Electrospun fibers and light interaction

Electrospinning is able to produce fibers from the nanometer diameter up to a few microns. In the nanometer range, it is smaller than the wavelength of visible light. The reductions in light scattering by nanofibrous fillers are able to give rise to transparent composite. For this, the diameter of the fibers should preferably be less than 100 nm. Bergshoef et al [1999] has successfully constructed transparent composite using electrospun nylon fibers with diameter of about 100 nm as fillers and epoxy as the matrix material. This reduction in light scattering has also enables the construction of electrospun filter media with high transparency. A thin electrospun PAN filter membrane was able to show removal efficiency of more than 95% while allowing 90% transparency [Liu et al 2015]. Similarly, Wu et al (2010) used copper acetate/polyvinyl acetate solution for electrospinning to give nanofibers. With a thin nanofiber layer, the optical transmittance is excellent in the visible and near-infrared ranges. Aligned Cu nanofibers was able to show 90% transmittance at sheet resistance of 25 ohm/sq[Wu et al 2010]. Transparent and flexible electrodes have been constructed by transferring the Cu nanofiber network to poly(dimethylsiloxane) (PDMS) substrate [Wu et al 2010].

While thin layer of nanofibers is able to exhibit good optical transmittance, a thicker layer favors light scattering. Fibers with diameter of about 100 nm and less has been electrospun and as an aggregate, they are easily visible once sufficient amount has been collected on a solid surface. Tang et al (2017) used the light scattering effect of electrospun fibers to improve the luminous efficiency of light emitting diode (LED). Using electrospun poly(lactic-co-glycolic acid) (PLGA) nanofiber films, they were able to achieve a reflectance of 98.8% compared to the BaSO₄ white plate. By depositing electrospun nanofibers average fiber diameter of 475 nm and a film thickness of 194.7 µm on the reflector surface of a LED lamp, the nanofiber films showed a correlated color temperature deviation decrease from 8880 K to 1407 K and a luminous efficiency improvement of 11.66% at 350 mA. Tang et al (2018) carried out further tests to determine the increase in luminous efficacy of LED with the use of electrospun polyacrylonitrile fibers. Their research showed that thicker and denser electrospun layer increases reflectance. A 65 µm layer was able to give a reflectance of 89% while a 212 µm layer raised the reflectance to 98% at wavelength of 450 nm for fiber diameter of 220 nm. This is consistent with the result obtained by Tang et al (2017) using electrospun PLGA fibers with similar thickness and fiber diameters. The reflectance from electrospun reflectance layer was greater than white oil coating circuit substrate (83%) and polymer reflector cup (85%) of the lamp. Their results showed that with 212 µm thickness layer electrospun fibers covering both the substrate and reflector of the LED lamp, there was significant radiant flux increases over reference lamp for blue LED lamps, white LED lamps, outwards remote phosphor layer (ORPL) lamps, and inwards remote phosphor layer (IRPL) lamps. White LED with electrospun layer was able to retain 97.89% luminous flux after a 96-hour aging process.

An advantage of electrospinning in the production of nanofibers is the ability to create ordered fiber alignment. Aligned nanofibers made of appropriate material have the potential to exhibit polarised light emission. The small area and volume it occupies make it possible for integration into microfluidic devices. Pagliara et al (2009) electrospun aligned light-emitting conjugated polymer poly[2-methoxy,5-(2- ethylhexyloxy)-phenylene-vinylene] (MEH-PPV)/poly(methylmethacrylate) (PMMA) by using parallel electrodes collector setup. A laser source shining on the nanofibers causes the embedded fluorophores to absorb and re-emit light isotropically. This polarised light-emitting array can be used to excite flowing dye chromophores in microchannels with a significant increase of imaging signal/excitation background (S/N) ratio with respect to unpolarised detection.

Electrospun material with good optical properties may be used for application such as organic solid state lasers. Christ et al (2022) electrospun bundles of highly aligned polymethyl methacrylate (PMMA) microfibers and showed its potential use as organic solid-state lasers. PMMA solution doped with rhodamine B (RhB) was electrospun into microfibers and excited by green laser. The excited RhB dye would emit red wavelengths which are transmitted by the PMMA fibers. At optimum dye concentration, the photoluminescence spectra of the excited with a laser showed an excitation peak with full-width-at-half-maximum of only 5.05?nm. The lasing threshold from pumping longitudinally was found to reduce with increasing RhB concentration, showing a low lasing threshold of 3.35?µJ at 1070?ppm where a low lasing threshold is more desirable. Pumping traversely, the lasing threshold was found to be 0.55 µm at 1070 and 535 ppm. At a low concentration of 53 ppm, no change in the slope was detected indicating that at traverse excitation, a minimum dye concentration is required.

In the interaction of electrospun fibers with light, another aspect is in the blocking of harmful ultraviolet (UV) rays. Sheng et al (2023) added UV absorber 329 (UV329) to PAN to produce electrospun UV protective membrane. With just 0.5 wt% of UV329 blended into PAN solution and electrospun into a membrane, the resultant membrane was able to exhibit an ultraviolet protection factor (UPF) of 777 and UVA transmittance (T(UVA)) of 1.1%>. This demonstrated excellent UV protection by the membrane as the Evaluation of UV Protection Performance of Textiles standard (GB/T 18830-2009), requires products with UPF > 40.0 and T(UVA) < 5.0% to be labeled as UV protection products. With 1 wt% UV329 added, the UPF increased to 854. An addition of 0.5 wt% TiO₂ nanoparticles with 1 wt% UV329 to the electrospun PAN membrane increased the UPF to 1352 although further increase in TiO₂ reduces the UPF. This has been attributed to the light scattering properties of TiO₂ nanoparticles especially at longer wavelengths. Higher concentration of TiO₂ nanoparticles result in greater aggregation and this reduces the light scattering ability of the nanoparticles.