Hollow-fiber membranes coated with polymerizable bicontinuous microemulsions

A new method has been successfully employed to prepare hollow-fiber membranes by coating and polymerizing bicontinuous microemulsions onto the internal surfaces of hollow-fiber membranes. The bicontinuous microemulsion consisting of water, a polymerizable zwitterionic surfactant of acryloyloxyundecyl dimethylammonio acetate, methyl methacrylate, and 2-hydroxylethyl methacrylate (HEMA) can form a transparent polymer thin film after polymerization. The hollow-fiber membranes as the supports for microemulsion coatings were fabricated from the spinning solution of polyethersulfone/diethylene glycol/N-methyl-2-pyrrolidone. The microemulsion coated hollow-fiber membranes were evaluated by the separation efficiency and the permeation rate of polyethylene glycol (PEG) solutions. The performance of coated membrane on the PEG separation is strongly dependent on the concentration of HEMA and water in precursor bicontinuous microemulsions. The pore size of the hollow-fiber membranes can be regulated between about 2 to 40 nm by varying the composition of precursor bicontinuous microemulsions. The characteristics of the coated membranes is believed to be directly related to the bicontinuous structures of precursor bicontinuous microemulsions. The use of polymerizable bicontinuous microemulsions enable one to better control the microstructures of coated membranes via in situ polymerization.     

Using breath figure patterns on structured substrates for the preparation of hierarchically structured microsieves

Microsieves are advanced filtration membranes characterized by a uniform pore size, a high pore density, and a thickness smaller than the pore diameter. The uniform pore size provides a high selectivity; the small thickness gives rise to a high flux and allows efficient removal of any filter cake by backflushing. However, microsieves are sensitive to mechanical stress. Thus, they need either an external macroporous support or a hierarchical structure that provides an integrated supportive structure. We prepare microsieves with a hierarchical pore structure by creating breath figure patterns within layers of solutions of polymers in a volatile solvent that are spread out on top of structured supports. For the formation of breath figure patterns, the volatile solvent is evaporated in a moist atmosphere. This cools the surface to such an extent that dew droplets form on the thin film, partially penetrate into the layer, and create a concave imprint in the final solid polymer layer. This procedure is usually done on flat surfaces; in our case the spreading of the polymer solution is done on a support decorated with protrusions. In this procedure, the dew droplets touch the protrusions of the structured support before the polymer solution vitrifies. At the same time, the trenches of the structured substrate are filled with polymer much deeper than the penetration depth of the dew droplets. After the separation of the vitrified layer from the substrate, we obtain thin polymer membranes with a hierarchical structure consisting of an ultrathin active separation layer with submicrometer pores and a supporting layer with larger pores.     

Porous polymer membranes via selectively wetted surfaces

Here, we show that porous polymeric membranes can be prepared using the principles of offset printing: an offset printing plate is structured into hydrophobic and hydrophilic regions with the help of photolithography and is selectively wetted with a solution of calcium chloride in water at the hydrophilic regions. Then, a polymer solution (poly(methyl methacrylate) in chloroform) is applied to this surface and forms a hydrophobic layer that is structured by the aqueous droplets. Deviating from standard offset printing, this layer is not transferred to another surface in its liquid state but is solidified and subsequently is separated from the printing plate. The thickness of the polymer film is chosen in such a way that the aqueous droplets on the surface protrude from the film. Thus, we obtain polymer membranes with pores in the size of the protruding aqueous droplets. These membranes are then characterized by the filtration of model dispersions.     

Nano-structured proton exchange membranes molded by polymerizing bi-continuous microemulsion

A unique type of nano-structured proton exchange membrane (PEM) has been fabricated through photo-polymerizing a bicontinuous microemulsion. This microemulsion is constituted by a polymerizable zwitterionic surfimer 3-((11-acryloyloxyundecyl)imidazoyl)propyl sulfonate (AIPS), 3-sulfopropylmethacrylate, potassium salt (SPM), acrylonitrile and water. As the resulting colloidal system maintains quasi-homogeneous state throughout the polymerization course, the inherent bicontinuous microemulsion structure was successfully transferred to the matrix of the polymer membrane. Such membranes are therefore composed of dual phase channels with ca. 1.5–2 nm of the hydrophilic channel breadth. This particular structural trait imparts to the membranes excellent proton conductivities of up to 10⁻¹ S cm⁻¹ as well as low methanol permeability. The DMFC single cell loaded with the demo PEM manifests ca. 20 mW cm⁻² of power output. The attributes of this PEM are elucidated from the bicontinuous structure of microemulsion.     

