Performance study of isoporous membranes with tailored pore sizes

Sponge-like Cryogels from Liquid-Liquid Phase Separation: Structure, Porosity, and Diffusional Gel Properties

Macroporous gels find application in several scientific fields, ranging from art restoration to wastewater filtration or cell entrapment. In this work, two-component sponge-like cryogels are challenged to assess their cleaning performances and to investigate how pores size and connectivity affect physico-chemical properties. The gels were obtained through a freeze-thaw process, exploiting a spontaneous polymer-polymer phase-separation occurring in the pre-gel solution. During the freezing step, a highly hydrolyzed polyvinyl alcohol (H-PVA) forms the hydrogel walls. The secondary components, namely a partially hydrolyzed polyvinyl alcohol (L-PVA) or polyvinyl pyrrolidone (PVP), act as modular porogens, being partially extracted during gel washing. H-PVA/L-PVA and H-PVA/PVP mixtures were studied by confocal laser scanning microscopy to unveil sols and gels morphology at the micron-scale, while small angle X-ray scattering was used to get insights about characteristic dimensions at the nanoscale. The gelation mechanism was investigated through rheology measurements, and the characteristic exponents were compared to De Gennes' scaling laws gathered from percolation. In the field of art conservation, these sponge-like gels are ideal systems for the cleaning of artistic painted surfaces. Their interconnected pores allow the diffusion of cleaning fluids at the painted interface, facilitating dirt uptake and/or detachment. This study uncovered a direct relationship linking a gel's cleaning performance to its apparent tortuosity. These findings can pave the way to fine-tuning systems with enhanced cleaning abilities, not restricted to the restoration of irreplaceable priceless works of art, but with possible application in diverse research fields.

Hybrid Organic-Inorganic-Organic Isoporous Membranes with Tunable Pore Sizes and Functionalities for Molecular Separation

Accomplishing on-demand molecular separation with a high selectivity and good permeability is very desirable for pollutant removal and chemical and pharmaceutical processing. The major challenge for sub-10 nm filtration of particles and molecules is the fabrication of high-performance membranes with tunable pore size and designed functionality. Here, a versatile top-down approach is demonstrated to produce such a membrane using isoporous block copolymer membranes with well-defined pore sizes combined with growth of metal oxide using sequential infiltration synthesis and atomic layer deposition (SIS and ALD). The pore size of the membranes is tuned by controlled metal oxide growth within and onto the polymer channels, enabling up to twofold pore diameter reduction. Following the growth, the distinct functionalities are readily incorporated along the membrane nanochannels with either hydrophobic, cationic, or anionic groups via straightforward and scalable gas/liquid-solid interface reactions. The hydrophilicity/hydrophobicity of the membrane nanochannel is significantly changed by the introduction of hydrophilic metal oxide and hydrophobic fluorinated groups. The functionalized membranes exhibit a superior selectivity and permeability in separating 1-2 nm organic molecules and fractionating similar-sized proteins based on size, charge, and hydrophobicity. This demonstrates the great potential of organic-inorganic-organic isoporous membranes for high-performance molecular separation in numerous applications.

Selective Swelling and Functionalization of Integral Asymmetric Isoporous Block Copolymer Membranes

SNIPS stands for a membrane fabrication technique that combines the evaporation induced self-assembly of the block copolymers and the classical nonsolvent induced phase separation. It is a one-step readily scalable technique to fabricate integral asymmetric isoporous membranes. The prominent developments in the last decade have carved out a niche for SNIPS as a potential technique to fabricate next generation isoporous membranes. In the last decade, a rich polymer library and variety of membrane postmodification routes have been successfully implemented to fabricate SNIPS membranes having the desired pore functionality. Some of these membranes form soft nanochannels in hydrated state due to swelling of the pore wall, i.e., the pore forming block of the block copolymer. These membranes having soft nanochannels have demonstrated the potential to perform several challenging separation tasks in ultrafiltration and nanofiltration. This paper highlights the currently accessible pore functionality, the strategies to tune the swelling of the soft nanochannels, the potential applications, and future perspectives of these membranes.

