Self-assembly And Non-solvent Induced Phase Separation Research Articles

Modular Synthesis of Functional Block Copolymers by Thiol–Maleimide “Click” Chemistry for Porous Membrane Formation

In this work, a modular strategy for the synthesis of block copolymers (BCPs) is presented using the quantitative reaction of a maleimide with a thiol. For this purpose, anionic polymerization and atom transfer radical polymerization were used to generate end-functionalized homopolymers. For proof of concept, polystyrene-b-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA) BCPs with different molecular weights and segment ratios were synthesized, leading to high molecular weights of 128 kg mol–1 with a polydispersity index of 1.18. Additionally, the synthesis of polystyrene-b-poly(2-aminoethyl methacrylate) and polystyrene-b-poly(2-hydroxyethyl methacrylate)-b-poly(2,2,2-trifluoroethyl methacrylate) terpolymer was investigated. The access of PS-b-PHEMA to isoporous integral membranes by the self-assembly and non-solvent-induced phase separation (SNIPS) process was confirmed by scanning electron microscopy. The produced SNIPS membranes featured a maximum water flux of 608 L bar–1 h–1 m–2 and a maximum transmembrane pressure of 2.7 bar. This modular synthesis system demonstrates the utility of scalable SNIPS membrane formation for biomedical and water filtration applications.

Modeling Microstructure Formation in Block Copolymer Membranes Using Dynamical Self-Consistent Field Theory.

Block copolymers have attracted recent interest as candidate materials for ultrafiltration membranes, due to their ability to form isoporous integral-asymmetric membranes by the combined processes of self-assembly and nonsolvent-induced phase separation (SNIPS). However, the dependence of surface layer and substructure morphologies on the processing variables associated with SNIPS is not well understood nor is the interplay between microphase and macrophase separation in block copolymers undergoing such coagulation. Here, we use dynamical self-consistent field theory to simulate the microstructure evolution of block copolymer films during SNIPS and find that such films form the desired sponge-like asymmetric porous substructure only if the solvent and nonsolvent have opposite block selectivities and that otherwise they form a dense nonporous microphase-separated film. Our results could have important implications for the choices of solvent and nonsolvent in the processing of block copolymer membranes.

Double stimuli-responsive isoporous block copolymer membranes upon phase separation strategies

Isoporous membranes from block copolymer commonly involves in the self-assembly and non-solvent induced phase separation (SNIPS), which could improve the selectivity and water permeance. However, such strategy face challenges: (1) to prepare poly (acrylic acid) PAA based block copolymer isoporous membrane with SNIPS method is difficult since PAA is extremely hydrophilic; (2) large consumption of block copolymer and non-solvent (water) might limit its application. Herein, isoporous membranes from Polystyrene-block-poly (acrylic acid) (PS-b-PAA) were well prepared with amine terminated poly(N-isopropylacrylamide) (pNIPAM-NH2) through the SNIPS and the airflow accelerated evaporation induced phase separation methods (acEIPS). Such membranes bear not only regularity in morphological structure, but also high water permeance (1500 L h−1 m−2 bar−1 for the SNIPS, and 3000 L h−1 m−2 bar−1 for the acEIPS). With the participation of pNIPAM-NH2 the consumption of the block copolymer is limited to 7.8 wt % for the SNIPS method, and even to 5.3 wt % for the acEIPS method without non-solvent precipitation process. Besides, the membrane exhibits pH and thermal responsiveness.

Asymmetric block copolymer membrane fabrication mechanism through self-assembly and non-solvent induced phase separation (SNIPS) process

In this paper, the concept of the functional mechanism of copolymer membrane formation is explained and analyzed from the theoretical and experimental points of view. To understand the phase inversion process and control the final membrane morphology, styrene-acrylonitrile copolymer (SAN) membrane morphology through the self-assembly phenomena is investigated. Since the analysis of the membrane morphology requires the study of both thermodynamic and kinetic parameters, the effect of different membrane formation conditions is investigated experimentally; In order to perceive the formation mechanism of the extraordinary structure membrane, a thermodynamic hypothesis is also developed based on the hydrophilic coil migration to the membrane surface. This hypothesis is analyzed according to Hansen Solubility Parameters and proved using EDX, SAXS, and contact angle analysis of SAN25. Moreover, the SAN30 membrane is fabricated under different operating conditions to evaluate the possibility of morphological prediction based on the developed hypothesis.

