Casting Solution Composition Research Articles

Ternary phase field model and characterization of water/NMP/polysulfone membrane prepared by non-solvent induced phase separation

A ternary phase field model is developed for water/N-methyl-2-pyrrolidone(NMP)/polysulfone system to simulate the non-solvent induced phase separation process. Our model succeeds to predict detailed asymmetric membrane structures, with dense skin layer and porous sublayer constructed from polymer network. It is found that the pore distribution and connectivity of polymer network change with the initial composition of casting solution and coagulation bath. Tortuosity and maximum pore size along the direction of thickness are calculated, with a typical trade-off relation found between them. Electrochemical separation performance was evaluated and found to have a notable correlation with structural characteristics.

Influence of initial composition of casting solution on morphology of porous thin polymer films produced via phase separation

Influence of initial composition of casting solution on morphology of porous thin polymer films produced via phase separation

Fabrication of Polyacrylonitrile UF Membranes by VIPS Method with Acetone as Co-Solvent.

For the first time, a systematic study was carried out of the replacement of the low-volatility solvents N-methyl-2-pyrrolidone (NMP) or dimethylsulfoxide (DMSO) with the high-volatility solvent acetone in the casting solution of polyacrylonitrile (PAN). The effect of acetone’s presence in the casting solution on the performance of ultrafiltration membranes fabricated via vapor-induced phase separation (VIPS) was investigated. It was possible to replace 40% of NMP and 50% of DMSO with acetone, which resulted in the reduction of the casting solution viscosity from 70.6 down to 41.3 Pa∙s (20% PAN, NMP), and from 68.3 down to 20.6 Pa∙s (20% PAN, DMSO). It was found that 20 min of exposure to water vapor (relative humidity—85%) was sufficient to govern the phase separation, which was mainly induced by the water vapor. Regardless of the casting solution composition (15 or 20% PAN; DMSO or NMP), all membranes formed via VIPS possessed a sponge-like porous structure. The addition of acetone to the casting solution allowed the reduction of the transport pore size from 35–48 down to 8.5–25.6, depending on the casting solution composition. By varying the acetone content at constant polymer concentration, it was possible to decrease the molecular weight cut-off (MWCO) from 69 to 10 kg/mol. Membranes prepared from 20% PAN solution in an acetone/DMSO mixture had the lowest MWCO of 10 kg/mol with a water permeance of 5.1 L/(m2·h·bar).

Modelling and optimization of a new complexing retardant-enhanced polymer inclusion membrane system for highly selective separation of Zn2+ and Cu2+

Polymer inclusion membranes (PIMs) have much application potential for metal ion separation. However, its separation selectivity highly depends on the extractant, and new efficient extractants are difficult to develop. This work proposes another approach, i.e., adding a complexing retardant to enhance the selectivity. The impurity ion (e.g., Cu2+) was retarded in the feed solution by the complexing retardant (e.g., citric acid), while the target ion (e.g., Zn2+) was transported across PIM. To reveal the enhancement mechanism, a mass transfer mathematical model was established. Contrast experiments proved that the complexing retardant can significantly enhance the overall separation coefficient βoverall,exp. The βoverall,exp of a conventional PIM system was 61.80, while βoverall,exp increased to 853.45 after the retardant was added. The effects of retardant concentration, initial pH of feed solution, membrane thickness, extractant dosage, and plasticizer dosage on the retardant-enhanced PIM system were investigated. Model calculation results agreed well with the experimental data. The maximal βoverall,exp 17525.68 was obtained at the optimal separation conditions of [CA] f,0 = 0.02 M, pHf,0 = 4.5, [H2SO4]s,0 = 0.5 M, and the optimal composition of the PIM casting solution, i.e., PVC: P507: DBP: THF = 0.125 g: 0.188 mL: 0.032 mL: 10 mL. Further studies indicated that the retardant mechanism was reflected in the difference increase of interfacial partition coefficients PZn2+,f and PCu2+,f. Finally, the optimization strategies of the separation effects for the retardant-enhanced PIM system are discussed in depth based on the simulated calculation.

