Electrospun mats based on poly(vinyl fluoride-co-hexafluoropropylene) and hybrid carbon nanofillers as high performance ultrafiltration membranes

Electrospun mats based on poly(vinyl fluoride-co-hexafluoropropylene) and hybrid carbon nanofillers as high performance ultrafiltration membranes | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Electrospun mats based on poly(vinyl fluoride-co-hexafluoropropylene) and hybrid carbon nanofillers as high performance ultrafiltration membranes Roberto Scaffaro, Michele Gammino, Andrea Maio This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2021850/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Graphene oxide (GO) and carbon nanotubes (CNTs) were integrated at different mutual ratios into poly(vinyl fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and electrospun to construct mats that were assessed for treating water contaminated by methylene blue (MB) via vacuum-assisted ultrafiltration. The materials were fully characterized from a morphological, physicochemical and mechanical point of view. The results revealed that such materials are suitable for being used as membranes for continuous processes, such as ultrafiltration. In particular, adding 2 wt.% of GO and CNTs gave the best performance, showing extremely high flux (800 L*m − 2 *h − 1 ), excellent rejection (99%) and flux recovery ratios (93.3%), along with antifouling properties (irreversible and reversible fouling below 6% and 25%, respectively), and reusability. These outstanding outcomes were ascribed to the peculiar microstructure achieved, which endowed polymeric membranes with high roughness, wettability, and mechanical robustness. Carbon nanotubes Graphene oxide Hybrid composites Multifunctional properties Electrospinning ultrafiltration membranes Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) copolymer, owing to its excellent resistance towards thermal, photo- and chemical degradation, as well as piezoelectric properties and hydrophobicity, is widely used for many functional applications, including cultural heritage conservation, wearable sensors, piezoelectric nanogenerators, biomedical devices, hydrogen production, and separation processes [ 1 – 6 ]. These latter ones are stimulating a rising interest, since environmental pollution and drinking water demand have become crucial global challenges [ 7 ]. Among the several water contaminants that are responsible for the adverse effects on environment and human health, aromatic dyes belong to the so-called class of emerging pollutants [ 8 , 9 ]. In fact, their removal from water is difficult due to their strong physicochemical stability in aqueous media, thus making them inefficacious most of the conventional water treatment techniques [ 8 ]. Hence, advanced membrane separation strategies are gaining momentum. Among them, nanofiltration and ultrafiltration are the most encouraging ones, both relying on the continuous operation of the process while differing from each other for the peculiar mechanism of rejection [ 8 ]. In fact, the former approach relies on size rejection, prompted by the presence of small pore diameters, below hundreds of nanometers [ 8 ]. This feature, while maximizing removal efficiency, inevitably affects the membrane permeance, thus decreasing the flux of permeate. The latter strategy, based on electrostatic rejection mechanism, is preferable in the perspective to obtain greater flow rates of water to be decontaminated [ 8 ]. In this context, materials science and technology is urged to develop novel materials and/or processing technologies that are suitable for achieving more and more efficient separation processes. For instance, the performance of PVDF-co-HFP membranes for the removal of molecular contaminants such as dyes, is affected by pore size (usually larger than those of contaminant molecules), low flow rate and surface fouling, due to their hydrophobic nature [ 10 ]. Hence, combining membrane distillation with adsorption is considered a promising approach to rapidly and efficiently treat large volumes of contaminated water [ 8 ]. In this case, active nanoparticles can be either embedded into the polymer matrix or even immobilized onto the membrane surface [ 8 , 11 , 12 ]. Indeed, integrating nanocarbon materials like graphene oxide (GO) or carbon nanotubes (CNTs) in a PVDF-co-HFP matrix has been recently explored to further improve the performance, by virtue of their interesting features [ 13 – 15 ]. In fact, beyond their well-known outstanding mechanical, electrical and antioxidant properties [ 18 , 19 ], such nanoparticles possess large surface area with plenty of potential active sites, thus being promising as nanoadsorbents [ 16 , 17 ]. Another critical issue relates to the mechanical performance of ultrafiltration membranes. In fact, while high porosity favors the rapidity of separation processes, on the contrary mechanical performance of materials tends to decrease [ 20 ]. In this context, electrospinning technique gives rise to nanofibrous membranes with interconnected pores that are undoubtedly more porous than those achieved via wet phase inversion, and other techniques but, on the other hand, their robustness is often unsatisfactory [ 20 ]. To solve this issue, mats prepared by electrospinning are usually stacked to each other and thus welded by compression molding, with ensuing increase of mechanical resistance at the expense of permeance [ 21 ]. A promising strategy to avoid the subsequent hot pressing step could be the promotion and control of blobbing phenomena during electrospinning by controlling the solvent-polymer phase separation rates. If the latter are slow enough to lead to the formation of an interconnected fibers network, the interlaminar adhesion of the mats could be improved, thus providing the materials with considerable mechanical strength despite their high porosity. Indeed, in a previous work [ 7 ], we demonstrated that hybrid network formed by GO and CNTs may give rise to polymer nanocomposites with emergent features, including the strong affinity to methylene blue and methyl orange, and mechanical properties exceeding the sum of those of constituents. Aim of this work is to explore the feasibility to construct highly porous electrospun mats that conjugate excellent performance in terms of permeance, pollutant rejection and anti-fouling features with outstanding mechanical robustness and durability. Hence, the structure-properties relationships of ternary nanocomposites based on a PVDF-co-HFP matrix and a hybrid loading of GO and CNTs were investigated and compared to those of binary systems containing solely GO or CNTs. 2. Experimental Part 2.1 Materials and reactants GO and CNTs used in this work were synthesized in our laboratories, following the protocols reported in our previous studies [ 7 , 16 , 22 – 25 ]. Briefly, GO (lateral size < 45 µm, thickness = 0.7–0.8 nm, C/O ratio = 1.1, density = 1.76 g/cm 3 ) was prepared by using Tour’s method, while CNTs (length = 1–2 µm, diameter = 10–20 nm, C/O ratio = 40.7, density = 2.24 g/cm 3 ) were synthesized by fluidized bed chemical vapour deposition. PVDF-co-HFP was a sample of Fluolyne HY kindly supplied by CTS Europe. It is an elastomeric thermoplastic fluorinated copolymer, having Mw = 400,000 g/mol and density equal to 1.77 g/cm 3 . Acetone and methylene blue (MB) reagent grade were purchased from Sigma Aldrich. 2.2 Preparation of electrospun mats The six formulations herein investigated are summarized in Table 1 . In order to obtain the designed formulation, nanoparticle(s) in the desired proportions were first dispersed in acetone through ultrasonication (4 h), thereafter PVDF-co-HFP was poured (10 wt.% with respect to the solvent) to the homogeneous dispersion and vigorously stirred until complete dissolution (6 hours). Table 1 Formulation of the samples investigated, amounts given in wt.% Sample codename C0-G0 C0-G1 C0-G2 C2-G0 C2-G1 C2-G2 PVDF-co-HFP 100 99 98 98 97 96 CNTs 0 0 0 2 2 2 GO 0 1 2 0 1 2 Each solution was then loaded to a glass syringe and electrospun by using a Linari equipment under the following operating parameters: flow rate, 1.8 ml/h; supplied high voltage, 15 kV; needle-to-collector distance, 12 cm; temperature, 25°C; and relative humidity, 50%. Nanofibers were collected on an aluminum-coated rotary drum (diameter = 25 mm, speed = 25 rpm), for 180 min in order to obtain ~ 50 µm thick mats. 2.3 Fabrication of devices for ultrafiltration Devices for ultrafiltration were assembled as depicted in Fig. 1 . Membrane septum and support grid were home-made fabricated by rapid prototyping. Thereafter, support grid was placed onto electrospun membranes (yet adherent to aluminum foil) and poured in ethanol to allow the easy detachment of mats from aluminum foil and their placement onto grid. 2.4 Characterization of nanohybrid mats Morphology was investigated by combining different techniques. Scanning electron microscopy (SEM) was carried out by using an ESEM FEI QUANTA 200 microscope (Thermo Fisher Scientific). Image analysis, performed by ImageJ software equipped with Diameter J plug-in, was used to measure size distribution of fiber and pore diameter of the mats [ 26 ]. Overall porosity of membranes was calculated via gravimetric method, as typically used for PVDF-co-HFP electrospun mats [ 27 ]. Briefly, all pores of a membrane sample of known weight and dimensions were totally filled with ethanol by soaking the mats for 2 hours. Porosity was then calculated according to Eq. 1 : $$Porosity \left(\%\right)=\frac{{W}_{s, wet}-{W}_{s,dry}}{{\rho }_{L}{V}_{s}} \times 100$$ 1 Where \({W}_{s, wet}\) and \({W}_{s, dry}\) respectively indicate the weight of wet and dry membrane sample, \({\rho }_{L}\) is the density of the wetting liquid, and \({V}_{s}\) is the sample volume. Atomic force microscopy (AFM) was employed to measure the roughness of membranes. AFM measurements were performed in air using a Bruker FAST-SCAN microscope equipped with a closed-loop scanner, in the soft tapping mode using a probe with an apical radius of about 5 nm. Arithmetic average roughness (Ra) and quadratic mean roughness (Rq) were calculated on squared areas (side length = 1 µm). Sessile drop water contact angle (WCA) testing was carried out to assess the surface wettability of membranes. Experiments were performed at room temperature by using an FTA 1000 (First Ten Ångstroms, U.K.) instrument. 