Study of the filtration performance of multilayer and multiscale fibrous structures

materials Article Study of the Filtration Performance of Multilayer and Multiscale Fibrous Structures Vânia Pais 1,2,*, Carlos Mota 1,2, João Bessa 1,2, JoséGuilherme Dias 3, Fernando Cunha 1,2 and Raul Fangueiro 1,2,4   Citation: Pais, V.; Mota, C.; Bessa, J.; Dias, J.G.; Cunha, F.; Fangueiro, R. Study of the Filtration Performance of Multilayer and Multiscale Fibrous Structures. Materials 2021,14, 7147. https://doi.org/10.3390/ma14237147 Academic Editor: Dubravko Rogale Received: 11 October 2021 Accepted: 19 November 2021 Published: 24 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Fibrenamics, Institute of Innovation on Fiber-based Materials and Composites, University of Minho, 4800 Guimarães, Portugal; [email protected] (C.M.); [email protected] (J.B.); [email protected] (F.C.); [email protected] (R.F.) 2Centre for Textile Science and Technology (2C2T), University of Minho, 4800 Guimarães, Portugal 3Poleva—Termoconformados, S.A. Rua da Estrada, 4610 Felgueiras, Portugal; [email protected] 4Department of Mechanical Engineering, University of Minho, 4800 Guimarães, Portugal *Correspondence: [email protected] Abstract: As the incidence of small-diameter particles in the air has increased in recent decades, the development of efficient filtration systems is both urgent and necessary. Nanotechnology, more precisely, electrospun nanofibres, has been identified as a potential solution for this issue, since it allows for the production of membranes with high rates of fibres per unit area, increasing the probability of nanoparticle collision and consequent retention. In the present study, the electrospinning technique of polyamide nanofibre production was optimized with the variation of parameters such as polymer concentration, flow rate and needle diameter. The optimized polyamide nanofibres were combined with polypropylene and polyester microfibres to construct a multilayer and multiscale system with an increased filtration efficiency. We observed that the penetration value of the multilayer system with a PA membrane in the composition, produced for 20 min in the electrospinning, is 2.7 times smaller than the penetration value of the system with the absence of micro and nano fibers. Keywords: electrospinning; nanofibres; filtration; particles retention; multilayer systems 1. Introduction The incidence of particles with a small diameter (lower than 2.5 µ m) in the air has risen in recent decades [ 1 ]. The rapid growth of urbanization and industrialization has led to a release of small particles to the atmosphere, such as solid particles and liquid droplets, which is concerning. Particles with diameters smaller than 2.5 µ m may cause considerable damage due to their ability to penetrate the human bronchi, lungs and even the extrapulmonary organs. Furthermore, these particles can be linked to bacteria or viruses and cause serious human health problems due to the development of acute and chronic diseases [ 2 , 3 ]. Developing a solution to this problem is extremely important, and certain approaches such as filtration membranes have been identified as useful. These filter membranes can be applied in various products, such as face masks and NBC suits (protection against nuclear, biological, and chemical warfare agents) [4]. There are several types of particles that require filtering and each one of them has unique properties. The particle’s diameter is one such property, and it can span from a few nanometers, as is the case for antibodies and viruses, to microns, such as for pollen [ 5 ]. Figure 1 shows the previously mentioned variation in particle size retention. Therefore, since there are different particles with distinct sizes, the filtration process should be optimized, depending on the objectives. Permeability, filtration performance and the uniformity of the structure are the three principal factors to consider when developing or applying a filtration process. Permeability, mostly related to breathability and water vapour transmission, should be optimized to make the structure wearable without compromising the Materials 2021,14, 7147. https://doi.org/10.3390/ma14237147 https://www.mdpi.com/journal/materials Materials 2021,14, 7147 2 of 15 filtration efficiency [ 4 ]. Concerning the filtration performance, the filtration theory provides that the efficiency of this process increases with a decrease in the dimension of the fibres that compose the filter. This statement relates to the increase in fibres per unit area, which leads to an increase in the probability of impact between the filter and particles that need to be filtered. The uniformity is related to efficacy [6]. Materials 2021, 14, x FOR PEER REVIEW 2 of 15 and water vapour transmission, should be optimized to make the structure wearable without compromising the filtration efficiency [4]. Concerning the filtration performance, the filtration theory provides that the efficiency of this process increases with a decrease in the dimension of the fibres that compose the filter. This statement relates to the increase in fibres per unit area, which leads to an increase in the probability of impact between the filter and particles that need to be filtered. The uniformity is related to efficacy [6]. Figure 1. Different particle sizes to be filtered. The filtration process occurs through different methods, dependent on the size of the particle requires filtering, and the parameter that most influences this. In Table 1 the different mechanisms of filtration are described, corresponding with the particles size being filtered. Larger particles are usually trapped by gravity sedimentation, since the pore sizes of filters are smaller than the particles size, blocking the particles outside the porous structure. Inertial impaction is also a possibility in the retention of