Effects of coagulation-bath conditions on polyphenylsulfone ultrafiltration membranes

Polyphenylsulfone (PPSU) ultrafiltration membrane with different structures was prepared by non-solvent-induced phase separation. The effects of coagulation bath conditions (concentration and temperature) on membrane morphology, pure water flux, pore size, porosity, and contact angle were studied and discussed based on ternary-phase diagrams. Results indicated that water had stronger coagulant power than ethanol, and that the morphology of the membrane prepared from the polyphenylsulfone/1-methyl-2-pyrrolidinone/H2O (PPSU/NMP/H2O) system had finger-like structures. Conversely, sponge-like structures were observed for the PPSU/NMP/(NMP-H2O) and PPSU/NMP/(70NMP-EtOH-H2O) systems. Ethanol also greatly influenced on membrane structures. According to the Scanning electronic microscopy (SEM) image, the composition (mass fraction) of casting solution is 16% PPSU-84% NMP and the coagulation bath consisting of 70% NMP-26% H2O-4% C2H5OH. Meanwhile, the PPSU ultrafiltration membrane with spong-like was prepared under 8 ℃ coagulation bath. The formation of sponge-like structure reduces the pure water flux of ppsu membrane from 488.39 L·m−2·h−1 to 36.04 L·m−2·h−1. It also reduces the gas permeability, porosity, and pore size of the membrane. The addition of ethanol and NMP into the coagulation bath increases the roughness of the PPSU ultrafiltration membrane and reduces the hydrophilicity of the membrane.

Anti-fouling behavior of micro-patterned PVDF membranes prepared via spray-assisted phase inversion: Influence of pattern shapes and flow configuration

One of the key issues in membrane technology is to avoid fouling formed on the membrane surface. Patterned surfaces have been suggested as one of the ways to reduce fouling. In this report, both experimental and simulation studies were performed using membranes with rectangular and triangular patterns, synthesized via the recently developed s-NIPS method. A casting solution composition of 20 wt% PVDF, 6.7 wt% PVP and 1 wt% H2O in DMF was used to create homogeneous patterns. Well-patterned membranes showed 95–140% higher PWP attributed to the additional effective surface and change in intrinsic membrane morphology associated with the non-solvent spraying. Reduced particles deposition after 3 h continuous filtrations and delayed cake layer formation were observed as compared to the counterpart flat, un-patterned membrane. CFD analysis for different feed to pattern directions (θf) showed the local flow distribution near the patterned grooves. For θf = 90°, vortices in the upper part of the patterns could potentially reduce the particle depositions, while aggregates of foulants can be easily formed in vortices developed in the lower part of the pattern, leading to a favorable fouling formation. On the other hand, for θf = 0°, the flow gave rise to effective slip velocities near the patterned membrane surface, hence reducing the particle adhesion. This indicates the importance of flow characteristics as well as the stress distribution to reduce the fouling in the patterned membrane system. Higher inter-pattern distances, i.e., Rec2000_w and Tri2000_w, led to unfavorable BSA deposition. This combined effect of patterning and s-NIPS resulted in higher effective surface area, reduced fouling and increased membrane porosity which substantially increased the membrane permeance and anti-fouling potential. These results suggest promising application of patterned PVDF membranes prepared via spray modified-NIPS for filtration.

Tuning the structure and performance of polyelectrolyte complexation based aqueous phase separation membranes

Aqueous Phase Separation (APS) provides a new and sustainable platform to fabricate polymeric membranes entirely in water. Still, little is known on how the casting solution and coagulation bath compositions can be used to tune membrane structure and performance. This work comprises a detailed investigation on the tuning parameters avaliable to tailor the morphology, pore size distribution, and water permeability of polyelectrolyte complex membranes prepared from poly(sodium 4-styrenesulfonate) (PSS) and polyallylamine hydrochloride (PAH). To avoid complexation of PAH and PSS in the casting solution, an optimum amount of base (NaOH) must be added to deprotonate PAH. In addition, the monomer mixing ratio of PSS to PAH significantly influences membrane morphology by modulating the interactions between the two polyelectrolytes. Coagulation bath pH can be used to control the driving force for complexation. Decreasing bath pH facilitates the formation of denser membranes, allowing ~97% protein retentions, whereas increasing bath pH leads to more open membrane structures. Changing the concentration of crosslinker in the coagulation bath allows tuning of membrane pore size from ~2 nm to ~46 nm, while simultaneously influencing membrane mechanical properties. Overall, this work highlights several key parameters to tune APS membrane morphology, demonstrating the versatility of APS to prepare optimized sustainable membranes for specific applications.