4 µL of deionized water were dropped onto the surface of each sample by way of an automatic liquid drop dosing system. Images of the drops onto the surface were acquired after 20 s. Mechanical testing was performed by using an Instron 3365 dynamometer (UK) onto prismatic specimens having width = 10 mm; length = 90 mm; thickness ~ 50 µm (measured before each measurement). The tests were performed onto at least 10 replicates, under the following conditions: distance between the jaws = 30 mm, crosshead speed = 1 mm min − 1 until failure [ 28 ]. The salient data were provided as mean values ± standard deviations. Elastic modulus (E 1 ) was calculated as the slope of stress-strain curve in the initial linear range (E 1 ). The slope of each curve was also calculated in the strain-hardening region (E 2 ). Toughness was measured as the integrated area of each curve. Tensile strength (TS) and elongation at break (EB%) were respectively calculated as the ordinate and the abscissa of stress-strain curve at failure. 2.5 Flux measurements and antifouling test of the devices Water permeance of the membranes was assessed at room temperature by performing flux experiments in a home-made setup. Membranes having 45 mm diameter were fixed into a test cell and a dead-end vacuum filtration system was used at an absolute pressure of 85 kPa and pure water flux (J, L/m 2 h) was calculated using Eq. 2 : $$J=\frac{V}{A t }$$ 2 where V is the permeate volume, A is the effective cross-sectional area of the membrane, t is the elution time (h). In order to quantify MB removal, vacuum filtration tests under the same conditions were carried out on 100 mL aqueous MB solutions (5 mg/L). MB concentration in the feeding solution and in the filtrate were then measured via UV-vis spectroscopy (at λ = 667 nm) and the ultimate separation efficiency, R (%), was calculated according to Eq. ( 3 ): $$R \left(\%\right)=\left(1-\frac{{A}_{2}}{{A}_{1}}\right)\times 100$$ 3 Where A 2 and A 1 are the values of absorbance of methylene blue recorded at the end and at the beginning of the vacuum-filtration experiments, respectively. Each membrane was subjected to continuous vacuum-filtration runs up to 50 minutes before washing, and removal efficiency of each sample was also measured at predetermined time intervals, in order to investigate its time-dependent behaviour. The analysis of anti-fouling properties was conducted by calculating water flux recovery ratio (FRR), along with reversible ( \({R}_{r}),\) irreversible ( \({R}_{ir})\) and total ( \({R}_{t})\) fouling rations, which are provided by using Equations (4–7): \(FRR \left(\%\right)=\frac{{J}_{w2}}{{J}_{w1}}\times 100\) (4) \({R}_{r} \left(\%\right)=\frac{{J}_{w2}-{J}_{p}}{{J}_{w1}}\times 100\) (5) \({R}_{ir} \left(\%\right)=\frac{{J}_{w1}-{J}_{w2}}{{J}_{w1}}\times 100\) (6) \({R}_{t} \left(\%\right)=\frac{{J}_{w1}-{J}_{p}}{{J}_{w1}}\times 100\) (7) Where \({J}_{w1}\) is the flux of pure water passing through the membrane, \({J}_{p}\) is the flux of water containing pollutant, and \({J}_{w2}\) is the flux of pure water passing through the membrane after it has been cleaned. Reusability was evaluated by subjecting the membranes to 10 process cycles. At the end of each cycle, lasting 50 minutes, the membranes were regenerated via washing with methanol, and eventual changes in separation efficiency or mechanical damaging were recorded. 3. Results And Discussion The morphology of the mats was investigated by SEM and image analysis. Figure 2 reports SEM micrographs at different magnifications of C0-G0 (a-a”), C0-G1 (b-b”), C0-G2 (c-c”), whereas Fig. 3 provides those of C2-G0 (a-a”), C2-G1 (b-b”), C2-G2 (c-c”). Table 2 provides the salient features of fibrous architectures, that is, mean fiber and pore diameter, calculated by image analysis (see size distributions, Fig. S1), and membrane porosity, evaluated by gravimetric method. As one can see, all the mats, i.e. neat polymer and its nanocomposites, show either weaker or pronounced blobbing phenomena, which led to the formation of an interconnected fibrous network. This occurrence can be ascribed to the use of acetone only to prepare polymeric solutions, which resulted in slow phase separation rates. Neat polymer (Fig. 2 a-a”) exhibits fibers with smooth surface and a discrete diameter distribution, with 45% fibers displaying submicrometric size, 43% having diameter between 1.6 and 3.2 µm, and 12% thicker than 4 µm (Fig. S1). This aspect can be ascribed to the high charge density of PVDF-co-HFP, which is known to promote the splitting of the jet, thus forming both thicker and thinner fibers [ 29 ]. GO addition, while not significantly altering the fiber diameter, was found to promote the formation of blobs at a higher extent, likely because of its well-known tendency to hinder the solvent evaporation [ 30 ]. Moreover, filler dispersion proved to be inhomogeneous with well-visible aggregation events (see Fig. 2 b-c). However, it is worth noting that GO foils display a broad distribution of lateral size, ranging from hundreds of nanometers to tens of micrometers. Larger GO foils were found to protrude out of the fibrous network, with an unfurled sail-like configuration. A closer inspection of these latter in the case of C0-G1 and C0-G2 is provided in the detailed micrographs of Fig. 2 b’-c’, respectively, which show GO sheets eventually surrounded by polymer Submicrometric sheets, instead, were small enough to be integrated within fibrous cage (Fig. 2 b”-c”), and were found either emergent from the fiber surface or embedded inside the fibers. Hence, both samples show an extremely variegated microstructure, comprising a cage made of fibers of different diameters, with GO particles of different lateral sizes, which in turn can be found either outside or integrated within the fibrous structure. The morphology of the system C2-G0 is shown in Figs. 3 a-a”. In this case, the addition of CNTs only, did not significantly alter the fibrous architecture of the neat polymer, which retained random orientation and discrete, multimodal size distribution, with substantially similar values of average fiber diameter, although the filler dispersion proved to be inhomogeneous, with well-detectable nanoparticle clusters likely arising from their scarce dispersability in acetone. It is worth noting that such aggregates were localized in both intra- and inter-fiber regions (see Fig. 3 a’). As visible from the micrographs of C2-G1 (Fig. 3 b-b”) and C2-G2 (Fig. 3 c-c”), hybrid loading of CNTs and GO determined some changes in the mat morphology. Differently from the systems containing solely GO or CNTs, a drastic reduction of size and number of aggregation events was observed in hybrid electrospun mats. Fiber diameter distribution of C2-G1 and C2-G2 (Fig. S1) proved to be narrower than those of the other systems, and the fibers displayed the lowest values of mean diameter (Table 2 ). The presence of CNTs also affected the configuration of larger GO sheets, which were found either to stretch like unfurled sails (Figs. 3 b-c) or even to roll-up around their axis, thus wrapping the polymeric fibers (Figs. 3 b’-c’). These two phenomena were observed in both hybrid samples, although with a dose-dependent trend: the former proved to prevail at low GO loading (C2-G1, Fig. 3 b-b”), while the latter governed the morphology of the samples containing the highest GO content (C2-G2, Fig. 3 c-c”). Is should also be noted that hybrid samples showed a higher concentration of nanoparticles emerging from the fiber surface and that the unfurled sail-like GO lamellae displayed a thinner thickness (Fig. 3 c-c”), likely because the presence of CNTs disturbed the typical stacking of GO layers. In order to get more information about the surface morphology of the fibers, AFM analysis was carried out and the images recorded on squared regions of 5 µm and 1 µm per side are respectively reported in Figure S2 and Fig. 4 a, whereas the values of arithmetic average and squared mean roughness of the samples are reported in Fig. 4 b. It can be seen that the presence of nanocarbons alters the surface features of fibers, although at different extents depending on formulation. As already envisaged from SEM analysis, in fact, nanoparticles could be found either emerging from fibers or covered with polymeric layers. In C0-G1, it is possible to well-recognize the typical rough texture imparted by two distinct GO foils emerging from the surface of a smooth fiber. In C0-G2, the fiber surface appears as smooth as that of neat polymer, likely because most of the foils proved to be surrounded by a thick polymeric layer, in agreement with the results of SEM imaging. In the case of C2-G0, instead, AFM was able to detect the presence of CNTs emerging from fiber surface, which resulted in a brush-like morphology. C2-G1 and C2-G2 samples displayed an extremely rough texture, likely due to the formation of an extensive GO-CNT hybrid framework that might have promoted a higher dispersion degree and thus the localization of a larger amount of nanoparticles in the surface of fibers., Table 2 Characterization of fibrous architecture of the mats: mean values of fiber and pore diameter calculated via image analysis, along with porosity assessed by gravimetric analysis. C0-G0 C0-G1 C0-G2 C2-G0 C2-G1 C2-G2 mean fiber diameter (µm) 1.8 1.9 1.82 1.6 0.6 0.8 Mean pore diameter (µm) 2.38 0.44 0.3 2.51 1.73 2.74 Porosity (%) 87.8 81.2 82.4 90.1 89 91.6 In fact, as visible in Fig. 4 b, all membranes containing GO possess fiber roughness similar or moderately larger than that of pristine PVDF-co-HFP, likely due to the bad filler dispersion, which results in smooth polymeric fibers and discrete aggregates. On the other hand, C2-series samples display a rougher texture, with C2-G2 sample showing the largest Rq and Ra values, almost equal to double those of C2-G0, and 700% higher than those of neat C0-G0. Surface wettability was studied via WCA testing and the results are provided in Fig. 5 . In C0-series systems, the hydrophobic character of PVDF-co-HFP (WCA = 108°) is substantially maintained even in the presence of GO, despite the well-known hydrophilicity of this latter. Again, this result could be likely ascribed to the poor extent of filler dispersion, also testified by the high data scattering of C0-G1 and C0-G2, and to the scarce presence of nanoparticles emerging from the surface, in full agreement with SEM and AFM observations. Predictably, hydrophobic character of the polymer was further enhanced by adding CNTs, due to their hydrophobicity, even boosted by the brush-like morphology of fibers. However, incorporating GO together with CNTs led to a different behavior. WCAs were found to decrease as a function of the GO content, until the hydrophilic character of the latter prevailed over the hydrophobic one of the other components at the highest GO dose (C2-G2), thus resulting in a WCA as low as 71°. These results are in strong agreement with those of SEM and AFM measurements (see again Figs. 2 – 4 ). Table 3 Salient tensile properties of the samples Property C0-G0 