Halloysite

Mar 18, 2015 - It could be concluded that the improvement of thermal stability of PAN/HNTs electrospun membranes, especially at 1% w/w HNTs loading, i...
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Electrospun Nanofibrous Membranes of Polyacrylonitrile/Halloysite with Superior Water Filtration Ability Maziyar Makaremi,† Rangika Thilan De Silva,† and Pooria Pasbakhsh*,† †

Mechanical Engineering Discipline, School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, 47500, Selangor Darul Ehsan, Malaysia S Supporting Information *

ABSTRACT: The necessity of benefiting a breakthrough in filtration technology has led to increasing attention in advanced functional nanosized materials such as nanofibers for filtering devices as a solution for providing water at lower energy costs. In this study, electrospun polyacrylonitrile (PAN) nanofibrous membranes were reinforced by 1, 2, and 3% w/w of halloysite nanotubes (HNTs) in order to improve their mechanical properties, thermal stability and water filtration performance for the possible application as water filtration membranes. Morphological analysis revealed the highly porous and nanofibrous structure of membranes which further confirmed by surface area analysis (BET). Incorporation of HNTs enhanced the mechanical properties of the membranes such as tensile strength and elongation at break (especially at 1% w/w HNTs) while resulted in significant improvement of their thermal properties. Moreover, PAN/HNTs membranes showed excellent oil/ water separation performance, while incorporation of HNTs led to increase in water flux rate, which is considered as a key point in water filtration membranes. Additionally, heavy metal ion adsorption performance of the membranes showed a significant increase by incorporation of 3% w/w HNTs. These results signified the potential of electrospun PAN/HNTs nanofibrous membranes to be used for water filtration applications.

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INTRODUCTION Polyacrylonitrile (PAN), a polymer with acrylonitrile as the repeating unit, gets increasing attention as precursor toward carbon materials while applications for carbon-based material are expanding rapidly due to their excellent mechanical and thermal properties.1 PAN-based nanofibers have been used in many applications such as electrically conductive nanofibers, wound dressing, biocatalyst, tissue scaffolding, and drug delivery systems.2 Furthermore, PAN nanofibers have been widely used in ultrafiltration, nanofiltration, and reverse osmosis membranes due to their high chemical resistant, thermal stability, and excellent wetability with water.3 Electrospinning as a method for fabrication of nanofibers relies on electrostatic forces to fabricate fibers in nano- to micrometer diameter range and has been broadly explored as a simple and low cost method to make fibers from polymer solutions.4,5 Although electrospinning of fibers from polymer solutions and melts has been known since the 1930s, this technique has received great attention in recent years due to its potential to fabricate nanofibers with unique characteristics such as small diameter, high surface to volume ratio and high porosity.6 PAN can be electrospun into very thin and uniform nanofibers7−13 while it is convertible to carbon material.14−17 In a study done by Gu et al.,18 influence of electrospinning parameters on diameter of PAN nanofibers were investigated © 2015 American Chemical Society

and their results indicated that solution viscosity plays an important role while applied voltage has no significant impact. Although PAN nanofibers have been used in various fields, their small diameter and unoptimized molecular orientation has hindered their usage in some applications.19 Furthermore, improvement in mechanical and thermal properties of these nanofibers is also desirable. In order to enhance these properties, PAN composite nanofibers consisting of nanoscale inorganic fillers and PAN matrix can be fabricated to combine the advantages of polymer materials such as lightweight, flexibility, and good moldability with properties of inorganic materials such as high strength, heat stability, and chemical resistance.20,21 Studies have shown that these electrospun composite nanofibers exhibit enhanced mechanical, electrical, optical, thermal and magnetic properties compared to neat PAN nanofibers.21−23 For instance, Qiao et al.24 fabricated PAN/Fe−montmorillonite fibrous nanocomposite membrane and reported that loading of Fe−montmorillonite can increase the tensile strength and thermal stability of PAN nanofibrous membrane. In another study, Shami et al.25 prepared PAN/ Na−montmorillonite fibrous nanocomposite membrane and Received: January 22, 2015 Revised: March 5, 2015 Published: March 18, 2015 7949