Influence of membrane structure on fouling layer morphology during apple juice clarification

The flux behavior of 0.2 μm nylon, polysulfone (PS), polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes was examined during dead-end microfiltration of commercial apple juice. On nylon membranes, a 0.1 μm thick surface fouling layer rapidly formed that acted as a secondary membrane. The colloidal particles retained by this surface layer aggregated to form a thick loose gel structure, producing an anisotropic fouling structure. In contrast, the 4 μm thick surface fouling layer of PES was slower to form and had a more open structure with a lower flux resistance per unit thickness. The morphology of the PES surface layer also did not differ dramatically from the loose gel structure that subsequently formed on top of this secondary membrane. The PS surface fouling layer was similar in structure to nylon whereas the PVDF layer more closely resembled that found with PES. The density of the surface fouling layer did not directly correlate to membrane surface hydrophobicity or pure water flux. Atomic force microscopy (AFM) indicated that surface roughness strongly influenced surface fouling layer morphology. The membrane surface appears to act as a template for the fouling process; therefore, smooth membranes (nylon and PS) produce a dense surface fouling layer whereas this same layer on rough membranes (PES and PVDF) is much more open. Consequently, the fluxes of PES and PVDF membranes are less affected by fouling formation.     

Direct imaging of nanoscale acidic clusters in a polymer electrolyte membrane

One of the factors hindering the development of technologies that rely on the use of proton-conducting polyelectrolyte membranes is the lack of control over the membrane morphology on the nanoscale. Of particular importance is the rearrangement and clustering of acidic groups, which may seriously degrade the electrical properties. Although electron microscopy is capable of imaging the morphology of the clusters, images of unmodified membranes with sufficient quality to discriminate between different proposed cluster morphology models have not been presented. Here we show the first determination of the cluster size distribution in a model polymer electrolyte membrane from electron micrographs of individual acidic clusters. Imaging of the sulfur-rich clusters by dark-field microscopy was facilitated by the spontaneous formation of thin, cluster-containing layers on the top and bottom surfaces of free-standing films with a thickness of ~35 nm.     

Synthesis and characterization of palladium containing membranes based upon polyacrylic acid

Different methods are described to synthesize a highly porous polymer membrane with fine dispersed metal-nanoparticles. The preparation of the porous catalytic membranes happens by crosslinking of polyacrylic acid dispersions with bifunctional crosslinker in presence of palladium particles. Palladium-nanoparticles, stabilized with the block copolymer polystyrene-block-polyethyleneoxide, can be immobilized in the polymer network in different ways. The polymer/metal network can be prepared in the form of thin flat membranes and dried under retention of the porosity and three-dimensional network structure. Different reduction and preparation methods were applied in order to obtain differences in particle size and distribution of the palladium. The morphology of the material was investigated by scanning electron microscopy. Transmission electron microscopy was employed to show the size and distribution of the metal-nanoparticles in the polymeric matrix. The catalytic activity of the obtained membranes was investigated for the gas phase hydrogenation of cyclohexene and propyne.     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units III. Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance

Flat sheet asymmetric reverse osmosis membranes were successfully prepared from N,N-dimethylacetamide (DMAc) solutions of a series of novel wholly aromatic polyamide-hydrazides that contained different amounts of para- and meta-phenylene rings. These polyamide-hydrazides were synthesized by a low temperature solution polycondensation reactions of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous DMAc as a solvent. All the polymers have the same structural formula except of the way of linking phenylene units inside the polymer chains. The content of para- to meta-phenylene moieties was varied within these polymers so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0-100 mol%. All the membranes were characterized for their salt rejection (%) and water permeability (cm³ cm⁻² day⁻¹) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effects of polymers structural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were studied. Effects of various processing parameters of the membranes on their transport properties were investigated by varying the temperature and period of the solvent evaporation of the cast membranes, coagulation temperature of the thermally treated membranes, annealing of the coagulated membranes, casting solution composition, membrane thickness and the operating pressure. During the thermal treatment step, the asymmetric structure of the membranes with a thin dense skin surface layer supported on a more porous layer was established. The former layer seems to be responsible for the separation performance. The results obtained showed that membrane performance was very much influenced by all of the examined processing variables and that membranes with considerably different properties could be obtained from the same polymer sample by using different processing parameters. Thus, the use of higher temperatures and longer exposure times in the protomembrane forming thermal treatment step would result in a membrane of lower solvent content and with a thicker skin layer and consequently led to higher salt rejection at lower water permeability. Most significantly, the membrane properties clearly depended on the polymer structure. Under identical processing condition, substitution para-phenylene rings for meta-phenylene ones within the polymer series resulted in an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with increased molecular packing and rigidity through enhanced intermolecular hydrogen bonding. This produces a barrier with much smaller pores that would efficiently prevent the solute particles from penetration. Coagulation temperature controls the structure (porosity) of the membrane particularly its supported layer and consequently its water permeability. Moreover, annealing of the prepared membranes in deionized water at 100 °C was found essential for useful properties in the single-stage separation applications, which required optimum membrane selectivity. Upon annealing, the membrane shrinks resulting in reducing its pore size particularly in the skin layer and consequently improving the salt rejection. Addition of lithium chloride to the casting solution produced a membrane with increased porosity and improved water permeability. Salt rejection capability of the membranes is clearly affected by the applied pressure, reaching its maximum at nearly 4000 kPa. Furthermore, the water permeability is inversely proportional to the membrane thickness, while the salt rejection is not substantially influenced.     

