Non-solvent Induced Phase Separation Process Research Articles

Controlling the polymorphs of PVDF membranes: Effects of a salt additive on PVDF polarization during phase separation processes

Polyvinylidene fluoride (PVDF) membranes, acclaimed for their outstanding chemical resistance, thermal stability, and mechanical properties, play a crucial role in diverse industrial purification applications. While various methods have been employed for fabricating PVDF membranes such as thermally induced, evaporation-induced, vapor-induced, and nonsolvent-induced phase separation processes, controlling the polarity of PVDF chains using these techniques remains challenging. Recent studies have shown that the nonpolar α-phase PVDF is susceptible to organic acid fouling owing to hydrophobic interactions, whereas the β-phase PVDF has demonstrated polarity and offers antifouling properties. Herein, porous PVDF membranes predominantly composed of the β-phase were successfully fabricated by incorporating a small amount of LiCl salt into the PVDF dope solution. The addition of LiCl influences ion–dipole moments between ions and PVDF during phase separation, simultaneously affecting the PVDF chain arrangement and membrane morphology. Since the LiCl additive alters both the pore size and structure of PVDF membranes, both freestanding and non-woven supported membranes were prepared and characterized to assess the effects of polarity and pore size on fouling tendency. Characterizations of prepared PVDF membranes revealed considerable changes in the PVDF polymorph, membrane morphology, pore size, and antifouling properties upon introducing the salt additive. Our findings imply that β-phase–dominant PVDF membranes can be effectively utilized in water purification applications.

Facile Lithium Densification Kinetics by Hyperporous/Hybrid Conductor for High-Energy-Density Lithium Metal Batteries.

Lithium metal anode (LMA) emerges as a promising candidate for lithium (Li)-based battery chemistries with high-energy-density. However, inhomogeneous charge distribution from the unbalanced ion/electron transport causes dendritic Li deposition, leading to "dead Li" and parasitic reactions, particularly at high Li utilization ratios (low negative/positive ratios in full cells). Herein, an innovative LMA structural model deploying a hyperporous/hybrid conductive architecture is proposed on single-walled carbon nanotube film (HCA/C), fabricated through a nonsolvent induced phase separation process. This design integrates ionic polymers with conductive carbon, offering a substantial improvement over traditional metal current collectors by reducing the weight of LMA and enabling high-energy-density batteries. The HCA/C promotes uniform lithium deposition even under rapid charging (up to 5mA cm-2) owing to its efficient mixed ion/electron conduction pathways. Thus, the HCA/C demonstrates stable cycling for 200 cycles with a low negative/positive ratio of 1.0 when paired with a LiNi0.8Co0.1Mn0.1O2 cathode (areal capacity of 5.0mAhcm-2). Furthermore, a stacked pouch-type full cell using HCA/C realizes a high energy density of 344Whkg-1 cell/951WhL-1 cell based on the total mass of the cell, exceeding previously reported pouch-type full cells. This work paves the way for LMA development in high-energy-density Li metal batteries.

Formation of Polyimide Membranes via Non-Solvent Induced Phase Separation: Insight from Molecular Dynamics Simulations.

Molecular dynamics simulations were applied to investigate the formation of P84 polyimide membranes through the non-solvent induced phase separation (NIPS) process, considering two scenarios: one using a conventional organic solvent like n-methyl-2-pyrrolidone (NMP) and the other a greener alternative, γ-butyrolactone (GBL), with water serving as the non-solvent. Different compositions of polymer solutions were established along the binodal boundaries of the respective systems, derived from experimental cloud point data on the ternary phase diagram. The resulting polymer membranes were analyzed and compared in terms of their morphology. The wettability of their surfaces was notably affected by the polymer content in the initial casting solution and demonstrated a correlation with the Brunauer-Emmet-Teller (BET) specific surface area of the associated polymer nanostructures. The GBL solvent systems produced porous polymers qualitatively similar to those obtained with NMP, albeit with slightly narrower pore size distributions.

Efficient Dye Extraction from Wastewater Using Indium-MOF-Immobilized Polyvinylidene Fluoride Membranes with Selective Filtration for Enhanced Remediation.

