Electrospun Liquid-Infused Membranes for Emulsified Oil/Water Separation

From an environmental perspective, microfiltration membranes are attractive for the separation of emulsified oils from contaminated water. However, fouling of the membrane is a major drawback of the technology. “Liquid-infused membranes” (LIMs) have the potential to eliminate membrane fouling. Here, we demonstrate the practical application of LIMs for the separation of oil from a stable oil-in-water emulsion and characterize their resistance to fouling. The base membrane is an electrospun nonwoven fibrous layer of the fluorinated copolymer poly­(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP). The surface energy of the PVDF-co-HFP fibers was lowered by the covalent attachment of a fluorinated silane (PFOCTS), and then, the membrane was infused with a perfluoropolyether. The membrane was then challenged with model emulsions of dodecane in water in a cross-flow configuration. This PFOCTS-modified LIM showed better infused liquid stability, permeation selectivity, higher permeate flux than the unmodified LIM, and better anti-fouling properties than the bare membrane without infused liquid. We also examine the mechanism for transport of the dispersed oil phase through the liquid-infused membrane. We find a linear relationship between the dodecane flux and dodecane concentration in the feed and a higher dodecane flux through the PFOCTS-modified membrane than the unmodified one, which suggests that the capture of dodecane droplets from the feed plays an important role in determining the overall rate of permeation. Other factors such as lower viscosity of the infused liquid, larger pore size, and higher operating pressure also improved the permeate flux through the LIMs. Overall, this work provides some guidelines on the design of composite membranes comprising infused liquids and the choice of operating conditions for the filtration process

Shape-Stable Composites of Electrospun Nonwoven Mats and Shear-Thickening Fluids

To improve the flexibility of the fabric stacks used in protective clothing, shear-thickening fluids (STFs) have previously been incorporated into woven microfiber fabrics to enhance their impact resistance. However, the microfiber–STF composites can exhibit loss of the STF from the composite over time due to the large interstitial spaces between fibers, resulting in limited long-term shape stability. In this study, nonwoven mats of electrospun ultrafine fibers (UFFs) were used in place of woven microfiber fabrics to improve the STF retention within the fiber–STF composites by taking advantage of high specific surface area, small pore size, and large capillary force. The UFF–STF composite, comprising an electrospun polyamide (PA 6,6) UFF mat and a fumed silica (FS) STF, exhibited excellent shape stability with high breakthrough pressure and improved STF retention compared to composites based on conventional microfiber fabrics. The mechanical response of the composite is shown to depend on the rate of deformation. At strain rates lower than the shear-thickening threshold of the STF, the introduction of STF resulted in no stiffening or strengthening of fiber mats, allowing the composite to remain flexible. At high deformation rates above the onset of shear thickening, the incorporation of STF improved both the elasticity and the viscosity of the material. In addition, the shape stability and the mechanical properties of the composite were influenced by the STF viscosity and the UFF morphology. STF with high particle loading and UFF with small fiber diameter resulted in a more pronounced enhancement to membrane performance

Direct Three-Dimensional Visualization of Membrane Fouling by Confocal Laser Scanning Microscopy

Membrane-based separation is an important technique for removing emulsified oil from water. However, the mechanisms of fouling are complex because of the deformability and potential for coalescence and break-up of the oil droplets. Here, we report for the first time direct, three-dimensional (3D) visualization of oil droplets on electrospun fiber microfiltration membranes after a period of membrane-based separation of oil-in-water emulsions. High-resolution 3D images were acquired by a dual-channel confocal laser scanning microscopy (CLSM) technique in which both the fibers and the oil (dodecane) were fluorescently labeled. The morphology of dodecane as the foulant was observed for two different types of fibers, an oleophobic nylon (PA6(3)­T), and oleophilic polyvinylidene fluoride (PVDF). Through direct visualization, the rejected oil was found to form droplets of clam-shell shape on the PA6(3)­T fibers, whereas the oil tended to wet the PVDF fibers and spread across the membrane. The morphology was also analyzed as a function of separation time (i.e., “4D” imaging), as the oil accumulated within and upon the membranes. The observations are qualitatively consistent with a transition from blocking of individual pores in the membrane to coalescence of oil droplets into coherent liquid films with increasing filtration time. Analysis of permeate flux using blocking filtration models corroborate the transition of fouling modes indicated by the 3D images. This direct, 3D visualization CLSM technique is a powerful tool for characterizing the mechanisms of fouling in membranes used for liquid emulsion separations

