Heidi Schreuder-Gibson

Transport properties of porous membranes based on electrospun nanofibers

Abstract Electrospinning is a process by which high voltages are used to produce an interconnected membrane-like web of small fibers (10–500 nm in diameter). This novel fiber spinning technique provides the capacity to lace together a variety of types of polymers, fibers, and particles to produce ultrathin layers. Of particular interest are electrospun membranes composed of elastomeric fibers, which are under development for several protective clothing applications. The various factors influencing electrospun nonwoven fibrous membrane structure and transport properties are discussed. Performance measurements on experimental electrospun fiber mats compare favorably with transport properties of textiles and membranes currently used in protective clothing systems. Electrospun layers present minimal impedance to moisture vapor diffusion required for evaporative cooling. There may be special considerations in the application of elastomeric membranes for protective clothing. Effects of membrane distortion upon transport behavior of the structure might be significant. Preliminary measurements have found that changes in elastomeric membrane structure under different states of biaxial strain were reflected in measurements of air flow through the membrane. Changes in membrane structure are also evident in environmental scanning electron microscope (SEM) images of the pore/fiber rearrangement as the membrane is stretched. Experimental measurements and theoretical calculations show electrospun fiber mats to be extremely efficient at trapping airborne particles. The high filtration efficiency is a direct result of the submicron-size fibers generated by the electrospinning process. Electrospun nanofiber coatings were applied directly to an open cell polyurethane foam. The air flow resistance and aerosol filtration properties correlate with the electrospun coating add-on weight. Particle penetration through the foam layer, which is normally very high, was eliminated by extremely thin layers of electrospun nanofibers sprayed on to the surface of the foam. Electrospun fiber coatings produce an exceptionally lightweight multifunctional membrane for protective clothing applications, which exhibits high breathability, elasticity, and filtration efficiency.

Different electrostatic methods for making electret filters

Three charging techniques (viz., corona charging, tribocharging, and electrostatic fiber spinning) were used to charge fibers or fabrics of different polymer types. Corona charging is suitable for charging monopolymer fiber or fiber blend, or fabrics. Tribocharging is only appropriate for charging fibers with dissimilar electronegativity. Electrostatic fiber spinning combines the charging of polymer and the spinning of the fibers as a one-step process. It was observed that two dissimilar fibers following tribocharging had higher filtration efficiency than the corona-charged polypropylene fibers. An electrostatic spinning process produced nanofibers exhibiting extremely high efficiency by mechanical filtration mechanisms. Little charge was retained in electrospun polyethylene oxide fibers; however, polycarbonate and polyurethane retained a great amount of charge.

Organophophorous Ester Degradation by Chromium(III) Terephthalate Metal-Organic Framework (MIL-101) Chelated to N,N-Dimethylaminopyridine and Related Aminopyridines

Porous materials based on chromium(III) terephthalate metal organic frameworks (MOF) MIL-101(Cr) and their complexes with dialkylaminopyridines (DAAP) were synthesized via a DAAP-MOF complexation, and tested for hydrolytic degradation of organophosphorous esters such as diethyl 4-nitrophenyl phosphate (paraoxon). Elemental analysis, TGA, XRD, FT-IR, TEM, SEM, and nitrogen adsorption measurements indicated that the DAAP units were incorporated into MIL-101 pores by complexation, keeping the parent framework intact. The DAAP-MOF enabled facile paraoxon hydrolysis in water/acetonitrile mixtures under ambient conditions (100% conversion after 24 h at pH 10). The MOF-DAAP complexes showed synergistic effects, being 7-fold and 47-fold more active than the parent MIL-101 or DAAP materials, respectively. The high hydrolysis reaction turnover was realized by simultaneous action of the Lewis acid Cr(III) center of the MOF as well as the electron-rich nucleophile, DAAP. This study demonstrates a simple and efficient method of generating catalytically active MOF materials for environmental detoxification as well as defensive applications.

Electrospinning Technology: Direct Application of Tailorable Ultrathin Membranes

that offer advantages for protective outerwear for military applications. A materials evaluation program was initiated by the Army Survivability Directorate in late 1985 to identify potential, alternative commercial waterproof and moisture vapor permeable materials capable of meeting military needs for the Extended Cold Weather Clothing System. After a market survey identified many new semipermeable laminated materials, screen tests for fabric weight, strength, and hydrostatic resistance were used

ELECTROSPUN PHOTOVOLTAIC CELLS

ABSTRACT Electrospinning is a simple technique to form high surface area membranes using large static electric potentials. An application, taking advantage of such a high surface area, is a dye-sensitized photovoltaic cell, wherein a chromophore molecule absorbs light before being oxidized to generate a photocurrent. We report functioning photovoltaic cells made from polyacrylonitrile fibers infused with the azo-dye, Congo Red in a liquid-junction cell assembly. Cell performance of the electrospun cells compared favorably to cells made by spin-coating thin films of polyacrylonitrile and Congo Red. Also reported are the effects of adding nanoparticles of titanium dioxide to the cells. Finally, the maximum amount of dye that can be incorporated into a fiber is presented.

