology mentioned. We anticipate that this novelty enhances the specific function of nanoporous films.     

A comparison of redox processes for polypyrrole/dodecylsulfate films in aqueous and non-aqueous media

Polypyrrole/dodecylsulfate (PPy/DDS) films were synthesized in aqueous and ethanolic solutions and investigated in aqueous, ethanolic, methanolic and acetonitrile solutions by cyclic voltammetry (CV). The amounts of anions and cations in the films before and after electrochemical treatment were determined by electron probe microanalysis (EPMA); the film morphology was studied by scanning electron microscopy (SEM). The results prove that the mobility of bulky DDS^– ions in PPy increases in the order: water<acetonitrile<ethanol<methanol. It was found that dopant DDS^– ions can be easily removed from PPy matrix swollen in alcohols or acetonitrile by electrochemical reduction or by soaking in electrolyte solutions of these solvents. The influence of electrochemical treatment on the change of doping level in aqueous solution is essentially less and depends on the cations in the test solution. Although the electroneutrality of PPy/DDS films during redox cycling is realized mainly by movement of the cations in aqueous solution and by movement of the anions in organic solvents, nevertheless the participation of anions in aqueous and cations in organic solvents is also established. The redox properties of PPy/DDS are more dependent on the solvent of the test solutions than of the synthesis solutions. Electronic Publication     

Hydrophobic and hydrophilic aggregation of tailor‐made urethane acrylate anionomers in various solvents and their network structures

Tailor‐made urethane acrylate anionomer (UAA) chains show higher viscosity and polyelectrolyte behavior in dimethyl sulfoxide (DMSO) than in water and toluene. Water is a nonsolvent for the hydrophobic soft segment but a good solvent for the hydrophilic hard segments, so hydrophobic segments are aggregated and form particles in the water phase, resulting in a smaller viscosity. Also, the fact that the viscosity of UAA chains is lowest in toluene can be interpreted as a result of ionic aggregation due to the nonpolarity of toluene. The structures of UAA networks dramatically change with the nature of the solvents used (i.e., the interaction between the UAA chains and the solvents used changes); this is confirmed by the results of tensile property, morphology, and wide‐angle X‐ray scattering data. Ionic aggregation formed in UAA/toluene (UATG networks) and hydrophobic aggregation formed in UAA/water (UAAG networks) are locked in by a chemical crosslinking reaction and result in a greater modulus and X‐ray scattering intensity. The greater elongation and swelling ratio in methylene chloride of UATG networks prepared in a UAA/toluene solution indicates that toluene is a better solvent than DMSO for the hydrophobic segments of UAA chains. Also, the greater swelling ratio in a pH 11 buffer solution and greater modulus of UAAG networks show that water is a better solvent than DMSO for hydrophilic ionic segments. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1903–1916, 2000     

Characterization of pore structure in a nanoporous low-dielectric-constant thin film by neutron porosimetry and X-ray porosimetry

A small-angle neutron scattering (SANS) porosimetry technique is presented for characterization of pore structure in nanoporous thin films. The technique is applied to characterize a spin-on organosilicate low dielectric constant (low-k) material with a random pore structure. Porosimetry experiments are conducted using a "contrast match" solvent (a mixture of toluene-d8 and toluene-h8) having the same neutron scattering length density as that of the nanoporous film matrix. The film is exposed to contrast match toluene vapor in a carrier gas (air), and pores fill with liquid by capillary condensation. The partial pressure of the solvent vapor is increased stepwise from 0 (pure air) to P0 (saturated solvent vapor) and then decreased stepwise to 0 (pure air). As the solvent partial pressure increases, pores fill with liquid solvent progressively from smallest to largest. SANS measurements quantify the average size of the empty pores (those not filled with contrast match solvent). Analogous porosimetry experiments using specular X-ray reflectivity (SXR) quantify the volume fraction of solvent adsorbed at each step. Combining SXR and SANS data yields information about the pore size distribution and illustrates the size dependence of the filling process. For comparison, the pore size distribution is also calculated by application of the classical Kelvin equation to the SXR data.     

