Robust preparation of tubular PTFE/FEP ultrafine fibers-covered porous membrane by electrospinning for continuous highly effective oil/water separation

PTFE Stent Membrane Based on the Electrospinning Technique and Its Potential for Replacing ePTFE

Expanded poly(tetrafluoroethylene) (ePTFE), obtained by the paste extrusion-stretching method, is a commonly used stent membrane material for the treatment of arterial stenosis or aneurysm in clinical practice. However, the structure of ePTFE is nonfibrous, which is not friendly to cells, and the equipment consumes a lot of energy and often requires the use of flammable and toxic lubricants. In this study, electrospinning was used to prepare PTFE vascular stent membranes, following plasma treatment, dopamine, and heparin grafting to obtain an anticoagulant surface. The morphology, structure, axial and circumferential tensile strength, porosity, water penetration pressure, and heparin-releasing behaviors of the samples were studied at first. Then, the experiments of blood compatibility, cytotoxicity, and in vivo subcutaneous implantation were conducted. Results showed that the PTFE electrospun tubular membrane has submicrometer to nanoscale fiber structures similar to the extracellular matrix. The axial and circumferential tensile strengths can reach 8.12 and 6.10 MPa, respectively, and the axial and circumferential elongations at break can reach 328.75% and 285.28%, respectively. It maintains higher porosity and water penetration pressure as well as a longer heparin-releasing period. It has a suitable hemolysis rate and superior anticoagulant properties. Dopamine and heparin modifications can facilitate the adhesion and proliferation of endothelial cells. Histological analysis of the PTFE electrospun tubular membrane showed no difference from the commercially available ePTFE graft.

High-Performance Membranes Based on Spherical-Beaded Nanofibers and Nanoarchitectured Networks for Water-in-Oil Emulsion Separation

High-performance separation materials for oil-water emulsions are crucial to environmental protection and resource recovery; however, most existing fibrous separation materials are subject to large pore size and low porosity, resulting in limited separation performance. Herein, we create high-performance membranes consisting of spherical-beaded nanofibers and nanoarchitectured networks (nano-nets) using electrostatic spinning/netting technology, for water-in-oil emulsion separation. By manipulating the nonequilibrium stretching of jets, spherical-beaded nanofibers capable of generating a robust microelectric field are fabricated as scaffolds, on which charged droplets are induced to eject and phase separate to self-assemble nano-nets with small pores. Benefiting from 3D undulating networks with cavities originating from 2D nano-nets supported by 1D spherical-beaded nanofibers, the membranes exhibit under-oil superhydrophobicity (>152°), a striking separation performance with an efficiency of >99.2% and a flux of 5775 L m-2 h-1, together with wide pressure applicability, antifouling, and reusability. This work may open up new horizons in developing fibrous materials for separation and purification.

Facile Approach for the Potential Large-Scale Production of Polylactide Nanofiber Membranes with Enhanced Hydrophilic Properties

Polylactide (PLA) nanofiber membranes with enhanced hydrophilic properties were prepared through electrospinning. As a result of their poor hydrophilic properties, common PLA nanofibers have poor hygroscopicity and separation efficiency when used as oil-water separation materials. In this research, cellulose diacetate (CDA) was used to improve the hydrophilic properties of PLA. The PLA/CDA blends were successfully electrospun to obtain nanofiber membranes with excellent hydrophilic properties and biodegradability. The effects of the additional amount of CDA on the surface morphology, crystalline structure, and hydrophilic properties of the PLA nanofiber membranes were investigated. The water flux of the PLA nanofiber membranes modified with different CDA amounts was also analyzed. The addition of CDA improved the hygroscopicity of the blended PLA membranes; the water contact angle of the PLA/CDA (6/4) fiber membrane was 97.8°, whereas that of the pure PLA fiber membrane was 134.9°. The addition of CDA enhanced hydrophilicity because it tended to decrease the diameter of PLA fibers and thus increased the specific surface area of the membranes. Blending PLA with CDA had no significant effect on the crystalline structure of the PLA fiber membranes. However, the tensile properties of the PLA/CDA nanofiber membranes worsened due to the poor compatibility between PLA and CDA. Interestingly, CDA endowed the nanofiber membranes with improved water flux. The water flux of the PLA/CDA (8/2) nanofiber membrane was 28,540.81 L/m²·h, which was considerably higher than that of the pure PLA fiber membrane (387.47 L/m²·h). The PLA/CDA nanofiber membranes can be feasibly applied as an environmentally friendly oil-water separation material because of their improved hydrophilic properties and excellent biodegradability.

Fluoropolymer Membranes for Membrane Distillation and Membrane Crystallization

Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

Design of Robust FEP Porous Ultrafiltration Membranes by Electrospinning-Sintered Technology

Perfluoropolymer membranes are widely used because of their good environmental adaptability. Herein, the ultrafine fibrous FEP porous membranes were fabricated with electrospinning-sintered technology. The effects of PVA content and sintering temperature on the fabricated membranes’ morphologies and properties were investigated. The results indicate that a kind of dimensionally stable network structure was formed in the obtained ultrafine fibrous FEP porous membranes after sintering the nascent ultrafine fibrous FEP/PVA membranes. The optimal sintering conditions were obtained by comparing the membranes’ performance in terms of membrane morphology, hydrophobicity, mechanical strength, and porosity. When the sintering temperature was 300 °C for 10 min, the porosity, water contact angle, and liquid entry pressure of the membrane were 62.7%, 124.2° ± 2.1°, and 0.18 MPa, respectively. Moreover, the ultrafine fibrous FEP porous membrane at the optimal sintering conditions was tested in vacuum membrane distillation with a permeate flux of 15.1 L·m⁻²·h⁻¹ and a salt rejection of 97.99%. Consequently, the ultrafine fibrous FEP porous membrane might be applied in the seawater desalination field.