larger particles. This mechanism occurs when the particles do not follow the direction of the airflow due to their large inertia. Thus, when associating high speed with larger particles, there is an increase in the probability of collision between particles and fibres. After collision, the particles can adhere to the fibres, and are retained in the filter. The interception mechanism is related to the retention of particles below 0.6 µm and occurs when the particles follow the airflow. Eventually, the particles come into contact with fibres that compose the filter and remain connected by Brownian forces. The efficiency of this process increases with the decrease in particle size. For nano-sized particles (below 0.2 µm), diffusion is the predominant mechanism. The particles do not follow the streamline direction and have a very slow and random movement. At some point, the particles and fibres collide and remain attached. Electrostatic attraction occurs in particles of different dimensions and occurs when the fibres are electrically charged and can capture the particles that are oppositely charged [5,7,8]. Table 1. Mechanisms of filtration and respective particle sizes to be filtered [7]. Mechanisms of Filtration Size of Particles Gravity sedimentation Between 1 and 10 µm Inertial impaction Above 0.6 µm Interception Below 0.6 µm Diffusion Below 0.2 µm Electrostatic attraction Charged particles Concerning the retention of small particles, diffusion is the predominant mechanism of filtration. The diffusion efficiency increases with a decrease in the diameter of the fibres that constitute the filter. However, when the particles have a size of around 0.3 µm, the retention process can be harder to achieve because the diffusion mechanism may not occur. Therefore, to increase retention via interception, a multi-layer approach should be applied [5]. To promote nanoparticles’ retention, two factors should be considered: the Figure 1. Different particle sizes to be filtered. The filtration process occurs through different methods, dependent on the size of the particle requires filtering, and the parameter that most influences this. In Table 1the different mechanisms of filtration are described, corresponding with the particles size being filtered. Larger particles are usually trapped by gravity sedimentation, since the pore sizes of filters are smaller than the particles size, blocking the particles outside the porous structure. Inertial impaction is also a possibility in the retention of larger particles. This mechanism occurs when the particles do not follow the direction of the airflow due to their large inertia. Thus, when associating high speed with larger particles, there is an increase in the probability of collision between particles and fibres. After collision, the particles can adhere to the fibres, and are retained in the filter. The interception mechanism is related to the retention of particles below 0.6 µ m and occurs when the particles follow the airflow. Eventually, the particles come into contact with fibres that compose the filter and remain connected by Brownian forces. The efficiency of this process increases with the decrease in particle size. For nano-sized particles (below 0.2 µ m), diffusion is the predominant mechanism. The particles do not follow the streamline direction and have a very slow and random movement. At some point, the particles and fibres collide and remain attached. Electrostatic attraction occurs in particles of different dimensions and occurs when the fibres are electrically charged and can capture the particles that are oppositely charged [5,7,8]. Table 1. Mechanisms of filtration and respective particle sizes to be filtered [7]. Mechanisms of Filtration Size of Particles Gravity sedimentation Between 1 and 10 µm Inertial impaction Above 0.6 µm Interception Below 0.6 µm Diffusion Below 0.2 µm Electrostatic attraction Charged particles Concerning the retention of small particles, diffusion is the predominant mechanism of filtration. The diffusion efficiency increases with a decrease in the diameter of the fibres that constitute the filter. However, when the particles have a size of around 0.3 µ m, the retention process can be harder to achieve because the diffusion mechanism may not occur. Therefore, to increase retention via interception, a multi-layer approach should be applied [ 5 ]. To promote nanoparticles’ retention, two factors should be considered: the use of fibres with very small diameters in the filter membrane and the application of a multi-layer system [9–11]. Concerning the relationship between fibres with lower diameters and higher efficiencies, nanofibres have been identified as a solution with great potential. Nanofibre filters Materials 2021,14, 7147 3 of 15 have a controllable small diameter, low basis weight, high permeability values, reduced thickness and a porous structure [ 7 , 12 ]. The electrospinning technique is a simple and effective method used to produce fibres at a nanoscale. In this methodology, a high electric field is applied which promotes repulsive interactions among the polymeric solution, and the Taylor cone is formed. When the electrostatic forces overlap the repulsive interactions, a charged jet is ejected from the Taylor cone with a dynamic whipping. Concurrently, the solvent evaporates, and the jet is stretched into fibres with finer diameters that are deposited on a grounded collector [ 13 , 14 ]. The electrospun nanofibre diameters can range from a few nanometers to micrometres. Additionally, this filtration layer can be produced with distinct raw materials and by the application of different parameters, obtaining specific nanofibres with several functionalities. So, the nanoscale diameters and the interconnected porous structure of the electrospun nanofibres make the electrospinning technique a very attractive approach for filtration