Friedel–crafts reactions of pendant vinyl groups in macroporous monosized poly(meta‐divinylbenzene) and poly(para‐divinylbenzene) particles

Residual vinyl groups in macroporous monosized polymer particles of poly(meta‐DVB) and poly(para‐DVB) prepared with toluene and 2‐EHA as porogens have been reacted with aluminum chloride as Friedel–Crafts catalyst with and without the presence of lauroyl chloride. In the reaction between aluminum chloride and pendant vinyl groups a post‐crosslinking by cationic polymerization takes place. A reaction occurring simultaneously is the addition of HCl to the double bonds. The progress of these reactions was studied by characterization of vinyl group conversion, pore size distribution, specific surface area, morphology, and swelling behavior. In the reaction with aluminum chloride the poly(para‐DVB) particles showed a substantially higher conversion of pendant vinyl groups than the particles made of poly(meta‐DVB) independent of porogen type. The reaction with aluminum chloride led to a reduced swelling in organic solvents and an increased rigidity of the particles prepared with toluene as porogen. This is confirmed by an increase in the total pore volume in the dry state and a change in the pore size distribution of these particles. Also in the reaction with lauroyl chloride poly(para‐DVB) particles have shown a higher conversion of pendant vinyl groups than poly(meta‐DVB) particles and the acylation was almost complete at the early stage of the reaction. The swelling in organic solvents is reduced as a result of the incorporation of acyl groups into the particles prepared with toluene as porogen. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1366–1378, 2000     

煅烧温度对阳极氧化WO3多孔薄膜的形貌及光电化学性能的影响

采用一步式阶跃电压加压方法,在NH4F/(NH4)2SO4电解质溶液中对W片进行阳极氧化处理制备了WO3多孔薄膜,通过后续热处理温度的控制,制备了性能规律性变化的WO3多孔纳米薄膜材料.用场发射扫描电镜(FE-SEM)、X射线衍射(XRD)分析等手段考察了热处理温度对氧化钨晶体结构和形貌影响的规律,在450°C以下的煅烧温度下,薄膜保持50-100nm孔径;通过对光电化学性质、光催化降解甲基橙动力学行为的研究,考察了不同热处理温度对WO3多孔薄膜光电转换性能影响的规律.研究表明,450°C煅烧处理后的WO3薄膜在500W氙灯光源照射及1.2V偏压下,光电流密度达到5.11mA·cm-2;340及400nm单色光辐射下光电转换效率(IPCE)值分别达到87.4%及22.1%.电化学交流阻抗谱显示,450°C煅烧处理后的WO3薄膜表现出最佳的导电率及最小的界面电荷转移电阻.实验结果证明,高结晶度的多孔结构是WO3薄膜具有高光电转换效率的主要因素,控制热处理温度是实现薄膜具有高孔隙率、完整结晶度、低电阻的重要手段.     

Cross-linking of poly[1-(trimethylsilyl)-1-propyne] membranes using bis(aryl azides)

Cross-linkable poly[1-(trimethylsilyl)-1-propyne] (PTMSP) films were cast from toluene solutions containing PTMSP and either 4,4′-diazidobenzophenone or 4,4′-(hexafluoroisopropylidene)diphenyl azide. The composite films were clear and homogeneous and were cross-linked by UV irradiation at room temperature or thermal annealing at 180°C. Low levels of the bis(aryl azide) (1–5 wt %) were effective in rendering the films insoluble in toluene and THF, both good solvents for PTMSP. The process is simple and effective, and thus PTMSP can be readily converted to mechanically stable membranes with permeabilities and separation factors comparable or higher than those of poly(dimethylsiloxane). The films were characterized by measuring their density, their permeability toward O₂ and N₂, and their spectroscopic properties. Compared to PTMSP, films containing bis(aryl azide) cross-linkers had lower permeabilities and higher separation factors, consistent with a reduction in free volume. When the films were cross-linked photochemically, the permeabilities declined further and the separation factor increased. Films cross-linked thermally had permeabilities comparable to their PTMSP/azide precursors, and density and swelling measurements suggest that higher free volumes are obtained in thermally cross-linked films. All films stored in air suffered from a slow decline in permeability which may reflect slow surface oxidation of the films. When stored in vacuum, cross-linked films were stable and showed no loss in permeability, but the permeability of uncross-linked PTMSP films stored under the same conditions fell to 70% of their original value in 1 month. We attribute the permeability decline to densification accelerated by impurities and solvents. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 959–968, 1998     

Formation, structure, and reactivity of amino-terminated organic films on silicon substrates