Humidity-Dependent Air Permeability of Textile Materials1:

Changes in fabric structure as hygroscopic fibers swell at high humidities can have a large influence on the measured air permeability of fabrics such as cotton, wool, silk, and nylon. The variation of air permeability as a function of relative humidity is of practical importance in ranking and evaluating candidate textiles for protective clothing applications. This paper describes a test method used to determine the relative humidity dependence of the air permeability of hygroscopic woven textile fabrics. The instru mentation also permits dynamic measurements during a step change in relative humid ity. Typical results are shown for woven fabrics, nonwoven battings, and novel elec trospun fiber mats.

Chemical protection fabrics via surface oximation of electrospun polyacrylonitrile fiber mats

Polyacrylonitrile (PAN) submicron fiber mats were modified to become reactive components of self-detoxifying chemical protection fabrics. Oximation of the mats with excess hydroxylamine resulted in functionalization of the fibers to form polyacrylamidoxime (PAAO). The fiber morphology remains intact after oximation, with fiber diameters ranging from 250 to 500 nm. Nucleophilic amidoxime groups enable the fiber mats to react with organophosphate pesticides or chemical warfare agents (CWA), as demonstrated using diisopropyl fluorophosphate (DFP) as a CWA simulant. The DFP decomposition kinetics were investigated using 31P MAS NMR, which afforded measurement of the observed pseudo-first order reaction rate constant, kobs. The values of kobs with PAAO-functionalized fibers exceed those of the parent PAN fibers by as much as 80-fold at a water content of 130% by weight. The observed reaction rates depend on the amount of the reactive fiber mats, yielding an apparent second-order rate constant, k2 = 1.0 × 10−6 s−1mg−1. The hydrolytic degradation of DFP occurs only in the presence of free water, which serves as a medium to promote the nucleophilic action of the amidoxime groups in the fibers by facilitating proton transfer and stabilizing the transition state.

Chemical and biological decontamination functions of nanofibrous membranes

Novel composite membranes with poly(vinyl alcohol-co-ethylene-g-diallylmelamine) (PVA-co-PE-g-DAM) nanofibers layered on poly(propylene-g-diallylmelamine) (PP-g-DAM) meltblown nonwoven fabric were successfully developed as lightweight and breathable protective materials with chemical and biological decontamination functions. By controlling levels of coated nanofibers, three nanofibrous membranes with similar surface morphology, hydrophilicity and transport properties were prepared. The N-halamine precursor moieties in the membrane matrices can be converted to active N-halamine structures with a diluted sodium hypochlorite solution, and the active chlorine content on the membranes is rechargeable and durable. The chlorinated nanofibrous membranes demonstrated very powerful and rapid biocidal effects against both E. coli and S. aureus by contact, as well as excellent disinfection effect to wet bacterial penetration through the membrane. Furthermore, the efficient chemical detoxification functions of the halamine nanofibrous membranes were also observed by a total decontamination of aldicarb, a carbamate pesticide, within 30 min. These PVA-co-PE-g-DAM nanofiber composite membranes can serve as ideal ultra-light filtering media for chemical and biological protective clothing materials.

Moisture Transport for Reaction Enhancement in Fabrics

The role of water in protective fabrics is critical to comfort and material performance. Excessive perspiration in clothing causes discomfort, and bound water can adversely affect the ability of carbon to adsorb chemicals. Yet the presence of water can also improve the moisture vapor transport of protective polymer films, and is essential for the hydrolytic destruction of nerve agents. Reported here are the findings of wicking and drying experiments conducted on various hydrophilic and hydrophobic cover fabrics that demonstrate the influence of wetting on permeation through fabrics. The influence of water content on reactive polymers capable of degrading nerve agent simulant is also discussed, and the importance of a novel “delivery system” for water to the reactive components through the use of a wicking fabric is introduced.

Electrospinning of Polycarbonates and their Surface Characterization using the SEM and TEM

Unlike conventional spin methods, electrospinning is capable of yielding fibers with sub-micron range diameters and high specific surface areas. The use of such electrospun materials in applications, whose functionality depends on the area available, such as separation processes, will be useful. In this study a Bisphenol-A polycarbonate was dissolved in two solvents: Chloroform and a 1:1 mixture of Tetrahydrofuran (THF) and Dimethylformamide (DMF) and resulting polycarbonate solutions were then electrospun to produce polycarbonate fiber-mats. The morphological features of the electrospun polycarbonate fibers have been studied as a function of the solvent used and also as a function of the processing voltage. The studies were conducted using the SEM, TEM and Scion image analysis program. The results indicate bold differences in the fiber morphology and bead density trends with the solvent used. Electrospun polycarbonate fibers exhibit a “Raisin like” puckered structure. Such a structure will enhance the functional efficiency of an electrospun material when used in an area-based application. In addition, studies on crazing of bulk polycarbonate and the surface features of electrospun polycarbonate fibers have been conducted. Results indicate a relation between crazing and the topological features of electrospun polycarbonates.