Finger-like Macrovoids Research Articles

Effect of the addition of the surfactant Tetronic 1307 on poly(ether sulfone) porous hollow‐fiber membrane formation

AbstractPoly(ether sulfone) (PES) hollow‐fiber membrane was prepared via a nonsolvent‐induced phase‐separation method, and the effect of the addition of the surfactant Tetronic 1307 on the membrane performance and characteristics was investigated. The phase diagram of the PES/N‐methyl‐2‐pyrrolidone (NMP)/water system was clarified. When the polymer solution involved Tetronic 1307, the amount of water required to induce the phase separation decreased, which indicated that Tetronic 1307 was one kind of nonsolvent. The kinetics of phase separation for the PES/NMP/Tetronic 1307 system were studied by light‐scattering measurements. With the addition of Tetronic 1307, delayed phase separation was observed, and the structure growth rate decreased. Scanning electron microscopy images for all of the membranes showed the formation of fingerlike macrovoids through the cross section. Membrane surface morphologies were measured by atomic force microscopy. The obtained results indicated that membrane with 7 wt % Tetronic 1307 had higher roughness parameters than original membrane without the addition of surfactant. Ultrafiltration experiment results showed that the addition of Tetronic 1307 brought about an increase in water permeability and decreased the rejection of dextran with a molecular weight of about 10,000. The contact angles of water on the membrane outer surface decreased with the addition of Tetronic 1307. This mean the membrane surface became more hydrophilic. Thus, the addition of Tetronic 1307 was useful for improving the water permeability and for obtaining a hydrophilic membrane surface. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Dual-layer P84/polyethersulfone hollow fibers for pervaporation dehydration of isopropanol

We have developed BTDA-TDI/MDI (P84)/polyethersulfone (PES) dual-layer hollow fibers for pervaporation dehydration of isopropanol (IPA). The effects of spinning conditions, e.g. air gap distances, outer- and inner-layer dope flow rates on membrane formation, morphology and pervaporation performance have been investigated. Compared to the wet-spun dual-layer hollow fibers, macrovoids in the outer-layer are significantly suppressed in the dry-jet wet-spun fibers. It is found that the intrusion of non-solvent from the outer-layer may propagate the formation of finger-like macrovoids into the inner-layer. Because of different phase-inversion rates in the dual-layers, the effects of elongational stresses in the air gap region on the separation performance of dual-layer hollow fibers are much more complicated than that of single-layer hollow fibers. Both thermal treatment at elevated temperatures and chemical cross-linking modification by p-xylenediamine were investigated. Heat treatment at 200 °C increases separation factor with reduced flux. However, unlike single-layer P84 hollow fibers, further increased heat treatment temperature do not enhance separation property of P84/PES dual-layer hollow fibers because of the enhanced sub-layer resistance via the densification of PES layer. The performance enhancement by p-xylenediamine cross-linking shows very promising for the P84/PES dual-layer hollow fiber because p-xylenediamine induces cross-linking reactions only in the P84 outer-layer. The best separation factor/selectivity is reached at 2 h cross-linking time. This study demonstrates that a significant material cost saving can be achieved without sacrificing separation performance by choosing the dual-layer hollow fiber approach with the aid of p-xylenediamine cross-linking modification.

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.

Formation and morphology studies of different polysulfones-based membranes made by wet phase inversion process

Different polysulfones were sulfonated and were used to prepare asymmetric membranes by wet phase inversion. The morphology of the porous structure of the membranes obtained in different coagulation conditions was studied and discussed. Nanofiltration membranes can be formed by coagulating a concentrated solution (e.g. 40 wt.%) of sulfonated cardo polyethersulfone by water. The higher the sulfonation degree, the better the rejection of nanosize solutes, and the bigger the pores and the macrovoids. The data on binodal phase separation suggest that such a behavior is due to a longer time required for the polymer to be vitrified at higher sulfonation degree. When the sulfonated polymer is blended with the original polysulfone, the morphology of the coagulated membranes is different from that of single polymer membranes: irregular macrovoids, and eventually polymer nodules, instead of finger-like macrovoids. Wrinkles were observed in the case of asymmetric membranes made of PSU-based blends. They would be generated by compressive strains that appear in the skin layer bonded to a soft substrate (liquid polymer dope) during the departure of the solvent from the nascent membrane towards the coagulation bath. A decrease in the coagulation temperature or in the total polymer in the casting dope led to larger and more numerous macrovoids, probably due to an increase in the phase separation time before the gelling of the system.