Synthesis of patterned PVDF ultrafiltration membranes: Spray-modified non-solvent induced phase separation

Patterned membranes have been attractive for the mitigation of fouling in membrane processes, but existing patterning methods are rather in-effective in terms of scalability and performance due to pore-size reduction and patterning at the non-selective membrane side. Polyvinylidene fluoride (PVDF) was used now to synthesize patterned membranes via the spray-modified non-solvent induced phase separation (s-NIPS), a recently developed technique that avoids the current drawbacks of patterned membranes. However, PVDF intrinsically suffers from a slow phase separation, while very fast fixation of the patterns is crucial with this synthesis method. Therefore, the casting solution was optimized towards an accelerated process of phase inversion by systematically studying the effect of polymer concentration, addition of poly(vinylpyrrolidone) (PVP), addition of non-solvent (H2O) and use of different casting solvents for adequate patterning of the membranes. SEM analysis and pure water permeance of the synthesized membranes assessed the homogeneity and pattern formation for each membrane. The most optimal casting solution composition comprised of 20 wt% PVDF, 6.7 wt% PVP and 1 wt% H2O which resulted in homogenously patterned PVDF membranes. The optimized patterned membrane showed a 9 fold increase in PWP as compared to the reference flat membrane thanks to the addition of PVP and H2O as well as the additional surface area. In the filtration of proteins, the water permeance of the membranes improved drastically (+140%) upon patterning with only a moderate loss of BSA rejection (from 90% to 71%) as compared to the corresponding flat membrane. Realisation of patterning via the s-NIPS method resulted in a higher effective membrane surface area combined with an increased membrane porosity. Reduced flux decline of the patterned membrane (52%) as compared to the flat membrane (62%) during continuous BSA filtration also proved the anti-fouling potential of the created patterns.

Fouling resistance improvement with a new superhydrophobic electrospun PVDF membrane for seawater desalination

With excellent water repellency, superhydrophobic membranes are good candidates for membrane distillation (MD). In this study, a new superhydrophobic electrospun poly(vinylidene difluoride) (PVDF) membrane was prepared and applied to the MD type desalination. The casting solution compositions were optimized to produce a rough surface with porous microspheres. The membranes exhibited an acceptable permeability with a contact angle of 152.4°, which were ideal for MD desalination applications. After desalinated real seawater for 120 h, the proposed membrane displayed more stable permeate flux than the conventional PVDF membrane. The desalinated water with the newly developed membrane had a lower conductivity owing to the fouling resistance of membrane caused by the air gap formed between the feed solution and the membrane. The developed membrane provides a possible strategy for improving the long-term efficiency of MD type seawater desalination.

Fabrication of proton permselective composite membrane for electrodialysis-based waste acid reclamation

One of difficulties in electrodialysis (ED)-based waste acid reclamation lies in how to separate protons from metal cations efficiently and economically. A proton permselective composite membrane (PPCM) is specially designed for the process as evidenced by a porous polysulfone membrane and a cross-linked chitosan layer, which perform functions of support and separation, respectively. Herein, a series of PPCM fabricating conditions are analyzed, including casting solution composition, cross-linking agents, and the coating process. Subsequently, the changes in chemical composition and microstructure are explored by ATR-FTIR spectroscopy, SEM and the measurement of porosity and average pore size. Furthermore, the investigations on the electrodialytic transport properties of PPCM are performed, including electrical resistance, limiting current, water transport behavior, and proton permselectivity. Notably, ED experiments show some PPCM performances are even better than those of conventional cation-exchange membranes (CEMs), such as relatively low leakage of metal cations and water migration behavior, relatively high energy efficiency and productivity. However, the proton permselectivity of PPCM for metal cations will weaken with increase of current density, and its proton permselectivity for anions is inferior to CEM. Nevertheless, in view of its many advantages, this paper indicates PPCMs have a promising application prospect.