Nanoporous Block Copolymer Membranes with Enhanced Solvent Resistance Via UV-Mediated Cross-Linking Strategies.

In this work, a block copolymer (BCP) consisting of poly((butyl methacrylate-co-benzophenone methacrylate-co-methyl methacrylate)-block-(2-hydroxyethyl methacrylate)) (P(BMA-co-BPMA-co-MMA)-b-P(HEMA)) is prepared by a two-step atom-transfer radical polymerization (ATRP) procedure. BCP membranes are fabricated applying the self-assembly and nonsolvent induced phase separation (SNIPS) process from a ternary solvent mixture of tetrahydrofuran (THF), 1,4-dioxane, and dimethylformamide (DMF). The presence of a porous top layer of the integral asymmetric membrane featuring pores of about 30nm is confirmed via scanning electron microscopy (SEM). UV-mediated cross-linking protocols for the nanoporous membrane are adjusted to maintain the open and isoporous top layer. The swelling capability of the noncross-linked and cross-linked BCP membranes is investigated in water, water/ethanol mixture (1:1), and pure ethanol using atomic force microscopy, proving a stabilizing effect of the UV cross-linking on the porous structures. Finally, the influence of the herein described cross-linking protocols on water-flux measurements for the obtained membranes is explored. As a result, an increased swelling resistance for all tested solvents is found, leading to an increased water flux compared to the pristine membrane. The herein established UV-mediated cross-linking protocol is expected to pave the way to a new generation of porous and stabilized membranes within the fields of separation technologies.

Ordered isoporous membranes from ionic diblock copolymers via SNIPS: Optimizing effective factors with a structural survey

The membrane formation directly through the self-assembly and nonsolvent induced phase separation (SNIPS) method without the aid of foreign additives has been more challenging in the case of ionic block copolymers, like polystyrene-block-poly (acrylic acid) (PS-b-PAA), in comparison with non-ionic amphiphilic systems. Herein, the influence of governing factors on the structure tuning, including physicochemical characteristics of polymer, solvent and nonsolvent, casting parameters and process conditions to prepare additive-free and ordered isoporous membranes was studied. Due to the high degree of ionization and hydrophilicity of PAA blocks, micellization in the form of “reverse micelles” was aimed to induce more stable micelles assembly. The well-ordered micelles assembly was generated by the suitable selection of solvent (1,4-dioxane), strongly segregated characteristic of PS-b-PAA (high χN) and proper adjustment of casting parameters including polymer concentration and solvent evaporation rate and time. The uniformity and regularity of the micelles assembly during phase inversion step was preserved owing to the high freezing point of solvent. Highly permeable asymmetric isoporous membranes prepared over a wide range of evaporation times (40–120 s) using relatively low polymer concentrations (15 wt%) with adjustable and monodispersed pore-size in the range of sub-10 up to 22 nm. FESEM and AFM images in addition to DLS measurements and solution SAXS analysis were applied to demonstrate the hexagonal close-packed (HCP) structure of micelles assembly and partially hexagonal arrangement of pores at the membrane selective-layer. It was also demonstrated that corona (PS) of adjacent micelles on the membrane surface fused together to form a continuous membrane matrix, while surface-pores were the nanoscale voids, formed between the densely packed spherical reverse micelles practically immobilized by immersion in low-temperature water bath. Such additive-free, asymmetric, iso- and nano-porous membranes have attractive potential practical applications in biomedical and pharmaceutical industries for size-based separations, in addition to their conventional applications in water purification.

