Highly porous PVDF hollow fiber membranes for VMD application by applying a simultaneous co-extrusion spinning process

A Comprehensive Review of Hollow-Fiber Membrane Fabrication Methods across Biomedical, Biotechnological, and Environmental Domains

This work presents methods of obtaining polymeric hollow-fiber membranes produced via the dry-wet phase inversion method that were published in renowned specialized membrane publications in the years 2010-2020. Obtaining hollow-fiber membranes, unlike flat membranes, requires the use of a special installation for their production, the most important component of which is the hollow fiber forming spinneret. This method is most often used in obtaining membranes made of polysulfone, polyethersulfone, polyurethane, cellulose acetate, and its derivatives. Many factors affect the properties of the membranes obtained. By changing the parameters of the spinning process, we change the thickness of the membranes' walls and the diameter of the hollow fibers, which causes changes in the membranes' structure and, as a consequence, changes in their transport/separation parameters. The type of bore fluid affects the porosity of the inner epidermal layer or causes its atrophy. Porogenic compounds such as polyvinylpyrrolidones and polyethylene glycols and other substances that additionally increase the membrane porosity are often added to the polymer solution. Another example is a blend of two- or multi-component membranes and dual-layer membranes that are obtained using a three-nozzle spinneret. In dual-layer membranes, one layer is the membrane scaffolding, and the other is the separation layer. Also, the temperature during the process, the humidity, and the composition of the solution in the coagulating bath have impact on the parameters of the membranes obtained.

Polyvinylidene Difluoride (PVDF) Hollow Fiber Membrane Incorporated with Antibacterial and Anti-Fouling by Zinc Oxide for Water and Wastewater Treatment

The addition of antibacterial material to hollow fiber membranes improves the membrane anti-biofouling characteristics. Antibacterial membranes were fabricated in this study to improve membrane function while also extending membrane lifetime. Neat polyvinylidene difluoride (PVDF) and PVDF hollow fiber membrane with the incorporation of antibacterial agent zinc oxide (ZnO) nanoparticles with various loading (2.5–7.5 wt.%) were fabricated by using dry/wet spinning method. The membrane structure, particle distribution, functional group, hydrophilicity, and pore size of each membrane were all assessed. The result shows that all ZnO/PVDF hollow fiber membranes have the asymmetric structure with even dispersion of ZnO nanoparticles throughout the membranes. The results showed that increased ZnO loadings considerably improved membrane hydrophilicity, and average pore size, in addition to good performance of pure water flux. Antibacterial testing shows that ZnO incorporated in the membrane matrix and membrane surfaces prevents bacteria that cause biofouling from adhering to the membrane. ZnO/PVDF membrane recorded excellent bovine serum albumin (BSA) rejection at 93.4% ± 0.4 with flux recovery rate at 70.9% ± 2.1. These results suggest that antibacterial ZnO/PVDF hollow fiber membranes are promising in relation to reducing biofouling for various water and wastewater treatment.

Highly-efficient PVDF adsorptive membrane filtration based on chitosan@CNTs-COOH simultaneous removal of anionic and cationic dyes

An adsorptive membrane filtration based on polyvinylidene fluoride (PVDF) with chitosan (CS) and carboxylated carbon nanotubes (CNTs-COOH) is prepared by method of phase conversion, and the PVDF-CS@CNTs-COOH membranes can effectively separate anionic and cationic dye wastewater. Compared to pure PVDF membranes, PVDF-CS@CNTs-COOH increases pure water flux from 36.39 (L·m^-2·h^-1) to 85.25 (L·m^-2·h^-1), an increase of nearly 230%. The membrane exhibits excellent rejection performance in the filtration of six types of dye wastewater. The modified membranes also performed well in terms of rejection of mixed anionic and cationic dyes and also have a high performance in recycling, with a flux of over 94% for both anionic and cationic dyes. In addition, the adsorption curve fitting results showed that the adsorption process was more consistent with the pseudo-second-order adsorption kinetic model and Langmuir mode.

A new generation of hollow polymeric microfibers produced by gas dissolution foaming

A new and straightforward route to produce polymeric hollow microfibers has been proposed. Polycaprolactone (PCL) hollow fibers are obtained for the first time using an environmentally friendly gas dissolution foaming approach, overcoming its limitations to induce porosity on samples in the micrometric range. Different porous morphologies are achieved from solid PCL microfibers with a well-controlled diameter obtained by conventional electrospinning. The optimization of the foaming parameters provides two sets of well-defined hollow fibers, one showing smooth surfaces and the other presenting an enhanced surface porosity. Accordingly, gas dissolution foaming proves to be not only suitable for the production of hollow polymeric microfibers, but is also capable of providing diverse porous morphologies from the same precursor, solid fibers. Moreover, a preliminary study about the suitability of this new generation of foamed hollow polymeric fibers for drug delivery is carried out, aiming to take advantage of the enhanced surface area and tunable morphology obtained by using the proposed new production method. It is found that the foamed microfibers can be loaded with up to 15 wt% of ibuprofen while preserving the morphology of each kind of fiber. Then, foamed PCL fibers presenting a hollow structure and surface porosity show a remarkable constant release of ibuprofen for almost one and a half days. In contrast, the original solid fibers do not present such behavior, releasing all the ibuprofen in about seven hours.