Effect of Casting Solution Composition on Properties of PIM-1/PAN Thin Film Composite Membranes

In this work, PIM-1 thin film composite membranes supported on PAN were developed. The influence of PIM-1 concentration and nature of solvent stabilizer on the structure and gas separation properties of TFC membranes were studied. It was shown that amylene stabilized chloroform as PIM-1 solvent allows membranes to be obtained with a uniform selective layer in the whole range of concentrations used, and the ethanol stabilized chloroform provides a uniform layer at 2 and 4 wt % PIM-1 concentration. The best CO2/N2 selectivities were 35,9 and 39,5 for 4 % wt PIM-1 solution in ethanol and amylene stabilized chloroform, respectively.

Two‐stage phase separation of cellulose acetate membranes modified with plasma‐treated natural zeolite: Response surface modeling

Cellulose acetate (CA) microfiltration membranes were prepared by two‐stage vapor‐induced phase separation (VIPS) and immersion precipitation. To improve the hydrophilicity and permeability of the membranes at low operating pressures, plasma‐treated natural zeolite was incorporated into the membranes. A response surface methodology based on the three‐level central composite design (CCD) was used to model and optimize the casting solution composition of the membranes with the aim of maximizing membranes permeability. Three independent variables for CCD optimization were concentration of CA, polyvinylpyrrolidone (PVP) pore former, and plasma‐treated zeolite additive. The results showed that a second‐order polynomial model could properly predict the response (pure water flux) at any input variable values with a satisfying determination coefficient (R2) of 0.954. Also, analysis of variance (ANOVA) confirmed the adequacy of the obtained model. The permeability of the prepared membranes increased by increasing zeolite loading from 0.10 to 0.50 wt%, which was related to the membranes morphology and porosity and confirmed by scanning electron microscopy (SEM) images. Pure water flux of the membranes decreased by increasing CA concentration while an optimum PVP amount was required to reach the maximum flux. The result of the bubble point analysis well matched with surface SEM images of the membranes and permeability trend predicted by CCD model. Also, the prepared CA membranes with different compositions showed no toxicity for mouse L929 fibroblast, which indicated their nontoxic and biocompatible nature.

Development of high flux ultrafiltration polyphenylsulfone membranes applying the systems with upper and lower critical solution temperatures: Effect of polyethylene glycol molecular weight and coagulation bath temperature

A novel phase inversion technique for preparation of high flux polyphenylsulfone (PPSU) ultrafiltration membranes employing the systems which feature upper critical solution temperature (UCST), gel point and lower critical solution temperature (LCST) is proposed. These systems include PPSU, polyethylene glycol (PEG) with molecular weight in the range of 6 ÷ 40 × 103 g mol−1 and N-methyl-2-pyrrolidone (NMP) as a solvent. According to the triangular phase diagrams obtained, the polymer solutions consisting of 20 wt% PPSU – 15 wt% PEG (Mw = 6 ÷ 40 × 103 g mol−1) – NMP are revealed to be two-phase systems at T > 100 °C and gels at T < 38–40 °C. The developed technique involves PPSU solution processing at the temperature region between LCST and UCST (gel point). Meanwhile, the coagulation bath temperature has to be between UCST (gel point) and LCST. The effect of PEG molecular weight, casting solution composition and coagulation bath temperature on membrane structure, performance and hydrophilic-hydrophobic balance of the membrane selective layer was studied. Applying the proposed technique flat-sheet membranes with pure water flux of 486 L m−2 h−1 bar−1 and human serum albumin rejection of 90% were prepared which is the best performance for PPSU ultrafiltration membranes reported up-to-date. It was found that introduction of PEG (Mw = 6 ÷ 40 × 103g mol−1) into the casting solution results in the efficient membrane hydrophilization, which is confirmed by FTIR spectroscopy of the membrane selective layer.