The Factors Determining Formation Dynamics and Structure of Ring-Shaped Deposits Resulting from Capillary Self-Assembly of Particles

The influence of different physicochemical parameters, such as particle concentration and size, droplet volume, dispersion medium composition, and substrate hydrophilicity, on the structure of deposits resulting from evaporating sessile droplets of colloidal dispersions has been studied. Parameters enabling one to targetedly control the structure of a deposit have been determined. The possibility of obtaining deposits having the shapes of thin rings and monolithic disks has been shown. The conditions have been found under which monolayer and multilayer deposits with ordered arrangement of particles are formed.     

Mono-layer of Ni(100-x)Fe(x) nanoparticles fabricated on a polyimide film under different curing atmospheres

A mono-layer of nano-sized metal particles was prepared on the surface of a polyimide film by simply depositing a thin film of Ni80Fe20 on top of the polyamic acid that was spin coated onto a Si wafer. During thermal imidization of the polyamic acid film, Fe was selectively etched by reacting with the carbonyl group of the polyamic acid to leave behind uniformly distributed Ni-rich metallic particles. The average diameter of the particles was 4 nm and the particles were confined into a single layer on top of the polymer film. Moreover, it was also shown that the morphology of the nanoparticles can be substantially altered by curing the precursor film in a hydrogen atmosphere, without significantly damaging the polymer film. Thus produced nanoparticles lay exposed on top of the electrically insulating and chemically stable polymer film so that it is possible that the nanoparticles can be directly used for fabricating a nonvolatile flash memory device or as a template for building functional nano-structures.     

Small-angle scattering as a tool for the study of microemulsion structure

Microemulsions are equilibrium dispersions of oil and water stabilized by a surfactant-rich sheet at the internal oil-water interfaces. The domains of oil and water have characteristic dimensions of a few hundred Ångstroms, and so are appropriate for study with small-angle scattering. The scattering from droplets of oil in water or water in oil may be modelled well with modern representations of the structure of colloidal suspensions, but the structure of microemulsions containing comparable amounts of oil and water is likely bicontinuous and is well represented as a disordered lamellar structure. Recent theoretical and experimental results are reviewed and extended.     

Particulate and bulk bicontinuous cubic phases obtained from mixtures of glyceryl monooleate and copolymers bearing blocks of lipid-mimetic anchors in water

Copolymers based on poly(ethylene glycol) bearing one or more lipid-mimetic anchors were mixed with glycerylmonooleate (GMO)-a lipid with nonlamellar propensity-to form bulk and particulate bicontinuous cubic phases in water. The particulate phase was obtained via a liquid precursor method. Three forms of copolymer/GMO mixtures were investigated-precursor dispersions in glycerol and bulk and particulate phases in water-by visual observations, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The bulk phases were found to very slowly develop a macroscopic appearance that can be associated with the bicontinuous cubic phase. They were prepared in a slight excess of water, which became opalescent in some of the preparations. Cryo-TEM investigation of the excess showed that vesicles and particles with a dense interior coexisted. The precursors were prepared as solutions in glycerol. The viscous liquid material was investigated by DLS. Diffusion coefficients and the corresponding hydrodynamic radii, ranging from about 10 to 30 nm, were calculated. The particles are presumably of a structure similar to that of conventional emulsion droplets with GMO in the interior and copolymer molecules in the outer regions. The particulate phase in water was obtained upon hydration of the liquid precursors. The dispersions were investigated by DLS and cryo-TEM. DLS revealed the formation of nanosized particles. The size was found to increase with increasing copolymer content for copolymers with only one lipid-mimetic anchor, whereas the opposite trend was observed for the formulations with copolymers bearing more than one lipid-mimetic anchor. The shape and interior of the particles were studied by cryo-TEM. It was found that most particles were globular. For some of the compositions, particles with a dense internal structure dominated. The texture of the internal structures was assigned to dispersed bicontinuous cubic or L3 phases. In other compositions, the interior seemingly consists of arrays of interlamellar attachments.     