The preparation and characterization of novel charged polyacrylonitrile/PES-C blend membranes used for ultrafiltration

2-Acrylamido-2-methylpropane-sulfonic acid (AMPS) was incorporated into polyacrylonitrile by water-phase precipitation copolymerization successfully. The chemical structure, molecular weight and chemical composition of the synthesized copolymer were characterized by IR, viscosity measurement and elemental analysis, respectively. Furthermore, blends of phenolphthalein poly(ether sulfone) (PES-C) and the synthesized copolymer with different composition dissolved in dimethylformamide (DMF) were used to prepared the ultrafiltration membranes by means of classical phase inversion process. And then, the effect of the blend composition on the hydrophilicity, electrical properties, morphologies and permeation properties of the blend membranes was investigated elaborately. The static water contact angle of the blend membrane surface decreased from 78.7 ± 0.9° to 42.6 ± 3.5° with the increase of PAN-co-AMPS from 0 to 20%. It was found that salt retention properties of the blend membranes were influenced intensely by both the content of the charged copolymer and the pH of feed solution. In addition, FESEM images of membrane surface and cross-section displayed that pore size decreases, the top layer became thicker and the finger-like macrovoids disappeared gradually as the PAN-co-AMPS was added. As a result, the water flux is decreased from 230 to 40 L/m 2 h and rejection of PEG20000 and PEG35000 was increased from 0.129 to 0.7389, and 0.5528 to 0.91, respectively (as measured at 100 kPa).

Influence of drying method on morphology and properties of asymmetric cellulose hollow fiber membrane

Using a dry/wet spinning process, asymmetric cellulose hollow fiber membranes (CHFM) were prepared from a dope composed of cellulose/ N-methylmorpholine- N-oxide/water. The formation mechanism for the finger-like macrovoids at the inner portion of as-spun fibers was explained. Naturally drying and three solvent exchange drying methods were tried to investigate their influence on morphology and properties of CHFM. It was found that the ethanol–hexane exchange drying was an appropriate method to minimize morphology change of the as-spun CHFM, whereas the naturally drying caused the greatest shrinkage of the fibers that made the porous membrane become dense. As a result, CHFM from ethanol–hexane exchange drying performed the highest gas permeation rate but gas permeation of the naturally dried membrane could not be detectable. The resultant CHFM from the ethanol–hexane exchange drying also showed acceptable mechanical properties, thus it was proposed to be an appropriate method for gas separation purpose. The experimental results supported the proposed drying mechanism of CHFM. The free water would evaporate or be replaced by a solvent that subsequently would evaporate but the bonded water would remain in the membrane. What dominated the changes of membrane morphology during drying should be the molecular affinities of cellulose–water, water–solvent and solvent–solvent.

Thickness Dependence of Macrovoid Evolution in Wet Phase-Inversion Asymmetric Membranes

Following an inspiration from Vogrin et al. (J. Membr. Sci. 2002, 207, 139), we have investigated the thickness dependence of macrovoid evolution during the phase inversion of asymmetric flat membranes and observed, for the first time, a critical structure-transition thickness, Lc, which indicates a transition of the membrane morphology from a spongelike to a fingerlike structure with an increase in membrane thickness. Below Lc, the membranes show a fully spongelike cross-sectional structure, whereas above Lc, the membranes exhibit a mainly fingerlike macrovoid structure. Two binary solutions are employed in this study: 20 wt % polyethersulfone/N-methyl-2-pyrrolidone (NMP) and 20 wt % BTDA−MDI/TDI co-polyimide/NMP solutions. Their corresponding critical thicknesses, Lc, are 1.5 ± 0.4 and 11 ± 2 μm, respectively.