DOI: 10.1021/acs.jpcc.5b00662 J. Phys. Chem. C 2015, 119, 7949−7958

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The Journal of Physical Chemistry C

6 h until the solution became homogeneous. The HNTs were then directly added into the solution and vigorously stirred for 12 h. The PAN/HNTs solution was electrospun onto a rotating drum (300 rpm) via an electrospinning device (NB-EN1, NanoBond), containing a metallic needle, a feed pump and a high voltage power supply. All samples were electrospun with a solution flow rate of 1.4 mL/h, having distance of 15 cm and voltage that ranged from 13 to 14 kV. Characterization of Membranes. Morphology Analysis. The morphologies of electrospun PAN membranes containing different percentage of HNTs were observed under ultrahigh resolution field emission scanning electron microscope (FESEM, Hitachi SU8010). To prevent electrostatic charging during observation, the samples were coated with a thin layer of platinum. Fiber diameter and porosity of the membranes were measured by ImageJ analysis software. Furthermore, the contribution and dispersion of HNTs in the fibers were observed under the STEM mode of the same FE-SEM instrument. Tensile Analysis. Mechanical properties of the membranes were calculated by a strip tensile test technique based on the ASTM D882-02 standard. Each film was cut into strips (10.0 × 2.5 cm) and stretched to break at rate of 2 mm/min suing a TA XT PlusTexture Analyzer (Stable Micro System, U.K.). The thickness of samples varied between 0.016 to 0.04 mm and thickness of individual samples was used in the calculation. One-way analysis of variance (ANOVA) together with Turkey’s post hoc test were implemented to evaluate statistical significance of the results and the difference was considered statistically significant at p < 0.05 (commercial software SPSS, v.18 was used). Fourier Transform Infrared (FTIR) Analysis. FTIR spectroscopy (Thermo Scientific IS10) was conducted to identify the chemical interactions occurring inside the PAN/HNTs membranes and the chemical compositional homogeneity of the samples. All spectra were obtained for wavelengths of 600 to 4000 cm−1 with 32 scans per specimen at 0.4 cm−1 resolution. X-ray Diffraction (XRD) Analysis. To investigate the crystalline structure of varied PAN/HNTs samples, Xraydiffraction (XRD) of both powder and membrane samples was obtained by D8 Discover X-ray diffractometer (Bruker, Germany) with nickel-filtered Cu Kα radiation. The data were collected at 0.02 intervals with counting for 0.5 s at each step in the 2θ range of 5−60° at 40 kV and 40 mA. BET Surface Area Analysis. The isothermal N2 gas adsorption−desorption of electrospun membranes were obtained using ASAP 2020 (Micromeritics Inc.). Specific surface areas were measured by the Brunauer−Emmett−Teller (BET) method using the adsorption−desorption data in the relative pressure (P/P0) ranges of 0.01 to 0.2. The pore volume and the pore-size distributions were computed by applying the Barrett−Joyner−Halenda (BJH) method in the relative pressure (P/P0) range of 0.01 to 0.95. The samples were degassed to 200 μm Hg with an evacuation rate of 5 mmHg s−1 and a heating procedure with a heating rate of 10 °C/min to 90 °C. The samples were heated to 90 °C with a heating rate of 10 °C/min and held at 100 °C and 100 mmHg for 2 h. Thermogravimetric Analysis (TGA). The thermal stability was studied with thermogravimetric analysis (TGA) using a thermogravimeter (Q50, TA Instruments) under the nitrogen flow of 60 mL/min for the sample and 40 mL/min for the balance at the heating rate of 10 °C/min from 25 to 600 °C.