Continuous, Strong, Porous Silk Firoin-Based Aerogel Fibers toward Textile Thermal Insulation

Aerogel fiber, with the characteristics of ultra-low density, ultra-high porosity, and high specific surface area, is the most potential candidate for manufacturing wearable thermal insulation material. However, aerogel fibers generally show weak mechanical properties and complex preparation processes. Herein, through firstly preparing a cellulose acetate/polyacrylic acid (CA/PAA) hollow fiber using coaxial wet-spinning followed by injecting the silk fibroin (SF) solution into the hollow fiber, the CA/PAA-wrapped SF aerogel fibers toward textile thermal insulation were successfully constructed after freeze-drying. The sheath (CA/PAA hollow fiber) possesses a multiscale porous structure, including micropores (11.37 ± 4.01 μm), sub-micron pores (217.47 ± 46.16 nm), as well as nanopores on the inner (44.00 ± 21.65 nm) and outer (36.43 ± 17.55 nm) surfaces, which is crucial to the formation of a SF aerogel core. Furthermore, the porous CA/PAA-wrapped SF aerogel fibers have many advantages, such as low density (0.21 g/cm³), high porosity (86%), high strength at break (2.6 ± 0.4 MPa), as well as potential continuous and large-scale production. The delicate structure of multiscale porous sheath and ultra-low-density SF aerogel core synergistically inhibit air circulation and limit convective heat transfer. Meanwhile, the high porosity of aerogel fibers weakens heat transfer and the SF aerogel cellular walls prevent infrared radiation. The results show that the mat composed of these aerogel fibers exhibits excellent thermal insulating properties with a wide working temperature from −20 to 100 °C. Therefore, this SF-based aerogel fiber can be considered as a practical option for high performance thermal insulation.

Impacts of sodium hydroxide and sodium hypochlorite aging on polyvinylidene fluoride membranes fabricated with different methods

This study compared the effects of chemical aging on the polyvinylidene fluoride (PVDF) membranes fabricated with the methods of non-solvent induced phase separation (NIPS) (named NIPS-PVDF) and thermally induced phase separation (TIPS) (named TIPS-PVDF). The chemical solutions of sodium hypochlorite (NaClO) and sodium hydroxide (NaOH) were chosen at the concentration of 5000mg/L. The equivalence of 5 and 10years was respectively selected as the time of aging. The physicochemical evolutions of membrane aging are characterized on the base of morphology analysis, chemical components, permeation ability and mechanical properties. The aging of NIPS-PVDF membrane led to the elimination of surface hydrophilic additives, while NaOH focused on the dehydrofluorination process resulting in the formation of conjugated chains of polyene on the skeleton structure. The chemical components of the surface of TIPS-PVDF membrane were removed continuously during the aging processes of both NaClO and NaOH, which was caused by the saponification of surface additives and the chain scissions of skeleton structure, but without producing any obvious conjugated chains of polyene. All the aging processes led to the increase of contact angle and the decrease of mechanical properties, and the permeability was reduced first and increased later due to the enlargement of surface membrane pores and membrane block. With the influence of membrane aging, selectivity of membrane was decreased (except coliform bacteria). At the beginning of filtration, the turbidity and particle count were at relatively high levels and declined with the filtration process.

A polyethersulfone-bisphenol sulfuric acid hollow fiber ultrafiltration membrane fabricated by a reverse thermally induced phase separation process

A novel antifouling polyethersulfone (PES) hollow fiber membrane was modified by the addition of bisphenol sulfuric acid (BPA-PS) using a reverse thermally induced phase separation (RTIPS) process. BPA-PS was synthesized by click chemistry and was blended to improve the hydrophilicity of PES hollow fiber membranes. The performance of PES/BPA-PS hollow fiber membranes, prepared with different contents of BPA-PS and at different temperatures of the coagulation water bath, was characterized by scanning electron microscopy (SEM), pure water flux (J w), BSA rejection rate (R), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and water contact angle measurements. SEM morphologies revealed that a finger-like cross-section emerged in the hollow fiber membrane by a non-solvent induced phase separation (NIPS) mechanism while a sponge-like cross-section appeared in the hollow fiber membrane via the RTIPS method. Both FTIR and XPS analysis indicated that the sulfate group in BPA-PS was successfully blended with the PES membranes. The results from AFM and water contact angle measurements showed that the surface roughness increased and the hydrophilicity of the PES/BPA-PS hollow fiber membrane was improved with the addition of BPA-PS. The results demonstrated that the PES/BPA-PS membrane with 1 wt% BPA-PS via RTIPS exhibited optimal properties.