Polypropylene

Article pubs.acs.org/IECR

Fabrication of Multiwalled Carbon Nanotube/Polypropylene Conductive Fibrous Membranes by Melt Electrospinning Li Cao,† Dunfan Su,† Zhiqiang Su,*,‡ and Xiaonong Chen*,† †

Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical Technology), Ministry of Education, Beijing 100029, China ‡ Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing University of Chemical Technology, Beijing 100013, China ABSTRACT: A novel multiwalled carbon nanotube/polypropylene (CNT/PP) conductive fibrous membrane with fiber diameter of 1−3 μm was fabricated by melt electrospinning. To improve the dispersibility of CNT and enhance the spinnability of PP fibers, CNTs were first mixed with small amounts of paraffin liquid (PL) and then melt-blended with PP for melt electrospinning. The morphology of fibrous membranes and the orientation of CNTs in PP fibers were observed via scanning and transmission electron microscopy (SEM and TEM). The effect of PL and CNTs on crystallization behavior of PP was studied by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Tensile test and impedance analysis were performed to investigate the mechanical and electrical properties of the fibrous membranes. The results indicated that CNT has a distinct nucleating effect on PP, and the addition of PL can improve the spinnability of the CNT/PP compound remarkably. This novel conductive fibrous membrane fabricated by melt electrospinning exhibits improved tensile strength and modulus, good electric conductivity, and enhanced dielectric constant and hydrophobicity.

1. INTRODUCTION Electrospinning is an efficient method to fabricate ultrafine fibers with diameter ranging from a few tens of nanometers to a few micrometers. In the past decades, electrospinning products have received great attention in terms of their practical applications, such as tissue engineering,1 air and water filtration,2 solar cell electrodes,3,4 ion battery materials,5 super capacitors, and sensors.6,7 Among all the electrospun products, most of them were prepared by solution electrospinning and the others by melt electrospinning.8 Compared with melt electrospinning, there exist some shortcomings for solution electrospinning: (a) some commonly used polymers like polyolefin (PP, PE), polyimide (PI), poly(ethylene terephthalate) (PET), and polyamide (PA) can only dissolve in some certain solvents and high temperature is required for complete dissolution; (b) most solvents for polymers are toxic, which is harmful for operators, and the products are not safe when they are applied in tissue engineering, drug delivery, self-healing coatings, and so forth; (c) the evaporation of solvents in the process of electrospinning leads to defect formation on fiber surface;9 (d) for preparation of scaffolds, the size of the membrane pore is supposed to be several micrometers, which is much easier for melt electrospinning than for solution elecrospinning;10 (e) low output; it is reported that only 2− 10% of the total volume in solution electrospinning was collected compared to 100% in melt electrospinning.11 Melt electrospinning was first reported in 1981, when Larrondo and Manley designed an electrospinning device and prepared polypropylene fibers with diameters of about 50 μm.12 In the past decades, melt electrospinning has been successfully applied to several polymers, including polycarbonate (PC), poly(lactic acid) (PLA), poly(ethylene glycol) (PEG), polycaprolactam (PCL), polyamides (PA), polyethylene (PE), and © 2014 American Chemical Society

polypropylene (PP). Polymer melts can be electrospun into fabrics varying in shape and fiber arrangement and can be used in versatile applications, like gas and liquid filtration, protective clothing, drug delivery, electronic circuit design, and sensor industry.8,13 Recently, Hutmacher and Dalton et al. prepared poly(E-caprolactone) (PCL) scaffolds via melt electrospinning in a direct writing mode. Because of the scaffolds’ own high porosity and interconnectivity, cells can present throughout and underneath the scaffolds.14,15 Besides, compared with traditional melt spinning method, melt electrospinning increases the draw ratio of fibers, and the fiber whipping in electrospinning is much more significant,16 which ensures melt electrospinning a promising future in textile manufacturing and relative fields. Polypropylene is a commercial polymer best known for its excellent performances in product forms, including fibers. A lot of work has been done focusing on the improvement of mechanical properties17,18 and on fabrication of products with versatile forms and applications. Till now, polypropylene can be electrospun into fibers with a diameter ranging from hundreds of nanometers to tens of micrometers through melt electrospinning. Cho et al.19 prepared polypropylene fibers with diameters of several micrometers through melt electrospinning at elevated temperatures and proved the superhydrophobicity of these fibers. It is argued that these hydrophobic fiber mats can be applied on the surface of solar cells to avoid contamination of the surface and expand lifetime of the solar cells.13 Lee and Obendorf20,21 prepared a kind of protective clothing with good barrier performance by melt electrospinning Received: Revised: Accepted: Published: 2308

