Rational design of phase inversion membranes by tailoring thermodynamics and kinetics of casting solution using polymer additives

Thermodynamic Modeling and Validation of the Temperature Influence in Ternary Phase Polymer Systems

The effect of the temperature, as a process variable in the fabrication of polymeric membranes by the non-solvent induced phase separation (NIPS) technique, has been scarcely studied. In the present work, we studied the influence of temperature, working at 293, 313 and 333 K, on the experimental binodal curves of four ternary systems composed of PVDF and PES as the polymers, DMAc and NMP as the solvents and water as the non-solvent. The increase of the temperature caused an increase on the solubility gap of the ternary system, as expected. The shift of the binodal curve with the temperature was more evident in PVDF systems than in PES systems indicating the influence of the rubbery or glassy state of the polymer on the thermodynamics of phase separation. As a novelty, the present work has introduced the temperature influence on the Flory-Huggins model to fit the experimental cloud points. Binary interaction parameters were calculated as a function of the temperature: (i) non-solvent/solvent (g₁₂) expressions with UNIFAC-Dortmund methodology and (ii) non-solvent/polymer (χ₁₃) and solvent/polymer (χ₂₃) using Hansen solubility parameters. Additionally, the effect of the ternary interaction term was not negligible in the model. Estimated ternary interaction parameters (χ₁₂₃) presented a linear relation with temperature and negative values, indicating that the solubility of the polymers in mixtures of solvent/non-solvent was higher than expected for single binary interaction. Finally, PES ternary systems exhibited higher influence of the ternary interaction parameter than PVDF systems.

Probing protein rejection behavior of blended PES-based flat-sheet ultrafiltration membranes: A density functional theory (DFT) study

Membrane fouling is a common problem in membrane technology and causes detrimental effects for the applied membranes such as loss of integrity and productivity. Henceforward, we devoted this work to fabricate membranes that pose favored criteria in the direction of alleviating membrane fouling incidence. Herein, the fabricated membranes were traced via an assortment of both experimental and molecular modeling verifications to understand the mechanism of interaction. To do so, firstly, three different ultrafiltration (UF) membranes had been prepared via facile wet phase inversion method thru dipping a casting solution composed of polyethersulfone-polyvinyl pyrrolidone (PES-PVP) and polyethersulfone-Pluronic P31R1 (PES-P31R1) in a water coagulation bath. Regarding the practical-based data, the pristine PES membrane exhibited the highest rejection of bovine serum albumin (BSA) protein (model foulant) compared with the modified PES-based membranes. The membrane chemical compositions were elucidated with ATR-FTIR Spectroscopy. On the other hand, molecular modeling has been carried out via calculating thermodynamic parameters, level parametric method, and density functional theory (DFT). Thermodynamic parameters analysis indicated that the noticeable difference of BSA rejection may be ascribed to different entropy behavior for the fabricated membranes. In addition, the level parametric method (PM6) and density functional theory DFT: B3LYP with 6-31g (d,p) basis set models clarified the interaction manner of BSA molecules to membrane surfaces.

Novel behavior in a polymer solution: the disappearance of the melting temperature (Tm) and enthalpy change (ΔHm) of the solvent

The phase change temperature and enthalpy change as a function of polystyrene (PS) concentration in dimethylformamide through a dynamic heating and quenching process were investigated. Cold crystallization, freezing and melting phenomena in a 10 wt% PS solution were all observed. Cold crystallization and melting phenomena were still observed in a 20 wt% solution. In a 30 wt% solution, all three phenomena disappeared without any solvent enthalpy changes, e.g., enthalpy changes at the melting temperature. The disappearance of both the melting temperature and the melting enthalpy change indicated that all polymer and solvent molecules in the 30 wt% solution existed only in the amorphous phase without any phase changes despite repeated heating and quenching processes. Thus, our results can provide a new approach for gelation through enthalpy changes and can be applied in the fabrication of porous membranes with a narrow distribution.