Industrial activities have led to releasing harmful substances into the environment, necessitating the elimination of these toxic compounds from wastewater. Organic dyes, commonly found in industrial effluents, pose a threat to ecosystems and human health. Conventional treatment methods often suffer from limitations such as high cost and poor efficiency. Metal-organic frameworks (MOFs) have emerged as promising materials for selective separation, including membrane filtration (MF). Mixed-matrix membranes (MMMs) combining MOFs with polymers offer improved filtration properties. In this study, MMMs were fabricated by incorporating synthesized In-MOF with a polyvinylidene fluoride (PVDF) polymer (In-MOF@PVDF MMMs) using the nonsolvent-induced phase separation process. The MMMs were evaluated for the MF of various organic dyes, achieving notable removal efficiencies. The membrane containing 20% In-MOF (M4) demonstrated exceptional performance, removing 99% of the methylene blue (MB) dye. Additionally, membrane M4 effectively filtered Azure A (AZA), Azure B (AZB), and toluidine blue O (TOLO) with a removal efficiency of 99%. However, for Rhodamine B (RHB) and methyl orange (MO), the removal efficiencies were slightly lower at 74 and 39%, respectively. Further, these membranes are utilized in selective dye filtration in the MB+/RHB+ and MB+/MO- systems, where the selectivity was found for MB. The isothermal and DFT studies revealed the membrane's behavior with dye mixtures, while water stability and regeneration studies confirmed its durability. Thus, these findings highlight the potential of In-MOF@PVDF MMMs for effective and selective dye removal in wastewater treatment applications.

Waste treats waste: Facile fabrication of porous adsorbents from recycled PET and sodium alginate for efficient dye removal

Dye-contaminated water and waste plastic both pose enormous threats to human health and the ecological environment, and simultaneously solving these two issues in a sustainable and resource-saving way is highly important. In this work, a sodium alginate-polyethylene terephthalate-sodium alginate (SA@PET) composite adsorbent for efficient dye removal is fabricated using wasted PET bottle and marine plant-based SA via simple and energy-efficient nonsolvent-induced phase separation (NIPS) method. Benefiting from its porous structure and the abundant binding sites, SA@PET shows an excellent methylene blue (MB) adsorption capacity of 1081 mg g−1. The Redlich–Peterson model more accurately describes the adsorption behavior, suggesting multiple adsorption mechanisms. In addition to the electrostatic attractions of SA to MB, polar interactions between the PET matrix and MB are also identified as adsorption mechanisms. It is worth mentioning that SA@PET could be recycled 7 times without a serious decrease in performance, and the trifluoroacetic acid-dichloromethane solvent involved in the NIPS process has the possibility of reuse and stepwise recovery. Finally, the discarded adsorbent could be completely degraded under mild conditions. This work provides not only a composite adsorbent with excellent cationic dye removal performance for wastewater treatment, but also an upcycling strategy for waste PET.

Porous Structure of Polymer Films Optimized by Rationally Tuning Phase Separation for Passive All-Day Radiative Cooling.

Passive all-day radiative cooling (PARC) films with porous structures prepared via nonsolvent-induced phase separation (NIPS) have attracted considerable attention owing to their cost-effectiveness and wide applicability. The PARC performances of the films correlate with their porous structures. However, the porous structure formed using the NIPS process cannot be finely regulated. In this study, we prepared polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) films with porous structures optimized by rationally tuning the phase separation, which was achieved by adjusting the proportions of two good solvents with varying solubility parameters. The optimized PVDF-HFP film with a hierarchically porous structure exhibited a high solar reflectance of 97.7% and an infrared emissivity of 96.7%. The film with excellent durability achieved an average subambient cooling temperature of approximately 5.4 °C under a solar irradiance of 945 W·m-2 as well as a temperature of 11.2 °C at nighttime, thus demonstrating all-day radiative cooling. The results indicate that the proposed films present a promising platform for large-scale applications in green building cooling and achieving carbon neutrality.