applications. Furthermore, the static charge, which is a result of the electrospinning process, may remain on the fibres that have been produced and enhance the filtration retention by electrostatic attraction [12,15,16]. The selection of the most suitable polymer to produce the fibres to be applied in particles retention should consider good mechanical properties, hydrophobicity, biocompatibility and compatibility with non-toxic solvents [ 17 – 19 ]. Polyamide (PA) stands out as a potential polymer since it has excellent chemical stability and thermal resistance. It is a synthetic polymer that is biodegradable and biocompatible. Usually, polyamide is dissolved in formic acid, and this combination can be electrospun to efficiently produce nanometric fibres [ 17 , 18 ], as opposed to polycaprolactone (PCL) (for example), which is usually dissolved in chloroform and dimethylformamide (DMF) [ 11 ]. According to EU directive 67/548/EEC, DMF is toxic [18]. In this study, electrospun nanofibres were produced with PA polymers to optimize systems with a higher filtration efficiency. Firstly, the parameters’ polymers concentration, flow rate and needle’s diameter were optimized to obtain fibres with very low diameters and a mat with controlled porosity. The morphology and intrinsic properties of the produced nanofibres were analyzed. In the second part of the study, several combinations of the optimized electrospun PA nanofibres with polypropylene (PP) and polyester (PES) microfibres were analyzed to obtain a multi-layer and multiscale system with a high retention capacity of small particles. The performance of the obtained combinations was evaluated by measuring the filtering material penetration, air permeability and breathing resistance. 2. Materials and Methods 2.1. Materials PA 6.6 pellets (with a molecular weight of 262.35 g/mol, Tm = 250–260 ◦C, density = 1.14 g/mL at 25 ◦ C, Sigma Aldrich, St. Louis, MO, USA) were used as a polymeric matrix. The solvent used was formic acid (FA) (98–100%, Fisher Scientific, Leics, UK). The PP microfibres membrane (weight = 50 g per square meter (gsm), thickness = 360 µ m, average fibre diameter = 3.7 µ m), applied as a substrate for PA nanofibres, were obtained from Protechnic S.A. (Cernay, France). 2.2. Production of Electrospun PA Membranes The polymeric solution was optimized after studying different PA concentrations (20% w/v and 25% w/v) to obtain fibres with small diameters and without defects. The solvent applied was FA in 1:1 proportion. The polymeric solution was prepared through the dissolution of PA pellets in FA, for at least 6 h at 30 ◦C, at constant stirring. PA nanofibre webs were produced by electrospinning NF-103 from MECC Co., Ltd. (Fukuoka, Japan). The electrospinning parameters were also optimized to obtain fibres with small diameters to increase filtration efficacy. The tested parameters include the flow rate (ranging from 0.4 to 2 mL/h), needle’s diameter (0.33, 0.41 and 0.61 mm) and the fibre deposition time (10, 20 and 30 min). The voltage applied was 28 kV and the collector-needle distance was 100 mm. The electrospinning process was conducted at 60% ± 5 RH and Materials 2021,14, 7147 4 of 15 20 ◦C±2 . The group’s previous studies were consulted to define certain fixed parameters, including voltage, collector-needle distance and the applied solvent [ 20 , 21 ]. The studied conditions and the corresponding produced membranes are presented in Table 2. Table 2. Operational conditions tested during electrospinning production. Sample Solution Parameters Electrospinning Parameters Concentration (% (w/v)) Solvent Voltage (kV) Collector-Needle Distance (mm) Flow Rate (mL/h) Needle-Diameter (mm) A 20 100% FA 28 100 0.4 0.33 B 0.8 C 1 D 0.4 0.41 E 0.8 F 1 G 1 0.61 H 2 I 25 0.4 0.41 After optimizing the ideal conditions to produce the nanofibres, deposition was performed over PP microfibres to construct a multilayer and multiscale system with higher performance in terms of small particles retention. Another layer of PP microfibres were added to the PA nanofibres deposited above the PP microfibres. Thus, the filtration layer is composed of 3 layers: PP microfibres, PA nanofibres and PP microfibres. Two different polyester (PES) nonwovens were added to the filtration layer, one for the inner layer— PES IL—and the other for the outer layer—PES OL. A schematic representation of the multilayer system is presented in Figure 2. A total of 4 different combinations were obtained, as represented in Table 3. The difference between the multiple combinations relates to the nanofibre production time, of 0, 10, 20 and 30 min. The membranes were combined through a thermoforming process using a mould with a face-mask shape. However, it is important to mention that the structure can be molded to other shapes for application in other types of products. 2.3. Electrospun PA Membranes Characterization The morphology of the produced PA nanofibres was investigated using a scanning electron microscope (SEM). The analyses were performed using a NOVA 200 Nano SEM from the FEI Company (Hillsboro, OR, USA). Due to the polymeric nature of the analysed specimens, the samples were vacuum metalized with a thin film of gold-palladium (Au-Pd) before the analysis. The average diameters and the porous distribution of the fibres were calculated by taking measurements from different regions using ImageJ software (1.52a). The chemical composition and the structural aspects of the electrospun nanofibres were analyzed using Fourier transform infrared spectroscopy (FTIR) coupled with the attenuated reflection (ATR) technique using IRAffinity-1S, SHIMADZU equipment ( Kyoto, Japan ). All spectra were obtained in the transmittance mode with 45 scans over a wavenumber range of 4000–400 cm−1. The thermal behaviour of the electrospun nanofibres was assessed via a thermogravimetric analysis (TGA) of