November 5, 2013 January 22, 2014 January 23, 2014 January 23, 2014 dx.doi.org/10.1021/ie403746p | Ind. Eng. Chem. Res. 2014, 53, 2308−2317

Industrial & Engineering Chemistry Research

Article

has some significant advantages, such as nontoxicity, easy processing, excellent tensile strength and modulus, good electric conductivity, good dielectric constant, and hydrophobicity, and can be used to prepare electric devices, antistatic protective clothing, coatings on solar cells, tissue engineering scaffolds, biosensors, and electrochemical detectors.

of polypropylene, and the protective clothing can be used for agriculture workers to protect them from liquid pesticide. Fang et al.22 designed a needless melt electrospinning setup with a rotary metal disc spinneret. By optimizing the electrospinning apparatus and by adding a cationic surfactant, PP fibers with diameters of 400 ± 290 nm were obtained, and the application of these fine fibers can be anticipated. Polypropylene electrospun fibrous membranes can be applied as battery separators due to their innate features of a large surface area and fine pore sizes. Of all the above work on the melt electrospinning of PP, most of them focus on preparation of fine fibers, and the investigation on the crystal structure, tensile properties, and electrical properties is lacking. CNTs have been well known for their unique onedimensional and curly graphite structure, which makes CNTs an excellent filler candidate for fabricating multifunctional high performance materials. A lot of work has been done on the CNTs filled polymer fibers prepared through solution electrospinning, and good mechanical and electrical properties were obtained.23,24 It is also reported that CNTs can act as nucleating agent in crystalline polymers and help improve the mechanical properties of the nanocomposites.25 For CNTbased hybrid composites, the dispersion and the alignment of CNTs in polymer matrix as well as the interfacial interaction of CNTs with polymer have significant influence on the final performances of the nanocomposites. Because of the strong van der Waals interactions among CNTs, they usually exist as stable bundles and their dispersion and alignment in polymer matrix are very poor. Methods to solve these problems include functionalizing of CNTs, adding coupling agent, and adopting different processing methods that have been reviewed by Sahoo26 and Spitalsky.27 In their review, electrospinning was listed as an effective method to disperse and align carbon nanotubes in polymer matrix. It was also concluded that functionalization benefits the dispersion of CNTs in polymer matrix and improves the mechanical properties of the nanocomposites, though it disrupts the extended π conjugation of CNTs and reduces the conductivity of the functionalized CNTs. Therefore, to maintain good conductivity of CNT/polymer nanocomposites, efforts were contributed to improve the mixing methods during nanocomposites preparation. A onestep mixing approach was introduced by Müller where polyethylene glycol (PEG) was employed as the additive to disperse CNTs in PE matrix, and highly improved electrical properties were achieved.28 In another work,29 CNT was first mixed with ethylene-co-vinyl acetate copolymer (EVA) in solution phase to form a master batch with high CNT concentration, was melt mixed with polycarbonate (PC), and finally was melt mixed with polyethylene (PE). A good dispersion and improved tensile strength and elastic modulus were obtained in the CNT/EVA/PC microfibrillar-filled PE composites. In the present work, melt electrospinning was adopted to prepare CNT/PP fibrous membrane. In the process of melt electrospinning, to improve the dispersibility of CNT and enhance the spinnability of PP fibers, a small amount of PL was blended in PP matrix as dispersant and diluents. The influence of the addition of CNTs and PL on the crystallization characteristics, mechanical properties, electrical properties, and hydrophobicity of the CNT/PP fibrous membranes was investigated. Compared with traditional fibrous membranes, this novel fibrous membrane fabricated by melt electrospinning