reported that the addition of nanoclay can delay the thermal degradation temperature and affect the distribution and the fiber diameter. Furthermore, Wang et al.26 reported that PAN fibers showed better adsorption of methylene blue (MB) with addition of organic-modified montmorillonite (O-MMT) while the average fiber diameter decreased as O-MMT loading increased. During the past decade an economically viable clay mineral known as halloysite nanotubes (HNTs) with hollow tubular structure, high surface area and high adsorption properties has attracted great interest. Halloysite is a 1:1 layer silicate clay mineral where each layer is composed of a tetrahedral (Si−O) and an octahedral (Al−OH) sheet with stoichiometry of Al2Si2O5(OH)4·nH2O.27 Individual tubes vary from submicrometer to several micrometers in length and diameter between 30 and 100 nm. HNTs have potential applications in a wide range of areas to improve strength and/or fire resistance of polymers, as carriers for the supply and controlled release of active agents for drug delivery and anticorrosion coatings and for adsorption of contaminants or pollutants.28 Several studies have been focused on HNTs as an adsorbent material for hydrophobic compounds29,30 and heavy metal pollutants.31 For instance, Cavallaro et al.32 fabricated HNT/ sodium alkanoates hybrids with an enhanced hydrophilic surface and a hydrophobic core. Results indicated an improvement in adsorption of oils from vapor and liquid phases. Furthermore, HNTs have been utilized effectively as nanofillers to improve mechanical, thermal and chemical properties of polymers for applications such as drug delivery,33 tissue engineering34 and protein absorption.35 For instance, Zhao et al.36 reported significant improvement in mechanical properties of poly(lactic-co-glycolic acid) (PLGA) nanofibers by incorporation of HNTs. Furthermore, Cai et al.37 fabricated electrospun poly(L-lactic acid) (PLLA) reinforced by HNTs and observed enhancement in mechanical and thermal properties of nanofibrous membranes. In this study for the first time we evaluated the effect of incorporation of HNTs on morphological, chemical, mechanical and thermal properties of electrospun PAN nanofibrous membranes. Chemical structure and morphological properties of nanofibrous membranes were examined by field emission scanning electron microscope (FE-SEM), scanning transmission electron microscope (STEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Mechanical properties were investigated by tensile tests while thermal stability was studied by thermogravimetric analysis (TGA). Furthermore, heavy metal ion adsorption and oil/water separation performance of the membranes were evaluated by copper photometer and ultraviolet−visible spectroscopy (UV− vis), respectively.

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EXPERIMENTAL SECTION Materials. Polyacrylonitrile (PAN, Mw = 150 kDa) was supplied from Sigma-Aldrich. The spinning solvent dimethyl sulfoxide (DMSO) was provided by Fischer Scientific and HNTs (Dragonite) possessing length between 0.5 and 1.5 μm, inner diameter between 5 and 30 nm, outer diameter around 20−150 nm and aspect ratio (L/D) of 928 were donated by Applied Minerals Inc. Preparation of Electrospun PAN/HNTs Nanofibrous Membranes. In order to make 8% w/w PAN solutions containing 0, 1.0, and 3.0% w/w of HNTs, 0.86 g of polymer pellets were dissolved in 10 mL of 99.6% DMSO and stirred for 7950

DOI: 10.1021/acs.jpcc.5b00662 J. Phys. Chem. C 2015, 119, 7949−7958

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Figure 1. FE-SEM micrographs of electrospun PAN nanofibers containing the following: (a) 0% w/w HNTs; (b) 1% w/w HNTs; (c) 3% w/w HNTs.

Figure 2. FE-SEM micrographs along with diameter distribution chart of electrospun PAN nanofibers containing the following: (a) 0% w/w HNTs; (b) 1% w/w HNTs; (c) 3% w/w HNTs.

Water Filtration Evaluation. The oil/water separation performance of membranes was evaluated using a dead-end filtration module (Syringe Filter Holder 25 mm, Sartorius) at a constant flow rate of 0.4 mL/min on electrospun membranes with 200 μm thickness. 1350 ppm vegetable oil/water/125 ppm surfactant (Tween20) emulsion was prepared by stirring at 13,600 rpm for 15 min with a homogenizer. The feed solution and filtrates were introduced into a glass cuvette and the transmittance was measured from 400 to 900 nm using a UV− vis spectrometer (Cary 300, Agilent Technologies, U.S.A). Permeation flux was calculated by J=

Q A(Δt )

curve of oil/water emulsion obtained in the range of 0−100 ppm. In the experiments of adsorbing Cu(II), the Cu(II) solution with concentration of 10 mg/L was prepared by dissolving Cu(NO3)2·3H2O (Sigma-Aldrich) in deionized water. A 10 mL aliquat of Cu(II) solution was driven through 60 μm thick electrospun membrane at a rate of 1 mL/min using the method discussed previously. The Cu(II) concentrations of the feed and permeate solution were determined by a low range copper photometer (HI 96747, Hanna Instruments) and R value was calculated using eq 2 given earlier.