Ultrafiltration hollow fiber membranes of sulfonated polyetherimide/polyetherimide blends: preparation, morphologies and anti-fouling properties

Sulfonated polyetherimide/polyetherimide (SPEI/PEI) ultrafiltration (UF) hollow fiber membranes were prepared with phase inversion method and SPEI was used as a polymer additive. Scanning electron microscope (SEM) study revealed that the addition of SPEI could induce the formation of nodular structure and suppress the formation of finger-like macrovoids. With the increase of air gap distance, it was found that the micropores were formed in the outer fiber surface and the pore size increased accordingly. On the other hand, the pure water flux of the membrane reached a maximum and then decreased while the solute retention decreased at first and leveled off at longer air gap. Through bovine serum albumin (BSA) solution filtration test, it was found that the use of SPEI in the blend membranes increased its hydrophilicity. When SPEI with ion exchange capacity (IEC) of 2.88 mmol/g was blended with PEI by 15/85 weight ratio, 75% of the membrane flux reduction in the BSA solution (pH=8.0) filtration process was caused by reversible BSA adsorption, compared with 32% for PEI membrane. Increasing the pH value of the BSA solution, the membrane fouling tendency reduced and the reversible flux reduction ratio increased.

Membranes with a particulate morphology prepared by a dry–wet casting process

The effect of the evaporation step on the occurrence of particles in poly(ethylene-co-vinyl alcohol) (EVAL) membranes cast from DMSO solutions via the dry/wet process were studied. The structure of the EVAL membranes can be changed from an asymmetrical structure consisting of a dense skin layer and finger-like macrovoids in the sublayer to a skinless and symmetric structure by constituent particles bonded to each other. From the analysis of the membrane formation mechanism, the competition of different phase separation process during membrane formation is most important. The onset of the phase separation may be either liquid–liquid demixing or solid–liquid demixing, which determines the resulting membrane properties. Directly immersing the casting solution into a water bath, liquid–liquid demixing is of considerable importance during the phase separation of the solutions. As a result of the lower activation energy for nucleation, liquid–liquid demixing can precede solid–liquid demixing even in cases where solid–liquid demixing is favored thermodynamically. By using the evaporation process, the phase separation proceeds slowly via solid–liquid demixing and thus leads to a particulate morphology in the membrane. This suggests that the evaporation step cause crystallization of EVAL molecules from the casting solution to inhibit the macrovoid formation. In addition, the duration of the evaporation step is shown to have a strong influence on the disappearance of particles. The results presented here offer a qualitative basis for the development of membranes with a particulate morphology.

Formation of crystalline EVAL membranes by controlled mass transfer process in water–DMSO–EVAL copolymer systems

The effect of solvent content in the coagulation bath on the formation of poly(ethylene-co-vinyl alcohol) (EVAL) membranes by isothermal immersion–precipitation in water–dimethylsulfoxide (DMSO)–EVAL system has been investigated. As a homogeneous EVAL dope is immersed in a harsh bath, e.g., water, instant precipitation occurs initiated by the liquid–liquid demixing process. The formed membrane exhibits an asymmetric morphology with extensive finger-like macrovoids, similar to that observed in most amorphous polymeric membranes. On the other hand, if precipitation takes place in a soft bath containing a substantial amount of solvent, crystallization, rather than liquid–liquid demixing, starts to dominate and a skinless, bi-continuous, particulate membrane becomes the precipitated product. The immersion–precipitation process for the present system has been modeled as a ternary mass transfer problem. The calculated diffusion trajectories and concentration profiles illustrate reasonably the membrane morphologies formed in various immersion conditions.