the electrospun fibres, which was performed with an STA 700 from HITACHI (Tokyo, Japan). The samples were tested in a temperature range from 25 ◦ C to 500 ◦C, at a heating rate of 10 ◦C/min. Materials 2021,14, 7147 5 of 15 Materials 2021, 14, x FOR PEER REVIEW 4 of 15 solvent applied was FA in 1:1 proportion. The polymeric solution was prepared through the dissolution of PA pellets in FA, for at least 6 h at 30 °C, at constant stirring. PA nanofibre webs were produced by electrospinning NF-103 from MECC Co., Ltd. (Fukuoka, Japan). The electrospinning parameters were also optimized to obtain fibres with small diameters to increase filtration efficacy. The tested parameters include the flow rate (ranging from 0.4 to 2 mL/h), needle’s diameter (0.33, 0.41 and 0.61 mm) and the fibre deposition time (10, 20 and 30 min). The voltage applied was 28 kV and the collectorneedle distance was 100 mm. The electrospinning process was conducted at 60% ± 5 RH and 20 °C ± 2. The group’s previous studies were consulted to define certain fixed parameters, including voltage, collector-needle distance and the applied solvent [20,21]. The studied conditions and the corresponding produced membranes are presented in Table 2. Table 2. Operational conditions tested during electrospinning production. Sample Solution Parameters Electrospinning Parameters Concentrati on (% (w/v)) Solvent Voltage (kV) CollectorNeedle Distance (mm) Flow Rate (mL/h) NeedleDiameter (mm) A 20 100% FA 28 100 0.4 0.33 B 0.8 C 1 D 0.4 0.41 E 0.8 F 1 G 1 0.61 H 2 I 25 0.4 0.41 After optimizing the ideal conditions to produce the nanofibres, deposition was performed over PP microfibres to construct a multilayer and multiscale system with higher performance in terms of small particles retention. Another layer of PP microfibres were added to the PA nanofibres deposited above the PP microfibres. Thus, the filtration layer is composed of 3 layers: PP microfibres, PA nanofibres and PP microfibres. Two different polyester (PES) nonwovens were added to the filtration layer, one for the inner layer—PES IL—and the other for the outer layer—PES OL. A schematic representation of the multilayer system is presented in Figure 2. A total of 4 different combinations were obtained, as represented in Table 3. The difference between the multiple combinations relates to the nanofibre production time, of 0, 10, 20 and 30 min. The membranes were combined through a thermoforming process using a mould with a face-mask shape. However, it is important to mention that the structure can be molded to other shapes for application in other types of products. Figure 2. Multilayer system produced. Figure 2. Multilayer system produced. Table 3. Multilayer systems tested in terms of filtration efficiency. Reference Inner Layer Filtration Layer Outer Layer PA Deposition Time Multilayer_0 min PES IL PP microfibres/PP microfibres (Note: without PA nanofibres) PES OL 0 min Multilayer _10 min PP microfibres/ PA (10 min)/ PP microfibres 10 min Multilayer _20 min PP microfibres/ PA nanofibres (20 min)/ PP microfibres 20 min Multilayer _30 min PP microfibres/ PA nanofibres (30 min)/ PP microfibres 30 min 2.4. Electrospun PA + Nonwoven Fabrics Characterization The PP microfibres, PES IL and PES OL purchased from Protechnic S.A. (Cernay, France) were also studied. Morphology was analyzed through brightfield microscopy using a Microscope Leica DM750 M (brightfield) (Leica, Wetzlar, Germany) with a coupled camera. The areal mass of several layers from the multilayer system was calculated by dividing the weight of the sample mat by its effective area. The thicknesses of the different membranes and multilayers systems were measured by analyzing the cross-section at the corresponding SEM images. The areal mass and thickness were obtained by performing measurements on 10 different regions of the samples. The air permeability, filtration and respiratory evaluations were performed according to the standard EN 149:2001+A1:2009., For the air permeability evaluation, 40 Pa pressure was applied, using an air permeability tester from TEXTEST instruments (Zurich, Switzerland), model FX 3300. The tests related to filtration and respiratory evaluation were performed at Aitex—textile research institute, in Spain. The penetration of sodium chloride aerosol was tested by applying a flow rate of 95 L/min, and the maximum value was registered after 3.5 min of exposure. To perform the respiratory resistance evaluation, the pressure at 3 different conditions was registered, namely, inhalation at 30 L/min, inhalation at 95 L/min and exhalation at 160 L/min. 3. Results 3.1. Electrospun Fibre Characterization 3.1.1. Fourier Transform Infrared Spectroscopy and Thermogravimetric Analysis The PA nanofibres were produced using an electrospinning technique according to the conditions reported in Table 2. The FTIR spectra of the nanofibres produced with PA polymer are shown in Figure 3. From the figure, it is possible to identify one significant band at 3300 cm −1 , typically attributed to the N-H stretching vibration. The asymmetric and symmetric stretching of CH 2 appears at 2931 and 2860 cm −1 , respectively. In this spectrum, amide bands of the PA 6.6. appear at 1637 and 1537 cm −1 , and the bands located at 1145 cm −1 correspond to the CO–CH symmetric bending vibration when combined with CH 2 twisting. The bands at 935 and 688 cm −1 are typically attributed to the stretching and Materials 2021,14, 7147 6 of 15 bending vibrations of C–C bonds and the band at 580 cm −1 may be a result of O=C–N bending. The FTIR spectra obtained agree with those of other authors as a PA 6.6. FTIR spectrum [22–25]. Materials 2021, 14, x FOR PEER REVIEW 6 of 15 3. Results 3.1. Electrospun Fibre Characterization 3.1.1. Fourier Transform Infrared Spectroscopy and Thermogravimetric Analysis The PA nanofibres were produced using an electrospinning