C0-G1 C0-G2 C2-G0 C2-G1 C2-G2 E 1 (MPa) 0.104 ± 0.00 0.323 ± 0.01 0.824 ± 0.01 0.197 ± 0.01 0.485 ± 0.03 4.0 ± 0.2 E 2 (MPa) 0.43 ± 0.01 0.95 ± 0.07 2.4 ± 0.06 1.8 ± 0.02 7.2 ± 0.2 3.81 ± 0.03 TS (MPa) 0.41 ± 0.01 0.84 ± 0.01 1.48 ± 0.03 1.56 ± 0.04 4.05 ± 0.1 3.85 ± 0.16 EB (%) 178 ± 18 175 ± 11 215 ± 10 190 ± 15 180 ± 19 183 ± 22 Toughness (MJ/m 3 ) 0.4 ± 0.01 0.88 ± 0.01 2.21 ± 0.03 1.21 ± 0.02 3.47 ± 0.06 3.22 ± 0.07 The representative stress-strain curves collected during tensile tests are provided in Fig. 6 , whereas the salient mechanical properties measured via tensile tests are summarized in Table 3 . It can be noted that neat polymer and its nanocomposites, with the only exception of C2-G2 sample, display a J-shaped strain-stiffening behavior, being soft and compliant at small strains, while becoming rapidly stiffer at higher strains. In fact, as visible in Table 3 , such samples show an initial slope of stress-strain curve (E 1 ) which is quite lower than that calculated in strain-hardening region (E 2 ). Indeed, this peculiar mechanical behavior, likely attributable to the multimodal distribution of fiber diameters (Figs. S1), was already detected for other fibrous systems based on PVDF and PVDF-co-HFP[ 31 ]. The mechanical behavior of C0-G1 and C0-G2 proved to be dominated by the matrix, although the presence of nanofillers imparted strengthening and stiffening effects, without compromising ductility. C2-G0 shows ultimate resistance and deformability similar to those of C0-G2 but the J-shape of the curve is more pronounced, likely because of the orientation of CNTs along deformation axis with ensuing strain-hardening. In C2-G1, which displayed the highest TS, the formation of a GO-CNT hybrid network and the prevalent unfurled sail-like configuration of GO reasonably resulted in a remarkable aliquot of GO-CNT hybrids oriented along the deformation axis, with consequent strain hardening that led to the most remarkable J-shaped behavior. In C2-G2, instead, the mechanical behavior seems to be governed by the GO-CNT hybrid framework, which endowed the fibers with higher stiffness even at low strains, thus suppressing the J-shaped strain-stiffening behavior. Anyhow, the addition of GO and/or CNTs determined a significant stiffening and strengthening of the PVDF-co-HFP matrix, without compromising its ductility. In particular, hybrid membranes retain the good deformability of neat PVDF-co-HFP (EB close to 200%) while experiencing an outstanding increase of tensile strength (20-times higher than that of neat polymer and 2–3 times higher than those of the nanocomposites containing solely GO or CNTs), and toughness (almost ​​9 times higher than that of PVDF-co-HPF and about double or triple those of the composites containing only one type of nanofiller), thus possessing mechanical prerequisites good enough to be used for ultrafiltration. Indeed, the values of tensile strength of these materials match those of less porous membranes prepared via wet phase inversion or compression molding-aided assembly [ 32 – 34 ]. The possibility to use such mats as membranes for ultrafiltration of MB was assessed. Water flux and MB retention experiments give useful information about permeability and selectivity of the membranes. Unfortunately, because of their poor mechanical performance, C0-G0 and C0-G1 experienced mechanical failure during preliminary flux tests in pure water, whereas C0-G2 underwent perforation after 3 minutes of filtration experiments with water contaminated by MB dye. Therefore, the characterization was restricted to C2-G0, C2-G1 and C2-G2. Figure 7 a provides the permeance of membranes toward pure water (empty circles), and solution flux (filled circles), along with the final removal rate of MB dye (histograms). The outcomes point out that all these samples display good water permeance, due to the high porosity levels of electrospun fibrous mats, along with suitable pore size. However, a clear trend was observed as a function of GO content, reasonably ascribed to the different surface characteristics of each sample, including wettability and roughness. Noteworthy, C2-G2 granted the highest flux of pure water (as large as 820 L*m − 2 *h − 1 ), far superior to other vacuum-ultrafiltration membranes recently proposed for treating water contaminated by MB dye [ 35 , 36 ]. C2-G1 showed slightly lower permeance (780 L*m − 2 *h − 1 ), whereas C2-G0 gave the worst performance (450 L*m − 2 *h − 1 ). This aspect put into evidence the crucial role of oxygen-rich GO lamellae, which boost permeability of the membranes owing to their hydrophilicity (Fig. 5 ), and the wrinkled texture of GO foils, with consequent enhancement of roughness (Fig. 4 ). When subjected to the flux of water contaminated by MB dyes, the differences between C2-G1 (660 L*m − 2 *h − 1 ) and C2-G2 (770 L*m − 2 *h − 1 ) tend to augment. Nevertheless, both samples displayed J p values quite higher than that of C2-G0 (320 L*m − 2 *h − 1 ). Thus, incorporating GO and CNTs into a PVDF-co-HFP electrospun membrane results in extremely robust materials that can withstand water flux without failure despite their high porosity and thin fibers. This latter aspect allows outstanding permeability, granted by the combination of hydrophilicity (Fig. 5 ) and adequate pore size (Fig. S3 and Table 2 ). Often, increasing permeability can lead to a worsening of dye rejection performance. In this case, instead, C2-G2 showed also the highest capacity to reject MB cationic dyes (99%), followed by C2-G1 (89%), while C2-G0 (51%) displayed the worst results. Furthermore, the analysis of evolution of removal efficiency upon treatment time (Fig. 7 b) points out that C2-G0 membranes achieved a plateau within 20 minutes, thus suggesting the possible insurgence of fouling issues, whereas the removal efficiency of hybrid membranes proved to monotonically increase upon time. Indeed, one of the most critical design parameters is the fouling resistance of the membranes for water treatment. This aspect was assessed and the results are shown in Fig. 8 , which reports the flux recovery ratio, FRR (Fig. 8 a), together with irreversible, reversible and total fouling rations (Fig. 8 b). The trend observed for FRR follows what seen for WCA, with the most hydrophilic C2-G2 membranes displaying the strongest anti-fouling properties, i.e. the highest flux recovery ratio (93.33%), followed by C2-G1 (88%), whereas C2-G0 gave the worst performance (64%). Noteworthy, the aliquot of irreversible fouling is sensationally low for C2-G2 (6%), which displays promising reusability. By contrast, the mats containing CNTs only (C2-G0) are prone to irreversible fouling, likely due to their hydrophobicity and to the strong tendency of CNTs to form stable complexes with MB molecules via π-π stacking [ 7 ]. The reusability of the systems is another crucial prerequisite in the perspective of reducing economic and environmental costs. The results, provided in Fig. 9 , put into evidence that C2-G0 experienced damaging after 3 cycles, whereas C2-G1 and C2-G2 proved to withstand mechanical stresses without failure in the whole time investigated. These outcomes, in strong agreement with those of mechanical testing, once again point out, on one hand, the difficulty to achieve efficient membranes via electrospinning and, on the other hand, the surprising reinforcing effect exerted by the hybrid GO-CNT network integrated within the polymer matrix. Beyond the considerations on mechanical durability, it is worth noting that hybrid membranes substantially retained a constant flux and a high separation efficiency even after 10 cycles, while C2-G0 sample has undergone a dramatic decline in separation performance, reasonably because of its poor anti-fouling properties. In fact, in this latter sample an initial flux decrease was observed during the second cycle, likely because of fouling, while the raise of flux detected in the third cycle, associated with a further loss of separation efficiency, presumably indicates the incipient structural failure of the mats. Ultimately, a synoptic view of overall performance of the three systems is provided as a radar plot in Fig. 10 . As one can see, hybrid membranes (especially C2-G2) displayed the best performance in terms of mechanical robustness, permeance, removal capacity, anti-fouling properties, and reusability, showing values at least double those of the mats containing CNTs only. 4. Conclusions Electrospun membranes based on PVDF-HFP loaded with GO and/or CNTs were constructed by avoiding toxic solvents and surfactants, exploiting the slow phase separation of polymer in acetone to promote blobs formation. Morphological analysis pointed out that hybrid loading of GO-CNTs led to a uniform dispersion with the formation of a well-structured nanocarbon framework throughout the matrix, with several nanoparticles emerging from the fiber surface. By virtue of this feature, hybrid membranes displayed the highest values of roughness and hydrophilicity, beyond outstanding mechanical robustness coupled with high porosity and micrometric pore diameter. Hence, when tested as membranes for vacuum-assisted ultrafiltration, they showed the highest values of permeance (800 L*m − 2 *h − 1 ) and capability of removing methylene blue (99%), with negligible fouling issues (irreversible and reversible fouling below 6% and 25%, respectively) and remarkable reusability up to at least 10 cycles. Declarations Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding PO-FESR-2014-2020-SICILIA, SAWE, “Support, Alerting, Early Warning” cod. no. 08PA9511000101 CUP G78I18000960007 Azione 1.1.5 Regione Siciliana. Author's contribution All the authors conceptualized the work, M.G. and A.M. carried out experimental work, A.M. and R.S. wrote the main manuscript text, A.M. and M.G. prepared the figures, R.S. supervised the work, and all the authors reviewed the manuscript. Acknowledgements The authors are grateful to Prof. G. Buscarino for technical help in performing AFM measurements. References Sha Z, Boyer C, Li G, Yu Y, Allioux F-M, Kalantar-Zadeh K, et al. Electrospun liquid metal/PVDF-HFP nanofiber membranes with exceptional triboelectric performance. Nano Energy 2022;92:106713. https://doi.org/https://doi.org/10.1016/j.nanoen.2021.106713 . Wang Y, Huang K, Zhang P, Li H, Mi H. PVDF-HFP based polymer electrolytes with high Li + transference number enhancing the cycling performance and rate capability of lithium metal batteries. 