Molecular tailoring of polystyrene-block-poly (acrylic acid) block copolymer toward additive-free asymmetric isoporous membranes via SNIPS

Polystyrene-block-poly (acrylic acid) (PS-b-PAA) diblock copolymers with different compositions were systematically synthesized by the reversible addition-fragmentation chain transfer (RAFT) polymerization and used to prepare asymmetric isoporous membranes directly through self-assembly and nonsolvent induced phase separation (SNIPS) process, without using foreign additives. A simple and efficient way, for the first time, to generate additive-free asymmetric isoporous PS-b-PAA membranes was enabled just by choosing the right blocks composition. Dynamic light scattering (DLS) measurements at dilute concentrations and solution small angle X-ray scattering (solution SAXS) analysis plus rheological measurements in concentrated regime as well as field emission scanning electron microscopy (FE-SEM) were employed to investigate the relationship between properties of different block copolymer casting solutions and final membrane structures. DLS measurements and FE-SEM micrographs revealed that only micellar solutions with adequate micelle dimensions were able to produce membranes with more uniform surface pores. Micelles with short PS- lengths led to the formation of mesostructures with adequate structural characteristics in the casting solution, whereas rheological characteristics were the worst. A combination of both appropriate structural and rheological characteristics for the casting solution was necessary to achieve perfect membrane structures with ordered, high density and mono size-dispersed surface pores that all were dependent on PS/PAA blocks ratio. In addition to surface pores, membrane substructures were also influenced by the PS- block length. Sublayers with mostly finger-like structure along with higher permeability were obtained for the block copolymer with adequate PS- block length. Although dispersity (Ð) value of the synthesized block copolymers was acceptable (≤1.2) to form well-defined ordered isoporous structures, a slight difference in Ð had a decisive role on micellization, casting solution characteristics and final membrane structures. The most appropriate membrane structure showed a ≈150 nm thin selective-layer with ≈14 nm mean pore diameter and relatively high pore density of about 4 × 1014 pores/m2 at top surface, supported by the asymmetric substructure composed of a finger-like upper layer (66% of total thickness) and interconnected open pores at underneath. Considering its free-additive nature and high performances, these PS-b-PAA membranes are promising candidates for a wide range of size-based separation applications including biomedical and pharmaceutical industries.

Preparation of isoporous membranes from low χ block copolymers via co-assembly with H-bond interacting homopolymers

Self-assembly and non-solvent induced phase separation (SNIPS) was extensively used in the fabrication of isoporous membranes for its compatibility with massive production. However, for the well-defined SNIPS, preparation of isoporous membranes from low χ block copolymers is challenging because self-assembly of these polymers is usually hard to induce. In this work, we showed that such a problem could be effectively overcome through the co-assembly strategy by blending with homopolymers that interacted with block copolymers (BCPs). For the polystyrene-block-poly(N-isopropylacrylamide) (PS-b-PNIPAM) which has relatively small Flory-Huggins interaction parameter (χ ≈ 0.05), the formation of the isopores was readily induced when polyacrylic acid (PAA) was added as an additive. Three PS-b-PNIPAM with the PNIPAM mass fractions ranging from 5.9 to 36.5 wt% were used as membrane-forming materials, and three PAA with different molecular weights (MWs) were used to investigate how MWs of homopolymer affected the isopore formation. The selective H-bonding of PAA with PNIPAM was in-situ determined by infrared spectroscopy, which was further supported by the small angle X-ray scattering that indicated the microphase separation of PS-b-PNIPAM in solution was remarkably enhanced by PAA. The contribution of PAA on isopore formation was also found for the casting solutions of a wide range of concentrations and different solvent compositions. Our finding showed herein actually provided a facile route to prepare isoporous membranes from low χ BCPs under general conditions.

Structure of Nonsolvent‐Quenched Block Copolymer Solutions after Exposure to Electric Fields during Solvent Evaporation

AbstractIn this work, an electric field is incorporated during the solvent evaporation step of the block copolymer self‐assembly and nonsolvent induced phase separation (SNIPS) process. This study investigates the structure of nonsolvent‐quenched polystyrene‐block‐poly(2‐vinylpyridine) (PS‐b‐P2VP) and polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer solutions after exposure to electric fields during solvent evaporation. The results indicate an emphasizing effect of the electric field, enhancing self‐assembly and compensating mediocre experimental parameters to a certain degree. However, the impact is found to be different for the two diblock copolymer membranes depending on the selection of experimental parameters as well as the segregation strength of the systems. In addition, an increasing length of the vertically aligned cylindrical pores by the electric field is observed for both membranes, scaling with the applied voltage. This effect is primarily attributed to the suppression of defects and ill‐alignment of polymer domains in the course of structure formation during SNIPS by the electric field. Scanning electron microscopy is used to image the surface and cross‐sectional morphology of the integral asymmetric membranes.