A robust air superhydrophilic/superoleophobic diatomite porous ceramic for high-performance continuous separation of oil-in-water emulsion

Three-dimensional (3D) porous architecture has attracted considerable attention in remediation of oil/water emulsion. In present work, an air superhydrophilic/superoleophobic diatomite porous ceramic (AS-DC) was prepared, using SiO₂ whiskers modified diatomite ceramic as the substrate and FS-50 as the modifier. The interconnected SiO₂ whiskers intertwined on the skeleton of ceramic block forming a 3D network structure, which not only improved the wettability of AS-DC, but also reinforced its mechanical property (about 2.5 MPa of compressive strength). The as-prepared AS-DC with intrinsically superoleophobicity (154°) and superhydrophilicity (0°) exhibited an underwater oil contact angle of 161°, suggesting a multifunctional separation capability. By simply assembling AS-DC with pipes and a pump, it could not only separate the surfactant-stabilized oil-in-water emulsion in a permeation flux as high as 107.8 kg min^-1 m^-2 with a selectivity of >95%, but also collect the clean water from the floating oil/water mixture in a flux of 197.4 kg min^-1 m^-2 and a selectivity of ∼99%. In addition, the AS-DC was resistant to the salt/acid/alkaline corrosion and temperature fluctuation. The mechanical/chemical firmness of AS-DC renders it tremendous potential as a robust 3D architecture in real application for purification of oil/water mixture.

Asymmetric Cellulose/Carbon Nanotubes Membrane with Interconnected Pores Fabricated by Droplet Method for Solar-Driven Interfacial Evaporation and Desalination

Solar-driven interfacial water purification and desalination have attracted much attention in environmentally friendly water treatment field. The structure design of the photothermal materials is still a critical factor to improve the evaporation performance such as evaporation rate and energy conversion efficiency. Herein, an asymmetric cellulose/carbon nanotubes membrane was designed as the photothermal membrane via a modified droplet method. Under 1 sun irradiation, the evaporation rate and energy efficiency of pure water can reach up to 1.6 kg m−2 h−1 and 89%, respectively. Moreover, stable reusability and desalination performance made the cellulose/carbon nanotubes membrane a promising photothermal membrane which can be used for solar-driven desalination.

Advances in Micro/Nanoporous Membranes for Biomedical Engineering

Porous membrane materials at the micro/nanoscale have exhibited practical and potential value for extensive biological and medical applications associated with filtration and isolation, cell separation and sorting, micro-arrangement, in-vitro tissue reconstruction, high-throughput manipulation and analysis, and real-time sensing. Herein, an overview of technological development of micro/nanoporous membranes (M/N-PMs) is provided. Various membrane types and the progress documented in membrane fabrication techniques, including the electrochemical-etching, laser-based technology, microcontact printing, electron beam lithography, imprinting, capillary force lithography, spin coating, and microfluidic molding are described. Their key features, achievements, and limitations associated with micro/nanoporous membrane (M/N-PM) preparation are discussed. The recently popularized applications of M/N-PMs in biomedical engineering involving the separation of cells and biomolecules, bioparticle operations, biomimicking, micropatterning, bioassay, and biosensing are explored too. Finally, the challenges that need to be overcome for M/N-PM fabrication and future applications are highlighted.

Formation and characterization of polytetrafluoroethylene nanofiber membranes for high-efficiency fine particulate filtration

Polytetrafluoroethylene (PTFE) porous membranes are widely used for high-temperature filtration. The polytetrafluoroethylene nanofiber membranes for fine particulate filtration were prepared by sintering the precursor electrospun polytetrafluoroethylene/polyvinyl/boric acid alcohol composite membranes. The effects of PTFE/PVA mass ratio and sintering temperature on the morphology and properties of the prepared membranes were investigated to obtain the PTFE nanofibers with different diameters, and the film has been characterized by SEM, TG, XRD, FT-IR, and EDS, and the mechanical and hydrophobic properties of the membranes were also investigated. The PTFE nanofiber membranes after sintering had nanofiber and nanowire structures. Moreover, the membranes were tested in air filtration. The filtration efficiency and pressure drop were tested to evaluate the membrane permeability and separation properties. The results showed a high filtration efficiency (98%) and a low pressure drop (90 Pa) for 300 nm sodium chloride aerosol particles at a 30 L min-1 airflow velocity and the hydrophobic membranes showed durable self-cleaning properties, which suggested that the PTFE nanofiber membranes were a promising candidate for high temperature filtration applications.

Preparation of an electrospun tubular PU/GE nanofiber membrane for high flux oil/water separation

A simple, tubular structure polyurethane/graphene (PU/GE) nanofiber membrane for continuous oil/water separation has been prepared.