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Maio A, Botta L, Tito AC, Pellegrino L, Daghetta M, Scaffaro R. Statistical study of the influence of CNTs purification and plasma functionalization on the properties of polycarbonate-CNTs nanocomposites. Plasma Process Polym 2014;11. https://doi.org/10.1002/ppap.201400008 . Maio A, Scaffaro R, Lentini L, Palumbo Piccionello A, Pibiri I. Perfluorocarbons–graphene oxide nanoplatforms as biocompatible oxygen reservoirs. Chem Eng J 2018;334:54–65. https://doi.org/10.1016/j.cej.2017.10.032 . She FH, Tung KL, Kong LX. Calculation of effective pore diameters in porous filtration membranes with image analysis. Robot Comput Integr Manuf 2008;24:427–34. https://doi.org/https://doi.org/10.1016/j.rcim.2007.02.023 . Ejaz Ahmed F, Lalia BS, Hilal N, Hashaikeh R. Underwater superoleophobic cellulose/electrospun PVDF–HFP membranes for efficient oil/water separation. Desalination 2014;344:48–54. https://doi.org/https://doi.org/10.1016/j.desal.2014.03.010 . Scaffaro R, Maio A. Influence of oxidation level of graphene oxide on the mechanical performance and photo-oxidation resistance of a polyamide 6. Polymers (Basel) 2019;11:857. https://doi.org/10.3390/polym11050857 . Li Z, Xu Y, Fan L, Kang W, Cheng B. Fabrication of polyvinylidene fluoride tree-like nanofiber via one-step electrospinning. Mater Des 2016;92:95–101. https://doi.org/https://doi.org/10.1016/j.matdes.2015.12.037 . Maio A, Fucarino R, Khatibi R, Rosselli S, Bruno M, Scaffaro R. A novel approach to prevent graphene oxide re-aggregation during the melt compounding with polymers. Compos Sci Technol 2015;119:131–7. https://doi.org/http://dx.doi.org/10.1016/j.compscitech.2015.10.006 . Scaffaro R, Maio A, Citarrella MC. Ionic tactile sensors as promising biomaterials for artificial skin: Review of latest advances and future perspectives. Eur Polym J 2021;151:110421. https://doi.org/https://doi.org/10.1016/j.eurpolymj.2021.110421 . Zhu X, Loo H-E, Bai R. A novel membrane showing both hydrophilic and oleophobic surface properties and its non-fouling performances for potential water treatment applications. J Memb Sci 2013;436:47–56. https://doi.org/https://doi.org/10.1016/j.memsci.2013.02.019 . Wu Y, Huang Q, Xiao C, Chen K, Li X, Li N. Study on the effects and properties of PVDF/FEP blend porous membrane. Desalination 2014;353:118–24. https://doi.org/https://doi.org/10.1016/j.desal.2014.09.010 . Nauman S, Lubineau G, Alharbi HF. Post Processing Strategies for the Enhancement of Mechanical Properties of ENMs (Electrospun Nanofibrous Membranes): A Review. Membranes (Basel) 2021;11. https://doi.org/10.3390/membranes11010039 . Parakala S, Moulik S, Sridhar S. Effective separation of methylene blue dye from aqueous solutions by integration of micellar enhanced ultrafiltration with vacuum membrane distillation. Chem Eng J 2019;375:122015. https://doi.org/https://doi.org/10.1016/j.cej.2019.122015 . Ren Y, Li T, Zhang W, Wang S, Shi M, Shan C, et al. MIL-PVDF blend ultrafiltration membranes with ultrahigh MOF loading for simultaneous adsorption and catalytic oxidation of methylene blue. J Hazard Mater 2019;365:312–21. https://doi.org/https://doi.org/10.1016/j.jhazmat.2018.11.013 . Additional Declarations No competing interests reported. Supplementary Files SI1.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2021850","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":134592097,"identity":"80a02a55-bd22-4f11-a098-0d20fd3ab1ba","order_by":0,"name":"Roberto Scaffaro","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABC0lEQVRIiWNgGAWjYFACHgTzQwWQ4AfiA6ji2LQkgFmMM84ASckGqBbcetC1GBzAtB4FmLf3Hnxc+cOGQb798MOGAzX35Ixv5B48XFDDIGOPQ4vMmXPJhmcS0hgMzqQZNhw4VmxsdiMv4fCMY7gdJiGRYybZkHCYwUCCwfzxB7aExG03cgwO87Dh1WL+E6RFfgb7x4YD/xISN88AafmH3xZGkBaGGzyGDQfbEhI3SAC18Lbh0cJzLlmyIS2Nx+BMTmHDwb4EY4kzbwwOz+yT4OE5gEMLe+/Bjw02NnLy7cc3Nhz4liDH355j/Lngm409ewMOa6AA1RXMQLPwq8cAzCSqHwWjYBSMguENAC/TWNcPef2AAAAAAElFTkSuQmCC","orcid":"","institution":"University of Palermo","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Roberto","middleName":"","lastName":"Scaffaro","suffix":""},{"id":134592100,"identity":"9e3868ee-711c-4619-ad2a-edb15de18114","order_by":1,"name":"Michele Gammino","email":"","orcid":"","institution":"University of Palermo","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Michele","middleName":"","lastName":"Gammino","suffix":""},{"id":134592102,"identity":"5e05cacc-99f8-4e32-9966-ad3df2ddd981","order_by":2,"name":"Andrea Maio","email":"","orcid":"","institution":"University of Palermo","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Andrea","middleName":"","lastName":"Maio","suffix":""}],"badges":[],"createdAt":"2022-09-01 12:44:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2021850/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2021850/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":26356433,"identity":"457f456f-0d4f-4baa-a24e-8013e3c6dc20","added_by":"auto","created_at":"2022-09-12 17:52:52","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":236416,"visible":true,"origin":"","legend":"\u003cp\u003eSchematics and digital photograph of experimental setup and device used.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/7bb3523d2d84ca5c3b34c9b1.png"},{"id":26356432,"identity":"f7de119a-d579-4eae-909c-892076503c7a","added_by":"auto","created_at":"2022-09-12 17:52:52","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":470396,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrographs at different magnifications of C0-G0 (a-a”), C0-G1 (b-b”), C0-G2 (c-c”). Arrows indicate peculiar morphologies imparted by nanoparticles.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/e92efc8cc0e941673bc60b1a.png"},{"id":26356546,"identity":"358baf47-8024-41b2-8029-96884327e598","added_by":"auto","created_at":"2022-09-12 17:57:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":501155,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrographs at different magnifications of C2-G0 (a-a”), C2-G1 (b-b”), C2-G2 (c-c”). Arrows indicate peculiar morphologies imparted by nanoparticles.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/454b63f2ae485d46998bf6f0.png"},{"id":26356441,"identity":"7af0d000-4db2-4014-afca-05d44a5d7f9d","added_by":"auto","created_at":"2022-09-12 17:52:52","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":270641,"visible":true,"origin":"","legend":"\u003cp\u003eAFM images of the samples together with quadratic (left Y-axis) and arithmetic (right Y-axis) mean roughness plotted as a function of GO content for C0-series and C2-series nanocomposites.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/6354bc5114524a18a3a49452.png"},{"id":26357028,"identity":"18cc4b14-8d65-4c2f-91e9-067b00b69d55","added_by":"auto","created_at":"2022-09-12 18:07:52","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":93914,"visible":true,"origin":"","legend":"\u003cp\u003eHydrophilicity of the samples evaluated by WCA testing.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/86058fb4b8da108f69bc9c73.png"},{"id":26356434,"identity":"b318de14-27e9-44d1-952a-dc1012d4d495","added_by":"auto","created_at":"2022-09-12 17:52:52","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":473518,"visible":true,"origin":"","legend":"\u003cp\u003eRepresentative stress-strain curves of the membranes during tensile testing.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/0b2e4380bafb9c07f579ddb0.png"},{"id":26357030,"identity":"0a2628ef-4271-417f-9e4d-a818b5fd86bc","added_by":"auto","created_at":"2022-09-12 18:07:52","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":71110,"visible":true,"origin":"","legend":"\u003cp\u003ea) Water and solution flux, along with final removal of MB, as a function of GO content of hybrid membranes; b) removal as a function of filtration time for the three systems investigated.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/9a0e9aa539749b09a752d1f8.png"},{"id":26357189,"identity":"eba77f6f-721a-4d1d-ba6e-1bac7962bbe3","added_by":"auto","created_at":"2022-09-12 18:12:52","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":36185,"visible":true,"origin":"","legend":"\u003cp\u003eAnti-fouling performance of the membranes: a) flux recovery ratio (FRR); b) rations of reversible (Rr), irreversible (Rir), and total (Rt) fouling\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/c5276123b88e681a4ce1c758.png"},{"id":26356881,"identity":"610fff35-43ea-41c8-b88d-7f9f017c2789","added_by":"auto","created_at":"2022-09-12 18:02:52","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":64399,"visible":true,"origin":"","legend":"\u003cp\u003eReusability of the membranes in terms of flux (a) and removal (b) up to 10 cycles.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/b5003a3467c9162b939be39c.png"},{"id":26356551,"identity":"d53a53eb-d908-4458-a459-a9db8f03df93","added_by":"auto","created_at":"2022-09-12 17:57:52","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":521978,"visible":true,"origin":"","legend":"\u003cp\u003eAnalysis of the overall performance for the membranes investigated\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/2288a2e7beea70cce067a536.png"},{"id":26357190,"identity":"f2b98158-a776-4e34-98d6-57c6337f524a","added_by":"auto","created_at":"2022-09-12 18:12:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2706509,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/4b13c754-a768-4218-9aaa-010eda2464a6.pdf"},{"id":26356547,"identity":"1655f1bb-eeb5-495a-8c2a-834350702440","added_by":"auto","created_at":"2022-09-12 17:57:52","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1252159,"visible":true,"origin":"","legend":"","description":"","filename":"SI1.docx","url":"https://assets-eu.researchsquare.com/files/rs-2021850/v1/375156d013f0002a204b607d.