Organic acids interacting with block copolymers have broadened the window that retains isoporous structures

Evaporation is significantly important for the preparation of isoporous membranes from self-assembly and non-solvent induced phase separation (SNIPS). Unfortunately, the time of solvent evaporation that retains isoporous structures is usually rather short, which causes many challenges to fabricate membranes with repeatable and controllable isoporous structures. In the current work, we used organic acids to tailor and enlarge the time window that led to isoporous structures, and their effects on membrane formation were investigated. It was found that binary and ternary organic acids all worked effectively and were able to extend the time window that produced isopores to longer than 200 s. In previous publications, this time duration was commonly in the range from several to tens of seconds. The nuclear magnetic resonance (NMR) determination implied that molecular cross-linking between acids and copolymers was existed in solution, and results from small angle X-ray scattering (SAXS) analysis demonstrated that the microphase separation of the block copolymer in casting solution was enhanced by the selected acids. The results proved that binary and ternary acids added to casting solution had not only enhanced the microphase separation but also stabilized the phase separated domains, which was ascribed as the reason for the extension of the time window. The effectiveness of organic acids on broadening the time window was also found for casting solutions of different concentrations and solvent compositions. Besides the role on stabilization of isoporous structure, the flux of isoporous membrane was also greatly increased by the organic acids.

Amine-Containing Block Copolymers for the Bottom-Up Preparation of Functional Porous Membranes

The remarkable ability of biological systems to specifically recognize, sort, and process molecules encourages the development of novel bio-inspired membranes and interfaces. Herein, we focus on the functionalization of amphiphilic polystyrene-block-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA) with the amino acid glycine yielding polar block segments with primary amine functionalities. Excellent control over the tailored block copolymer (BCP) synthesis and functionalization is proven by size exclusion chromatography (SEC), 1H solution NMR, and 15N solid state NMR spectroscopy. For the first time, BCPs containing primary amines were used for the self-assembly and non-solvent induced phase separation (SNIPS) process for the preparation of isoporous and integral asymmetric membranes. Hexagonally arranged open pores with diameters of 19 nm were obtained as shown by scanning electron microscopy (SEM), atomic force microscopy (AFM), and pH-dependent water flux measurements. Additionally, the primary amine was...

Isoporous membranes with sub-10 nm pores prepared from supramolecular interaction facilitated block copolymer assembly and application for protein separation

Isoporous membranes with pore size smaller than 10 nm are significantly important for molecular separation. However, preparation of such membranes from process containing self-assembly and non-solvent induced phase separation (SNIPS) is challenging, and only few works were conducted until now. Herein, we described that membranes with pore size down to 8.6 nm and pore density up to 1.3 × 1015/m2 were generated from the well-defined SNIPS with the specially designed additive, which interacted selectively with poly(4-vinylpyridine) and facilitated the phase separation when membranes were prepared from the polystyrene-block-poly (4-vinylpyridine) (S4VP). Interaction of the additive with S4VP in casting solutions was probed by nuclear magnetic resonance (NMR), which was ascribed to the reason for the enhanced phase separation that was inspected by small angle X-ray scattering (SAXS). The effect of preparation conditions, such as the ratios of the additive to P4VP, the compositions of solvent, and the polymer concentrations on membrane formation were systematically investigated. The morphologies and pore size on membranes were analyzed by scanning electron microscope (SEM), and their rejections to proteins of varied size and charges were finally determined. Our method showed herein has opened an access to the preparation of isoporous membranes with sub-10 nm pores that were desirable for molecular separation.