technique according to the conditions reported in Table 2. The FTIR spectra of the nanofibres produced with PA polymer are shown in Figure 3. From the figure, it is possible to identify one significant band at 3300 cm−1, typically attributed to the N-H stretching vibration. The asymmetric and symmetric stretching of CH2 appears at 2931 and 2860 cm−1, respectively. In this spectrum, amide bands of the PA 6.6. appear at 1637 and 1537 cm−1, and the bands located at 1145 cm−1 correspond to the CO–CH symmetric bending vibration when combined with CH2 twisting. The bands at 935 and 688 cm−1 are typically attributed to the stretching and bending vibrations of C–C bonds and the band at 580 cm−1 may be a result of O=C–N bending. The FTIR spectra obtained agree with those of other authors as a PA 6.6. FTIR spectrum [22–25]. Figure 3. Fourier transform infrared spectra of electrospun PA nanofibres. 3.1.2. Thermogravimetric Analysis To study the polymer behaviour at different temperatures, a TGA analysis was performed. The results are represented in Figure 4. At the TGA plot, a rapid decline in values starting at 350 °C is visible. No significant changes are observed preceding this temperature value. In this way, the PA polymer is found to have a very resistant thermal profile, that is only able to support temperatures up to approximately 320 °C. After this, a single-step degradation of the electrospun fibres is visible. The obtained results are in accordance with the results of other authors [26–28]. Figure 4. Thermogravimetric analysis curves of electrospun PA nanofibres. 0 50 100 0 50 100 150 200 250 300 350 400 450 500 Weight (%) Temperature (°C) Figure 3. Fourier transform infrared spectra of electrospun PA nanofibres. 3.1.2. Thermogravimetric Analysis To study the polymer behaviour at different temperatures, a TGA analysis was performed. The results are represented in Figure 4. At the TGA plot, a rapid decline in values starting at 350 ◦ C is visible. No significant changes are observed preceding this temperature value. In this way, the PA polymer is found to have a very resistant thermal profile, that is only able to support temperatures up to approximately 320 ◦ C. After this, a single-step degradation of the electrospun fibres is visible. The obtained results are in accordance with the results of other authors [26–28]. Materials 2021, 14, x FOR PEER REVIEW 6 of 15 3. Results 3.1. Electrospun Fibre Characterization 3.1.1. Fourier Transform Infrared Spectroscopy and Thermogravimetric Analysis The PA nanofibres were produced using an electrospinning technique according to the conditions reported in Table 2. The FTIR spectra of the nanofibres produced with PA polymer are shown in Figure 3. From the figure, it is possible to identify one significant band at 3300 cm−1, typically attributed to the N-H stretching vibration. The asymmetric and symmetric stretching of CH2 appears at 2931 and 2860 cm−1, respectively. In this spectrum, amide bands of the PA 6.6. appear at 1637 and 1537 cm−1, and the bands located at 1145 cm−1 correspond to the CO–CH symmetric bending vibration when combined with CH2 twisting. The bands at 935 and 688 cm−1 are typically attributed to the stretching and bending vibrations of C–C bonds and the band at 580 cm−1 may be a result of O=C–N bending. The FTIR spectra obtained agree with those of other authors as a PA 6.6. FTIR spectrum [22–25]. Figure 3. Fourier transform infrared spectra of electrospun PA nanofibres. 3.1.2. Thermogravimetric Analysis To study the polymer behaviour at different temperatures, a TGA analysis was performed. The results are represented in Figure 4. At the TGA plot, a rapid decline in values starting at 350 °C is visible. No significant changes are observed preceding this temperature value. In this way, the PA polymer is found to have a very resistant thermal profile, that is only able to support temperatures up to approximately 320 °C. After this, a single-step degradation of the electrospun fibres is visible. The obtained results are in accordance with the results of other authors [26–28]. Figure 4. Thermogravimetric analysis curves of electrospun PA nanofibres. 0 50 100 0 50 100 150 200 250 300 350 400 450 500 Weight (%) Temperature (°C) Figure 4. Thermogravimetric analysis curves of electrospun PA nanofibres. 3.1.3. Morphological Analysis The PA nanofibre morphology was studied using SEM images. The images and the corresponding fibre diameters and porous distribution are represented in Table 4. Materials 2021,14, 7147 7 of 15 Table 4. SEM images of the produced PA nanofibres and corresponding characteristics. Sample and Production Parameters SEM Images (×1000) SEM Images (×5000) Fiber Diameter ±STDEV (nm) Porous Distribution (%) A Flow rate = 0.4 mL/h Needle-diameter = 0.33 mm Materials 2021, 14, x FOR PEER REVIEW 7 of 15 3.1.3. Morphological Analysis The PA nanofibre morphology was studied using SEM images. The images and the corresponding fibre diameters and porous distribution are represented in Table 4. The SEM images reveal that randomly deposited fibres were produced. This feature is important, as filtration aims to produce a structure composed of innumerable pores with very small dimensions. This way, the probability of promoting the retention of nanoparticles is enhanced. The porous distribution, based on size, was measured using imageJ software. The obtained values ranged from 21 to 56%. To promote higher filtration efficiencies, a perfect combination of the fibres diameter and porous distribution should be achieved—a small fibre diameter allows for a lot of tinny pores, but the percentual distribution based on the size of these pores should be as small as possible, to increase the retention of particles probability. The PA nanofibres with the highest polymer concentration have the highest value of porous distribution. Most of the samples obtained percentage