Preparation and application of microporous TPX membranes

In the present work, poly(4-methyl-1-pentene) (TPX) was used to prepare hydrophobic microporous membranes, and the application of the prepared membranes to pervaporation and osmotic distillation was also investigated. The TPX/cyclohexane solution inclines to undergo solid-liquid demixing and form polymer particles at room temperature. The solid-liquid demixing is strongly related to the crystallization process. During membrane formation, the competition between solid-liquid demixing and polymer precipitation determines if particulate membranes can be prepared. By using suitable coagulant, such as propanol, the solid-liquid demixing process occurs before polymer precipitation, particulate TPX membranes with interconnected pores can thus be successfully fabricated. By adjusting the coagulation environment, the pore size of the porous TPX membrane can be tailored. Experiments were performed to evaluate the performance of the prepared membranes in pervaporation and osmotic distillation. The results indicate that the performance of the microporous TPX membranes prepared in the present work is comparable to the commercial PTFE membranes.     

Structure analysis of microporous plane membranes by a Monte Carlo method

Plane microporous PTFE membranes, obtained by stretching, have been modelled by layers of cylinders representing the polymer filaments. The concept of the simulation of Knudsen diffusion in the networks by a Monte Carlo method is used to estimate the complexity of the three-dimensional structure of these membranes. A tortuosity parameter can be calculated from a relation depending only upon the volumic porosity and the mean filament diameter. The results obtained are compared to data obtained for real membranes.     

Preparation and characterization of antibacterial microporous membranes fabricated by poly(AMS-co-DMAEMA) grafted polypropylene via melt-stretching method

Functional polypropylene (PP) is prepared by graft poly(AMS-co-DMAEMA) onto PP chains through melt blending, and then microporous membranes of functional PP are made by casting and stretching, and finally the polycation microporous membranes are obtained via quaternization. The obtained microporous membranes show good hydrophilicity and antibacterial properties.     

Synthesis of microporous membranes and films on various substrates by novel electrospinning method

The introduction of electrospinning technique in synthesis of supported microporous membranes and films opens bright prospects for mass production and practical applications. This novel and promising strategy has wide suitable range for various substrates with the possibility of large-area processing. We successfully synthesized several kinds of microporous materials into high quality membranes and films on different shaped supports by this method, such as zeolite NaA and pure-silica-zeolite Beta membranes on porous Al₂O₃ tube, zeolite NaY membrane on stainless steel net and a metal-organic framework Eu(BTC)(H₂O)·DMF (JUC-32) film on porous silica disc. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used as characterization means. The results verified the effectiveness of this new approach in fabrication of membranes and films.     

Structural and performance study of microporous polyetherimide hollow fiber membranes made by solvent-spinning method

The morphology of the inner and the outer surfaces of polyetherimide (PEI) hollow fibers were studied by atomic force microscopy (AFM). The effect of the spinning conditions, namely, the bore liquid flow rate on the surface characteristics was investigated. Bore liquid flow rate was varied between 0.1 and 0.4 ml/min. The pore sizes at the outer and inner surfaces of the hollow fibers were affected by the bore liquid flow rates. Bore liquid flow rates also influenced the ultrafiltration performances of the hollow fiber membranes. The alignment of nodule aggregates to the direction of bore liquid flow was observed.     

Aldehyde activated microporous membranes

An affinity membrane was designed for manufacturability, starting from novel polymer synthesis, through casting dope formulation, and finally membrane casting. Aldehyde functionality, introduced through acrolein, provided activation by Schiff base formation with proteins. Acrolein copolymers and their affinity applications were reviewed. Solution polymerization of acrolein and acrylonitrile produced a copolymer of sufficient mechanical strength to constitute a casting dope. Membranes cast from DMSO by the phase inversion process proved microporous when allowed to gel in warm (40–50°C), humid air. Membranes cast from other solvents or at ambient temperature or those immersed directly under water were all tighter. No other supporting non-functional membrane was needed, although a scrim reinforcement improved robustness. Protein-binding studies using alkaline phosphatase enzyme and enzyme-linked immunoassay (ELISA) studies using an IgG antibody-antigen complex were conducted. After various extensive washings to remove unbound protein, the polyaldehyde membranes were demonstrated to be comparable to the commercially available affinity membrane controls.     