Systematic variation of membrane casting parameters to control the structure of thermo-responsive isoporous membranes

Fouling is a critical issue in membrane process operation as it greatly compromises the efficiency of the treatment processes. A promising approach to overcome this problem is the production of easy-to-clean membranes by incorporating stimuli-responsive pores. In this study, we fabricated thermo-responsive polystyrene-b-poly(N-isopropyl acrylamide) (PS-b-PNIPAM) block copolymer membranes using the self-assembly and non-solvent induced phase separation (SNIPS) method and systematically varied several membrane casting parameters, i.e. evaporation time, polymer concentration, solvent type and water content, to obtain nano- and isoporous membranes. Membranes with a disordered surface were obtained for PS selective solvents, whereas isoporous membranes were obtained when the block copolymers were dissolved in PNIPAM selective solvent mixtures. Using 1,4-dioxane/ tetrahydrofuran mixtures resulted in isoporous membranes for a large parameter space, indicating the robustness of structure formation in the PS-b-PNIPAM system. Permeability tests at various temperatures demonstrated fully reversible thermo-responsive behavior of the membranes.

Biocatalytic Stimuli-Responsive Asymmetric Triblock Terpolymer Membranes for Localized Permeability Gating.

The functionalization with phosphotriesterase of poly(isoprene-b-styrene-b-4-vinylpyridine)-based nanoporous membranes fabricated by self-assembly and nonsolvent induced phase separation (SNIPS) is shown to enable dynamically responsive membranes capable of substrate-specific and localized gating response. Integration of the SNIPS process with macroporous nylon support layers yields mechanically robust textile-type films with high moisture vapor transport rates that display rapid and local order-of-magnitude modulation of permeability. The simplicity of the fabrication process that is compatible with large-area fabrication along with the versatility and efficacy of enzyme reactivity offers intriguing opportunities for engineered biomimetic materials that are tailored to respond to a complex range of external parameters, providing sensing, protection, and remediation capabilities.

Solution self‐assembly behavior of A‐B‐C triblock polymers and the implications for nanoporous membrane fabrication

ABSTRACTSelf‐assembled block polymer membranes are rising to the fore of next‐generation separations technologies, and a particular fabrication process of significant interest is the self‐assembly and non‐solvent induced phase‐separation (SNIPS) method. This is because the SNIPS process allows for the facile manipulation of independent variables to produce simultaneously size‐ and chemically‐selective membranes in a scalable manner. Despite significant advances in the SNIPS membrane fabrication procedure, the early‐stage solution behavior of many of the polymer–solvent systems employed is still poorly understood. This work addresses the challenges posed by these systems and illuminates guiding relationships for the optimization of SNIPS membrane casting solutions. Specifically, polymer aggregate structures and critical micelle concentration (CMC) values are determined for an A‐B‐C triblock polymer dissolved in 1,4‐dioxane using a combination of scattering and atomic force microscopy techniques. The CMC of this system ranges from 20 to 80 μM and shows a strong dependence on the polymer composition and solution processing method. Additionally, the shape of the self‐assembled structures in solution can be altered by varying these parameters. These studies establish the relationships between polymer solution properties (i.e., the block polymer composition and the processing method) and assembly behavior as they impact membrane fabrication through SNIPS processing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45531.

Effect of humidity on surface structure and permeation of triblock terpolymer derived SNIPS membranes

Block copolymer (BCP) ultrafiltration (UF) membranes derived from a hybrid process of BCP self-assembly and non-solvent induced phase separation (SNIPS) exhibit an asymmetric structure consisting of ordered pores in a selective skin layer above a highly permeable substructure. In this work, we investigate relative humidity (RH) as a casting condition that influences poly(isoprene-b-styrene-b-4-vinylpyridine) (ISV) membrane surface structure in terms of order and uniformity. An optimum RH of 40–45% is determined to produce membranes characterized by a high density of square packed pores with a narrow pore size distribution. Membranes cast at lower and higher RH show a drop off in order and uniformity. The RH dependent structural changes correlate well with small molar mass dye solute (methyl orange; molar mass of 327 g/mol) diffusion rates. Membranes cast at 40% RH reveal higher diffusivity than when cast at 75% RH. The rate of permeation is further controlled via pH dependent pore closure as well as through ISV terpolymer molecular architecture with increasing solute diffusivity observed in membranes cast from increasing terpolymer molar mass. Experimental findings on small molar mass solute permeability can only be accounted for when compared with theoretical predictions from a hydrodynamic theory combined with the effects of tortuosity when a simplified membrane structure is assumed. Results suggest that SNIPS derived BCP membranes may have potential for applications in drug delivery.

