In this article, the transparency of hydrophilic electrospun mats is studied. Results showing how transmittance varies under the action of water are presented. It is observed that swelling plays a crucial role in the transmittance of the material, changing it from opaque to transparent when it is wet. Atomic force microscope measurements show that the diameter distribution of nanofibers is modified during both the wetting and the drying of the mats. Using these distributions and modeling the change in the relative refractive index as a composite material, a qualitative explanation of the mat scattering behavior by using the Mie scattering theory for cylinders has been done. The obtained results indicate that changes on the optical response produced by water contact are different according to the mat thickness: samples with smaller thicknesses can act as a water sensor with a persistent response over time, while samples with thicknesses greater than 7 µm can act as sensors for drying time.
This work presents an electrospun nanofibrous membrane for water treatment, designed to reduce its pores through polymer swelling to retain contaminants, and after that, reopening them for easy cleaning. It consists of a polyvinyl alcohol (PVA) mat crosslinked with a natural agent to avoid water solubility but allow a high swelling of its nanofibers. Then, when the membrane is brought into contact with water, a transitory state occurs during which nanofibers increase their diameter 68%, closing the pores between them. For studying the swelling reversal effect, distilled water filtering was used observing a decrease in the permeate flow over time until a steady state is reached. This phenomenon is explained by the closure of pores, and it is described with a fouling model widely used in the literature. Thanks to this decrease in the pore size, the membrane achieves a rejection rate much higher than conventional PVA electrospun membranes, being capable to retain 20 nm nanoparticles with a rejection rate up to 99%. Swelling is reversible through a simple drying process, which allows reopening the pores and cleaning the fouling easily. Both the membrane and its use strategy extend the capacity of electrospun mats in a sustainable way.
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