Direct Three-Dimensional Visualization of Membrane Fouling by Confocal Laser Scanning Microscopy

Membrane-based separation is an important technique for removing emulsified oil from water. However, the mechanisms of fouling are complex because of the deformability and potential for coalescence and break-up of the oil droplets. Here, we report for the first time direct, three-dimensional (3D) visualization of oil droplets on electrospun fiber microfiltration membranes after a period of membrane-based separation of oil-in-water emulsions. High-resolution 3D images were acquired by a dual-channel confocal laser scanning microscopy (CLSM) technique in which both the fibers and the oil (dodecane) were fluorescently labeled. The morphology of dodecane as the foulant was observed for two different types of fibers, an oleophobic nylon (PA6(3)­T), and oleophilic polyvinylidene fluoride (PVDF). Through direct visualization, the rejected oil was found to form droplets of clam-shell shape on the PA6(3)­T fibers, whereas the oil tended to wet the PVDF fibers and spread across the membrane. The morphology was also analyzed as a function of separation time (i.e., “4D” imaging), as the oil accumulated within and upon the membranes. The observations are qualitatively consistent with a transition from blocking of individual pores in the membrane to coalescence of oil droplets into coherent liquid films with increasing filtration time. Analysis of permeate flux using blocking filtration models corroborate the transition of fouling modes indicated by the 3D images. This direct, 3D visualization CLSM technique is a powerful tool for characterizing the mechanisms of fouling in membranes used for liquid emulsion separations

Direct Three-Dimensional Visualization of Membrane Fouling by Confocal Laser Scanning Microscopy

Membrane-based separation is an important technique for removing emulsified oil from water. However, the mechanisms of fouling are complex because of the deformability and potential for coalescence and break-up of the oil droplets. Here, we report for the first time direct, three-dimensional (3D) visualization of oil droplets on electrospun fiber microfiltration membranes after a period of membrane-based separation of oil-in-water emulsions. High-resolution 3D images were acquired by a dual-channel confocal laser scanning microscopy (CLSM) technique in which both the fibers and the oil (dodecane) were fluorescently labeled. The morphology of dodecane as the foulant was observed for two different types of fibers, an oleophobic nylon (PA6(3)­T), and oleophilic polyvinylidene fluoride (PVDF). Through direct visualization, the rejected oil was found to form droplets of clam-shell shape on the PA6(3)­T fibers, whereas the oil tended to wet the PVDF fibers and spread across the membrane. The morphology was also analyzed as a function of separation time (i.e., “4D” imaging), as the oil accumulated within and upon the membranes. The observations are qualitatively consistent with a transition from blocking of individual pores in the membrane to coalescence of oil droplets into coherent liquid films with increasing filtration time. Analysis of permeate flux using blocking filtration models corroborate the transition of fouling modes indicated by the 3D images. This direct, 3D visualization CLSM technique is a powerful tool for characterizing the mechanisms of fouling in membranes used for liquid emulsion separations

Polyvinylferrocene for Noncovalent Dispersion and Redox-Controlled Precipitation of Carbon Nanotubes in Nonaqueous Media

