Self‐Assembled Asymmetric Block Copolymer Membranes: Bridging the Gap from Ultra‐ to Nanofiltration

The self‐assembly of block copolymers is an emerging strategy to produce isoporous ultrafiltration membranes. However, thus far, it has not been possible to bridge the gap from ultra‐ to nanofiltration and decrease the pore size of self‐assembled block copolymer membranes to below 5 nm without post‐treatment. It is now reported that the self‐assembly of blends of two chemically interacting copolymers can lead to highly porous membranes with pore diameters as small as 1.5 nm. The membrane containing an ultraporous, 60 nm thin separation layer can fully reject solutes with molecular weights of 600 g mol^?1 in aqueous solutions with a water flux that is more than one order of magnitude higher than the permeance of commercial nanofiltration membranes. Simulations of the membrane formation process by dissipative particle dynamics (DPD) were used to explain the dramatic observed pore size reduction combined with an increase in water flux.     

Poly(N-isopropylacrylamide) grafted into nanopores: Thermo-responsive behaviour in the presence of different salts

Surface initiated polymerization of N(isopropylacrylamide) (NIPAM) was performed by controlled radical polymerization on PET track-etched membranes presenting two different pore diameters (narrow pores: ∼80 nm and large pores: ∼330 nm). The opening and closing characteristics of the resulting PNIPAM-g-PET membranes were investigated by conductometric measurements carried out at different temperatures below and above the LCST of PNIPAM and in the presence of different salts. Depending on the membrane pore size, two types of permeation control mechanisms are observed. In large pore membranes, expanded PNIPAM chains conformations result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, in narrow pore membranes, the expanded PNIPAM brush presents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.     

Generation of Nanopores Down to 10 nm for Use in Deep‐Nulling Interferometry

Scanning electron microscope images show that it is easy to generate nanopores on polycarbonate membranes with well‐defined pore diameters by ion‐track perforation and subsequent magnetron sputtering with metal. The size reduction of the nanopores during sputtering with gold is a linear function of time. Images of different angles and from the bottom side of the membrane show that the channels are the smallest very close to the surface of the metal layer, have a conelike shape, and reach about half as much into the polymer membranes as the metal‐layer thickness. This topographical pore shape is ideal for use as optically coherent near‐field sources in deep‐nulling microscopy. We present the first results of significantly improved nulling stabilization in the presence (<2 nm optical pathway difference) and the absence (<0.6 nm optical pathway difference) of the nanoapertures in the focal region of a deep‐nulling microscope.     

Metal–Organic Framework (MOF) Nanorods,Nanotubes, and Nanowires

New mechanisms for the controlled growth of one‐dimensional (1D) metal–organic framework (MOF) nano‐ and superstructures under size‐confinement and surface‐directing effects have been discovered. Through applying interfacial synthesis templated by track‐etched polycarbonate (PCTE) membranes, congruent polycrystalline zeolitic imidazolate framework‐8 (ZIF‐8) solid nanorods and hollow nanotubes were found to form within 100 nm membrane pores, while single crystalline ZIF‐8 nanowires grew inside 30 nm pores, all of which possess large aspect ratios up to 60 and show preferential crystal orientation with the {100} planes aligned parallel to the long axis of the pore. Our findings provide a generalizable method for controlling size, morphology, and lattice orientation of MOF nanomaterials.     

Metal–Organic Framework (MOF) Nanorods,Nanotubes, and Nanowires

New mechanisms for the controlled growth of one‐dimensional (1D) metal–organic framework (MOF) nano‐ and superstructures under size‐confinement and surface‐directing effects have been discovered. Through applying interfacial synthesis templated by track‐etched polycarbonate (PCTE) membranes, congruent polycrystalline zeolitic imidazolate framework‐8 (ZIF‐8) solid nanorods and hollow nanotubes were found to form within 100 nm membrane pores, while single crystalline ZIF‐8 nanowires grew inside 30 nm pores, all of which possess large aspect ratios up to 60 and show preferential crystal orientation with the {100} planes aligned parallel to the long axis of the pore. Our findings provide a generalizable method for controlling size, morphology, and lattice orientation of MOF nanomaterials.     