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(1)

where J is the permeation flux (L/m2 h), Q is the permeation volume (L) of the testing solution, A is the effective area of the tested substrate (m2) and Δt is the sampling time (h). Total concentration rejection (R) in the filtration of oil/water emulsion was given by R=

Cf − Cp Cf

RESULTS AND DISCUSSION

Morphological Analysis. Morphology of PAN membranes are illustrated in Figure 1. Fabricated pure PAN nanofibers exhibit a randomly oriented, ultrafine, and well-uniformed structure (Figure 1a). The structural integrity of PAN nanofibers are well maintained with addition of a low amount of HNTs (1% w/w) (Figure 1b), where high HNTs loadings (3% w/w) led to form conical beads along the fiber axis which affected the uniformity of the fibrous structure (Figure 1c). This nonhomogeneous fibrous structure at high HNTs loadings could occur due to the HNTs aggregates resulting from a poor dispersion of HNTs within the PAN solution during the electrospinning process which caused nonuniformity in the viscosity of solutions. Furthermore, agglomeration of HNTs induced structural irregularity to the electrospun web and made localized charge accumulations in the driven jet which is resulting in an inhomogeneity of the electric field. This

× 100 (2)

where Cf and Cp represent the concentrations of the feed and permeate solution, respectively. Solute concentrations in the feed and permeate solutions were determined by the ultraviolet visible light spectroscopy (UV−vis) method. Calculation of concentration of the permeate was assisted by a calibration 7951

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Figure 3. High magnification FE-SEM micrographs of electrospun PAN nanofibers containing the following: (a) 0% w/w HNTs; (b) 1% w/w HNTs; (c) 3% w/w HNTs.

phenomenon led to the formation of beads and other structural irregularities due to the different extensional stresses induced by the electrified jet.38 Figure 2 shows a magnified area of the electrospun PAN/ HNTs membranes. Under the specified electrospinning conditions, the diameters of pure PAN nanofibers highly populated within the range of 450−550 nm with an average diameter of 484 nm. Addition of HNTs in to PAN nanofibers tend to increase the fiber diameter; for instance, incorporation of 1 and 3 (w/w %) of HNTs yielded to an average fiber diameter of 556 and 570 nm, respectively. It is known that the properties of an electrospinning solution could be significantly affected by the addition of an anionic or a cationic species such as clays. The introduction of negatively charged HNTs into the electrospinning solution would result in a decrease of surface charge density of the spinning jet. Consequently an increase will occur in electrical conductivity and viscosity of the solution, leading to formation of fibers with larger diameters. Similar observations have been reported in the literature.39,40 Examination of the membrane at high magnification (Figure 3) indicated that the surface of PAN nanofibers holds a highly porous and rough structure. The porous structure on the surface of the fibers contributed to spinodal or binodal phases which could be induced by phase separation resulting from the rapid evaporation of solvent (DMSO) during the electrospinning process. These surface-pores are shapeless and oriented along the fiber axis, which is attributed to the mechanism of phase separation and fiercely whipping motion of the electrospinning jet. Parts b and c of Figure 3 show porous structure of PAN nanofibers at high magnification. It can be seen that incorporation of HNTs resulted in formation of fibers with larger diameter. These fibers contained more HNTs and called HNTs-rich fibers. Parts a−c of Figure 4 show the STEM and FE-SEM micrographs of the HNTs embedded along the electrospun PAN fibers containing 1% w/w HNTs. The HNTs were randomly oriented in the PAN solution, but due to elongation of the fluid jet in electrospinning process, most of the HNTs

Figure 4. STEM (a, c) and FE-SEM (b) micrographs of electrospun PAN nanofibers containing 1 w/w% HNTs; (d) FE-SEM micrograph of PAN nanofibers containing 3% w/w HNTs. (parts a and b are not taken from the same area, and each figure was taken from different samples).