Microstructuring of Polystyrene Surfaces with Nonsolvent Sessile Droplets

Herein, we study the microstructuring of toluene‐vapor‐softened polystyrene surfaces with nonsolvent sessile droplets. Arrays of microvessels are obtained by depositing non‐evaporating droplets of ethylene glycol/water on the original polystyrene surfaces and subsequently exposing them to saturated toluene vapor. The droplets act as a mask on the polymer, thereby impeding the toluene vapor to diffuse and soften the polystyrene surface below them. Alternatively, the formation of microcraters at random positions—with an average depth‐to‐width aspect ratio of 0.5 and a diameter as small as 1.5 μm—is achieved by condensing water droplets on a softened polystyrene surface. The cross‐sections of the microvessels and the contact angle of the sessile water droplets suggest that the structures are formed by the combined action of the Laplace pressure at the bottom of the droplet and the surface tension acting at the three‐phase contact line of the droplets. As a support, the rim height and the depth of the microvessels are fitted with an elastic theory to provide Young’s modulus of the softened polystyrene surface.     

Template synthesis of metal nanowires containing monolayer molecular junctions

Metal nanowires containing in-wire monolayer junctions of 16-mercaptohexanoic acid were made by replication of the pores of 70 nm diameter polycarbonate track etch membranes. Au was electrochemically deposited halfway through the 6 microm long pores and a self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid was adsorbed on top. A thin layer of Au was then electrolessly grown to form a metal cap separated from the bottom part of the wire by the SAM. Electron micrographs showed that the bottom and top metal segments were separated by an approximately 2 nm thick organic monolayer. Current-voltage measurements of individual nanowires confirmed that the organic monolayer could be contacted electrically on the top and bottom by the metal nanowire segments without introducing electrical short circuits that penetrate the monolayer. The values of the electrical properties for zero-bias resistance, current density, and breakdown field strength were within the ranges expected for a well-ordered alkanethiol SAM of this thickness.     

A review of: “Nineteenth Century Attitudes: Men of Science (Chemists and Chemistry Series,Vol. 13)”. Sydney Ross. Kluwer Academic Publishers; Dordrecht, 1991. pp. xi+235. $69.00.

Abstract

The potential of polytetrafluoroethylene (PTFE) membranes as water‐in‐oil (W/O) emulsification devices was investigated to obtain uniformly sized droplets and to convert them into microcapsules and polymer particles via subsequent treatments. Uniform W/O emulsion droplets have not been achieved using glass membranes unless the membrane was rendered hydrophobic by treatment with silanes. If a PTFE membrane is capable of providing uniform droplets for a W/O emulsion, a coordinated membrane emulsification system can be established since glass membranes have been so successful for O/W (oil‐in‐water) emulsification. In order to examine the feasibility of PTFE membrane emulsification, O/W and W/O emulsion characteristics prepared using PTFE membranes were compared with those prepared by the conventional SPG (Shirasu porous glass) membrane emulsification method. A 3 wt.% sodium chloride solution was dispersed in kerosene using a low HLB surfactant. Effects of the membrane pore size, permeation pressure, and the type of emulsifiers and concentration on the droplet size and on the size distribution (CV, coefficient of variation) were investigated. The CV of the droplets was fairly low, and the average droplet size was correlated with the critical permeation pressure of the dispersed phase, revealing that the PTFE membrane could be used as a one‐pass membrane emulsification device. Low CV values were maintained with a Span 85 (HLB = 1.8) concentration, 0.2–5.0 wt.% and a range of HLB from 1.8–5.0. For a brief demonstration of practical applications, nylon‐6,10 microcapsules prepared by interfacial polycondensation and poly(acrylamide) hydrogels from inverse suspension polymerization are illustrated.     

Preparation of alpha-alumina-supported mesoporous bentonite membranes for reverse osmosis desalination of aqueous solutions

In this study, mesoporous bentonite clay membranes approximately 2 microm thick were prepared on porous alpha-alumina substrates by a sol-gel method. Nanosized clay particles were obtained from commercial Na-bentonite powders (Wyoming) by a process of sedimentation, washing, and freeze-drying. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption-desorption were employed for membrane characterization. It was found that the content of solids, concentration of polymer binder, and pH value of the clay colloidal suspension had critical influences on membrane formation during the dip-coating process. The membranes were tested for reverse osmosis separation of a 0.1 M NaCl solution. Both water permeability and Na(+) rejection rate of the supported membranes were comparable to those of the compacted thick membranes reported in the literature. However, due to the drastically reduced membrane thickness, water permeance and flux of the supported membranes were significantly higher than those of the compacted thick membranes. It was also observed that the calcination temperature played a critical role in determining structural stability in water and desalination performance of the clay membrane.     