Tuning of the Temperature Window for Unit‐Cell and Pore‐Size Enlargement in Face‐Centered‐Cubic Large‐Mesopore Silicas Templated by Swollen Block Copolymer Micelles

The unit‐cell size and pore diameter as functions of temperature are investigated in the syntheses of FDU‐12 silicas with face‐centered cubic structure templated by Pluronic (PEO‐PPO‐PEO) block copolymer micelles swollen by toluene. The temperature range in which the unit‐cell size and pore size strongly increase as temperature decreases is correlated with the critical micelle temperature (CMT) of the surfactant. While Pluronic F127 affords a wide range of unit‐cell parameters (28–51 nm) and pore diameters (16–32 nm), it renders moderately enlarged pore sizes at 25 °C. The use of Pluronic F108 with higher CMT affords FDU‐12 with very large unit‐cell size (～49 nm) and large pore diameter (27 nm) at 23 °C. Large unit‐cell size (40–41 nm) and pore size (22 nm) were obtained even at 25 °C. The application of Pluronics F87 and F88 with much smaller molecular weights and higher CMTs also allows one to synthesize FDU‐12 with quite large unit‐cell parameters and pore sizes at room temperature. The present work demonstrates that one can judiciously select Pluronic surfactants with appropriate CMT to shift the temperature range in which the pore diameter is readily tunable.     

Hierarchically porous carbon membranes derived from PAN and their selective adsorption of organic dyes

Porous carbon membranes were favorably fabricated through the pyrolysis of polyacrylonitrile(PAN) precursors, which were prepared with a template-free technique-thermally induced phase separation. These carbon membranes possess hierarchical pores, including cellular macropores across the whole membranes and much small pores in the matrix as well as on the pore walls. Nitrogen adsorption indicates micropores(1.47 and 1.84 nm) and mesopores(2.21 nm) exist inside the carbon membranes, resulting in their specific surface area as large as 1062 m2/g. The carbon membranes were used to adsorb organic dyes(methyl orange, Congo red, and rhodamine B) from aqueous solutions based on their advantages of hierarchical pore structures and large specific surface area. It is particularly noteworthy that the membranes present a selective adsorption towards methyl orange, whose molecular size(1.2 nm) is smaller than those of Congo red(2.3 nm) and rhodamine B(1.8 nm). This attractive result can be attributed to the steric structure matching between the molecular size and the pore size, rather than electrostatic attraction. Furthermore, the used carbon membranes can be easily regenerated by hydrochloric acid, and their recovery adsorption ratio maintains above 90% even in the third cycle. This work may provide a new route for carbon-based adsorbents with hierarchical pores via a template-free approach, which could be promisingly applied to selectively remove dye contaminants in aqueous effluents.     

A novel method for the pore size control of the battery separator using the phase instability of the ternary mixtures

The battery separator plays a key role in determining the capacity of the battery. Since separator performance mainly depends on the pore size of membrane, development of a technique for the fabrication of the membrane having controlled pore size is essential in producing a highly functional battery separator. In this study, microporous membranes having the desired pore size were produced via thermally‐induced phase separation (TIPS) process. Control of the phase boundaries of polymer‐diluent blends is the main concern in manipulating pore size in TIPS process, because pore size mainly depends on the temperature gap between phase separation temperature of the blend and the crystallization temperature of polymer. Microporous membranes having controlled pore size were produced from polyethylene (PE)/dioctyl phthalate (DOP) blends, PE/isoparaffin blends, and polymer/diluent‐mixture ternary blends, that is, PE/(DOP/isoparaffin) blends. PE/DOP binary blends and PE/(DOP/isoparaffin) ternary blends exhibited typical upper critical solution temperature (UCST) type phase behavior, while PE formed a homogeneous mixture with isoparaffin above the crystallization temperature of PE. When the mixing ratio of polymer and diluent‐mixture was fixed, the phase separation temperature of PE/diluent‐mixture blend first increased with increasing DOP content in the diluent‐mixture, went through a maximum centered at about 80 wt % DOP and then decreased. Furthermore, the phase separation temperatures of the PE/diluent‐mixture blends were always higher than that of the PE/DOP blend when diluent‐mixture contained more than or equal to 20 wt % of DOP. Average pore size of microporous membrane prepared from PE/DOP blend and that prepared from PE/isoparaffin blend were 0.17 and 0.07 μm, respectively. However, average pore size of microporous membrane prepared from ternary blends was varied from 0.07 to 0.5 μm by controlling diluent mixing ratio. To understand the phase behavior of ternary blend, phase instability of the ternary mixture was also explored. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2025–2034, 2006     

