Application of electrospun membrane with inter-fibers nanowebs

Nanowebs are typically made out of nanofibers with diameters that are an order of magnitude smaller than the fibers surrounding it and typically about 20 to 30 nm in diameter. While the exact mechanism for the formation of nanowebs is still unclear, it has generally been shown that higher charges on the solution through the addition of salt facilitated in its formation [Zhang et al 2017]. The interfiber pore size for the nanoweb is also much smaller than regular electrospun fiber pore sizes. Such small fiber diameter gives it much higher specific surface area and it has the potential to significantly enhance functions that are dependent on surface area.

Electrospun nanofibers have been used as commercial air filtration media as the smaller fiber diameter facilitates particle capturing with low pressure drop. Model simulation using a 3-D structure resembling nanofiber (diameter less than 200 nm) filter media, low air flow and no-slip boundary conditions, nanoparticles less than 100 nm are shown to be mostly captured through Brownian diffusion while larger particles are captured through interception with the fiber. To further enhance the particle capturing ability of electrospun membrane, smaller diameter fibers may be constructed. Zhang et al (2017) electrospun a poly(m-phenylene isophthalamide) nanofiber/nanowebs for ultrathin high-efficiency air filter with the diameter of the fibers forming nanoweb having a diameter of 20 nm. The resultant electrospun air filter media has an ultra-low penetration air filter level of 99.999% and low pressure drop of 92Pa for 300 - 500nm particles by size exclusion.

Apart from air filtration, nanowebs may also be used in water filtration. Li et al (2016b) demonstrated the use of such membrane for microfiltration. Using electrospun polyvinylidene fluoride (PVDF) with tetrabutylammonium chloride (TBAC) added, the resultant membrane was able to reject 99.9% of 300 nm polystyrene particles and a high pure water flux of 2.88 x 104 L.m^-2h^-1 under the pressure of 25 psi. With electrospun PVDF membrane without the nanowebs, the rejection was only 46%. Despite the good rejection rate, more studies are needed to determine the drop in flux as the particles accumulates and the recovery capability of this membrane.

Breathable and waterproof membrane works by having pores that repelling water droplets while allowing water vapor to diffuse through it. Similar to the pores found in Gore-Tex, pores of electrospun membranes are also formed from interfiber spaces. To enhance moisture transport, a double layer system may be used with the inner layer hydrophobic and the outer layer hydrophilic to draw moisture away from the skin through the hydrophobic layer. Ju et al (2017) constructed an electrospun composite membrane with the top layer made of thermoplastic polyurethane (TPU) with tetrabutylammonium chloride (TBAC) and the bottom layer of TPU. The electrospun TPU/TBAC fiber layer was hydrophilic with nanowebs between the larger fibers. Electrospun TPU layer in contrast was hydrophobic. The resultant composite membrane showed significantly better water vapour transmission (WVT) rate, hydrostatic pressure and air permeability compared to commercial TPU membrane. The nanoweb of the TPU/TBAC layer with its much smaller interfiber pore size could prevent water from passing through even though the layer was hydrophilic.

With the right selection of material, electrospun fibers and nanowebs may be used to enhance the material water swelling ability. Zhao et al (2015) constructed a composite water swellable rubber (WSR) made of electrospun crosslinked poly(acrylic acid) (PAA) nanofibers with nanowebs and hydrophobic rubber. Common WSR is comprised of hydrophobic rubber and hydrophilic, water absorbent resin. However, due to poor interface between the two components, the water absorbent resin particles often detaches from the rubber and the water swelling function gets reduced. In the form of electrospun membrane, the network of water absorbent PAA embedded within the rubber makes it very difficult for PAA to escape. With the high specific surface area of electrospun cross-linked PAA with nanowebs, the swelling ability of the WSR was shown to improve.

High surface area of the ultra-fine nanofibers that forms the nanowebs makes it very responsive to the environment. As sensors, conventional electrospun nanofibers with diameters in the hundreds of nanometers have been shown to be much more sensitive to analytes compared to film, having nanowebs with fiber diameter in the tens of nanometers will certainly be even more sensitive. Wang et al (2011) electrospun polyethyleneimine (PEI) functionalized polyamide 6 (PA 6) (PEI-PA 6) nanowebs on a quartz crystal microbalance (QCM) as sensor for humidity detection. The resultant sensor showed fast response/recovery time to humidity and outperform current porous structure-based sensors. The same research team (Ding et al 2011) used the same setup, electrospun polyethyleneimine (PEI) functionalized polyamide 6 (PA 6) (PEI-PA 6) nanowebs on quartz crystal microbalance (QCM) for highly sensitive formaldehyde detection. The constructed sensor system showed low detection limit (50 ppb), rapid response, superior selectivity and good reproducibility.

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