Lithium chloride (LiCl)-modified polyethersulfone (PES) substrate surface pore architectures on thin poly(dimethylsiloxane) (PDMS) dense layer formation and the composite membrane's performance in gas separation

The use of pore forming agents has been notable for improving the water flux in a water-based separation membrane but are rarely being studied as a methodology to influence the substrate's surface architectures for composite membrane fabrication in gas separation. In this study, the influence of lithium chloride (LiCl) on the surface pore architectures and hence, the gas permeance, has been studied in both bare and composite forms with poly(dimethylsiloxane) (PDMS). 1-4 wt% of LiCl was mixed with the dope solution of PES/NMP in the ratio 0.19 and was casted via the dry-wet phase inversion method. Bare substrates were noted to possess increasingly larger surface pore sizes but at a diminishing surface pore density with maximum surface porosity at 2 wt% LiCl. The permeances were, however, significantly reduced with the increase in the LiCl content from 105 300 to 4300 GPU for N₂ gas, presumably due to the thicker skin layer. Nevertheless, the porous surface morphology was confirmed and exhibited Knudsen selectivity with a CO₂/N₂ selectivity of about 0.8, signifying minimal gas flow resistance by the substrates. Upon coating with a similar amount of thin PDMS layer, the composite permeances retain the same trend with values from 361.9 GPU for 0 wt% LiCl substrates to 68.8 GPU for 4 wt% LiCl substrates for CO₂ gas at a consistent selectivity of about 14. As the PDMS layer of the same volumes were used and no significant difference in the coating thickness was noted, the mixed influence of pore intrusion and lateral diffusion is hypothesised at the substrate-coating interface owing to the different surface pore architectures of the substrates.

Durability and Recoverability of Soft Lithographically Patterned Hydrogel Molds for the Formation of Phase Separation Membranes

Hydrogel-facilitated phase separation (HFPS) has recently been applied to make microstructured porous membranes by modified phase separation processes. In HFPS, a soft lithographically patterned hydrogel mold is used as a water content source that initiates the phase separation process in membrane fabrication. However, after each membrane casting, the hydrogel content changes due to the diffusion of organic solvent into the hydrogel from the original membrane solution. The absorption of solvent into the hydrogel mold limits the continuous use of the mold in repeated membrane casts. In this study, we investigated a simple treatment process for hydrogel mold recovery, consisting of warm and cold treatment steps to provide solvent extraction without changing the hydrogel mold integrity. The best recovery result was 96%, which was obtained by placing the hydrogel in a warm water bath (50 °C) for 10 min followed by immersing in a cold bath (23 °C) for 4 min and finally 4 min drying in air. This recovery was attributed to nearly complete solvent extraction without any deformation of the hydrogel structure. The reusability of hydrogel can assist in the development of a continuous membrane fabrication process using HFPS.

Electrohydrodynamic Patterning of Polyethersulfone Membranes

Microstructuring the surface of membranes is recognized as one of the effective strategies to mitigate the fouling phenomenon. Over the years, significant efforts have been undertaken to develop new techniques for altering the membrane surface topography at the micro- and nanoscale. However, all the previously suggested approaches suffer from some serious drawbacks that impede their widespread implementations, including cost, time, and cumbersomeness. In this study, we show that the electrohydrodynamic (EHD) patterning process can be successfully adopted to form surface patterns on polyethersulfone (PES) microfiltration membranes. The linear stability analysis and nonlinear numerical simulation are performed to theoretically predict the size of the created raised columnar structure (often called pillars). In contrast to the conventional EHD patterning process, the developed method works at room temperature and nonsolvent-induced phase separation is used to solidify the formed structures. An array of pillars was formed on the membrane surface, whose height and width were found to be as low as 31 ± 5 and 98 ± 12 μm, respectively. It is demonstrated that fabricating surface-patterned PES membranes does not require sophisticated facilities and precise control of process condition using this simple moldless method.