Effect of capsule diameter on the permeability to horseradish peroxidase of individual HEMA-MMA microcapsules

Poly (2-hydroxyethyl methacrylate-methyl methacrylate), (HEMA-MMA) microcapsules to be used for the transplantation of live mammalian cells were prepared by an interfacial precipitation process. A submerged jet coextrusion technique was used to make various sizes of capsules as small as 450 μm in outside diameter. A novel method based on an enzyme-chromogenic substrate assay was used to measure the capsule permeability at the individual capsule level. Capsule size, shearing frequency (number of capsules sheared per unit time), capsule permeability and the capsule-to-capsule variation in permeability were dependent, directly or indirectly, upon the shearing force (i.e. hexadecane flow-rate) applied to the nozzle during preparation. The average permeability coefficients of those capsules to horseradish peroxidase, (HRP, MW 40kDa, a model protein), were in the range of 2×10 −10 to 9×10 −10 cm 2/sec; the permeability varied from 1×10 −10 to 3×10 −10 cm 2/s for 500 μm capsules (the best) and 0.1×10 −10 to 6×10 −10 cm 2/s for 660 μm capsules (the poorest). Scanning electron microscopy illustrated that finger-like macrovoids were formed under the external skin layer of the capsule membrane, but that an interconnected open-cell structure was formed under the internal skin. Modelling the precipitation process in two dimensions (i.e. between two glass slides) suggested that the faster precipitation of HEMA-MMA at the external surface dominated the formation of the membrane structure, compared with the slower precipitation at the internal surface.

Observations on the dynamics of nonsolvent-induced phase inversion

The diffusion and gelation dynamics of nonsolvent-induced phase inversion in several polyethersulfone (PES)/solvent/nonsolvent systems are observed using a dark-ground optical technique. The observed dynamics are correlated with the resultant morphologies of the solidified gels obtained via scanning electron microscopy. In situ dynamic measurements show that rapid precipitations result in finger formation and delayed precipitations result in sponge formation. Rapid precipitations for some systems also exhibit an initial region of high, anomalous diffusion front motion which correlates well with the appearance of finger-like macrovoids in the film sublayer. Micrographs of both thin (200–300 μm) and thick (3 mm) films formed by liquid-liquid demixing clearly show that the overall morphologies scale with initial film thickness. However, as observed for the cellulose acetate (CA)/dimethylsulfoxide (DMSO)/H2O system, the possibility of crystallization can complicate the scaling analysis. A ternary diffusion model is also employed to describe the isothermal diffusion encountered during the formation of PES membranes. Binary thermodynamic and kinetic parameters needed for computations are determined from experimental data. Model results agree well with experimental observations. The model accurately predicts the transition from finger-to-sponge formation, as well as other observed trends in dynamics and morphology. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys, 35: 569–585, 1997

Effects of casting solution composition on performance of poly(ether sulfone) membrane

Flat-type poly(ether sulfone) (PES) membranes for ultrafiltration were prepared by the traditional phase-inversion technique. The effects of the concentration of PES and the combination of two solvents, dichloromethane (DCM) and N-methyl-2-pyrrolidone (NMP), with differences in volatility and solvating power on membrane performance were examined in terms of pure water flux (PWF) and solute rejection (SR) against poly(ethylene glycol) (PEG, MW 20,000). Changing the thermodynamic quality of PES/NMP casting solution by combining DCM, a volatile and weak solvent, affected the PWF of the resulting membrane. The SR of PES/DCM–NMP membrane, however, was more likely dependent upon the effect of evaporating the volatile solvent from the casting solution/air interface rather than the effect of changing the thermodynamic quality of the casting solution. Combining DCM in PES/NMP casting solution transformed the fingerlike macrovoids of PES/NMP membrane prepared without DCM into the isolated macrovoids. PES/DCM–NMP membrane prepared with PVP, a water soluble poreforming agent, showed an increased PWF while maintaining SR of over 90%, even under the reduced feeding pressure of 1 kg/cm2. It is necessary to measure molecular weight cutoff of membrane for demonstrating the potential of PVP for improving the membrane permeability without losing the selectivity.© 1996 John Wiley & Sons, Inc.