Fabrication of porous collagen/chitosan scaffolds with controlling microstructure for dermal equivalent

Two sets of homemade apparatus have been utilized to fabricate collagen/chitosan porous membranes by quenching its acetic solution and subsequently extracting the solvent with ethanol. The influence of chitosan concentration on the surface morphology of the collagen/chitosan membranes was studied using a quenching cold plate (apparatus 1). The pore size was enlarged along with an increase in the chitosan content, accompanied with the emergence of a sheet‐like microstructure. Due to the large thermal conductivity of the membrane‐forming platform (stainless steel), collagen/chitosan membranes prepared using apparatus 1 at freezing temperature between ?60 to ?20 °C present similar pore size (2–4 nm) and surface morphology. However, a large difference in pore size is generated using apparatus 2 (membrane preparation in a cold ethanol bath) and using a membrane‐forming platform of poor thermal conductivity (polymethylmethacrylate), e.g. ～10 to 20 μm at freezing temperature of ?60 to ?40 °C, and 265 μm at ?20 °C accompanied with the transformation from fiber‐ to sheet‐dominated morphology. The spongy collagen/chitosan membranes with pore sizes ranging from tens to hundreds of micrometers and porosity higher than 95%, which could be used as dermal regeneration template, have thus been fabricated. Copyright © 2003 John Wiley & Sons, Ltd.     

非对称性微孔复合陶瓷膜制备技术研究

采用压制-烧结法制备多孔氧化铝陶瓷片,并采用溶胶-凝胶法在多孔氧化铝陶瓷载体上依次沉积孔径逐渐减小的氧化铝陶瓷膜、氧化锆陶瓷膜和氧化钛陶瓷膜,从而得到具有非对称性结构的多孔复合陶瓷膜。采用压制-烧结法,将粉末造粒、压制、干燥,然后烧结,可分别得到平均孔径大约为2μm,6μm,10μm的多孔陶瓷片,该多孔陶瓷片的开口孔隙率大于40%,断裂强度大于50N。以此陶瓷片作载体,经溶胶-凝胶法多次涂敷不同的溶胶,可分别得到孔径大约为200nm,400nm和600nm,厚度大约为40μm的多孔复合陶瓷膜。此多孔复合膜可作为气体及液体的过滤材料,也可以作为钯合金膜的支撑体。     

Chiral separation of phenylalanine in ultrafiltration through DNA-immobilized chitosan membranes

Ultrafiltration experiments for the chiral separation of racemic phenylalanine were performed with DNA-immobilized chitosan membranes having various pore sizes. Atomic analysis on the membranes showed that the chitosan membranes covalently bound six times more DNA than the cellulose membranes used in our previous study [A. Higuchi, Y. Higuchi, K. Furuta, B.O. Yoon, M. Hara, S. Maniwa, M. Saitoh, K. Sanui, Chiral separation of phenylalanine by ultrafiltration through immobilized DNA membranes, J. Membr. Sci. 221 (2003) 207–218]. d-Phenylalanine preferentially permeated through DNA-immobilized chitosan membranes with a pore size <6.4 nm [molecular weight cut-off (MWCO) <67,000]. The binding affinity of a specific enantiomer due to the pore size of the DNA-immobilized membranes regulated the preferential permeation of the enantiomer through the membranes. l-Phenylalanine was adsorbed on the DNA-immobilized chitosan membranes with a pore size <6.4 nm (MWCO < 67,000), while there was little difference between the adsorption of d-phenylalanine and l-phenylalanine on the membranes with a pore size >6.4 nm (MWCO > 67,000). The DNA-immobilized chitosan membranes were categorized as channel type 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.     