The effect of different stabilizers on the formation of self‐assembled porous film via the breath‐figure technique

The Breath‐Figure technique was employed to imprint honeycomb structures in the polymer films via the condensation of water vapor on the surface of an evaporating polymer solution. Generally, the condensed water droplets can be stabilized by an end‐functional polymer or by particles added to the polymer solution. In this study, we carried out a systematic experiment on the effect of different stabilizers on the porous honeycomb structure under identical physical conditions. The end‐functional polymer produced a large area of regular spherical bubbles, whereas adding particles to the polymer solution leads to smaller arrays of the flattened bottom bubbles. The separation length between pores was larger for polymer/particle sample than that of the end‐functional polymer films. In the regular area of polymer/particle film many bubbles were not decorated by particles. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1430–1436, 2011     

热致相分离法偏二氯乙烯-氯乙烯共聚物多孔膜的制备及性能

以偏二氯乙烯-氯乙烯共聚物[P(VDC-co-VC)]为成膜聚合物, 邻苯二甲酸二甲酯(DMP)为稀释剂, 采用热致相分离(TIPS)法制备了具有多孔结构的P(VDC-co-VC)膜. 通过聚合物-稀释剂二元体系相图、 场发射扫描电镜(FESEM)、 差示扫描量热仪(DSC)、 X射线衍射(XRD)、 原子力显微镜(AFM)、 纯水通量、 接触角、 孔径及其分布、 截留率及力学性能等研究了聚合物含量对P(VDC-co-VC)多孔膜结构和性能的影响. 结果表明, P(VDC-co-VC)-DMP二元体系成膜过程以液-液(L-L)分相为主, 随着聚合物含量增加, 膜的横截面由类花瓣状结构向胞腔状结构转变, 膜的孔连通性降低, 结构变得较为致密, 同时膜上表面孔隙率降低, 粗糙度增大. L-L分相时间和聚合物含量的变化, 导致膜结晶度先降低后增大. 聚合物含量的增加使膜上表面接触角、 断裂强度及蛋白截留率增加, 但膜的平均孔径、 孔隙率及纯水通量先增加后减小. 当聚合物质量分数为30%时, 所得膜通透性较优, 断裂强度可达7.5 MPa.     

The characteristics of component transfer through barrier membrane layers

A uniform matrix with randomly distributed impurity centers, which create an effective field repulsing particles that diffuse in the membrane and prevent the formation of “percolation” paths in “thick” membranes, is considered as a barrier layer model. Two repulsive potential types were analyzed, one decreasing according to a power law depending on the distance between the localization center of an impurity and the diffusing particle and the other decreasing exponentially as a function of this distance. An exponential dependence of the permeability constants of the desired components on the membrane layer thickness was predicted. According to this dependence, the components to be separated effectively pass through membrane layers only in local regions, where the force field that retards particles is weakened because of a fluctuation decrease in the concentration of centers repulsing the diffusing particles. The process is then characterized by nonequilibrium transmembrane transfer conditions, under which particles have time to be effectively “sorbed” only in regions of increased membrane permeability. Under these conditions, the selectivity of membrane separation can be influenced by the state of the surface of membranes. For this reason, the modification of the surface of barrier structures can be used to control their selective permeability to the desired products. Equations for the rate of component transfer through barrier membrane layers under the most general boundary conditions were obtained. These equations can be used to analyze separation on membranes with barrier structures subjected to surface modification.     

Nanomechanical and optical studies on polymeric membranes

The nanomechanical properties of poly(ethylene terephthalate) (PET) membranes, were examined in light of nanoindentation experiments under conditions of maximum contact load in the range of 0.5-12 m?. Spectroscopic Ellipsometry (SE) from 1.5 to 6.5 ev (Vis-FUV range) was also applied to probe the dielectric function (ϵ(ω) of the industrially supplied membranes, as well as their geometrical structure. Mechanical stretching (uniaxial or biaxial) procedures are usually applied for the elongation of the polymeric membranes, their thickness reduction and enhancement of their mechanical and optical performance, causing a preferable orientation of the macromolecules close to the surface. Nanoindentation and se testing have revealed the existence of a two-layer geometrical structure of the pet membranes, consisting of a thick amorphous pet layer and a thin crystalline-like pet overlayer, with increased hardness (elastic modulus). The analyses of the experimental dataprovides quantitative information on the formed overlayer, which is ascribed to the processing history of the membrane.