Spraying of Ultrathin Isoporous Block Copolymer Membranes-A Story about Challenges and Limitations

Isoporous membranes can be prepared by a combination of self-assembly of amphiphilic block copolymers and the non-solvent induced phase separation process. As the general doctor-blade technique suffers from high consumption of expensive block copolymer, other methods to reduce its concentration in the casting solution are sought after. Decreasing the block copolymer concentration during membrane casting and applying the block copolymer solution on a support membrane to obtain ultrathin isoporous membrane layers with e.g., spraying techniques, can be an answer. In this work we focused on the question if upscaling of thin block copolymer membranes produced by spraying techniques is feasible. To upscale the spray coating process, three different approaches were pursued, namely air-brush, 1-fluid nozzles and 2-fluid nozzles as generally used in the coating industry. The different spraying systems were implemented successfully in a membrane casting machine. Thinking about future development of isoporous block copolymer membranes in application it was significant that a continuous preparation process can be realised combining spraying of thin layers and immersion of the thin block copolymer layers in water to ensure phase-separation. The system was tested using a solution of polystyrene-block-poly(4-vinylpyridine) diblock copolymer. A detailed examination of the spray pattern and its homogeneity was carried out. The limitations of this method are discussed.

Fabrication and Applications of 3D Nanoarchitectures for Advanced Electrocatalysts and Sensors

For the last few decades, nanoscale materials and structures have been extensively studied and developed, making a huge impact on human sustainability. For example, the introduction of nanostructures has brought substantial development in electrocatalysts and optical sensing applications. However, there are still remaining challenges that need to be resolved to further improve their performance, reliability, and cost-effectiveness. Herein, long-range ordered 3D nanostructures and their design principles are introduced with an emphasis on electrocatalysts for energy conversion and plasmonic nanostructures for optical sensing. Among the various fabrication techniques, sequential solvent-injection-assisted nanotransfer printing is suggested as a practical fabrication platform for tunable long-range ordered 3D nanostructures composed of ultrahigh-resolution building blocks. Furthermore, the importance of understanding and controlling the 3D design parameters is discussed to realize more efficient energy conversion as well as effective surface-enhanced Raman spectroscopy analyses, suggesting new solutions for clean energy and healthcare issues.

Facilitated Structure Formation in Isoporous Block Copolymer Membranes upon Controlled Evaporation by Gas Flow

The conventional fabrication of isoporous membranes via the evaporation-induced self-assembly of block copolymers in combination with non-solvent induced phase separation (SNIPS) is achieved under certain environmental conditions. In this study, we report a modification in the conventional fabrication process of (isoporous) flat sheet membranes in which the self-assembly of block copolymers is achieved by providing controlled evaporation conditions using gas flow and the process is introduced as gSNIPS. This fabrication approach can not only trigger and control the microphase separation but also provides isoporous structure formation in a much broader range of solution concentrations and casting parameters, as compared to fabrication under ambient, uncontrolled conditions. We systematically investigated the structure formation of the fabrication of integral asymmetric isoporous membranes by gSNIPS. A quantitative correlation between the evaporation conditions (causing solvent evaporation and temperature drop) and the self-assembly of block copolymers beginning from the top layer up to a certain depth, orientation of pores in the top layer and the substructure morphology has been discussed empirically.

Self-assembly of block copolymers during hollow fiber spinning: an in situ small-angle X-ray scattering study

We investigated the self-assembly of block copolymers during hollow fiber membrane (HFM) fabrication by conducting in situ small angle X-ray scattering (SAXS) and ex situ scanning electron microscopy (SEM) studies. SAXS enables us to follow the structural rearrangements after extrusion at different distances from the spinning nozzle. The kinetics of the spinning process is examined as a function of the composition of block copolymer solutions and the spinning parameters. We studied the influence of the extrusion rate on the block copolymer microdomains and their self-assembly in weakly segregated and ordered solutions. The addition of magnesium acetate (MgAc2) leads to the ordering of micelles in the block copolymer solution already at lower polymer concentrations and shows an increased number of micelles with larger domain spacing as compared to the pristine solution. The SAXS data show the effect of shear within the spinneret on the self-assembly of block copolymers and the kinetics of phase separation after extrusion. It is observed that the ordering of micelles in solutions is decreased as indicated by the loss of crystallinity while high extrusion rates orient the structures perpendicular to the fiber direction. The structural features obtained from in situ SAXS experiments are correlated to the structure in the block copolymer solutions in the absence of shear and the morphologies in flat sheet and HF membranes obtained by ex situ SEM. This allows a systematic and comparative study of the effects varying the microdomain ordering within different block copolymer solutions and the formed membrane structures.