Incorporating amphiphilic modifier into PVDF membrane through in-situ electrostatic crosslinking for efficient oil-water separation

Constructing stable amphiphilic membrane surfaces in a facile and controlled way is highly desired for fabrication of antifouling water purification membranes. Herein, inspired by adhesives of sandcastle worms, we proposed an in-situ electrostatic crosslinking method to incorporate amphiphilic modifiers into poly (vinylidene fluoride) membranes. Pluronic F127 grafted with positively charged quaternary ammonium groups are added into the casting solution, while perfluorosulfonic acid bearing negatively charged sulfonic acid groups and perfluorocarbon chains are dissolved in the coagulation bath. During non-solvent induced phase separation process, electrostatic crosslinking between quaternary ammonium and sulfonic acid led to the in-situ formation of amphiphilic modifiers on membrane surfaces and pore walls. The resulting membrane exhibits high water permeance up to 625 Lm−2h−1 bar−1 and superior antifouling performance with total flux decline ratio of 2 % and flux recovery ratio of 99.3 % for oil-water separation. Besides, the membrane displays excellent structural stability and self-cleaning properties against highly viscous crude oils.

Defect free pervaporation mixed matrix membranes fabricated on ZIF-8 surface segregation substrates

Mixed matrix membranes (MMMs), which incorporate porous particles into polymer matrix, were a promising approach to overcome the inherent trade-off effect of polymeric membranes. Controllable and easily scalable membrane fabrication strategies remained a challenge for the development of defect-free pervaporation MMMs. In this work, to avoid defects formed by particle exposure in MMMs, forced surface segregation of ZIF-8 on a substrate was achieved by vapor-assisted non-solvent induced phase separation (NIPS) process. Subsequently, ZIF-8 ‘islands’ on the substrate surface were sealed with PVA solution and defect-free dense MMMs were successfully prepared. The surface segregation of ZIF-8 was proved by scanning electron microscope (SEM), X-Ray diffraction (XRD) and fourier transform infrared (FTIR). The prepared MMMs showed an increase in flux of 162% and an increase in separation factor of 299% compared to the pristine membranes. It was expected that this strategy of inducing particles to undergo forced surface segregation could be extended to the preparation of other membranes.

Facile synthesis of ZIF-8 modified PES beads with improved sorption characteristics for elimination of Congo red from aquatic stream

The adsorption is a well know mass transfer operation that was employed for more than decades in water industry. Predominately carbon based, inorganic materials and MOF etc. have been widely employed as an adsorbent. In contrast in the present study polymeric nano beads incorporated with MOF were synthesized for efficient adsorption performances. The polymeric beads were obtained from Polyethersulfone (PES) and was incorporated with ZIF-8. In the inception phase, ZIF-8 incorporated PES polymeric beads were prepared through non-solvent induced phase separation (NIPS) process to discern the factors governing the adsorption mechanism. The zeta potential analysis showed that adding ZIF-8 into the PES matrix changed the charge from negative to positive. The ZIF-8@PES showed a maximum adsorption efficiency of 99.7% for CR dye removal compared to 14% of unmodified PES. The enhanced CR removal due to ZIF-8 addition was specifically due to π-π interactions between the linker and CR molecules, open Zn metal sites and electrostatic attraction. The sorption followed the Langmuir isotherm and Elovich kinetic model. The performance and regeneration studies showed that the modified beads were 91.1% efficient up to 4 cycles.

Insight into polydopamine coating on microfiltration membrane with controlled surface pore size for enhanced membrane rejection

Membrane modification with polydopamine (PDA) is a common practice to enhance membrane selectivity and improve membrane compatibility with nanoparticles or thin films. Depending on the membrane surface properties, dopamine can deposit or penetrate into microporous membranes, leading to alteration in the membrane's physicochemical properties and consequently its performance. In this study, microfiltration (MF) polyethersulfone membranes with varying surface pore sizes were fabricated by manipulating the precasting time (solvent evaporation time of 3s, 15s, 30s and 60s) prior to non-solvent induced phase separation (NIPS) process. Notably, the coating of PDA slightly reduced the membrane's surface pore size and membrane's porosity. M60 membrane which possessed the smallest mean surface pores (∼300 nm), exhibited PDA clusters predominantly formed on top of the membrane surface. With an increase in surface pore size, dopamine can penetrate the porous structure of the MF membrane, leading to the polymerization of dopamine into PDA aggregates on both the membrane surface and inner pores. The M30 membrane had the largest surface pore size (∼500 nm), showing the formation of large PDA globules in the finger-like pores. The membrane (M30P) experienced significant narrowing of the inner pores, resulting in reduced water permeability. Overall, all PDA-coated membranes exhibited improved sucrose rejection (85–95%) compared to the uncoated ones (64–83%). These finding provides insight into the polymerization mechanism of dopamine on porous membranes prior to the incorporation of nanomaterials or thin film coating.