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Electrospun mats based on poly(vinyl fluoride-co-hexafluoropropylene) and hybrid carbon nanofillers as high performance ultrafiltration membranes","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePolyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) copolymer, owing to its excellent resistance towards thermal, photo- and chemical degradation, as well as piezoelectric properties and hydrophobicity, is widely used for many functional applications, including cultural heritage conservation, wearable sensors, piezoelectric nanogenerators, biomedical devices, hydrogen production, and separation processes [\u003cspan additionalcitationids=\"CR2 CR3 CR4 CR5\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. These latter ones are stimulating a rising interest, since environmental pollution and drinking water demand have become crucial global challenges [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Among the several water contaminants that are responsible for the adverse effects on environment and human health, aromatic dyes belong to the so-called class of emerging pollutants [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. In fact, their removal from water is difficult due to their strong physicochemical stability in aqueous media, thus making them inefficacious most of the conventional water treatment techniques [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Hence, advanced membrane separation strategies are gaining momentum. Among them, nanofiltration and ultrafiltration are the most encouraging ones, both relying on the continuous operation of the process while differing from each other for the peculiar mechanism of rejection [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn fact, the former approach relies on size rejection, prompted by the presence of small pore diameters, below hundreds of nanometers [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. This feature, while maximizing removal efficiency, inevitably affects the membrane permeance, thus decreasing the flux of permeate. The latter strategy, based on electrostatic rejection mechanism, is preferable in the perspective to obtain greater flow rates of water to be decontaminated [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this context, materials science and technology is urged to develop novel materials and/or processing technologies that are suitable for achieving more and more efficient separation processes. For instance, the performance of PVDF-co-HFP membranes for the removal of molecular contaminants such as dyes, is affected by pore size (usually larger than those of contaminant molecules), low flow rate and surface fouling, due to their hydrophobic nature [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Hence, combining membrane distillation with adsorption is considered a promising approach to rapidly and efficiently treat large volumes of contaminated water [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In this case, active nanoparticles can be either embedded into the polymer matrix or even immobilized onto the membrane surface [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIndeed, integrating nanocarbon materials like graphene oxide (GO) or carbon nanotubes (CNTs) in a PVDF-co-HFP matrix has been recently explored to further improve the performance, by virtue of their interesting features [\u003cspan additionalcitationids=\"CR14\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. In fact, beyond their well-known outstanding mechanical, electrical and antioxidant properties [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], such nanoparticles possess large surface area with plenty of potential active sites, thus being promising as nanoadsorbents [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Another critical issue relates to the mechanical performance of ultrafiltration membranes. In fact, while high porosity favors the rapidity of separation processes, on the contrary mechanical performance of materials tends to decrease [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. In this context, electrospinning technique gives rise to nanofibrous membranes with interconnected pores that are undoubtedly more porous than those achieved via wet phase inversion, and other techniques but, on the other hand, their robustness is often unsatisfactory [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. To solve this issue, mats prepared by electrospinning are usually stacked to each other and thus welded by compression molding, with ensuing increase of mechanical resistance at the expense of permeance [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA promising strategy to avoid the subsequent hot pressing step could be the promotion and control of blobbing phenomena during electrospinning by controlling the solvent-polymer phase separation rates. If the latter are slow enough to lead to the formation of an interconnected fibers network, the interlaminar adhesion of the mats could be improved, thus providing the materials with considerable mechanical strength despite their high porosity. Indeed, in a previous work [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], we demonstrated that hybrid network formed by GO and CNTs may give rise to polymer nanocomposites with emergent features, including the strong affinity to methylene blue and methyl orange, and mechanical properties exceeding the sum of those of constituents.\u003c/p\u003e \u003cp\u003eAim of this work is to explore the feasibility to construct highly porous electrospun mats that conjugate excellent performance in terms of permeance, pollutant rejection and anti-fouling features with outstanding mechanical robustness and durability. Hence, the structure-properties relationships of ternary nanocomposites based on a PVDF-co-HFP matrix and a hybrid loading of GO and CNTs were investigated and compared to those of binary systems containing solely GO or CNTs.\u003c/p\u003e"},{"header":"2. Experimental Part","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials and reactants\u003c/h2\u003e \u003cp\u003eGO and CNTs used in this work were synthesized in our laboratories, following the protocols reported in our previous studies [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Briefly, GO (lateral size\u0026thinsp;\u0026lt;\u0026thinsp;45 \u0026micro;m, thickness\u0026thinsp;=\u0026thinsp;0.7\u0026ndash;0.8 nm, C/O ratio\u0026thinsp;=\u0026thinsp;1.1, density\u0026thinsp;=\u0026thinsp;1.76 g/cm\u003csup\u003e3\u003c/sup\u003e) was prepared by using Tour\u0026rsquo;s method, while CNTs (length\u0026thinsp;=\u0026thinsp;1\u0026ndash;2 \u0026micro;m, diameter\u0026thinsp;=\u0026thinsp;10\u0026ndash;20 nm, C/O ratio\u0026thinsp;=\u0026thinsp;40.7, density\u0026thinsp;=\u0026thinsp;2.24 g/cm\u003csup\u003e3\u003c/sup\u003e) were synthesized by fluidized bed chemical vapour deposition.\u003c/p\u003e \u003cp\u003ePVDF-co-HFP was a sample of Fluolyne HY kindly supplied by CTS Europe. It is an elastomeric thermoplastic fluorinated copolymer, having Mw\u0026thinsp;=\u0026thinsp;400,000 g/mol and density equal to 1.77 g/cm\u003csup\u003e3\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAcetone and methylene blue (MB) reagent grade were purchased from Sigma Aldrich.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Preparation of electrospun mats\u003c/h2\u003e \u003cp\u003eThe six formulations herein investigated are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In order to obtain the designed formulation, nanoparticle(s) in the desired proportions were first dispersed in acetone through ultrasonication (4 h), thereafter PVDF-co-HFP was poured (10 wt.% with respect to the solvent) to the homogeneous dispersion and vigorously stirred until complete dissolution (6 hours).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFormulation of the samples investigated, amounts given in wt.%\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample codename\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC0-G0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eC0-G1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC0-G2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC2-G0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eC2-G1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eC2-G2\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePVDF-co-HFP\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e96\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCNTs\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eEach solution was then loaded to a glass syringe and electrospun by using a Linari equipment under the following operating parameters: flow rate, 1.8 ml/h; supplied high voltage, 15 kV; needle-to-collector distance, 12 cm; temperature, 25\u0026deg;C; and relative humidity, 50%.\u003c/p\u003e \u003cp\u003eNanofibers were collected on an aluminum-coated rotary drum (diameter\u0026thinsp;=\u0026thinsp;25 mm, speed\u0026thinsp;=\u0026thinsp;25 rpm), for 180 min in order to obtain\u0026thinsp;~\u0026thinsp;50 \u0026micro;m thick mats.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Fabrication of devices for ultrafiltration\u003c/h2\u003e \u003cp\u003eDevices for ultrafiltration were assembled as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Membrane septum and support grid were home-made fabricated by rapid prototyping.\u003c/p\u003e \u003cp\u003eThereafter, support grid was placed onto electrospun membranes (yet adherent to aluminum foil) and poured in ethanol to allow the easy detachment of mats from aluminum foil and their placement onto grid.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Characterization of nanohybrid mats\u003c/h2\u003e \u003cp\u003eMorphology was investigated by combining different techniques. Scanning electron microscopy (SEM) was carried out by using an ESEM FEI QUANTA 200 microscope (Thermo Fisher Scientific). Image analysis, performed by ImageJ software equipped with Diameter J plug-in, was used to measure size distribution of fiber and pore diameter of the mats [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOverall porosity of membranes was calculated via gravimetric method, as typically used for PVDF-co-HFP electrospun mats [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Briefly, all pores of a membrane sample of known weight and dimensions were totally filled with ethanol by soaking the mats for 2 hours. Porosity was then calculated according to Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$Porosity \\left(\\%\\right)=\\frac{{W}_{s, wet}-{W}_{s,dry}}{{\\rho }_{L}{V}_{s}} \\times 100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({W}_{s, wet}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({W}_{s, dry}\\)\u003c/span\u003e\u003c/span\u003e respectively indicate the weight of wet and dry membrane sample, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\rho }_{L}\\)\u003c/span\u003e\u003c/span\u003e is the density of the wetting liquid, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({V}_{s}\\)\u003c/span\u003e\u003c/span\u003e is the sample volume. Atomic force microscopy (AFM) was employed to measure the roughness of membranes. AFM measurements were performed in air using a Bruker FAST-SCAN microscope equipped with a closed-loop scanner, in the soft tapping mode using a probe with an apical radius of about 5 nm. Arithmetic average roughness (Ra) and quadratic mean roughness (Rq) were calculated on squared areas (side length\u0026thinsp;=\u0026thinsp;1 \u0026micro;m).\u003c/p\u003e \u003cp\u003eSessile drop water contact angle (WCA) testing was carried out to assess the surface wettability of membranes. Experiments were performed at room temperature by using an FTA 1000 (First Ten \u0026Aring;ngstroms, U.K.) instrument. 4 \u0026micro;L of deionized water were dropped onto the surface of each sample by way of an automatic liquid drop dosing system. Images of the drops onto the surface were acquired after 20 s.