distribution values varying from 20 to 30%, thus emphasizing the applicability of PA nanofibres in the field of filtration. It is also important to produce fibre matts with higher densities, to ensure an increase in the collision points’ probability. For this purpose, samples A, D, E, F, G and H are identified as viable options. Regarding the diameters of the fibres, samples A and D were found to be significant. Both samples were produced with a flow rate of 0.4 mL/h, with a needle with a diameter of 0.33 mm applied on sample A, and a needle with a diameter of 0.41 mm applied on sample B. Although the two samples have the two lowest mean fibre diameters, sample B had a smaller value—293 nm. Furthermore, samples A and D also have the lowest standard deviation value, which means that the produced fibres are more uniform with each other in terms of size. Sample I was produced using the same ideal conditions as sample B, however, the polymer proportion was higher (25% w/v instead of 20% w/v). When comparing sample I with the others, it is clear that the number of fibres produced per unit area was considerably lower. The average diameters of the produced fibres, as well as the corresponding standard deviation, are higher when compared to the other samples. This result reveals that the application of 20% (w/v) PA is the best approach to produce homogeneous mats of PA nanofibres with smaller diameters.. Table 4. SEM images of the produced PA nanofibres and corresponding characteristics. Sample and Production Parameters SEM Images (×1000) SEM images (×5000) Fiber Diameter ± STDEV (nm) Porous Distribution (%) A Flow rate = 0.4 mL/h Needle-diameter = 0.33 mm 302 ± 46 21.66 B Flow rate = 0.8 mL/h Needle-diameter = 0.33 mm 442 ± 170 48.97 Materials 2021, 14, x FOR PEER REVIEW 7 of 15 3.1.3. Morphological Analysis The PA nanofibre morphology was studied using SEM images. The images and the corresponding fibre diameters and porous distribution are represented in Table 4. The SEM images reveal that randomly deposited fibres were produced. This feature is important, as filtration aims to produce a structure composed of innumerable pores with very small dimensions. This way, the probability of promoting the retention of nanoparticles is enhanced. The porous distribution, based on size, was measured using imageJ software. The obtained values ranged from 21 to 56%. To promote higher filtration efficiencies, a perfect combination of the fibres diameter and porous distribution should be achieved—a small fibre diameter allows for a lot of tinny pores, but the percentual distribution based on the size of these pores should be as small as possible, to increase the retention of particles probability. The PA nanofibres with the highest polymer concentration have the highest value of porous distribution. Most of the samples obtained percentage distribution values varying from 20 to 30%, thus emphasizing the applicability of PA nanofibres in the field of filtration. It is also important to produce fibre matts with higher densities, to ensure an increase in the collision points’ probability. For this purpose, samples A, D, E, F, G and H are identified as viable options. Regarding the diameters of the fibres, samples A and D were found to be significant. Both samples were produced with a flow rate of 0.4 mL/h, with a needle with a diameter of 0.33 mm applied on sample A, and a needle with a diameter of 0.41 mm applied on sample B. Although the two samples have the two lowest mean fibre diameters, sample B had a smaller value—293 nm. Furthermore, samples A and D also have the lowest standard deviation value, which means that the produced fibres are more uniform with each other in terms of size. Sample I was produced using the same ideal conditions as sample B, however, the polymer proportion was higher (25% w/v instead of 20% w/v). When comparing sample I with the others, it is clear that the number of fibres produced per unit area was considerably lower. The average diameters of the produced fibres, as well as the corresponding standard deviation, are higher when compared to the other samples. This result reveals that the application of 20% (w/v) PA is the best approach to produce homogeneous mats of PA nanofibres with smaller diameters.. Table 4. SEM images of the produced PA nanofibres and corresponding characteristics. Sample and Production Parameters SEM Images (×1000) SEM images (×5000) Fiber Diameter ± STDEV (nm) Porous Distribution (%) A Flow rate = 0.4 mL/h Needle-diameter = 0.33 mm 302 ± 46 21.66 B Flow rate = 0.8 mL/h Needle-diameter = 0.33 mm 442 ± 170 48.97 302 ±46 21.66 B Flow rate = 0.8 mL/h Needle-diameter = 0.33 mm Materials 2021, 14, x FOR PEER REVIEW 7 of 15 3.1.3. Morphological Analysis The PA nanofibre morphology was studied using SEM images. The images and the corresponding fibre diameters and porous distribution are represented in Table 4. The SEM images reveal that randomly deposited fibres were produced. This feature is important, as filtration aims to produce a structure composed of innumerable pores with very small dimensions. This way, the probability of promoting the retention of nanoparticles is enhanced. The porous distribution, based on size, was measured using imageJ software. The obtained values ranged from 21 to 56%. To promote higher filtration efficiencies, a perfect combination of the fibres diameter and porous distribution should be achieved—a small fibre diameter allows for a lot of tinny pores, but the percentual distribution based on the size of these pores should be as small as possible, to increase the retention of particles probability. The PA nanofibres with the highest polymer concentration have the highest value of porous distribution. Most of the samples obtained percentage distribution values varying from 20 to 30%, thus emphasizing the applicability of PA nanofibres in the field