Transport Properties, Mechanical Behavior, Thermal and Chemical Resistance of Asymmetric Flat Sheet Membrane Prepared from PSf/PVDF Blended Membrane on Gauze Supporting Layer

Asymmetric polysulfone (PSf) membrane is prepared using phase inversion method and blending with polyvinylidene fluoride (PVDF) on the gauze solid support. Casting solution composition optimization has been done to get PSf/PVDF membrane with best characteristics and permeability. The result shows that blending on PSf with PVDF polymer using phase inversion method has been very helpful in creating an asymmetric porous membrane. Increased level of PVDF in casting solution has increased the formation of asymmetry structure and corresponding flux membrane. The result from thermal test using Differential Scanning Calorimetry (DSC)-Thermal Gravimetric Analysis (TGA) shows the resistance of the membrane to temperature 460 °C. Membrane resistance against acid looks from undetectable changes on infrared spectra after immersion process in H2SO4 6–98 v/v%. While membrane color changes from white to brownish and black is detected after the immersion process in sodium hydroxide (NaOH) 0.15–80 w/v%.

Synthesis, characterization and performance studies of mixed-matrix poly(vinyl chloride)-bentonite ultrafiltration membrane for the treatment of saline oily wastewater

Abstract The present study deals with the improvement of surface morphology, hydrophilicity and antifouling behavior of polyvinyl chloride (PVC) membrane by adding bentonite for ultrafiltration of oil-in-water emulsion under wide range of salinity (0–35000 ppm). Flat-sheet UF membranes were prepared by phase inversion technique using N, N-dimethylacetamide (DMAc) and water. Viscoelastic properties of the membrane casting solution were measured to fix sonication frequency, membrane casting speed and casting solution composition. Field-emission-scanning-electron-microscope and atomic-force-microscope were used to measure surface-porosity, pore-size distribution, pore density and roughness parameters of the membranes. Hydrophilicity of the membranes was determined by measuring equilibrium water content, contact angle and work of adhesion, whereas antifouling character was quantified by flux recovery ratio using saline oily-wastewater. Casting solution with PVC:DMAc:bentonite = 12.0:87.23:0.77 (i.e., PVC/bentonite-6) exhibited the maximum loss tangent and in-phase dynamic viscosity and resulted enhanced surface porosity (56.42–58.62%), root mean square surface roughness (127.6 nm), surface pore density (42.29–32.05 pores μm−2), hydrophilicity (work-of-adhesion:111.08 mN m−1) and antifouling character (flux-recovery-ratio:81.97%). PVC/bentonite-6 also delivered improved performance to separate oily-wastewater with zero salinity (oil rejection: 97.0% and permeate flux: 186 L m−2 h−1) and 35000 ppm salinity (oil rejection: 92.5% and permeate flux: 94nL m−2 h−1) at 0.2 MPa trans-membrane pressure.

Polyethersulfone membranes prepared with Rhodiasolv®Polarclean as water soluble green solvent

Polyethersulfone (PESU) porous membranes with high pure water permeability were successfully produced by coupling non-solvent induced phase separation (NIPS) with vapor induced phase separation (VIPS) techniques by employing Rhodiasolv®Polarclean (Polarclean®) for the first time, as eco-friendly sustainable solvent. Membrane morphology and performance were tailored by varying the casting solution composition and the exposure time to controlled humidity and temperature. Polyvinylpyrrolidone (PVP) and poly(ethylene glycol) (PEG) were used as hydrophilic pore former agent and small-molecule liquid, respectively. The resulting membranes were characterized in terms of morphology, thickness, porosity, contact angle, mechanical features, pore size and pure water permeability. The obtained data indicated that the exposure time to humid air as well as the polymer and the PEG concentration in the casting solution represent the most relevant parameters to obtain hydrophilic membranes with different structure and properties. Both ultrafiltration (UF) and microfiltration (MF) membranes, with a pore size ranging from ~0.04 to ~0.4µm, were efficiently prepared by using the investigated novel solvent, offering the possibility to replace commonly used toxic diluents in polysulfones’ membrane fabrication.

PES-Kaolin Mixed Matrix Membranes for Arsenic Removal from Water.