are oriented along the streamlines of PAN electrospun nanofibers. Hence, improvement in mechanical properties of electrospun membranes at 1% w/w is expected due to this phenomenon. However, Figure 4d shows an FE-SEM micrograph of a PAN nanofiber which has been supported by an agglomeration of HNTs alongside of the fiber. This confirms that by adding more HNTs (3% w/w) HNTs started to agglomerate which deteriorate the mechanical and thermal properties of the PAN membranes. BET Surface Area Measurements. N2 adsorption− desorption isotherms of PAN nanofibers containing different percentages of HNTs are shown in Figure 5. All samples showed type IV isotherms with H3 type hysteresis which is a characteristic of mesoporous structure and indicates the 7952

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Table 1. BET Surface Area, Pore Volume, and Pore Width Data of PAN Nanofibers Containing Different Percentages of HNTs sample

BET surface area (m2/g)

pore volume (cm3/g)

pore width (nm)

PAN PAN/1%HNTs PAN/3%HNTs

6.85 8.35 13.6

0.032 0.034 0.065

23.1 22.7 23.3

surface area of electrospun PAN nanofibrous membrane was 6.85 m2/g while the surface area increased to 8.35 and 13.6 m2/ g for membranes containing 1% and 3% w/w HNTs, respectively. Meanwhile, pore volume of PAN nanofibers showed an increase of 103% from 0.032 to 0.065 cm3/g by incorporation of 3% w/w HNTs. It can be seen that BET surface area and pore volume of the PAN nanofibers increases as HNTs concentration increases. This can be attributed to increase in internal porosity and surface roughness of PAN/ HNTs nanofibers due to presence of HNTs on their surface as well as formation of HNTs rich nanofibers. Although incorporation of HNTs resulted in increase in fiber diameter of electrospun fibers, but this enhancement did not lead to reduction in surface area. Fourier Transform Infrared (FTIR) Analysis. Figure 7 shows the FTIR spectra of pure PAN, pure HNTs and electrospun PAN membranes containing different concentrations of HNTs. As shown in Figure 7e, FTIR spectra of pure HNTs shows visible peaks of O−H stretching of inner-surface hydroxyl groups (3695 cm−1), O−H stretching of inner hydroxyl group (3621 cm−1), in-plane Si−O−Si stretching (999 cm−1) and O−H deformation of inner hydroxyl groups (905 cm−1),43 which confirms the presence of HNTs. FTIR spectra of PAN/HNTs electrospun membranes have been shown in Figure 7b−d. The spectrum within the range of 930−880 cm−1 (Figure 7, inset 1) shows the presence of O−H deformation of inner hydroxyl groups of HNTs and the peak has shifted toward to lower wave numbers in the electrospun membranes with the addition of HNTs. Furthermore, the vibrational bands of C−N groups of PAN at 1042 cm−1 has shifted to 1028 cm−1 with addition of 3% w/w of HNTs (Figure 7, inset 2). These shifts may be attributed to hydrogen bonding interactions between the C−N groups of PAN and O−H groups of HNTs. Evidently, effective adhesion between HNTs and PAN matrix will be obtained due to these interactions which affect the mechanical and thermal properties of PAN/HNTs electrospun membranes. Tensile Analysis. Mechanical properties of electrospun PAN/HNTs nanofibrous membranes have been summarized in Table2. It can be seen that incorporation of 1 w/w% HNTs increased the tensile strength and elongation at break by 7 and 30%, respectively. This could be attributed to orientation of the HNTs along the longitude of PAN nanofibers due to shear force during electrospinning process, as discussed in the Morphology Analysis earlier. However, further increase in HNTs loading led to decrease in mechanical properties of the membrane, indicating that HNTs are no longer been reinforcement to the polymer at high loadings. On the other hand, elastic modulus of the membranes did not increase with addition of HNTs. In fact, incorporation of 3% w/w HNTs reduced the elastic modulus of electrospun PAN membranes by 14%. According to the statistical analysis (one-way ANOVA) there is no significant difference between the values obtained

Figure 5. N2 adsorption−desorption isotherms of PAN nanofibers containing different percentages of HNTs.