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.     

SBA‐15 pore‐width decrease via a one‐ or a two‐step covalent bonding of a Fischer tungsten carbene as measured by N2 sorption

The chemical attachment of Fischer tungsten carbene molecules around the cylindrical walls of SBA‐15 causes a pore‐width diminution with respect to the diameters of the voids existing in the pristine substrate. The attachment is made via a bridging unit of aminopropyltriethoxysilane (APTES) that, at its ethoxysilane end, anchors on the SBA‐15 surface while, at its amino end, reacts with the tungsten carbene. The attachment can either be made in a single stage by adding the whole molecule, i.e. bridging + carbene units, or stepwise, i.e. the bridging unit is firstly attached to the silica surface and afterward the carbene group is linked to the amine end of the previous species. A remarkable result is that a uniform pore‐width decrease of 0.30 nm is achieved when the ethoxysilane bridging units are anchored on the SiO₂ surface; this decrease being independent of the pore size. An additional decrease of 0.37 nm is observed when the tungsten carbene is fixed to the amine end of the bridging molecule; the total pore‐width decrease is thus 0.67 nm. In turn, the one‐stage insertion of the whole carbene molecules causes a uniform pore‐width decrease of 0.78 nm. The central cores of the functionalized pore entities remain free for the molecular transport of species that are taking place in catalytic, separation, and controlled release processes. The chemical anchoring of molecules on the surface of SBA‐15 pores can be tailor sized and occlude the existing micropores, thus representing important and fundamental industrial applications. Copyright © 2008 John Wiley & Sons, Ltd.     

Temperature-controlled flow switching in nanocapillary array membranes mediated by poly(N-isopropylacrylamide) polymer brushes grafted by atom transfer radical polymerization

We report actively controlled transport that is thermally switchable and size-selective in a nanocapillary array membrane (NCAM) prepared by grafting poly(N-isopropylacrylamide) (PNIPAAm) brushes onto the exterior surface of a Au-coated polycarbonate track-etched membrane. A smooth Au layer on the membrane surface, which is key to obtaining a uniform polymer film, was prepared by thermal evaporation of approximately 50 nm Au on both exterior surfaces. After evaporation, the inner diameter of the pore is reduced slightly, but the NCAM retains a narrow pore size distribution. PNIPPAm brushes with 10-30 nm (dry film) thickness were grafted onto the Au surface through surface-initiated atom transfer radical polymerization (ATRP) using a disulfide initiator, (BrC(CH3)2COO(CH2)11S)2. Molecular transport through the PNIPAAm polymer brush-modified NCAMs was investigated by real-time fluorescence measurements using fluorescein isothiocyanate (FITC)-labeled dextrans ranging from 4.4 to 282 kDa in membranes with variable initial pore diameters (80, 100, and 200 nm) and different PNIPAAm thicknesses. Manipulating the temperature of the NCAM through the PNIPAAm lower critical solution temperature (LCST) causes large, size-dependent changes in the transport rates. Over specific ranges of probe size, transport is completely blocked below the LCST but strongly allowed above the LCST. The combination of the highly uniform PNIPAAm brush and the monodisperse pore size distribution is critical in producing highly reproducible switching behavior. Furthermore, the reversible nature of the switching raises the possibility of using them as actively controlled filtration devices.     

Enhanced microfiltration of γ-globulin solution upon treatment of NaCl addition and/or DNase digestion

Microfiltration of a γ-globulin solution has been investigated through the virus removal membranes having different pore sizes (i.e. r=15, 35 and 75 nm) and a dialysis membrane (r=3.4 nm), which were all made of the same regenerated cellulose material. The addition of NaCl in the γ-globulin feed solution was effective to enhance the flux and transmission through the membranes having a pore size ranging from 15 to 75 nm. DNase treatment of a γ-globulin solution with Micrococcal nuclease enhanced the flux and transmission of γ-globulin through the membranes either with or without NaCl. The membranes having a pore size of 35 nm showed dramatically enhanced flux in the microfiltration of a γ-globulin solution containing NaCl and/or being treated with Micrococcal nuclease. This can be explained as a DNase treatment and NaCl addition in the protein solution dissociate protein aggregates of DNA–γ-globulin complex, which plugs the pores in the microfiltration membranes.     

