Porous Hydrophobic-Hydrophilic Composite Hollow Fiber and Flat Membranes Prepared by Plasma Polymerization for Direct Contact Membrane Distillation

A Review of the Preparation of Porous Fibers and Porous Parts by a Novel Micro-Extrusion Foaming Technique

This review introduces an innovative technology termed "Micro-Extrusion Foaming (MEF)", which amalgamates the merits of physical foaming and 3D printing. It presents a groundbreaking approach to producing porous polymer fibers and parts. Conventional methods for creating porous materials often encounter obstacles such as the extensive use of organic solvents, intricate processing, and suboptimal production efficiency. The MEF technique surmounts these challenges by initially saturating a polymer filament with compressed CO2 or N2, followed by cell nucleation and growth during the molten extrusion process. This technology offers manifold advantages, encompassing an adjustable pore size and porosity, environmental friendliness, high processing efficiency, and compatibility with diverse polymer materials. The review meticulously elucidates the principles and fabrication process integral to MEF, encompassing the creation of porous fibers through the elongational behavior of foamed melts and the generation of porous parts through the stacking of foamed melts. Furthermore, the review explores the varied applications of this technology across diverse fields and imparts insights for future directions and challenges. These include augmenting material performance, refining fabrication processes, and broadening the scope of applications. MEF technology holds immense potential in the realm of porous material preparation, heralding noteworthy advancements and innovations in manufacturing and materials science.

Hydrophobic modification of a PVDF hollow fiber membrane by plasma activation and silane grafting for membrane distillation

Polyvinylidene fluoride (PVDF) hollow fibers were hydrophobically modified using a simple and scalable method of plasma activation and silane grafting. The effects of plasma gas, applied voltage, activation time, silane type, and concentration were investigated according to the membrane hydrophobicity and direct contact membrane distillation (DCMD) performance. Two kinds of silane were used, including methyl trichloroalkyl silane (MTCS) and 1H,1H,2H,2H-perfluorooctane trichlorosilane silanes (PTCS). The membranes were characterized by techniques such as Fourier transform infrared (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and contact angle. The contact angle of the pristine membrane was 88°, which increased to 112°-116° after modification. Meanwhile, the pore size and porosity decreased. In DCMD, the maximum rejection reached 99.95% by the MTCS-grafted membrane, while the flux decreased by 35% and 65% for the MTCS- and PTCS-grafted membranes, respectively. Treating humic acid-contained solution, the modified membrane showed steadier water flux and higher salt rejection than the pristine membrane, and 100% flux recovery was achieved by simple water flushing. This two-step method of plasma activation and silane grafting is very simple and effective to improve the hydrophobicity and DCMD performance of PVDF hollow fibers. However, further study on improving the water flux should be carried out.

Plasma-Induced Superhydrophobicity as a Green Technology for Enhanced Air Gap Membrane Distillation

Superhydrophobicity has only recently become a requirement in membrane fabrication and modification. Superhydrophobic membranes have shown improved flux performance and scaling resistance in long-term membrane distillation (MD) operations compared to simply hydrophobic membranes. Here, we introduce plasma micro- and nanotexturing followed by plasma deposition as a novel, dry, and green method for superhydrophobic membrane fabrication. Using plasma micro- and nanotexturing, commercial membranes, both hydrophobic and hydrophilic, are transformed to superhydrophobic featuring water static contact angles (WSCA) greater than 150° and contact angle hysteresis lower than 10°. To this direction, hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic cellulose acetate (CA) membranes are transformed to superhydrophobic. The superhydrophobic PTFE membranes showed enhanced water flux in standard air gap membrane distillation and more stable performance compared to the commercial ones for at least 48 h continuous operation, with salt rejection >99.99%. Additionally, their performance and high salt rejection remained stable, when low surface tension solutions containing sodium dodecyl sulfate (SDS) and NaCl (down to 35 mN/m) were used, showcasing their antiwetting properties. The improved performance is attributed to superhydrophobicity and increased pore size after plasma micro- and nanotexturing. More importantly, CA membranes, which are initially unsuitable for MD due to their hydrophilic nature (WSCA ≈ 40°), showed excellent performance with stable flux and salt rejection >99.2% again for at least 48 h, demonstrating the effectiveness of the proposed method for wetting control in membranes regardless of their initial wetting properties.