Permporometry characterization of microporous ceramic membranes

An evaluation of nano-order pore size of membranes was carried out using permporometry, the basic principle of which is based on capillary condensation of vapor and the blocking effect of permeation of a non-condensable gas. A computer-controlled apparatus was constructed, where liquid was injected by a syringe pump and nitrogen was used as a carrier, and was applied to the evaluation of the pore size of ceramic membranes prepared from a silica–zirconia composite. The pore size distribution, based on the Kelvin equation (Kelvin diameter), was evaluated over a range of 0.5–30 nm, using water as a condensable vapor. Vapors used in the present study were water, methanol, ethanol, isopropanol, carbon tetrachloride, and hexane. For the case of relatively large pore sizes (larger than 1 nm, based on water vapor), pore size distribution obtained by water vapor agreed very well with those by carbon tetrachloride and hexane. However, pore sizes measured using alcohols were found to be smaller than those determined by water vapor. For the case of pore sizes smaller than 1 nm, the adsorption layer before capillary condensation appears to play an important role.     

Preparation and performance evaluation of carboxylic acid containing polyamide incorporated microporous ultrafiltration PES membranes

Improved ultrafiltration membranes were prepared by the phase inversion technique via immersion precipitation of synthesized carboxylic acid containing polyamide (CPA) and polyethersulfone (PES) in dimethylacetamide. The CPA was synthesized and characterized by Fourier transform infrared (FTIR), nuclear magnetic resonance, thermogravimetric analysis, and differential scanning calorimetry analyses. Next, the influence of CPA adding and its different concentrations on the performances and membrane structure were investigated. The obtained membranes were characterized by means of FTIR in the attenuated total reflection mode, scanning electron microscopy, and contact angle. The membrane performance studies revealed that the presence of CPA in the membrane structure increased water permeability while reducing protein fouling. It turned out that the PES/CPA membranes had better porosity, more hydrophilic surface, and more vertically finger‐like pores in comparison with the bare PES membrane. When the CPA concentration in the blending solution reached 1 wt%, the water permeability increased from 7.3 to 153.6 L/m² h¹. The attenuated total reflection‐FTIR analysis confirmed that CPA was captured in the membrane matrix.     

Preparation and characterization of microporous PVDF/PMMA composite membranes by phase inversion in water/DMSO solutions

PMMA/PVDF composite membranes were prepared by isothermal immersion-precipitation of dope solutions consisting of PMMA, PVDF, and DMSO into both harsh and soft nonsolvent baths. The effects of PMMA and DMSO contents on the membrane morphology, crystal structure, thermal behavior and tensile strength of the formed membrane were investigated. For a PMMA-free casting dope immersed in a harsh bath, such as pure water, the formed membrane exhibited a typical asymmetric morphology characterized by skin, finger-like macrovoids, and cellular pores. In contrast, when a soft 70% DMSO bath was adopted, PVDF crystallized to form a membrane packed by spherulitic globules. Incorporation of PMMA gave rise to interesting morphological features; e.g., PVDF globules were observed to adhere to the interlocked polymer branches coexisting with the continuous porous channels, as revealed by high resolution FESEM imaging. XPS analysis of the surfaces of the composite membranes suggested the occurrence of a surface segregation phenomenon, wherein PVDF preferentially migrated to the top surface region of the membrane such as to minimize the interfacial energy. XRD analyses indicated that PVDF crystallized into ‘α’ structure in both PVDF and PMMA/PVDF composite membranes. The crystallinity of the membranes was found to decrease with increasing PMMA content, which was confirmed by DSC thermal analyses. The latter results also indicated a significant decrease in membrane’s melting temperature as the PMMA content was increased. Tensile strengths of the membranes were improved by inclusion of PMMA in either harsh or soft baths. However, elongation at break showed a reversed trend.     

Polyurethane microporous membranes as pericardial substitutes

Pericardial substitutes were prepared from stable and degradable segmented polyurethanes and/or polyurethane/polyhydroxybutyrate composites.Polyurethane membranes implanted as pericardial substitute in the rabbit, did not activate adhesion and epicardial reaction over 3 months.Polyurethane/polyhydroxybutyrate membranes induced minimal adhesion or epicardial reaction, yet stimulated the growth of epithelium on the polymeric substrate and reduced the incidence of infection.     