existence of narrow slit-like pores.41,42 As a typical characteristic of mesoporous materials, the hysteresis loop performed in the isotherms confirms the porous structure of the PAN fibers. The hysteresis is associated with the filling and emptying of the mesopores by capillary condensation.27 It can be seen that elecrospun PAN membrane reinforced by 3% w/w HNTs has the highest volume adsorbed and widest desorption branch. This is attributed to increase in content of mesopores of fibers due to incorporation of HNTs. Furthermore, during the adsorption and desorption process, existence of pores with complex shapes might trap the meniscus, leading to slower desorption process and wider desorption branch. Figure 6 shows pore size distribution curves of PAN nanofibers containing different percentage of HNTs. The

Figure 6. Pore size distribution curves of PAN nanofibers containing different percentages of HNTs.

pores are mainly distributed within two ranges, narrow peak ranging from 0 to 15 nm (peak 1) and broad peak ranging from 30 to 80 nm (peak 2). Peak 1 and peak 2 could be attributed to the fiber surface pores and pores between the fibers, respectively. Furthermore, a significant increase can be seen in the pore volume from 5 to 30 nm (between peak 1 and peak 2) associated with the PAN membranes reinforced by 1% and 3% w/w HNTs, attributed to the presence of lumen space inside HNTs. It can be seen that pore volume of membrane with 3% w/w HNTs significantly increased at peak 1 and 2, which could be attributed to existence of HNTs rich porous fibers, discussed in morphological analysis section beforehand. The results of surface area and pore volume of PAN/HNTs nanofibrous membranes with different HNTs loading were summarized in Table 1. The results indicated that the BET 7953

DOI: 10.1021/acs.jpcc.5b00662 J. Phys. Chem. C 2015, 119, 7949−7958

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Figure 7. FTIR spectra of (a) PAN powder, (b)as-spun PAN nanofibers, (c) as-spun PAN nanofibers with 1% w/w HNTs, (d) as-spun PAN nanofibers with 3% w/w HNTs, and (e) Pure HNTs, Inset 1: spectra within the range of 930−880 cm−1, Inset 2: spectra within the range of 1110− 990 cm−1.

Hence for HNTs in PAN nanofibers, the load transfer from PAN nanofibers to HNTs can be explained by hydrogen bonding between the nanotubes and the matrix. As discussed in Morphological Analysis section, HNTs at high loading tend to be interacted to each other along the fiber axis in the membranes. Existence of these aggregated nanotubes in the membranes impaired the effective load transfer from the polymer matrix to the fillers, result in reduction of mechanical properties. Moreover, higher percentage of HNTs agglomeration led to higher disruption of PAN polymeric chains. This may also attribute to degradation of mechanical properties at high HNTs loading. X-ray Diffraction (XRD) Analysis. X-ray diffractograms (XRD) of the PAN membranes reinforced by different percentages of HNTs are represented in Figure 8 to study their crystallinity. PAN powder (Figure 8a) exhibited two equatorial peaks. The primary equatorial (100) peak at 2θ = 16.8° corresponds to a spacing of 5.25 Å while the weaker reflection (110) at 2θ = 29.5° corresponds to a spacing of 3.05

Table 2. Mechanical Properties of Electrospun PAN/HNTs Nanofibrous Membranes HNTs content (% w/w) 0 1 2 3

tensile strength (MPa) 13 13.9 13.6 12.0

± ± ± ±

0.7 0.9 0.7 0.4

elastic modulus (Gpa) 0.56 0.52 0.50 0.48

± ± ± ±

0.1 0.4 0.6 0.8

elongation at break (%) 9.0 11.7 10.9 9.6

± ± ± ±

1.6 1.5 1.6 2.1

for mechanical properties of electrospun membranes containing different percentage of HNTs (p < 0.05), which implies that the addition of HNTs up to 3% w/w does not have any significant effect on the mechanical properties of membranes. Stress− strain curves associated with the membranes has been provided in the Supporting Information. According to Schadler et al.44 for a simple composite system without micromechanical interlocking between the filler and matrix, the load transfer from a matrix to filler was realized through a chemical bonding between the filler and the matrix. 7954

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Figure 8. X-ray diffraction patterns of (a) PAN powder, (b) HNTs, (c) as-spun PAN nanofibers, (d) PAN nanofibers with 1% w/w HNTs, and (e) PAN nanofibers with 3% w/w HNTs.

