Transport properties of porous membranes based on electrospun nanofibers
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.