\u003c/p\u003e \u003cp\u003eMechanical testing was performed by using an Instron 3365 dynamometer (UK) onto prismatic specimens having width\u0026thinsp;=\u0026thinsp;10 mm; length\u0026thinsp;=\u0026thinsp;90 mm; thickness\u0026thinsp;~\u0026thinsp;50 \u0026micro;m (measured before each measurement). The tests were performed onto at least 10 replicates, under the following conditions: distance between the jaws\u0026thinsp;=\u0026thinsp;30 mm, crosshead speed\u0026thinsp;=\u0026thinsp;1 mm min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e until failure [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The salient data were provided as mean values\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviations. Elastic modulus (E\u003csub\u003e1\u003c/sub\u003e) was calculated as the slope of stress-strain curve in the initial linear range (E\u003csub\u003e1\u003c/sub\u003e). The slope of each curve was also calculated in the strain-hardening region (E\u003csub\u003e2\u003c/sub\u003e). Toughness was measured as the integrated area of each curve. Tensile strength (TS) and elongation at break (EB%) were respectively calculated as the ordinate and the abscissa of stress-strain curve at failure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Flux measurements and antifouling test of the devices\u003c/h2\u003e \u003cp\u003eWater permeance of the membranes was assessed at room temperature by performing flux experiments in a home-made setup. Membranes having 45 mm diameter were fixed into a test cell and a dead-end vacuum filtration system was used at an absolute pressure of 85 kPa and pure water flux (J, L/m\u003csup\u003e2\u003c/sup\u003eh) was calculated using Eq.\u0026nbsp;\u003cspan refid=\"Equ2\" class=\"InternalRef\"\u003e2\u003c/span\u003e:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$J=\\frac{V}{A t }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere V is the permeate volume, A is the effective cross-sectional area of the membrane, t is the elution time (h).\u003c/p\u003e \u003cp\u003eIn order to quantify MB removal, vacuum filtration tests under the same conditions were carried out on 100 mL aqueous MB solutions (5 mg/L).\u003c/p\u003e \u003cp\u003eMB concentration in the feeding solution and in the filtrate were then measured via UV-vis spectroscopy (at λ\u0026thinsp;=\u0026thinsp;667 nm) and the ultimate separation efficiency, R (%), was calculated according to Eq.\u0026nbsp;(\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e):\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$R \\left(\\%\\right)=\\left(1-\\frac{{A}_{2}}{{A}_{1}}\\right)\\times 100$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere A\u003csub\u003e2\u003c/sub\u003e and A\u003csub\u003e1\u003c/sub\u003e are the values of absorbance of methylene blue recorded at the end and at the beginning of the vacuum-filtration experiments, respectively.\u003c/p\u003e \u003cp\u003eEach membrane was subjected to continuous vacuum-filtration runs up to 50 minutes before washing, and removal efficiency of each sample was also measured at predetermined time intervals, in order to investigate its time-dependent behaviour.\u003c/p\u003e \u003cp\u003eThe analysis of anti-fouling properties was conducted by calculating water flux recovery ratio (FRR), along with reversible (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({R}_{r}),\\)\u003c/span\u003e\u003c/span\u003eirreversible (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({R}_{ir})\\)\u003c/span\u003e\u003c/span\u003e and total (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({R}_{t})\\)\u003c/span\u003e\u003c/span\u003efouling rations, which are provided by using Equations (4\u0026ndash;7):\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e \u003ccolgroup cols=\"2\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(FRR \\left(\\%\\right)=\\frac{{J}_{w2}}{{J}_{w1}}\\times 100\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(4)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({R}_{r} \\left(\\%\\right)=\\frac{{J}_{w2}-{J}_{p}}{{J}_{w1}}\\times 100\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(5)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({R}_{ir} \\left(\\%\\right)=\\frac{{J}_{w1}-{J}_{w2}}{{J}_{w1}}\\times 100\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(6)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({R}_{t} \\left(\\%\\right)=\\frac{{J}_{w1}-{J}_{p}}{{J}_{w1}}\\times 100\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e(7)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({J}_{w1}\\)\u003c/span\u003e\u003c/span\u003e is the flux of pure water passing through the membrane, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({J}_{p}\\)\u003c/span\u003e\u003c/span\u003eis the flux of water containing pollutant, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({J}_{w2}\\)\u003c/span\u003e\u003c/span\u003eis the flux of pure water passing through the membrane after it has been cleaned.\u003c/p\u003e \u003cp\u003eReusability was evaluated by subjecting the membranes to 10 process cycles. At the end of each cycle, lasting 50 minutes, the membranes were regenerated via washing with methanol, and eventual changes in separation efficiency or mechanical damaging were recorded.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results And Discussion","content":"\u003cp\u003eThe morphology of the mats was investigated by SEM and image analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e reports SEM micrographs at different magnifications of C0-G0 (a-a\u0026rdquo;), C0-G1 (b-b\u0026rdquo;), C0-G2 (c-c\u0026rdquo;), whereas Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e provides those of C2-G0 (a-a\u0026rdquo;), C2-G1 (b-b\u0026rdquo;), C2-G2 (c-c\u0026rdquo;). Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e provides the salient features of fibrous architectures, that is, mean fiber and pore diameter, calculated by image analysis (see size distributions, Fig. S1), and membrane porosity, evaluated by gravimetric method.\u003c/p\u003e \u003cp\u003eAs one can see, all the mats, i.e. neat polymer and its nanocomposites, show either weaker or pronounced blobbing phenomena, which led to the formation of an interconnected fibrous network. This occurrence can be ascribed to the use of acetone only to prepare polymeric solutions, which resulted in slow phase separation rates.\u003c/p\u003e \u003cp\u003eNeat polymer (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-a\u0026rdquo;) exhibits fibers with smooth surface and a discrete diameter distribution, with 45% fibers displaying submicrometric size, 43% having diameter between 1.6 and 3.2 \u0026micro;m, and 12% thicker than 4 \u0026micro;m (Fig. S1). This aspect can be ascribed to the high charge density of PVDF-co-HFP, which is known to promote the splitting of the jet, thus forming both thicker and thinner fibers [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. GO addition, while not significantly altering the fiber diameter, was found to promote the formation of blobs at a higher extent, likely because of its well-known tendency to hinder the solvent evaporation [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Moreover, filler dispersion proved to be inhomogeneous with well-visible aggregation events (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb-c). However, it is worth noting that GO foils display a broad distribution of lateral size, ranging from hundreds of nanometers to tens of micrometers. Larger GO foils were found to protrude out of the fibrous network, with an unfurled sail-like configuration. A closer inspection of these latter in the case of C0-G1 and C0-G2 is provided in the detailed micrographs of Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb\u0026rsquo;-c\u0026rsquo;, respectively, which show GO sheets eventually surrounded by polymer Submicrometric sheets, instead, were small enough to be integrated within fibrous cage (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb\u0026rdquo;-c\u0026rdquo;), and were found either emergent from the fiber surface or embedded inside the fibers.\u003c/p\u003e \u003cp\u003eHence, both samples show an extremely variegated microstructure, comprising a cage made of fibers of different diameters, with GO particles of different lateral sizes, which in turn can be found either outside or integrated within the fibrous structure.\u003c/p\u003e \u003cp\u003eThe morphology of the system C2-G0 is shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea-a\u0026rdquo;. In this case, the addition of CNTs only, did not significantly alter the fibrous architecture of the neat polymer, which retained random orientation and discrete, multimodal size distribution, with substantially similar values of average fiber diameter, although the filler dispersion proved to be inhomogeneous, with well-detectable nanoparticle clusters likely arising from their scarce dispersability in acetone. It is worth noting that such aggregates were localized in both intra- and inter-fiber regions (see Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea\u0026rsquo;).\u003c/p\u003e \u003cp\u003eAs visible from the micrographs of C2-G1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb-b\u0026rdquo;) and C2-G2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-c\u0026rdquo;), hybrid loading of CNTs and GO determined some changes in the mat morphology. Differently from the systems containing solely GO or CNTs, a drastic reduction of size and number of aggregation events was observed in hybrid electrospun mats. Fiber diameter distribution of C2-G1 and C2-G2 (Fig. S1) proved to be narrower than those of the other systems, and the fibers displayed the lowest values of mean diameter (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The presence of CNTs also affected the configuration of larger GO sheets, which were found either to stretch like unfurled sails (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb-c) or even to roll-up around their axis, thus wrapping the polymeric fibers (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb\u0026rsquo;-c\u0026rsquo;). These two phenomena were observed in both hybrid samples, although with a dose-dependent trend: the former proved to prevail at low GO loading (C2-G1, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb-b\u0026rdquo;), while the latter governed the morphology of the samples containing the highest GO content (C2-G2, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-c\u0026rdquo;). Is should also be noted that hybrid samples showed a higher concentration of nanoparticles emerging from the fiber surface and that the unfurled sail-like GO lamellae displayed a thinner thickness (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec-c\u0026rdquo;), likely because the presence of CNTs disturbed the typical stacking of GO layers.