of filtration. It is also important to produce fibre matts with higher densities, to ensure an increase in the collision points’ probability. For this purpose, samples A, D, E, F, G and H are identified as viable options. Regarding the diameters of the fibres, samples A and D were found to be significant. Both samples were produced with a flow rate of 0.4 mL/h, with a needle with a diameter of 0.33 mm applied on sample A, and a needle with a diameter of 0.41 mm applied on sample B. Although the two samples have the two lowest mean fibre diameters, sample B had a smaller value—293 nm. Furthermore, samples A and D also have the lowest standard deviation value, which means that the produced fibres are more uniform with each other in terms of size. Sample I was produced using the same ideal conditions as sample B, however, the polymer proportion was higher (25% w/v instead of 20% w/v). When comparing sample I with the others, it is clear that the number of fibres produced per unit area was considerably lower. The average diameters of the produced fibres, as well as the corresponding standard deviation, are higher when compared to the other samples. This result reveals that the application of 20% (w/v) PA is the best approach to produce homogeneous mats of PA nanofibres with smaller diameters.. Table 4. SEM images of the produced PA nanofibres and corresponding characteristics. Sample and Production Parameters SEM Images (×1000) SEM images (×5000) Fiber Diameter ± STDEV (nm) Porous Distribution (%) A Flow rate = 0.4 mL/h Needle-diameter = 0.33 mm 302 ± 46 21.66 B Flow rate = 0.8 mL/h Needle-diameter = 0.33 mm 442 ± 170 48.97 Materials 2021, 14, x FOR PEER REVIEW 7 of 15 3.1.3. Morphological Analysis The PA nanofibre morphology was studied using SEM images. The images and the corresponding fibre diameters and porous distribution are represented in Table 4. The SEM images reveal that randomly deposited fibres were produced. This feature is important, as filtration aims to produce a structure composed of innumerable pores with very small dimensions. This way, the probability of promoting the retention of nanoparticles is enhanced. The porous distribution, based on size, was measured using imageJ software. The obtained values ranged from 21 to 56%. To promote higher filtration efficiencies, a perfect combination of the fibres diameter and porous distribution should be achieved—a small fibre diameter allows for a lot of tinny pores, but the percentual distribution based on the size of these pores should be as small as possible, to increase the retention of particles probability. The PA nanofibres with the highest polymer concentration have the highest value of porous distribution. Most of the samples obtained percentage distribution values varying from 20 to 30%, thus emphasizing the applicability of PA nanofibres in the field of filtration. It is also important to produce fibre matts with higher densities, to ensure an increase in the collision points’ probability. For this purpose, samples A, D, E, F, G and H are identified as viable options. Regarding the diameters of the fibres, samples A and D were found to be significant. Both samples were produced with a flow rate of 0.4 mL/h, with a needle with a diameter of 0.33 mm applied on sample A, and a needle with a diameter of 0.41 mm applied on sample B. Although the two samples have the two lowest mean fibre diameters, sample B had a smaller value—293 nm. Furthermore, samples A and D also have the lowest standard deviation value, which means that the produced fibres are more uniform with each other in terms of size. Sample I was produced using the same ideal conditions as sample B, however, the polymer proportion was higher (25% w/v instead of 20% w/v). When comparing sample I with the others, it is clear that the number of fibres produced per unit area was considerably lower. The average diameters of the produced fibres, as well as the corresponding standard deviation, are higher when compared to the other samples. This result reveals that the application of 20% (w/v) PA is the best approach to produce homogeneous mats of PA nanofibres with smaller diameters.. Table 4. SEM images of the produced PA nanofibres and corresponding characteristics. Sample and Production Parameters SEM Images (×1000) SEM images (×5000) Fiber Diameter ± STDEV (nm) Porous Distribution (%) A Flow rate = 0.4 mL/h Needle-diameter = 0.33 mm 302 ± 46 21.66 B Flow rate = 0.8 mL/h Needle-diameter = 0.33 mm 442 ± 170 48.97 442 ±170 48.97 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 402 ±141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 293 ±60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 402 ±84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 363 ±80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 399 ±193 25.09 Materials 2021,14, 7147 8 of 15 Table 4. Cont. Sample and Production Parameters SEM Images (×1000) SEM Images (×5000) Fiber Diameter ±STDEV (nm) Porous Distribution (%) H Flow rate = 2 mL/h Needle-diameter = 0.61 mm Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 356 ±70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is Materials 2021, 14, x FOR PEER REVIEW 8 of 15 C Flow rate = 1 mL/h Needle-diameter = 0.33 mm 402 ± 141 37.50 D Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 293 ± 60 25.27 E Flow rate = 0.8 mL/h Needle-diameter = 0.41 mm 402 ± 84 22.71 F Flow rate = 1 mL/h Needle-diameter = 0.41 mm 363 ± 80 28.57 G Flow rate = 1 mL/h Needle-diameter = 0.61 mm 399 ± 193 25.09 H Flow rate = 2 mL/h Needle-diameter = 0.61 mm 356 ± 70 21.11 I Flow rate = 0.4 mL/h Needle-diameter = 0.41 mm 25% (w/v) PA 755 ± 711 56.05 It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is 755 ±711 56.05 The SEM images reveal that randomly deposited fibres were produced. This feature is important, as filtration aims to produce a structure composed of