Plasma Polymerized Coatings on Hollow Fiber Membranes-Applications and Their Aging Characteristics in Different Media

In the past 30 years, plasma polymerization has emerged as a versatile technique for depositing ultrathin nanocoating on a variety of substrates for applications that range from providing lubricity to the substrate, protection from harsh environments, promoting adhesion, surface modification to applications of coating in ultrafiltration and gas separation membranes. Applications in the field of volatile organic compound (VOC) recovery and membrane distillation have also gained importance in recent years. Most of these applications use silicone and fluorosilicone-based plasma polymers that provide versatility, good separation characteristics, and long-term stability to the membrane. However, plasma polymers are known to age with time. The current study focuses on the aging behavior of silicone and fluorosilicone plasma polymers in different environments that include air, ionized air, heat, aqueous solutions of inorganic chemicals, as well as harsh solvents such as hexane, dichloromethane (DCM), and toluene. Membrane gas permeance and gas selectivity were used to quantitatively measure the aging behavior of the coatings on gas separation membranes, while water and VOC flux were used to measure the effect of aging for membranes designed for membrane distillation and VOC separation. It was found that while all plasma polymers of this study showed changes in membrane gas permeance on exposure to air, they fundamentally retained their membrane separation characteristics in all the studied environments. Significant changes in gas permeability characteristics were observed on exposure of the membranes to organic solvents like dichloromethane, 2-propanol, hexane, and toluene, which are attributed to dimensional changes in the hollow fiber substrate rather than changes in plasma polymer characteristics. Ionized air was also found to have a significant effect on the gas permeability characteristic of the membranes, reducing the gas permeance by as much as 50% in some cases. This is attributed to accelerated oxidation and crosslinking of the polymer in ionized air. XPS studies showed an increase in the oxygen content of the polymer on aging. Differences were found in the aging behavior of polymer coatings made from different monomers with long-chain monomers such as hexamethyltrisiloxane offering more stable coatings. The cross-link density of the polymer also influenced the aging behavior, with the more cross-linked polymer showing a lesser influence on aging in a chemical environment. No significant effect of aging was found on applications of these polymer coatings in the field of membrane distillation, pervaporation, and VOC removal, and a stable performance was observed over a long period of time. It was also noted that the selection of co-monomers played a significant role in membrane distillation, with polymers forming fluoro co-monomers giving better results. The current study also demonstrated the usefulness of plasma polymers in controlling the pore size of microporous membranes that can find useful applications in bio-filtration and VOC recovery.

State-of-the-Art Organic- and Inorganic-Based Hollow Fiber Membranes in Liquid and Gas Applications: Looking Back and Beyond

The aggravation of environmental problems such as water scarcity and air pollution has called upon the need for a sustainable solution globally. Membrane technology, owing to its simplicity, sustainability, and cost-effectiveness, has emerged as one of the favorable technologies for water and air purification. Among all of the membrane configurations, hollow fiber membranes hold promise due to their outstanding packing density and ease of module assembly. Herein, this review systematically outlines the fundamentals of hollow fiber membranes, which comprise the structural analyses and phase inversion mechanism. Furthermore, illustrations of the latest advances in the fabrication of organic, inorganic, and composite hollow fiber membranes are presented. Key findings on the utilization of hollow fiber membranes in microfiltration (MF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), pervaporation, gas and vapor separation, membrane distillation, and membrane contactor are also reported. Moreover, the applications in nuclear waste treatment and biomedical fields such as hemodialysis and drug delivery are emphasized. Subsequently, the emerging R&D areas, precisely on green fabrication and modification techniques as well as sustainable materials for hollow fiber membranes, are highlighted. Last but not least, this review offers invigorating perspectives on the future directions for the design of next-generation hollow fiber membranes for various applications. As such, the comprehensive and critical insights gained in this review are anticipated to provide a new research doorway to stimulate the future development and optimization of hollow fiber membranes.