Amino-functionalized organic films were prepared by self-assembling 3-aminopropyltriethoxysilane (APTES) on silicon wafers in either anhydrous toluene or phosphate-buffered saline (PBS) for varied deposition times. Fourier transform infrared spectroscopy (FTIR) and ellipsometry have shown that the structure and thickness of APTES films are governed by the deposition time and reaction solution. Deposition from an anhydrous toluene solution produces APTES films ranging from 10 to 144 A in thickness, depending on the reaction time. FTIR spectra indicate that film growth initially proceeds by adsorption of APTES to the silicon surface followed by siloxane condensation, and after an extended period of time APTES molecules accumulate on the underlying APTES film by either covalent or noncovalent interactions. In contrast, spectroscopically indistinguishable APTES films in thickness ranging from 8 to 13 A were formed when deposition was conducted in aqueous solutions. Measured water contact angles indicate that APTES films deposited in aqueous solutions are more hydrophilic compared to those prepared in toluene solutions. Fluorescence measurements revealed that APTES films prepared in toluene solutions contain more reactive surface amino groups by ca. 3 to 10 times than those prepared in aqueous solutions for the identical reaction time.     

Effect of trace amounts of water on organic solvent transport through gamma-alumina membranes with varying pore sizes

The transport behavior of toluene and n-hexane in gamma-alumina membranes with different pore diameters was studied. It was shown that the permeability of water-lean hexane and toluene is in agreement with Darcy's law down to membrane pore diameters of 3.5 nm. The presence of molar water fractions of 5-8 x 10(-4) in these solvents led to a permeability decrease of the gamma-alumina layer by a factor of 2-4 depending on pore size. In general, a lower permeability was found for hexane than for toluene. Moreover, in the presence of water a minimum applied pressure of 0.5-1.5 bar was required to induce net liquid flow through the membrane. These phenomena were interpreted in terms of capillary condensation of water in membrane pores with a size below a certain critical diameter. This is thought to lead to substantial blocking of these pores for transport, so that the effective tortuosity of the membrane for transport of hydrophobic solvents increases.     

不同溶剂制备的聚乳酸多孔微球的形成机理

利用改进的双乳液溶剂挥发法制备了多孔聚乳酸( PLA)微球.通过采用具有不同沸点和水溶性的有机溶剂制备得到不同多孔结构的PLA微球.结果发现以二氯甲烷、氯仿和甲苯为溶剂制备的微球具有相似的均匀多孔结构,而以乙酸乙酯制备的微球却具有中空结构和多孔的壳层.通过进一步的实验研究了溶剂种类对于微球多孔结构的影响.结果表明溶剂的...     

Highly sensitive and quantitative analysis of polyeptides using a new gradient system based on an abrupt change in adsorption of polypeptide to the reversed-phase column packing

During a study of 100 microL aliquots of urocortin containing various acetonitrile contents, we hypothesized that a change in the acetonitrile content in the solution across a specific content of acetonitrile (critical threshold) causes an abrupt change in adsorption capacity to the column packing. Circular dichroism measurements suggest that the conformational change induced by acetonitrile in the solution causes the abrupt change in adsorption capacity, and this solvent-induced conformational change is reversible across the critical threshold. This hypothesis can apply to various polypeptides with molecular weights range from 1007 to 6789 and to other organic solvents. A new gradient system utilizing the instant recovery of the adsorption capacity across the critical threshold was designed, and applied to the analysis of a 100 microL aliquot of various polypeptide solutions. The results suggest that use of a solution containing organic solvents more than the critical threshold allows successful dilution of polypeptides up to picomolar concentration range without any loss due to its adsorption during handling procedures and loading onto the LC system, and that a new gradient system enables quantitative analysis of polypeptides at picomolar concentrations in such solutions.     

PEC films prepared from Chitosan-Alginate coacervates

Chitosan-alginate polyelectrolyte complex (PEC) have been prepared in situ in beads and microspheres. This study examines the preparation of suitable chitosan-alginate coacervates for casting into homogeneous PEC films for potential applications in packaging, controlled release systems and wound dressings. Coacervation between chitosan and alginate was rapid, but the rate may be controlled with the addition of water miscible organic solvents. Compared with ethanol and PEG200, acetone was the more promising solvent moderator. Suspensions of fine, uniformly dispersed coacervates were produced by a dropwise addition of 0.25% w/v chitosan solution (solvent: 1: 1 v/v of 2% acetic acid and acetone) into 0.25% w/v sodium alginate solution in water under rapid agitation. The PEC films were transparent and flexible. They exhibited high permeability to water vapor, but resisted complete dissolution in 0.1 M HCI, distilled water and pH 7.4 phosphate buffer solution. Microscopic heterogeneity in the films could be reduced by immersion in aqueous media, but this was accompanied by modifications in the thickness, permeability and mechanical property of the films.     