Microporous silica membranes deposited on porous supports by filtration

Water based particulate silica sols have been coated onto Anodisc® filters by filtration. The membranes prepared by this technique are more uniform than those formed by slip-casting. The average diameter of the silica particles used in these studies is 6 nm. Unsupported silica membranes formed from these sols have a microporous structure. The adhesion between silica and the alumina support is influenced by the sol pH. Coating thickness can be controlled by the concentration and volume of the sol filtered. Polyvinyl alcohol (PVA) was used to improve adhesion and to prevent cracking during drying. When the PVA/SiO₂ ratio by weight is less than 20%, the membranes retain their microporosity after firing. The membranes prepared by this filtration method have their pore size in Knudsen diffusion range.     

Nanofiltration membranes synthesized from hyperbranched polyethyleneimine

Four nanofiltration membranes, two negatively and two positively charged, were fabricated by interfacial polymerization. Three different amines, ethylenediamine (EDA), diethylenetriamine (DETA), and hyperbranched polyethyleneimine (PEI) were selected to react with two acyl chlorides, trimesoyl chloride (TMC) and terephthaloyl chloride (TPC). The two membranes containing hyperbranched PEI, PEI/TPC and PEI/TMC, are positively charged at the operational pH. But the other two membranes, EDA/TMC and DETA/TMC, are negatively charged. It is found that the two PEI membranes own special rejection characters during nanofiltration. The PEI/TPC membrane has a similar pore size to the EDA/TMC membrane but owns simultaneously the higher salt rejection and permeation flux. The PEI/TMC has a pore size as large as 1.5 nm and still has a higher NaCl rejection than the EDA/TMC membrane of which the pore size as small as 0.43 nm. We consider that the special rejection characters are derived from the special structure of PEI. The hyperbranched structure allows some of the charged amine groups drifting inside the pores and interacting with the ions in the pathway. The drifting amines increase salt rejection but have little effect on water permeation. It implies that a high flux and high rejection membrane for desalting can be obtained by attaching freely rotating charged groups.     

Nanoporous metals with controlled multimodal pore size distribution

A simple two-step dealloying strategy is described to make free-standing metal membranes with hierarchical porous architecture. This structure has a bimodal pore size distribution composed of large porosity channels and small porosity channel walls, where each pore size can be tailored independently of the others. A new gas-phase electroless plating technique was also developed here that could be used to uniformly fill porous structures with pore size as small as 10 nm.     

Hydraulic and surface characteristics of membranes with parallel cylindrical pores

Pore size distributions and pore densities of track-etched polycarbonate ultrafiltration (UF) membranes with pore sizes ranging from 10 to 100 nm (0.01–0.10 μm) were characterized by image analysis of field emission scanning electron micrographs (FESEM) of membranes. Porosity data obtained from image analysis compared well with those derived from manufacturer's specifications, but this may have been coincidental, as pore size and pore density results differed by 20–40% and 25–70%, respectively. The experimentally determined flux through each membrane type varied by up to 30–45% within a batch, and were about 8–46 times higher than the theoretical over the range of membranes. The disparity between theoretical and experimental flux was beyond the bounds of physical variability of the membranes. The membranes with smaller pore size tended to show a greater disparity. Water flux of all membranes increased with increasing temperature, generally in accord with the decreasing viscosity of water. However, unlike the linear increase for the membranes with larger pores (> 50 nm), the membranes with smaller pores (10 and 30 nm) showed exponential increase with temperature. Water flux also increased with a pressure increase from 50 to 300 kPa. Raised pressure appear to enlarge pores resulting in exponential flux enhancement at higher pressure, particularly for membranes with smaller pores (PC10). The pores may have stretched open under pressure to deliver the higher than expected fluxes due to flexibility of polycarbonate films, although FESEM showed no visible evidence of fracturing or tearing of the membranes. The flux results from filtration of aqueous protein solution were a little lower and correlated well with water permeability of the membranes, but remained in discord with the pore size distribution results.