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. Commercial isotactic PP, PP-H-GD-150 with an isotacticity of 94% and melt flow index of 15 g/10 min (obtained at 230 °C with a load of 2.16 kg according to ASTM D1238) was kindly supplied by Shijiazhuang Refining Branch of China Petroleum & Chemical Corporation (Shijiazhuang, China). Multiwalled carbon nanotubes (CNTs) with a purity of 98% were purchased from Sigma-Aldrich. The diameter of the CNTs ranged from 10 to 30 nm. The length of CNTs was about 5−15 μm. Paraffin liquid (PL) was purchased from Beijing Fine Chemicals Co., Ltd. (Beijing, China). For preparation of CNT/PP compounds, CNTs were melt blended with PP in a miniature single screw extruder (Dynisco LME-230) with a screw speed of 60 r/min at a temperature of 185 °C for 30 min. The concentration of CNTs in the mixtures was 0.01, 0.025, 0.05, 0.1, and 0.25% by weight. A pure PP sample was also prepared as the control sample. The compounds were collected as particles and were ready for electrospinning. For preparation of CNT/PL/PP compounds, proportioned CNTs were first mixed with PL to form a nano sol-gel, and then the mixture was blended with PP in the microextruder. The mass ratio of PL to PP in each sample was 0.03:1. 2.2. Electrospinning. The melt electrospinning device used in this work has been reported in our previous work.30 The compound particles prepared in section 2.1 were transferred to the melt electrospinning device to prepare fibrous membranes.30 All samples were first loaded in the cylinder of the melt electrospinning device and heated at 265 °C for about 5 min to allow a complete melt of PP compound particles. When several Taylor cones formed around the spray spinneret, a maximum voltage output of +60 kV and a maximum current output of 2 mA were charged to the cylinder. Polymer melt flowed out as lines and formed ultrafine fibers when they landed on the collector. The distance from the spinneret to the aluminum collector was 12 cm according to our previous studies.31 Fibers spun in the first 10 min were collected and a series of fibrous membranes with a thickness of 0.1−0.2 mm were obtained. 2.3. Morphological Characterizations. The morphology of CNT/PP electrospun fibrous membranes were examined with a field emission environment scanning electronic microscope FEI XL-30 at 20 kV. All samples were first sputter-coated with a gold layer under a vacuum. 2.4. Dispersion and Alignment of CNTs. The dispersion and alignment of CNTs in PP fibers was observed by means of a transmission electronic microscope (TEM, JEM 2100) at an accelerating voltage of 100 kV. Fibers were first embedded in a special epoxy resin, and after a curing process, the cured sample was cut into ultrathin pieces with a thickness of ca. 100 nm by a Leica Ultracut at room temperature. The sample pieces were collected on carbon-coated copper grids for TEM observation. 2.5. Crystallization Behavior Analyses. Crystallization characteristics of the fibrous membranes were characterized by differential scanning calorimetry (DSC) and wide-angle X-ray 2309

dx.doi.org/10.1021/ie403746p | Ind. Eng. Chem. Res. 2014, 53, 2308−2317


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Figure 1. Morphologies of melt electrospun (A, B, and C) CNT/PP and (D, E, and F) CNT/PL/PP fibrous membranes. CNT concentration in each sample is (A and C) 0 wt %, (B and E) 0.05 wt %, and (C and F) 0.25 wt %. Scale bar is 100 μm.

mechanical properties of the fibrous membranes. At least six specimens with a length of 50 mm and a width of 10 mm were prepared for each sample. The testing rate was 5 mm/min. 2.7. Electrical Property Analyses. Electrical properties of the fibrous membranes were measured by using an impedance analyzer (Agilent 4294A) over a frequency range of 100 Hz to 1.0 MHz. Fibrous membranes of each sample with an area of 1.0 cm2 were sputter-coated by a K-550X sputter coater with Au on both sides before testing. Five specimens were prepared for each sample. 2.8. Hydrophobicity Analyses. Hydrophobicity of the fibrous membranes was investigated by measuring the contact angle (CA) of water droplets on the membranes. The contact angle test was conducted on a JC2000C1 instrument. For each sample, 8−10 data points were collected, and an average CA was calculated.

diffraction (WAXD). The calorimetric measurements were performed on a DSC (STARe system, Mettler & Toledo) from 80 to 200 °C at a heating rate of 10 °C/min in a flowing N2 atmosphere. The weight of each sample was about 6 mg. Indium was used as the standard sample. The crystallinity (Xc) of the CNT/PP electrospun fibrous membranes can be evaluated according to the relative enthalpy of the DSC curves during the heating process. And the Xc can be calculated according to the following equation: Xc =