Robust Polymer Nanocomposite Membranes Incorporating Discrete TiO2 Nanotubes for Water Treatment

Polyethersulfone (PES) is a polymeric permeable material used in ultrafiltration (UF) membranes due to its high thermomechanical and chemical stability. The hydrophobic nature of PES membranes renders them prone to fouling and restricts the practical applications of PES in the fabrication of water treatment membranes. The present study demonstrates a non-solvent-induced phase separation (NIPS) approach to modifying PES membranes with different concentrations of discrete TiO₂ nanotubes (TNTs). Zeta potential and contact angle measurements showed enhanced hydrophilicity and surface negative charge in TNTs/PES nanocomposite membranes compared to unmodified PES membranes. To discern the antifouling and permeation properties of the TNTs/PES membranes, steam assisted gravity drainage (SAGD) wastewater obtained from the Athabasca oil sands of Alberta was used. The TiO₂ modified polymer nanocomposite membranes resulted in a higher organic matter rejection and water flux than the unmodified PES membrane. The addition of discrete TNTs at 1 wt% afforded maximum water flux (82 L/m² h at 40 psi), organic matter rejection (53.9%), and antifouling properties (29% improvement in comparison to pristine PES membrane). An enhancement in fouling resistance of TNTs/PES nanocomposite membranes was observed in flux recovery ratio experiments.

In situ-generated yttrium-based nanoparticle/polyethersulfone composite adsorptive membranes: Development, characterization, and membrane formation mechanism

In this study, a series of in situ-generated yttrium-based nanoparticle (NP)/polyethersulfone (PES) composite adsorptive membranes were prepared by the phase inversion method for the first time. The Y(NO₃)₃·6H₂O as precursor, uniformly dispersed at the molecular level in casting solution, reacted with OH^- in a coagulation bath and ambient CO₂ during the phase inversion process. The Y(CO₃)_0.5(OH)₂ NPs were formed in situ and distributed homogeneously in a PES matrix, which was confirmed by X-ray photoelectron spectroscopy (XPS) and Energy Dispersive X-Ray Spectroscopy (EDS) results. The compatibility of the nanocomposite membranes was improved by an in situ preparation method. With the increase in content of Y-based NPs in composite membranes, the surface hydrophilicity and water permeability first increased from M1 to M2, and then slightly decreased from M3 to M5, which was mainly related to membrane structure. From M1 to M5, the demixing way changed from instantaneous demixing to delayed demixing process as a result of thermodynamic enhancement and viscosity hindrance in the phase inversion process. A higher demixing rate led to a structure with large finger-like macro-voids, i.e., M1, whereas a lower demixing rate caused the suppression of finger-like macro-voids, i.e., M5. More importantly, the adsorption study indicated that the nanocomposite adsorptive membranes were stable in the treatment of fluoride-containing water, with no leakage of Y-based NPs from membrane matrix to solution. It is expected that the in situ preparation technique could be used to produce next-generation nanocomposite adsorptive membranes with improved comprehensive properties for application in water treatment.

Utilization of DES-Lignin as a Bio-Based Hydrophilicity Promoter in the Fabrication of Antioxidant Polyethersulfone Membranes

Enhancement of membrane permeability at no detriment of its other performances, e.g., selectivity, is a goal-directed objective in membrane fabrication. A novel antioxidant DES-lignin (lignin extracted from birch wood by using a deep eutectic solvent) polyethersulfone (PES) membrane, containing 0–1 wt % DES-lignin, was fabricated with the phase inversion technique. The performance and morphology of the fabricated membranes were characterized by a pure water flux, polyethylene glycol (PEG) retention, Fourier transform infrared spectroscopy, scanning electron microscopy, and contact angle measurements. Membranes with less negative charge and better hydrophilicity were obtained when the DES-lignin content in the polymer solution was increased. With the highest dosage, the incorporation of DES-lignin in the membrane matrix improved the membrane permeability by 29.4% compared to a pure PES membrane. Moreover, no leakage of DES-lignin from the membrane structure was observed, indicating good compatibility of DES-lignin with the PES structure. It was also found that the improvement of both rejection and pure water flux could be achieved by using a small dosage of DES-lignin (0.25 wt %) in membrane fabrication. The membranes incorporated with DES-lignin showed higher DPPH (2,2-diphenyl-1-picrylhydrazyl) and ABTS (2,2-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid)) scavenging activity compared to the pure membrane, where 2.6 and 1.1 times higher DPPH and ABTS scavenging activity was observed with the highest DES-lignin content (1 wt %). Thus, the results of this study demonstrate well the feasibility of utilizing DES-lignin as an antioxidant bio-based hydrophilicity promoter in the fabrication of ultrafiltration membranes.