Poly(2-hydroxyethyl methacrylate)-Based Amphiphilic Block Copolymers for High Water Flux Membranes and Ceramic Templates

Microphase separation drives the structure formation in block copolymers (BCP), and it is a promising and convenient way for the preparation of well-defined hierarchically structured materials. In this work, we focus on the synthesis of amphiphilic block copolymers, i.e., polystyrene-block-poly(2-hydroxyethyl methacrylate) (PS-b-PHEMA), via sequential anionic polymerization. Molecular weights of the prepared BCPs were in the range of 103–308 kg mol–1 with low polydispersity values (Đ = 1.03–1.25) and a PHEMA content of 17–25 vol % as proven by size exclusion chromatography (SEC) and 1H NMR spectroscopy measurements. For the first time, the PS-b-PHEMA BCPs were subjected to the self-assembly and non-solvent-induced phase separation (SNIPS) process for the generation of integral asymmetric films on tailor-made porous paper substrates. After water immersion, the final BCP membranes reveal an isoporous open structure at the surface featuring adjustable pore diameters of 20–60 nm. It is demonstrated that a hig...

Tuning substructure and properties of supported asymmetric triblock terpolymer membranes

Asymmetric poly(isoprene-b-styrene-b-4-vinylpyridine) (ISV) block copolymer membranes fabricated via self-assembly and non-solvent induced phase separation (SNIPS) process have drawn significant attention due to the simple processing method and the generation of high-quality isoporous ultrafiltration membranes. With the present study on SNIPS membrane substructure, we systematically varied membrane casting parameters to tune the cross-sectional morphologies of SNIPS membranes while simultaneously preserving top surface structure. Parameters such as polymer concentration, evaporation time, solvent ratio, and coagulation bath temperature were investigated to control transformation of commonly produced sponge-like cross-sectional morphologies into more open and permeable finger-like substructures. Membranes with sponge-like and finger-like substructures were then integrated onto nylon supports for enhanced mechanical properties. Hydraulic permeability tests at various pH conditions gave distinct open-state flux values for SNIPS membranes with different sublayer morphologies, while maintaining pH responsive functionality resulting from the poly(4-vinylpyridine) block.

Symmetrical Permeable Membranes Consisting of Overlapped Block Copolymer Cylindrical Micelles for Nanoparticle Size Fractionation

Free-standing symmetrical porous membranes consisting of uniform cylindrical micelles were prepared from a polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymer following a process combining self-assembly and nonsolvent-induced phase separation (SNIPS). The fabricated membranes displayed an impressive ultrastructure by the overlapping of the cylindrical micelles throughout the whole membrane profile (∼65 μm). The effects of copolymer molecular weight and membrane-formation condition on membrane morphology were investigated and discussed in detail. It was concluded that the cylindrical micellar membranes could be obtained in a wide window of fabrication conditions as low molecular weight of PS-b-P4VP was used as the raw material. The permeation tests indicated that the water permeability of the symmetrical membranes reached 400 L/(h m2 bar) in neutral condition, which was comparable to that of the block copolymer (BCP) isoporous membrane reported in the literature. Significantly, the permea...

Block Copolymer Membranes for Aqueous Solution Applications

Block copolymers are known for their intricate morphology. We review the state of the art of block copolymer membranes and discuss perspectives in this field. The main focus is on pore morphology tuning with a short introduction on nonporous membranes. The two main strategies for pore formation in block copolymer membranes are (i) film casting and selective block sacrifice and (ii) self-assembly and nonsolvent-induced phase separation (SNIPS). Different fundamental aspects involved in the manufacture of block copolymer membranes are considered, including factors affecting the equilibrium morphology in solid films, self-assembly of copolymer in solutions, and macrophase separation by solvent–nonsolvent exchange. Different mechanisms are proposed for different depths of the SNIPS membrane. Block copolymer membranes can be prepared with much narrower pore size distribution than homopolymer membranes. Open questions and indications of what we consider the next development steps are finally discussed. They inc...