Interaction of superhydrophobic materials with aqueous solutions and organic solvents

The interaction of a fluorine-containing superhydrophobic (SHP) coating on the surface of a siloxane rubber with aqueous solutions and organic solvents was studied by wettability analysis. The long-term durability of the coating against the damaging action of atmosperic precipitates saturated with salt ions, acid rains, as well as a number of organic solvents was demonstrated. The resistance of the coating to highly alkaline media is lower than to a neutral medium because of the hydrolysis of Si—O—Si bonds and amide groups. The low resistance of the fluorine-containing SHP coating to acetone is due to the swelling of the substrate and mechanical destruction of the fluoroorganic component.     

A Drying‐Free,Water‐Based Process for Fabricating Mixed‐Matrix Membranes with Outstanding Pervaporation Performance

Despite much progress in the development of mixed matrix membranes (MMMs) for many advanced applications, the synthesis of MMMs without particle agglomeration or phase separation at high nanofiller loadings is still challenging. In this work, we synthesized nanoporous zeolitic imidazole framework (ZIF‐8) nanoparticles with a particle size of 60 nm and a pore size of 0.34 nm in water and directly added them into an aqueous solution of the organic polymer poly(vinyl alcohol) (PVA) without an intermediate drying process. This approach led to a high‐quality PVA/ZIF‐8 MMM with enhanced performance in ethanol dehydration by pervaporation. The permeability of this MMM is three times higher than that of pristine PVA, and the separation factor is nearly nine times larger than that of pristine PVA. The significantly improved separation performance was attributed to the increase in the fractional free volume in the membranes.     

A Drying‐Free,Water‐Based Process for Fabricating Mixed‐Matrix Membranes with Outstanding Pervaporation Performance

Despite much progress in the development of mixed matrix membranes (MMMs) for many advanced applications, the synthesis of MMMs without particle agglomeration or phase separation at high nanofiller loadings is still challenging. In this work, we synthesized nanoporous zeolitic imidazole framework (ZIF‐8) nanoparticles with a particle size of 60 nm and a pore size of 0.34 nm in water and directly added them into an aqueous solution of the organic polymer poly(vinyl alcohol) (PVA) without an intermediate drying process. This approach led to a high‐quality PVA/ZIF‐8 MMM with enhanced performance in ethanol dehydration by pervaporation. The permeability of this MMM is three times higher than that of pristine PVA, and the separation factor is nearly nine times larger than that of pristine PVA. The significantly improved separation performance was attributed to the increase in the fractional free volume in the membranes.     

Adsorption studies of acetic acid dimers on activated charcoal from organic solvents

Acetic acid exists as dimers in organic solvents like benzene, toluene and xylene. Adsorption of dimeric acetic acid on activated charcoal (AC) at various temperatures from benzene, toluene and xylene solutions have been studied. The system obeys Langmuir isotherm, thus signifying a monolayer adsorption of dimers. Corrections on AC-solvent pore volume fillings, molecular cross sectional surface area of acetic acid dimers, the adsorption equilibrium constants, the free energy change and the enthalpy change values are computed at different temperatures for the three solvents. The adsorption process has been found to be physisorption type. The FTIR measurements show that the adsorbed acetic acid dimer seems to retain the cyclic structure against the open chain non-cyclic structure.     

Peculiar swelling properties of PVC films grafted with acrylic and methacrylic acids

It was found that PVC films grafted with methacrylic acid do not swell in either water or methanol, two solvents of poly(methacrylic acid), even for high grafting ratios. The swelling of these films was examined in mixtures of methylene chloride with methanol and curves of different shapes were obtained depending on the grafting ratio. PVC films grafted with acrylic acid readily swell in both water and methanol but they remain hard in the swollen state. The equilibrium swelling increases with swelling temperature but this process is not reversible; films swollen at high temperature keep a high degree of swelling even when the system is cooled.