We report noncovalent dispersion of carbon nanotubes (CNTs) in organic liquids with extremely high loading (∼2 mg mL^–1) using polyvinylferrocene (PVF). In contrast to common dispersants, PVF does not contain any conjugated structures or ionic moieties. PVF is also shown to be effective in controlling nanotube dispersion and reprecipitation because it exhibits redox-switchable affinity for solvents, while maintaining stable physical attachment to CNTs during redox transformation. This switchability provides a novel approach to creating CNT-functionalized surfaces. The material systems described here offer new opportunities for applications of CNTs in nonaqueous media, such as nanotube–polymer composites and organic liquid-based optical limiters, and expand the means of tailoring nanotube dispersion behavior via external stimuli, with potential applications in switching devices. The PVF/CNT hybrid system with enhanced redox response of ferrocene may also find applications in high-performance biosensors and pseudocapacitors

Development of Electrospun Polyacrylonitrile Aerosol Filter Media for Respiratory Personal Protective Equipment

The COVID-19 pandemic highlighted the importance of respiratory personal protective equipment (PPE) as a means of reducing the spread of disease via aerosolized droplets. For years, N95-type filtering facepiece respirators based on meltblown polypropylene nonwovens have been the technology of choice for healthcare professionals and personal use. However, their reliance on electrostatic charges to achieve an acceptable trade-off between filtration efficiency and pressure drop has led to concerns about shelf life, reusability, quality control, and versatility of materials. In this study, we show that media in which an electrospun polyacrylonitrile (PAN) nonwoven serves as the active layer, comprising fibers of much smaller diameter than typical meltblown fibers, can achieve high levels of filtration efficiency combined with low pressure drop without the assistance of electrostatic charging. Moreover, the aerosol filtration data is well-described by the slip flow-modified Kuwabara model for pressure drop and a single fiber efficiency model that takes into account particle collection via diffusion, interception, and impaction. These models may be used to guide the further design of nanofiber filters. Combined with a spunbond substrate, the proposed filtration media resolves practical concerns regarding mechanical robustness and residual solvent, and it has been fabricated into filtering facepiece respirators that meet N95 filtration standards when tested by standard methods

Atomistic Simulation of the Structure and Mechanics of a Semicrystalline Polyether

We report the use of atomistic simulation to study semicrystalline poly­(tetramethylene oxide) (PTMO), which is one of the major components of thermoplastic polyurethanes. This work reports the first application of an Interphase Monte Carlo model previously developed for polyethylene to a more complex chemistry involving heteroatoms, about which much less is known experimentally. The interface between the crystalline and amorphous domains of PTMO has been modeled in detail, complete with the equilibrium distributions of tails, loops and bridges. In doing so, a criterion has been established for selecting the relevant interface between domains, and a methodology developed that identifies the energetically most favorable interface in a heterogeneous material. A representative sample of configurations was then simulated by molecular dynamics, and analysis of deformation to small strains at different strain rates is described. Estimation of the full stiffness matrix of semicrystalline PTMO is reported for the first time

Ultra-Wide-Range Electrochemical Sensing Using Continuous Electrospun Carbon Nanofibers with High Densities of States

Carbon-based sensors for wide-range electrochemical detection of redox-active chemical and biological molecules were fabricated by the electrospinning of polyacrylonitrile fibers directly onto a polyacrylonitrile-coated substrate followed by carbonization at 1200 °C. The resulting electrospun carbon nanofibers (ECNFs) were firmly attached to the substrate with good mesh integrity and had high densities of electronic states (DOS), which was achieved without need for further modifications or the use of any additives. The mass of ECNFs deposited, and thus the electroactive surface area (ESA) of the sensor, was adjusted by varying the electrospinning deposition time, thereby enabling the systematic manipulation of the dynamic range of the sensor. A standard redox probe (Fe­(CN)₆^3–/4–) was used to demonstrate that the ECNF sensor exhibits strong electrocatalytic activity without current saturation at high analyte concentrations. Dopamine was used as a model analyte to evaluate the sensor performance; we find that the ECNF device exhibits a dynamic range ∼10⁵ greater than that of many existing carbon-based sensors. The ECNF sensors exhibited excellent sensitivity, selectivity, stability, and reproducibility for dopamine detection