ΔH × 100% ΔH θ × F

(1)

where ΔH represents the thermal enthalpy of the test sample, ΔHθ expresses the thermal enthalpy of perfectly crystallized sample, which is 190 J/g.32 F is the PP percentage in each sample. Wide-angle X-ray diffraction (WAXD) experiments were conducted with PaNalytical (Holland) X’pert ProMRD diffraction meter at 40 kV and 40 mA. The X-ray source is Cu Kα radiation with a wavelength of 0.154 nm. Electrospun CNT/PP fibrous membrane of 15 × 15 mm were prepared for WAXD test. Scans were performed with a 2θ range of 10−30°, at a scanning rate of 2 °/min and scanning step of 0.02°. The crystallinity (Xc) can be calculated according to the following equation:33 Xc = 1 −

3. RESULTS AND DISCUSSION 3.1. Morphologies of the Melt Electrospun CNT/PP Fibrous Membranes. The morphologies of the melt electrospun fibrous membranes are shown in Figure 1. In Figure 1, it is obvious that fibers in both CNT/PP (Figure 1A, B, C) and CNT/PL/PP (Figure 1D, E, F) fibrous membranes show smooth surface, indicating a good fluidity and consistency of the polymer melt during the electrospining process, which is a significant advantage for producing batch-tobatch products. At the same time, it can be observed that the diameter of CNT/PP electrospun fibers ranges from 5 to 10 μm, whereas CNT/PL/PP electrospun fibers show a smaller diameter of about 1−3 μm, indicating that PL improves the fluidity of PP melt and helps to reduce the diameter of the electrospun fibers. The mechanism of PL improving the fluidity of the polymer melt is that PL acts as lubricant among PP chains and brings more free volume in PP melt and, thus, reduces the viscosity of PP melt. A similar result was also

A amorphous ∑ (A amorphous + Acrystallization )

(2)

where Xc expresses the crystallinity, Aamorphous is the area of the amorphous phase, and Acrystallization is the area of the crystallization phase in the WAXD patterns. Curve-fitting software was used to calculate the peak intensities of WAXD profiles. The deconvoluted peaks can be obtained by using the mixed function of Gauss and Lorentz. 2.6. Mechanical Tests. Mechanical tests were conducted using a Gotech TCS-2000 tensile tester to study the static 2310



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Figure 2. TEM photomicrographs of the alignment and distribution of CNTs within PP fiber. (A and C) Fibers of CNT/PP fibrous membranes and (B and D) fibers of CNT/PL/PP fibrous membranes. The concentration of CNTs is (A, B) 0.1 wt % and (C, D) 0.25 wt %. The arrows indicate flow direction.

Figure 3. Thermal diagram of the melt electrospun PP fibrous membranes (A) without PL and (B) with PL. Concentration of CNTs in the membrane increases from 0 to 0.25 wt %.

inside and aligned along the electrospun fibers.35−37 Jose et al.38 investigated the alignment of CNTs in PP fibers prepared by melt spinning and found that the significant improvement on tensile strength and tensile modulus was ascribed to the highly aligned CNTs. A model was also given by Jose to better disclose the high orientation of CNTs along the fiber axis. In this work, the alignment of CNTs in PP matrix was observed by TEM and the results are shown in Figure 2. During electrospinning, once the electrostatic force was applied, polymer melt was stretched into fibers and CNTs were forced to align along the fiber axis, as Figure 2 shows. It is obvious that, under the external electrostatic force, both singledispersed CNT and CNT bundles are parallel to the fiber axis. At low CNT concentration, a single CNT can be observed, and there are large gaps between CNTs (Figure 2A); with improving the dispersion of CNTs, the uniform arrayed CNTs can be found inside the fiber (Figure 2B). In Figure 2C, when the concentration of CNTs increases to 0.25%, it is very hard to find the dispersed CNTs and only some aggregated CNTs are found to align along the fibers. However, in Figure 2D, though the concentration of CNT is same as