\u003c/p\u003e \u003cp\u003eIn order to get more information about the surface morphology of the fibers, AFM analysis was carried out and the images recorded on squared regions of 5 \u0026micro;m and 1 \u0026micro;m per side are respectively reported in \u003cb\u003eFigure S2\u003c/b\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea, whereas the values of arithmetic average and squared mean roughness of the samples are reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. It can be seen that the presence of nanocarbons alters the surface features of fibers, although at different extents depending on formulation. As already envisaged from SEM analysis, in fact, nanoparticles could be found either emerging from fibers or covered with polymeric layers. In C0-G1, it is possible to well-recognize the typical rough texture imparted by two distinct GO foils emerging from the surface of a smooth fiber. In C0-G2, the fiber surface appears as smooth as that of neat polymer, likely because most of the foils proved to be surrounded by a thick polymeric layer, in agreement with the results of SEM imaging. In the case of C2-G0, instead, AFM was able to detect the presence of CNTs emerging from fiber surface, which resulted in a brush-like morphology. C2-G1 and C2-G2 samples displayed an extremely rough texture, likely due to the formation of an extensive GO-CNT hybrid framework that might have promoted a higher dispersion degree and thus the localization of a larger amount of nanoparticles in the surface of fibers.,\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharacterization of fibrous architecture of the mats: mean values of fiber and pore diameter calculated via image analysis, along with porosity assessed by gravimetric analysis.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC0-G0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eC0-G1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC0-G2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC2-G0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eC2-G1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eC2-G2\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003emean fiber diameter (\u0026micro;m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMean pore diameter (\u0026micro;m)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e2.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePorosity (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e87.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e81.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e82.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e90.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e91.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn fact, as visible in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, all membranes containing GO possess fiber roughness similar or moderately larger than that of pristine PVDF-co-HFP, likely due to the bad filler dispersion, which results in smooth polymeric fibers and discrete aggregates. On the other hand, C2-series samples display a rougher texture, with C2-G2 sample showing the largest Rq and Ra values, almost equal to double those of C2-G0, and 700% higher than those of neat C0-G0.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSurface wettability was studied via WCA testing and the results are provided in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. In C0-series systems, the hydrophobic character of PVDF-co-HFP (WCA\u0026thinsp;=\u0026thinsp;108\u0026deg;) is substantially maintained even in the presence of GO, despite the well-known hydrophilicity of this latter. Again, this result could be likely ascribed to the poor extent of filler dispersion, also testified by the high data scattering of C0-G1 and C0-G2, and to the scarce presence of nanoparticles emerging from the surface, in full agreement with SEM and AFM observations. Predictably, hydrophobic character of the polymer was further enhanced by adding CNTs, due to their hydrophobicity, even boosted by the brush-like morphology of fibers. However, incorporating GO together with CNTs led to a different behavior. WCAs were found to decrease as a function of the GO content, until the hydrophilic character of the latter prevailed over the hydrophobic one of the other components at the highest GO dose (C2-G2), thus resulting in a WCA as low as 71\u0026deg;. These results are in strong agreement with those of SEM and AFM measurements (see again Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSalient tensile properties of the samples\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eProperty\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eC0-G0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eC0-G1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eC0-G2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC2-G0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eC2-G1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eC2-G2\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE\u003csub\u003e1\u003c/sub\u003e (MPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.104\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e0.323\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e0.824\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e0.197\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e0.485\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e \u003cp\u003e4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE\u003csub\u003e2\u003c/sub\u003e (MPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e0.95\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e1.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e7.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e \u003cp\u003e3.81\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTS (MPa)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.41\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e0.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e1.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e1.56\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e4.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e \u003cp\u003e3.85\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEB (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e178\u0026thinsp;\u0026plusmn;\u0026thinsp;18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e175\u0026thinsp;\u0026plusmn;\u0026thinsp;11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e215\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e190\u0026thinsp;\u0026plusmn;\u0026thinsp;15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e180\u0026thinsp;\u0026plusmn;\u0026thinsp;19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e \u003cp\u003e183\u0026thinsp;\u0026plusmn;\u0026thinsp;22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eToughness (MJ/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e0.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e2.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e1.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e3.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e \u003cp\u003e3.22\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe representative stress-strain curves collected during tensile tests are provided in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, whereas the salient mechanical properties measured via tensile tests are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It can be noted that neat polymer and its nanocomposites, with the only exception of C2-G2 sample, display a J-shaped strain-stiffening behavior, being soft and compliant at small strains, while becoming rapidly stiffer at higher strains. In fact, as visible in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, such samples show an initial slope of stress-strain curve (E\u003csub\u003e1\u003c/sub\u003e) which is quite lower than that calculated in strain-hardening region (E\u003csub\u003e2\u003c/sub\u003e). Indeed, this peculiar mechanical behavior, likely attributable to the multimodal distribution of fiber diameters (Figs. S1), was already detected for other fibrous systems based on PVDF and PVDF-co-HFP[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The mechanical behavior of C0-G1 and C0-G2 proved to be dominated by the matrix, although the presence of nanofillers imparted strengthening and stiffening effects, without compromising ductility. C2-G0 shows ultimate resistance and deformability similar to those of C0-G2 but the J-shape of the curve is more pronounced, likely because of the orientation of CNTs along deformation axis with ensuing strain-hardening. In C2-G1, which displayed the highest TS, the formation of a GO-CNT hybrid network and the prevalent unfurled sail-like configuration of GO reasonably resulted in a remarkable aliquot of GO-CNT hybrids oriented along the deformation axis, with consequent strain hardening that led to the most remarkable J-shaped behavior. In C2-G2, instead, the mechanical behavior seems to be governed by the GO-CNT hybrid framework, which endowed the fibers with higher stiffness even at low strains, thus suppressing the J-shaped strain-stiffening behavior. Anyhow, the addition of GO and/or CNTs determined a significant stiffening and strengthening of the PVDF-co-HFP matrix, without compromising its ductility. In particular, hybrid membranes retain the good deformability of neat PVDF-co-HFP (EB close to 200%) while experiencing an outstanding increase of tensile strength (20-times higher than that of neat polymer and 2\u0026ndash;3 times higher than those of the nanocomposites containing solely GO or CNTs), and toughness (almost ​​9 times higher than that of PVDF-co-HPF and about double or triple those of the composites containing only one type of nanofiller), thus possessing mechanical prerequisites good enough to be used for ultrafiltration. Indeed, the values of tensile strength of these materials match those of less porous membranes prepared via wet phase inversion or compression molding-aided assembly [\u003cspan additionalcitationids=\"CR33\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe possibility to use such mats as membranes for ultrafiltration of MB was assessed. Water flux and MB retention experiments give useful information about permeability and selectivity of the membranes. Unfortunately, because of their poor mechanical performance, C0-G0 and C0-G1 experienced mechanical failure during preliminary flux tests in pure water, whereas C0-G2 underwent perforation after 3 minutes of filtration experiments with water contaminated by MB dye. Therefore, the characterization was restricted to C2-G0, C2-G1 and C2-G2. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea provides the permeance of membranes toward pure water (empty circles), and solution flux (filled circles), along with the final removal rate of MB dye (histograms). The outcomes point out that all these samples display good water permeance, due to the high porosity levels of electrospun fibrous mats, along with suitable pore size. However, a clear trend was observed as a function of GO content, reasonably ascribed to the different surface characteristics of each sample, including wettability and roughness.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNoteworthy, C2-G2 granted the highest flux of pure water (as large as 820 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), far superior to other vacuum-ultrafiltration membranes recently proposed for treating water contaminated by MB dye [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. C2-G1 showed slightly lower permeance (780 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), whereas C2-G0 gave the worst performance (450 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). This aspect put into evidence the crucial role of oxygen-rich GO lamellae, which boost permeability of the membranes owing to their hydrophilicity (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), and the wrinkled texture of GO foils, with consequent enhancement of roughness (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). When subjected to the flux of water contaminated by MB dyes, the differences between C2-G1 (660 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and C2-G2 (770 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) tend to augment. Nevertheless, both samples displayed J\u003csub\u003ep\u003c/sub\u003e values quite higher than that of C2-G0 (320 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). Thus, incorporating GO and CNTs into a PVDF-co-HFP electrospun membrane results in extremely robust materials that can withstand water flux without failure despite their high porosity and thin fibers. This latter aspect allows outstanding permeability, granted by the combination of hydrophilicity (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) and adequate pore size (Fig. S3 and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Often, increasing permeability can lead to a worsening of dye rejection performance. In this case, instead, C2-G2 showed also the highest capacity to reject MB cationic dyes (99%), followed by C2-G1 (89%), while C2-G0 (51%) displayed the worst results. Furthermore, the analysis of evolution of removal efficiency upon treatment time (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb) points out that C2-G0 membranes achieved a plateau within 20 minutes, thus suggesting the possible insurgence of fouling issues, whereas the removal efficiency of hybrid membranes proved to monotonically increase upon time. Indeed, one of the most critical design parameters is the fouling resistance of the membranes for water treatment. This aspect was assessed and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, which reports the flux recovery ratio, FRR (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea), together with irreversible, reversible and total fouling rations (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb). The trend observed for FRR follows what seen for WCA, with the most hydrophilic C2-G2 membranes displaying the strongest anti-fouling properties, i.e. the highest flux recovery ratio (93.33%), followed by C2-G1 (88%), whereas C2-G0 gave the worst performance (64%). Noteworthy, the aliquot of irreversible fouling is sensationally low for C2-G2 (6%), which displays promising reusability. By contrast, the mats containing CNTs only (C2-G0) are prone to irreversible fouling, likely due to their hydrophobicity and to the strong tendency of CNTs to form stable complexes with MB molecules via π-π stacking [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe reusability of the systems is another crucial prerequisite in the perspective of reducing economic and environmental costs. The results, provided in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, put into evidence that C2-G0 experienced damaging after 3 cycles, whereas C2-G1 and C2-G2 proved to withstand mechanical stresses without failure in the whole time investigated. These outcomes, in strong agreement with those of mechanical testing, once again point out, on one hand, the difficulty to achieve efficient membranes via electrospinning and, on the other hand, the surprising reinforcing effect exerted by the hybrid GO-CNT network integrated within the polymer matrix. Beyond the considerations on mechanical durability, it is worth noting that hybrid membranes substantially retained a constant flux and a high separation efficiency even after 10 cycles, while C2-G0 sample has undergone a dramatic decline in separation performance, reasonably because of its poor anti-fouling properties. In fact, in this latter sample an initial flux decrease was observed during the second cycle, likely because of fouling, while the raise of flux detected in the third cycle, associated with a further loss of separation efficiency, presumably indicates the incipient structural failure of the mats.\u003c/p\u003e \u003cp\u003eUltimately, a synoptic view of overall performance of the three systems is provided as a radar plot in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. As one can see, hybrid membranes (especially C2-G2) displayed the best performance in terms of mechanical robustness, permeance, removal capacity, anti-fouling properties, and reusability, showing values at least double those of the mats containing CNTs only.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eElectrospun membranes based on PVDF-HFP loaded with GO and/or CNTs were constructed by avoiding toxic solvents and surfactants, exploiting the slow phase separation of polymer in acetone to promote blobs formation. Morphological analysis pointed out that hybrid loading of GO-CNTs led to a uniform dispersion with the formation of a well-structured nanocarbon framework throughout the matrix, with several nanoparticles emerging from the fiber surface. By virtue of this feature, hybrid membranes displayed the highest values of roughness and hydrophilicity, beyond outstanding mechanical robustness coupled with high porosity and micrometric pore diameter. Hence, when tested as membranes for vacuum-assisted ultrafiltration, they showed the highest values of permeance (800 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and capability of removing methylene blue (99%), with negligible fouling issues (irreversible and reversible fouling below 6% and 25%, respectively) and remarkable reusability up to at least 10 cycles.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePO-FESR-2014-2020-SICILIA, SAWE, \u0026ldquo;Support, Alerting, Early Warning\u0026rdquo; cod. no. 08PA9511000101 CUP G78I18000960007 Azione 1.1.5\u0026nbsp;Regione Siciliana.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor\u0026apos;s contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll the authors conceptualized the work, M.G. and A.M. carried out experimental work, A.M. and R.S. wrote the main manuscript text, A.M. and M.G. prepared the figures, R.S. supervised the work, and all the authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are grateful to Prof. G. Buscarino for technical help in performing AFM measurements.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSha Z, Boyer C, Li G, Yu Y, Allioux F-M, Kalantar-Zadeh K, et al. Electrospun liquid metal/PVDF-HFP nanofiber membranes with exceptional triboelectric performance. Nano Energy 2022;92:106713. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/https://doi.org/10.1016/j.nanoen.2021.106713\u003c/span\u003e\u003cspan address=\"10.1016/j.nanoen.2021.106713\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang Y, Huang K, Zhang P, Li H, Mi H. PVDF-HFP based polymer electrolytes with high Li + transference number enhancing the cycling performance and rate capability of lithium metal batteries. 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J Hazard Mater 2019;365:312\u0026ndash;21. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/https://doi.org/10.1016/j.jhazmat.2018.11.013\u003c/span\u003e\u003cspan address=\"10.1016/j.jhazmat.2018.11.013\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Carbon nanotubes, Graphene oxide, Hybrid composites, Multifunctional properties, Electrospinning, ultrafiltration membranes","lastPublishedDoi":"10.21203/rs.3.rs-2021850/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2021850/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eGraphene oxide (GO) and carbon nanotubes (CNTs) were integrated at different mutual ratios into poly(vinyl fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and electrospun to construct mats that were assessed for treating water contaminated by methylene blue (MB) via vacuum-assisted ultrafiltration. The materials were fully characterized from a morphological, physicochemical and mechanical point of view. The results revealed that such materials are suitable for being used as membranes for continuous processes, such as ultrafiltration. In particular, adding 2 wt.% of GO and CNTs gave the best performance, showing extremely high flux (800 L*m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e*h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), excellent rejection (99%) and flux recovery ratios (93.3%), along with antifouling properties (irreversible and reversible fouling below 6% and 25%, respectively), and reusability. These outstanding outcomes were ascribed to the peculiar microstructure achieved, which endowed polymeric membranes with high roughness, wettability, and mechanical robustness.\u003c/p\u003e","manuscriptTitle":"Electrospun mats based on poly(vinyl fluoride-co-hexafluoropropylene) and hybrid carbon nanofillers as high performance ultrafiltration membranes","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-09-12 17:52:49","doi":"10.21203/rs.3.rs-2021850/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ff6f7e38-e4d8-4d9d-9f30-101303a2f3f5","owner":[],"postedDate":"September 12th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2022-09-12T17:57:51+00:00","versionOfRecord":[],"versionCreatedAt":"2022-09-12 17:52:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2021850","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2021850","identity":"rs-2021850","version":["v1"]},"buildId":"FbvkV6FR0MCFSLy54lSbu","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}