Surface modification of self-assembled isoporous polymer membranes for pressure-dependent high-resolution separation

Polymer membranes with narrow pore size distribution demonstrate great performance in high-resolution and high-efficiency separation.

Setting the Stage for Fabrication of Self-Assembled Structures in Compact Geometries: Inside-Out Isoporous Hollow Fiber Membranes

Fabrication of evaporation-induced self-assembled structures on easily accessible surfaces is an established strategy, while achieving such microphase-separated structures in compact geometries has been a long-standing goal. The requirement of comparatively less concentrated block copolymer (BCP) solution to pass through the compact geometries significantly reduces the stimulations required for self-assembly. The high polymer relaxation rates and decreased thermodynamic driving forces, as well as high capillary suction of dilute solutions in the porous substrates, complicates the BCP self-assembly and fabrication of the uniform coated layer, respectively. In this study, highly permeable robust poly(ether sulfone) hollow fiber membranes (PES HFM) with an inner diameter of approximately 1 mm are selected as compact geometries, and the isoporous structures are developed on top of ≤10 μm thin coated layer. This fabrication process introduces a technologically favored inside-out configuration for isoporous composite HFM with large bore diameters.

Perfluorinated Compounds as Test Media for Porous Membranes

We suggest a failure-free method of porous membranes characterization that gives the researcher the opportunity to compare and characterize properties of any porous membrane. This proposal is supported by an investigation of eight membranes made of different organic and inorganic materials, with nine different perfluorinated compounds. It was found that aromatic compounds, perfluorobenzene, and perfluorotoluene, used in the current study show properties different from other perfluorinated aliphatics. They demonstrate extreme deviation from the general sequence indicating the existence of π-π-interaction on the pore wall. The divergence of the flow for cyclic compounds from ideal e.g., linear compounds can be an indication of the pore dimension.

Investigate the Effect of Thawing Process on the Self-Assembly of Silk Protein for Tissue Applications

Biological self-assembly is a process in which building blocks autonomously organize to form stable supermolecules of higher order and complexity through domination of weak, noncovalent interactions. For silk protein, the effect of high incubating temperature on the induction of secondary structure and self-assembly was well investigated. However, the effect of freezing and thawing on silk solution has not been studied. The present work aimed to investigate a new all-aqueous process to form 3D porous silk fibroin matrices using a freezing-assisted self-assembly method. This study proposes an experimental investigation and optimization of environmental parameters for the self-assembly process such as freezing temperature, thawing process, and concentration of silk solution. The optical images demonstrated the possibility and potential of -80ST48 treatment to initialize the self-assembly of silk fibroin as well as controllably fabricate a porous scaffold. Moreover, the micrograph images illustrate the assembly of silk protein chain in 7 days under the treatment of -80ST48 process. The surface morphology characterization proved that this method could control the pore size of porous scaffolds by control of the concentration of silk solution. The animal test showed the support of silk scaffold for cell adhesion and proliferation, as well as the cell migration process in the 3D implantable scaffold.

CO2-expanded liquid assisted self-assembly between Disperse Red 1 and PS-b-P4VP

This work shows that CO₂-expanded liquids facilitate the modulation of morphology and photoluminescence performance of the self assembled fluorescent composite formed between DR1 and PS-b-P4VP in CO₂-expanded ethanol.

Thin Isoporous Block Copolymer Membranes: It Is All about the Process

The combination of the self-assembly of amphiphilic block copolymers and the nonsolvent induced phase inversion process offers an efficient way to isoporous integral-asymmetric membranes. In this context we report fast, easily upscalable and material reducing ways to thin self-assembled membranes. Therefore, we succeeded to implement a spray or dip coating step into the membrane formation process of different diblock copolymers like polystyrene-block-poly(4-vinylpyridine), poly(α-methylstyrene)-bock-poly(4-vinylpyridine), and polystyrene-block-poly(iso-propylglycidyl methacrylate). The formation of hexagonal pore structures was possible using a highly diluted one solvent system allowing the reduction of diblock copolymer consumption and therefore the production costs are minimized compared to conventional blade casting approaches. The broad applicability of the process was proven by using different flat and hollow fiber support materials. Furthermore, the membranes made by this new method showed a more than 6-fold increase in water flux compared to conventional polystyrene-block-poly(4-vinylpyridine) membranes with similar pore sizes prepared by blade casting. The membranes could be proven to be stable at transmembrane pressures of 2 bar and showed a pH responsive flux behavior over several cycles.