The aim of this work was the fabrication and the characterization of mixed matrix membranes (MMMs) for arsenic (As) removal from water. Membrane separation was combined with an adsorption process by incorporating the kaolin (KT2) Algerian natural clay in polymeric membranes. The effects of casting solution composition was explored using different amounts of polyethersufone (PES) as a polymer, polyvinyl-pyrrolidone (PVP K17) and polyethylene glycol (PEG 200) as pore former agents, N-methyl pyrrolidone (NMP) as a solvent, and kaolin. Membranes were prepared by coupling Non-solvent Induced Phase Separation and Vapour Induced Phase Separation (NIPS and VIPS, respectively). The influence of the exposure time to controlled humid air and temperature was also investigated. The MMMs obtained were characterized in terms of morphology, pore size, porosity, thickness, contact angle and pure water permeability. Adsorption membrane-based tests were carried out in order to assess the applicability of the membranes produced for As removal from contaminated water. Among the investigated kaolin concentrations (ranging from 0 wt % to 5 wt %), a content of 1.25 wt % led to the MMM with the most promising performance.

TamiSolve® NxG as novel solvent for polymeric membrane preparation

Poly(vinylidene fluoride-hexafluoropropylene) P(VDF-HFP) porous membranes with a pore size ranging in the ultra- and micro-filtration range were successfully prepared for the first time by employing a Tamisolve® NxG as novel, non-reprotoxic solvent via non-solvent induced phase inversion. Membrane morphology and performance were tailored by acting on the casting solution composition, more specifically on the presence and the concentration of polyvinylpyrrolidone (PVP) and/or poly(ethylene glycol) (PEG) as pore former agents and by changing the polymer content in the dope solution. The resulting membranes were characterized in terms of morphology, thickness, porosity, contact angle, mechanical features, pore size, liquid entry pressure and pure water permeability (PWP). Direct Contact Membrane Distillation (DCMD) experiments were performed in order to evaluate the applicability of the novel membrane in water treatment by desalination operations. The obtained data indicated as both the additive and the polymer concentration in the casting solution played a crucial role for the final membrane structure and properties. Preliminary DCMD tests showed as the prepared P(VDF-HFP) membrane with a pore size comparable to that of a commercial polypropylene (PP), exhibited promising results in terms of permeate flux and salt rejection, opening new perspectives in the sustainable membrane technology.

A more sustainable membrane preparation using triethyl phosphate as solvent

Abstract In this work, hydrophobic poly(vinylidene fluoride) membranes for potential membrane distillation applications, with pore sizes ranging from ~ 0.05 to ~ 0.30 μm, were prepared by employing triethyl phosphate (TEP) as a substitute to the hazardous, commonly used, toxic solvents. Membrane morphologies and properties were tailored based on the main parameters, such as the casting solution composition and the operational conditions, which affect the phase inversion process. Experimental results showed that exposure time to controlled humid air and temperature, as well as additive content in the dope solution, strongly influence the membrane formation. The use of TEP as an alternative solvent may contribute to the development of sustainable separation operations.

The effect of casting solution composition on surface structure and performance of poly(vinylidene fluoride)/multi-walled carbon nanotubes (PVDF/MWCNTs) hybrid membranes prepared via vapor induced phase separation

Abstract In this work, the poly(vinylidene fluoride)/multi-walled carbon nanotubes (PVDF/MWCNTs) hybrid membranes were obtained through vapor induced phase separation (VIPS). In addition, the effects of solution composition on crystal behavior, elemental composite, morphology and hydrophilicity of membranes surface and the viscosity of casting solution were analyzed. The results of field emission scanning electron microscopy and X-ray photoelectron spectroscopy showed that the rough porous surface could be built in all hybrid membranes due to the slower double diffusion mechanism of the VIPS process, which also provided enough time for MWCNTs to move to the surface and be enriched there, to ensure the lowest surface free energy. Furthermore, other analysis displayed that both adding tiny amounts of MWCNTs and a low concentration of PVDF were favorable to the growth and aggregation of PVDF, which were put down to the adverse effect of hydrophobic MWCNTs on the thermodynamic compatibility of polar casting solution, the less nucleation points and the good fluidity with lower concentration of polymer. Then, the diameter of PVDF spherical particles and the membrane surface roughness increased, whereas the hydrophilicity of membrane surface declined effectively.