Ordered microporous membranes templated by breath figures for size-selective separation

Membranes with highly uniform pore size are important in various fields. Here we report the preparation and performance of ordered membranes, the pore diameter of which is on the micrometer scale. The ordered membranes fabricated at two-phase interfaces enable a high-resolution and energy-saving separation process. Moreover, a possible mechanism for the formation of through-pores has been proposed and experimentally verified.     

Flow-dependent rejection of polystyrene from microporous membranes

Dilute solutions of polystyrene (molecular weight 1 × 10⁵?2 × 10⁷) in a mixed solvent of 90% carbon tetrachloride-10% methanol were filtered through track-etched porous mica membranes. The reflection coefficient σ, defined as the fraction of polymer held back by the membrane, was measured as a function of polymer size r_s, pore radius r_o, and solvent flow rate q through each pore. Polymer size was characterized by the Stokes-Einstein radius, as determined from diffusion coefficients measured by quasielastic light scattering, and chain relaxation times τ were estimated from measured intrinsic viscosities. In the case of chains whose unperturbed radius was smaller than the pore, σ depended on the ratio r_s/r_o in the manner predicted by a hard-sphere theory, as long as \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma \tau < < 1 $\end{document}, where \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} is the mean rate of strain of solvent at the pore entrance. However, when the polymer chains exceeded the pore in size, σ depended on flow rate and decreased from almost unity, at small q, toward zero at high q. The relationship between σ and q was nearly independent of polymer and pore size, consistent with a theory based on scaling concepts of how polymer chains deform at the entrance of a pore, but the reduction in σ as q increased was very gradual and did not exhibit the sharp transition predicted by the theory. We were able to empirically correlate all the data for σ when r_s > r_o by a single similarity variable \documentclass{article}\pagestyle{empty}\begin{document}$ \theta = {{({{r_s} \mathord{\left/ {\vphantom {{r_s} {r_0}}} \right. \kern-\nulldelimiterspace} {r_0}})} \mathord{\left/ {\vphantom {{({{r_s} \mathord{\left/ {\vphantom {{r_s} {r_0}}} \right. \kern-\nulldelimiterspace} {r_0}})} {(\dot \gamma \tau)^n}}} \right. \kern-\nulldelimiterspace} {(\dot \gamma \tau)^n}} \sim ({{r_s} \mathord{\left/ {\vphantom {{r_s} {r_0}}} \right. \kern-\nulldelimiterspace} {r_0}})^{1 - 3n} q^{- n} $\end{document}; a least-squares fit gave n = 0.33, showing that σ is insensitive to polymer size for large chains.     

Tertiary amine-mediated synthesis of microporous carbon membranes

Microporous carbon membranes were prepared on an -alumina support by a pyrolysis of cationic tertiary amine/anionic polymer composites. The precursor solutions contain a thermosetting resorcinol/formaldehyde (RF) polymer and a cationic tertiary amine. Three types of cationic tertiary amines with different chain lengths were used, such as tetramethlammonium bromide (TMAB), tetrapropylammonium bromide (TPAB) and cetyltrimethylammonium bromide (CTAB). A porous structure was produced by a decomposition of the amine and the resulting pores assisted the further gasification of the RF polymer at high temperature. The carbon/alumina membranes have thin and continuous carbon top layers with a thickness of 1 μm. Gas permeation tests were performed using single gases of CO₂, O₂, N₂, CF₄, n-C₄H₁₀ and i-C₄H₁₀, as well as binary mixtures of CH₄/n-C₄H₁₀ and N₂/CF₄ at different temperatures between 23 and 150 °C. The carbon membrane prepared using TMAB showed separation factors higher than 650 for the CH₄/n-C₄H₁₀ mixtures and higher than 8100 for the N₂/CF₄ mixture. From the permeation of pure gases with different molecular sizes, the pore sizes of the carbon membrane prepared using TMAB, TPAB and CTAB are estimated to be 4.0, 5.0 and larger than 5.5 Å, respectively, indicating that the micropore size of the carbon membranes is controllable by using different amines.     

Immobilisation of antibody on microporous silicon membranes

Microchimica Acta - The immobilisation of antibody on silicon membranes containing arrays of micropores was studied. The membranes are modified by the deposition of aminopropyl triethoxysilane to...     