Figure 9. TGA curves of PAN/HNTs nanofibrous membranes.

loading, where almost no agglomerations were observed. Therefore, in this membrane, uniformly dispersed HNTs along the nanofibers and/or inserted into the nanofibers acted like a barrier to the passage of the volatile pyrolized products of PAN, leading to retardation in the thermal degradation of membrane and their significant high thermal properties. Similar result has been reported by Du et al.49 where incorporation of 10% w/w HNTs into poly(propylene) (PP) greatly improved the thermal stability of the nanocomposite. Temperature at 5% weight loss, the maximum weight loss (%) and the temperature at maximum weight loss rate have been presented in Table 3. It can be seen that temperature at

Å. The ratio of the d-spacing of these two peaks (1.72) is very close to √3:1 indicating hexagonal packing of the rod-like PAN chains.45 For HNTs (Figure 8b) the major peak at 2θ = 11.54°corresponds to a spacing of 7.66 Å, which referred to a dehydrated form of halloysite ((7 Å)-halloysite). XRD patterns of PAN electrospun membranes containing 0, 1, and 3% w/w HNTs (Figure 8c−e) showed almost a similar pattern while percentage of crystallinity remained constant at 32%. The broad and weak nature of the fiber pattern reflections, due to lack of high intensity PAN equatorial peaks, indicates that the crystalline microstructures of the electro-spun fibers are not well developed. The rapid evaporation of the solvent from the jet accompanied by the rapid structure formation, which occurs within milliseconds (∼50 ms) leads to less developed structures in the fibers. The rapid solvent evaporation reduces the jet temperature. Thus, the molecules that are aligned along the fiber axis have less time to realign themselves, leading to less favorable packing. For most semicrystalline polymers, the stretched chains under high elongation rate do not get enough time to form crystalline lamellae, which yields lower crystallinity. Hence, the crystallinity in the fibers is thereby influenced by the rate of solvent evaporation.46 Furthermore, HNTs equatorial peaks were also not visible in XRD patterns of PAN/HNTs membranes, which could be attributed to their low intensity. It can be said that HNTs did not act as a nucleation agent, since the percentage of crystallinity remained constant for membranes containing different percentage of HNTs loading. Thermogravimetric Analysis. TGA curves of PAN/ HNTs membranes with different HNTs loading are depicted in Figure 9. It can be seen that onset temperature for decomposition shifted from 276 °C for the pure PAN to 292 and 282 °C for the PAN/HNTs membranes containing 1 and 3 w/w% HNTs, respectively. Membranes experienced a sharp weight loss at this stage due to the formation of the ring compounds among −CN groups.47 Furthermore, degradation of PAN/HNTs membranes containing 0 and 3 w/w% HNTs continued after 300 °C leading to their final weight loss of 61.4 and 51.6%, respectively. On the other hand, degradation trend of PAN/HNTs membrane containing 1% w/w HNTs was much lower than the other membranes, resulting in final weight loss of 31.5% at 600 °C. It was reported that thermal properties of the nanocomposite membranes can be strongly affected by dispersion of filler in polymer matrices.48 As discussed in morphological analysis section, HNTs were well dispersed in PAN nanofibrous membranes containing 1% w/w HNTs

Table 3. Thermal Stability Parameters of PAN/HNTs Nanofibrous Membranes HNTs content (% w/w)

temperature at 5% weight loss (°C)

residual matter at 600 °C (%)

temperature at maximum weight loss rate (°C)

0 1 3

280 296 286

38.8 68.6 48.4

283 298 283

5% weight loss increased at 1% w/w HNTs loading but decreased with further increase in HNTs content. However, temperature at 5% weight loss for PAN/HNTs membranes were higher than pure PAN membranes by 16 and 6 °C due to incorporation of 1% and 3% w/w HNTs, respectively. Furthermore, it can be seen from the table that incorporation of 1% and 3% w/w HNTs decreased the maximum weight losses of membranes by 48% and 15%, respectively. Consequently, char residue of PAN/HNTs membranes were increased compared to PAN membranes. It has been reported that thermal stability is enhanced by increasing the char residue as the formation of char hinders the diffusion of the volatile decomposition products.50 Furthermore, increase in the temperature at maximum weight loss rate was observed for membrane containing 1% w/w HNTs loading, indicating the enhancement in thermal stability of the membrane comparing to PAN membrane. It could be concluded that the improvement of thermal stability of PAN/HNTs electrospun membranes, especially at 1% w/w HNTs loading, is attributed to good dispersion of HNTs and entrapment of degradation products of PAN inside the lumen structure of the tubes51 while this improvement has been dominated by not very well dispersed HNTs at 3 w/w%. Enhancement in thermal stability of electrospun PAN membranes by incorporation of nanoclays 7955