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.

Experimental phase diagram for the PVDF – DMAc– water ternary system with new topology: Method of construction, thermodynamics, and structure formation of membranes

Ternary mixtures of a semicrystalline (SC) polymer with a solvent and a nonsolvent are widely used primarily for preparation of membranes by nonsolvent induced phase separation (NIPS). A detailed analysis of the published studies devoted to phase equilibrium in such systems showed that the phase diagrams were either incomplete or presented in a schematic form. In the present paper, we propose a new method of plotting the ternary phase diagrams for the SC polymer – solvent – nonsolvent mixtures. This method is based on finding the location of the isotherm intersection with the curves on the temperature–composition pseudobinary phase diagrams for the SC polymer mixtures with solvent and binary mixtures of solvent and nonsolvent of different composition. The coordinates of the intersection points were used to plot for the first time the complete ternary phase diagram for the widely used mixture of polyvinylidene fluoride (PVDF) with dimethyl acetamide (DMAc) and water. The curves on the phase diagram separate the regions of homogeneous mixtures, coexistence of two liquids, macroscopically uniform gels (swollen SC polymer) and coexistence of such gels with liquid. Using the plotted phase diagram, a new understanding of the NIPS mechanism was proposed and confirmed by direct microscopic observation of the NIPS process and SEM analysis of the membranes obtained following this way. An unexpected result of the study is discovery of the crystalline complexes in the mixtures of PVDF and DMAc, which was confirmed using both DSC and XRD techniques.

Design of Silicon Rubber/BN Film with High Through-plane Thermal Conductivity and Ultra-low Contact Resistance

Polymer-hexagonal boron nitride (BN) composite has become an ideal thermal interface material (TIM) for electronic devices because of its high thermal conductivity and superior electronic insulation. However, owing to the 2D shape and chemical inertness of BN filler, the vertical alignment of BN and the huge thermal resistance are current challenges, which hinder the efficient heat transfer of polymer/BN composites. Herein, by a novel non-solvent induced phase separation process combined “in-situ welding” strategy, we present the fabrication of silicone rubber film with finger-like continuous BN-welded filler skeleton, which reveals a high through-plane thermal conductivity of 15.4 W m−1K−1 at only ∼ 15 wt% BN. Finite element simulation and nonlinear model analyses theoretically confirm that the filler-to-filler interfacial thermal resistance (ITR) is halved after in-situ welding process. In addition, thanks to the excellent compressibility and conformability of silicon rubber matrix, the contact thermal resistance (<70 Kmm2W−1) of this composite film is much lower than that of the commercial thermal pad under different pressure. The proposed strategy opens up a novel and high-throughput preparation strategy for the high-performance TIM for modern electronic devices.

Nonsolvent-induced phase separation inside liquid droplets.

Nonsolvent-induced phase separation (NIPS) is a popular method for creating polymeric particles with internal microstructure, but many fundamental questions remain surrounding the kinetics of the complex coupled mass transfer and phase separation processes. In this work, we use simulations of a phase-field model to examine how (i) finite domain boundaries of a polymer droplet and (ii) solvent/nonsolvent miscibility affect the NIPS process. To isolate the effects of phase separation kinetics and solvent/nonsolvent mass transfer on the NIPS process, we study two different cases. First, we investigate droplet concentrations that originate inside the two-phase region, where phase separation kinetics alone governs the microstructure. Second, we investigate the effects of solvent/nonsolvent mass transfer by studying droplet concentrations that begin outside the two-phase region, where both phase separation kinetics and mass transfer play a role. In both cases, we find that qualitative NIPS behavior is a strong function of the relative location of the initial droplet composition with respect to the phase diagram. We also find that polymer/nonsolvent miscibility competes with solvent/nonsolvent miscibility in driving NIPS kinetic behavior. Finally, we examine polymer droplets undergoing solvent/nonsolvent exchange and find that the model predicts droplets that shrink with nearly Fickian diffusion kinetics. We conclude with a brief perspective on the state of simulations of NIPS processes and some recommendations for future work.