innumerable pores with very small dimensions. This way, the probability of promoting the retention of nanoparticles is enhanced. The porous distribution, based on size, was measured using imageJ software. The obtained values ranged from 21 to 56%. To promote higher filtration efficiencies, a perfect combination of the fibres diameter and porous distribution should be achieved—a small fibre diameter allows for a lot of tinny pores, but the percentual distribution based on the size of these pores should be as small as possible, to increase the retention of particles probability. The PA nanofibres with the highest polymer concentration have the highest value of porous distribution. Most of the samples obtained percentage distribution values varying from 20 to 30%, thus emphasizing the applicability of PA nanofibres in the field of filtration. It is also important to produce fibre matts with higher densities, to ensure an increase in the collision points’ probability. For this purpose, samples A, D, E, F, G and H are identified as viable options. Regarding the diameters of the fibres, samples A and D were found to be significant. Both samples were produced with a flow rate of 0.4 mL/h, with a needle with a diameter of 0.33 mm applied on sample A, and a needle with a diameter of 0.41 mm applied on sample B. Although the two samples have the two lowest mean fibre diameters, sample B had a smaller value—293 nm. Furthermore, samples A and D also have the lowest standard deviation value, which means that the produced fibres are more uniform with each other in terms of size. Sample I was produced using the same ideal conditions as sample B, however, the polymer proportion was higher (25% w/v instead of 20% w/v). When comparing sample I with the others, it is clear that the number of fibres produced per unit area was considerably lower. The average diameters of the produced fibres, as well as the corresponding standard deviation, are higher when compared to the other samples. This result reveals that the application of 20% (w/v) PA is the best approach to produce homogeneous mats of PA nanofibres with smaller diameters. It can be observed that different parameters in PA-nanofiber production affect the fibre’s diameter and porous distribution values, as presented in Figure 5. The values are related to the samples produced with 20% (w/v) PA. By analyzing the bars related to the diameter of the needles between 0.33 mm and 0.41 mm in Figure 5a, it is possible to conclude that nanofibres with lower diameters are always obtained when using a needle with a diameter equal to 0.41 mm. Additionally, by analyzing the flow rate values, it is possible to conclude that generally, lower flow rates promote the production of fibres with smaller diameters. The results of porous distribution are presented in Figure 5b, and it seems that a direct correlation between this value and the nanofiber construction parameters cannot Materials 2021,14, 7147 9 of 15 be established. However, lower values for porous distribution are related to smaller fibres diameters, which becomes apparent when looking at both Figure 5a,b . When combining lower flow rates with smaller needle diameters, homogeneous mats of nanofibres (low standard deviation values) with small diameters and a porous distribution are produced. These conditions are optimal filtration purposes [6]. Materials 2021, 14, x FOR PEER REVIEW 9 of 15 possible to conclude that generally, lower flow rates promote the production of fibres with smaller diameters. The results of porous distribution are presented in Figure 5b, and it seems that a direct correlation between this value and the nanofiber construction parameters cannot be established. However, lower values for porous distribution are related to smaller fibres diameters, which becomes apparent when looking at both Figure 5a,b. When combining lower flow rates with smaller needle diameters, homogeneous mats of nanofibres (low standard deviation values) with small diameters and a porous distribution are produced. These conditions are optimal filtration purposes [6]. Figure 5. Influence of needle diameter (a) and flow rate (b) on fibre diameter and porous distribution values. 3.2. Electrospun PA + Non-Woven Fabrics Characterization The above-mentioned optimal conditions for producing PA nanofibres, using electrospinning methods, were applied in this phase of the study The production and deposition of PA nanofibres onto the PP microfibres were conducted to obtain a multilayer structure for filtration purposes. The PP microfibres were morphologically and physically characterized, and are presented in the microscope images in Figure 6. PP fibres were found to have a random orientation. The average fibre diameter is 3.7 µm (Table 5). This value is much higher than the value obtained for PA nanofibres. Due to the conjugation of these 2 different structures, a multiscale system was constructed, and the filtration efficiency is expected to increase with minimal levels of breathability and/or permeability. The nanofibre membrane is much thinner (ranges from 1.16 to 2.64 µm Figure 5. Influence of needle diameter (a) and flow rate (b) on fibre diameter and porous distribution values. 3.2. Electrospun PA + Non-Woven Fabrics Characterization The above-mentioned optimal conditions for producing PA nanofibres, using electrospinning methods, were applied in this phase of the study The production and deposition of PA nanofibres onto the PP microfibres were conducted to obtain a multilayer structure for filtration purposes. The PP microfibres were morphologically and physically characterized, and are presented in the microscope images in Figure 6. PP fibres were found to have a random orientation. The average fibre diameter is 3.7 µ m (Table 5). This value is much higher than the value obtained for PA nanofibres. Due to the conjugation of these 2 different