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The Journal of Physical Chemistry C has been studied previously. Shami et al.25 reported that incorporation of montmorillonite into electrospun PAN membranes delayed the decomposition initiation of nanofibers and also the residual mass increased with an enhancement in nanoclay-content loadings, where addition of 4% w/w nanoclay increased the residual mass from 47% to 58%. In another study, Wang et al.52 incorporated montmorillonite and graphene oxide into electrospun PAN membranes and observed enhancement in decomposition temperature of membranes while residual mass remained constant at 45%. Filtration Analysis. Oil/Water Separation. Filtration performance of electrospun PAN/HNTs nanofibrous membranes loaded with different percentage of HNTs using emulsified oil/water mixture as the feed solution is shown in Figure 10. Significant reduction in oil concentration of the

Table 4. Flux Rate and Rejection Ratio Results of Electrospun PAN/HNTs Membrane Containing Different HNTs Loading HNTs content (% w/w)

flux (L/m2h)

rejection (%)

0 1 3

81 86 92

99.5 99.5 99.5

flux of electrospun membranes by fillers has been reported before. For instance, Wang et al.56 reported that incorporation of surface-oxidized multiwalled carbon nanotubes (MWNTs) increased the flux rate of electrospun PVA membranes from 67 to 445 (L/m2.h). Heavy Metal Ion Adsorption. Table 5 presents the evaluation of electrospun PAN nanofibrous membrane Table 5. Adsorption Performance of Electrospun PAN/ HNTs Nanofibrous Membranes HNTs content (w/w %)

removal efficiency (%)

0 1 3

4.7 10.6 31.1

containing different HNTs loading for the adsorption of Cu(II) ions. The results indicated that as the HNTs loading increases, the membrane offers better adsorption performance, while addition of 3 w/w% HNTs into the electrospun PAN membrane increased the adsorption properties by 561% from 4.7 to 31.1%. It has been reported that HNTs have removal efficiency higher than most of conventional adsorbents.57 Aluminum oxide surface chemistry provides positive charge at inner pores of HNTs, while outer face is negatively charged due to the presence of silica. Therefore, adsorption of Cu (II) ions can be explained by cation exchange on negative surface sites of HNTs. Previously, heavy metal ion adsorption from the flowing water using functionalized electrospun membranes has been explored in several studies. For instance, Taha et al.58 fabricated electrospun functionalized cellulose acetate/silica nanofibrous membrane for adsorption of Cr(VI) from flowing aqueous solution and achieved removal efficiency of 35% in 10 min from solution with concentration of 30 ppm by employing three stacked pieces of functionalized nanofibrous membrane. In another study, Nthumbi et al.59 achieved Cu(II) removal efficiency of 94% from pond water using chitosan/polyacrylamide electrospun membrane with glutaraldehyde cross-linking. It must be said that reaction time plays an important role in adsorption performance of membranes. Hence, higher flux of contaminated water will lead to lower reduction in metal ion adsorption of membranes. Although incorporation of HNTs led to significant increase in adsorption performance of electrospun PAN membranes, however, high flux rate of contaminated water reduced the reaction time between HNTs and Cu(II) ions, leading to lower adsorption performance of these membranes.

Figure 10. UV−vis spectra of feed oil/water emulsion, filtrate, and pure water for PAN nanofibrous membranes.

filtrates was observed compared to the feed solution. Oil concentration of the filtrate was less than 7 ppm, indicating that performance of the membranes was more than 99.5%. High rejection ratio was expected since the typical oil particle size in oil/water emulsions ranges from 0.2 to 5.0 μm,53 while pore size of electrospun membranes is in the rage of 30 to 80 nm. It can be seen from inset of Figure 10 that the filtrate became very transparent after filtration. Furthermore, rejection ratio was constant (