In situ observation of the structure and dynamics of a polymer solution through nonsolvent-induced phase separation by x-ray photon correlation spectroscopy

Nonsolvent-induced phase separation (NIPS) is a complex but important phenomenon involved in the manufacture of fibers and other polymeric industrial products. We performed in situ x-ray photon correlation spectroscopy (XPCS) measurements on a cuprammonium cellulose solution immersed in aqueous acetone solution to reveal the structure and dynamics involved in the NIPS process on the microscopic scale. A combination of the structural information from the small angle x-ray scattering profiles and the dynamic information from the XPCS analysis revealed that NIPS occurs via two steps: dilution of the polymer solution by the solvent and desorption of the solvent from the polymer solution. This study will provide an opportunity for the fundamental understanding of NIPS, which is important from both the scientific and industrial viewpoints.

ZIF-67 blended PVDF membrane for improved Congo red removal and antifouling properties: A correlation establishment between morphological features and ultra-filtration parameters

Dye effluents from various sectors have constantly imperilled the environment and ecosystem. Nano-composite membrane technology incorporating metal-organic frameworks (MOFs) has shown tremendous potential for toxic pollutant remediation. This study details the impact of ZIF-67 MOF nanoparticles on the structural properties of polyvinylidene fluoride (PVDF) ultrafiltration membrane during the non-solvent induced phase separation (NIPS) process. In order to outline the properties that determine the performance parameters in a MOF-modified mixed matrix membrane, the corresponding changes in mean pore size (MPS), surface porosity, solvent viscosity, and hydrophilicity have been discussed with appropriate surface characterization analysis. The suitability of ZIF-67 as filler nanoparticles were established based on polymer compatibility, dispersibility, and water stability studies. The ZIF-67 incorporated PVDF mixed matrix membranes (MMM) showed 99.5% CR dye removal with 2.6 times DI water permeability than the neat. The flux recovery ratio (FRR) improved by 1.9 times and the membranes were found suitable for up to 5 filtration cycles. Based on the overall results, a correlation analysis between the MMM surface properties and membrane performance parameters were established to determine the key performance parameters. It was observed that in comparison to MPS, surface porosity was more correlated to Jd/Jw (r = 0.96) and FRR (r = 0.95).

Fine pore tailoring of PSf-b-PEG membrane in sub-5 nm via phase-inversion

It is highly important but challenging to fabricate polymeric separation membranes with pore size in sub-5 nm via the non-solvent induced phase separation (NIPS) process. Herein, we reported a simple but effective method to precisely tailor the pore size of polysulfone-block-polyethylene glycol (PSf-b-PEG) membrane in sub-5 nm via NIPS. Selective solvents of tetramethylene sulfone (Ts) and tetrahydrofuran (THF) towards the corresponding PEG block and PSf block in PSf-b-PEG were mixed at various ratios to tailor the casting solution properties and to finely tailor the pore size of the PSf-b-PEG membranes. The membrane formation mechanism was also investigated to reveal that the PSf-b-PEG membrane exhibited intertwined and fused nanofibrous structure with relatively high porosity and extremely strong mechanical strength. As the content of evaporative THF in the casting solution increased from 42.5 wt% to 85.0 wt%, the MWCO of the PSf-b-PEG membranes decreased from 127 kDa to 6.6 kDa, and the mean effective pore size of the membranes shrank from 15 nm to 2 nm, greatly exceeding the tailorable range and the minimum threshold of the pore size of PSf and PES membranes. Impressively, even if the pore size of the membrane was 2 nm, the pure water permeance of the membrane was still as high as 7 L m−2 h−1 bar−1, indicating the great potential of the PSf-b-PEG membranes for practical applications.