Preparation of membranes from polyacrylonitrile—polyvinylpyrrolidone blends and the study of their behaviour in the pervaporation of water—organic liquid mixtures

Membranes were made by blending polyacrylonitrile (PAN) and polyvinylpyrrolidone (PVP) in dimethylformamide, followed by an evaporation—coagulation phase to solidify the blends. Membranes of different structures were obtained by changing the preparation conditions: asymmetric membranes with finger-like macrovoids (low PVP content in the polymer and low solvent content in the precipitant bath), asymmetric membranes with honeycomb-like sub-structure, unskinned, granulous membranes (long precipitation in tetrahydrofuran), and dense film membranes (long evaporation time). The formation of these structures was interpreted on the basis of known mechanisms, except in the case of unskinned granulous membranes, where other hypotheses were proposed. Only dense membranes were sufficiently selective for the dehydration of water—organic liquid mixtures. Highest selectivity was obtained at a 1:1 composition of the blend. This type of membrane showed good performance with water—tetrahydrofuran mixtures. Its selectivity was lower but its permeability was higher with water—ethanol mixtures. Generally speaking, the membrane consisting of the 1:1 blend had a similar behaviour to that obtained by radiochemical grafting.

Observations on the structure of first generation polybenzimidazole reverse osmosis membranes

The structure-property relationships of polybenzimidazole (PBI) reverse osmosis (RO) membranes are important in process optimization. The membrane transport properties are influenced by the morphology which, in turn, is established by the process variables. Microscopical observation of the in situ porous textures of experimental membranes required development of special sample preparation methods. The preparation involves gradual replacement of water, and infiltration by an embedding resin, which permits thin sectioning for microscopical observation without collapse of the structure. p]Optical microscopy observations using the newly developed method provide an excellent overview useful for screening process variables. Scanning electron microscopy (SEM) was used to obtain useful three-dimensional fracture views of samples, although the membrane structure is somewhat collapsed during specimen preparation. Pore size and distribution, particularly in the region of the dense surface layer critical for effective separation properties (the active layer), were analyzed using transmission electron microscopy (TEM). Membrane surfaces were characterized using SEM and high resolution scanning electron imaging (SEI). Preliminary study revealed major morphological variation as flux levels varied. A study of high-flux as-cast and annealed first generation membranes revealed finger-like macrovoids below the top 500 nm layer. The top 20 nm has no resolvable pores (resolution limit about 10 nm, limited by sample thickness), whereas the bulk membrane exhibits pores about 100 nm across. The annealing process appears to shrink the membrane and reduce the pores to half the as-cast size.

PP copolymers with controlled granulometry SEM investigation on replica factor and internal morphology

Innovation and effective wastewater treatment technology is still in great demand given the emerging contaminants frequently spotted from the aqueous environment. By blending with poly (vinylidene fluoride) (PVDF), the strong hydrophilic graphene oxide (GO) and antibacterial copper oxide (CuxO) were used as nanofillers to develop the novel, highly antifouling composite membranes via phase inversion process in our latest work. The existence and dispersion of GO and CuxO posed a significant role on morphologies, structures, surface composition and hydrophilicity of the developed composite membranes, confirmed by SEM, TEM, FTIR and XPS in depth characterization. The SEM images showed that the modified membranes presented a lower resistant structure with developed finger-like macrovoids and thin-walled even interconnected sponge-like pores after adding nanofillers, much encouraging membrane permeation. The XPS results revealed that CuxO contained Cu2O and CuO in the developed membrane and the Cu2O nanoparticles were dominant accounting for about 79.3%; thus the modified membrane specifically exhibited an efficient antibacterial capacity. Due to the hydrophilic and bactericidal membrane surface, the composite membranes demonstrated an excellent antifouling performance, including higher flux recovery rate, more resistant against accumulated contaminants and lower filtration resistance, especially lower irreversible resistance. The antifouling property, especially anti-irreversible fouling, was significantly improved, showing a significant engineering potential.