Anodic alumina supported dual-layer microporous silica membranes

We present a new processing scheme for the deposition of microporous, sol–gel derived silica membranes on inexpensive, commercially available anodic alumina (Anodisk™) supports. In a first step, a surfactant-templated mesoporous silica sublayer (pore size 2–6 nm) is deposited on the Anodisk support by dip-coating, in order to provide a smooth transition from the pore size of the support (20 or 100 nm) to that of the membrane (3–4 Å). Subsequently, the microporous gas separation membrane layer is deposited by spin-coating, resulting in a defect-free dual-layer micro-/mesoporous silica membrane exhibiting high permeance and high selectivity for size selective gas separations. For example, in the case of CO₂:N₂ separation, the CO₂ permeance reached 3.0 MPU (1 MPU = 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) coupled with a CO₂:N₂ separation factor in excess of 80 at 25 °C. This processing scheme can be utilized for laboratory-scale development of other types of microporous or dense inorganic membranes, taking advantage of the availability, low cost and low permeation resistance of anodic alumina (or other metal oxide) meso- and macroporous supports.     

Solvent-induced morphological change of microporous hollow fiber membranes

Microporous polyethylene hollow fiber membranes (EHF-1 and EHF-2) were subjected to solvent treatment, and the effects of this treatment on membrane morphology and permeating properties were studied. Membranes treated with various organic solvents exhibited enhanced permeability, enlarged pore size, and increased shrinkage in the longitudinal direction. These phenomena were found to depend on the surface tension of the solvent: the higher the surface tension of the solvent, the larger the change in morphology and permeation of the membrane. A mechanism to account for the effects of solvent treatment on the morphology of the membrane is proposed taking into consideration the influence of the type of solvent used for treatment. The enhanced morphological and permeation changes are ascribed to the formation of liquid bridges between two microfibrils of the membrane during drying followed by the deformation and adhesion of the adjacent microfibrils based on the surface tension of the solvent.     

High pressure gas permeability of microporous carbon membranes

Microporous carbon membranes are prepared, characterised structurally and tested in terms of high pressure CO₂ permeability at temperatures around the critical. A maximum in the permeance versus relative pressure curve is observed in close analogy to the case of mesoporous membranes. This weakens considerably as the temperature is increased above the critical. The results offer significant input for an improved understanding and theoretical modelling of the process and may be potentially useful for the identification of the optimal pressure and temperature conditions for efficient gas separations.     

Long-term pervaporation performance of microporous methylated silica membranes

The incorporation of methyl groups in microporous silica membranes was proven to enhance the service time in the dehydration of a butanol-water mixture at 95 degrees C from a few weeks to more than 18 months with a water flux of about 4 kg m(-2) h(-1) and a selectivity between 500 and 20 000.     

Three dimensional structure of celgard® microporous membranes

The unique three dimensional structure of Celgard® 2500 film has been shown by direct enhanced contrast transmission electron microscopy (TEM) of thin sections from the bulk and by high resolution scanning electron imaging (SEI) of the film surface. The TEM method developed includes treatment with an unsaturated surfactant, which then reacts with osmium tetroxide to form an electron dense stain. This is followed by standard microtomy and TEM observations made possible by the stability and contrast imparted by the heavy metal. Sectioning in all three dimensions — along the machine direction, across the machine direction and across the face of the membrane — gives a total view of the bulk microstructure. Preparation for high resolution SEI imaging includes metal coating with the Ion Beam Micro-Sputter gun, to form a coherent fine grain surface to dissipate charge, provide secondary electrons for imaging, and protect the sensitive polymer from beam damage. The SEI view of the surface completes the three dimensional characterization.The microporous structure of Celgard® 2500 is formed during extrusion, annealing and stretching of isotactic polypropylene. The elliptical pores formed during this process have been shown to be oriented with their major axes parallel to the machine direction and the film surface. The pores are grouped into rows or pore channels which traverse the thickness of the film at an angle to its surface and which oscillate in and out of the plane of a cross-section taken in the machine direction. The observed structure is morphologically uniform throughout the bulk of the film. However, a thin surface layer appears to exhibit a slightly smaller pore size and a lower two dimensional pore area than does the interior. Thus, the microporous structure provides a continuous, but tortuous or staggered, row configuration necessary for high fluid transport rates.