High flux zwitterionic hybrid cellulose acetate desalination membranes: Formation of permeability-effective macropores by self-organizing zwitterionic silanes

Zwitterionic hybrid cellulose acetate membranes with improved flux and antifouling performance were produced by a modified non-solvent induced phase separation (NIPS) process in which a zwitterionic trimethoxysilane compound, (3-sulfopropylbetaine-propyl)-trimethoxysilane (SPPT), was added to the casting solutions at 1.0, 2.0, and 3.0 wt%. The successful incorporation of SPPT into cellulose acetate (CA) membranes was proved by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and thermal gravimetric analysis (TGA). Permeate fluxes of the hybrid membranes increased significantly compared to the control cellulose acetate membrane prepared without the addition of SPPT (15 bar pressure; 2000 ppm NaCl or MgSO4 aqueous solution filtration). The hybrid membranes exhibited an increase in permeate flux up to 87 % while their salt rejections slightly decreased in NaCl solution filtration. In MgSO4 solution filtration, the flux was increased by 61 % without sacrificing salt rejection. The reason for the enhancement in the flux was the formation of macropores in the membrane bulk structures by self-organizing zwitterionic silane groups. The highest resistance to bovine serum albumin (BSA) or xanthan gum as a foulant was observed in the hybrid membranes produced with 1.0 wt% of SPPT.

Highly stretchable porous regenerated silk fibroin film for enhanced wound healing.

Silk fibroin (SF) has received interest in tissue engineering owing to its biocompatibility, biodegradability, and favorable mechanical properties. However, the complex preparation, brittleness, and lack of pores in the structure of the silk fibroin film limit its application. Herein, we show that facile dissolution of SF in aqueous phosphoric acid followed by regeneration in aqueous ammonium sulfate ((NH4)2SO4) could afford highly stretchable films with nano-pores formed in the nonsolvent-induced phase separation process. The named phase separation, which determines the morphology and mechanical properties of the regeneration silk fibroin (RSF) films, is highly dependent on the (NH4)2SO4 concentration as well as the initial concentration of the SF solution. Therefore, the RSF films exhibit a tunable pore size ranging from 230 to 510 nm and excellent stretchability with tensile strain up to 143 ± 16%. Most interestingly, the RSF films were shown to support the proliferation of human skin fibroblasts in vitro as well as speed up full-thickness skin wound healing in a rat model. This work establishes an easy and feasible method to access porous RSF membranes that can be used for wound dressing in clinical settings.

Towards superior permeability and antifouling performance of sulfonated polyethersulfone ultrafiltration membranes modified with sulfopropyl methacrylate functionalized SBA-15

A non-solvent induced phase separation (NIPS) process was used to fabricate a series of sulfonated polyethersulfone (SPES) membranes blending with different concentrations of SBA-15-g-PSPA with the applications in the ultrafiltration (UF) process. SBA-15 was modified with 3-methacrylate-propyltrimethoxysilane (MPS) to form SBA-15-g-MPS. It was further modified with the charge tailorable polymer chains by reacting with 3-sulfopropyl methacrylate potassium salt. The nanoparticles were uniformly dispersed and finger-like channels were developed within the membrane. The adding of surface modified SBA-15-g-PSPA nanoparticles has significantly improved membrane water permeability, hydrophilicity, and antifouling properties. The pure water fluxes of the composite SPES membranes were significantly higher than the pristine SPES membrane. For the membrane containing 5% (mass) of SBA-15-g-PSPA (MSSPA5), the pure water flux was increased dramatically to 402.15 L∙m –2 ∙h –1 , which is ∼ 1.5 times that of MSSPA0 (268.0 L∙m –2 ∙h –1 ). The high flux rate was achieved with 3% (mass) of SBA-15 nanoparticles with retained high rejection ratio 98% for natural organic matter. The results indicate that the fashioned composite membrane comprising SBA-15-g-PSPA nanoparticles have a promising future in ultrafiltration applications.