obtained by Yoon et al. They found that PEG could act as a plasticizer to reduce the viscosity of PLA melt and decrease the diameter of PLA fibers.34 Besides, the smooth surface of fibers and fiber continuity are also attributed to the improved dispersion of CNTs in PP matrix. For CNT-related composites, the dispersion of CNTs in the polymer matrix is very crucial for melt electrospinning. The uniform dispersion of CNTs helps to improve the fluidity of polymer melt and enhances its spinnability. Herein, we employed PL to improve the dispersion of CNTs in PP matrix and acquired fibers with a smoother surface and finer diameter. It is shown in Figure 1 that as the concentration of CNTs increases, the fiber diameter increases due to the increased viscosity of the polymer melt. The trend is more obvious for CNT/PP fibrous membrane because the diameter of the fibers become nonuniform when CNT concentration reaches 0.25 wt % (Figure 1C), whereas that of CNT/PL/PP membrane remain continuous and uniform (Figure 1F). 3.2. Alignment and Distribution of CNTs Inside PP Fiber. A lot of work has been done on the electrospinning of CNT/polymer blends, which proved that CNTs dispersed well 2311



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Figure 4. WAXD patterns of melt electrospun PP fibrous membranes at different CNT concentration (A) without PL and (B) with PL.

Figure 2C, well-dispersed CNTs could be found, indicating that PL helps to improve the dispersion of CNTs in PP matrix. It can be explained that the PL-modified PP shows improved fluidity and less chain entanglement, which favors the distribution of CNTs inside PP fibers. 3.3. Crystallization Behavior of the Electrospun CNT/ PP Fibrous Membranes. The crystallization behaviors of the electrospun fibers were studied by a combination of DSC and WAXD methods, which are shown in Figures 3 and 4. During the melt electrospinning, the flow of polymer chains is accelerated by the external electrostatic force and the inner shear between CNTs and PP chains. As we know, CNTs have great influence on the nucleation and growth of polymer crystals, which finally influence the mechanical properties of the electrospun CNT/PP fibrous membranes.25 Besides, PL was also reported to have some influences on the crystallization behavior of polypropylene.39 Therefore, in this work, when considering the factors that may influence the crystallization behavior of PP, both PL and CNTs were counted. For crystal growth kinetics, chain mobility and quench depth are the defining parameters.40,41 Because of the differences in molecular structure, the crystallization and chain diffusion of polymers are not as fast as that of micromolecules; thus, the microstructure inside polymers is governed by kinetics as well as thermodynamics factors. During the melt electrospinning process, primary crystals of polymers are formed on an intramolecular nucleating spot (PP chain entanglement) or on an exotic nucleating agent (CNTs), and the surrounding slack PP chains instantly reorganize and form a lateral crystal growth front to host the subsequent thickening and spread of PP crystals. The spreading process pulls out chains or chain segments from the amorphous PP region, which is very slow in such a strong stress field where chain diffusion is depressed and viscosity of polymer melts increases.42 In CNT/PL/PP system, PL (as diluents) helps the diffusion of PP chains from the restricted amorphous phase to crystal growth front. At the same time, a melting point depression could be observed due to the change of chain mobility in the diluents, which can be found by a comparison between Figure 3A and B that the melting point of the major peak shifted to low temperature after adding PL in PP. Besides, the melting peak in the CNT/PL/PP samples are wider than that in CNT/PP samples; the reason may be described as that the existence of PL allowed PP chains inside the amorphous region diffused to the crystalline region and formed many less-perfect crystal lamellas sharing low melting points.39

In Figure 3, it can also be concluded that, with the addition of CNTs, the content of crystals sharing a lower melting point (T1) first increases with the increase of CNT concentration and then decreases because of the aggregation of CNTs at high loading amount (Figure 3A).43,44 However, for the samples with PL (Figure 3B), CNTs disperse well in PP matrix even at high CNTs content and the nucleating effect of CNTs is still distinct. Thus, the content of crystal layers sharing low melting point is still high when CNT content reaches 0.25%. In the melt electrospinning process, the molecular chains of PP are given only a few seconds to cool down from 265 °C to room temperature, and the chain diffusion is far from finished and a lot of inner stress exists in the fibers. In Figure 4, as the WAXD patterns of the fibrous membranes show, polymer chains are highly stretched by the electrostatic force in the electrospinning process and highly ordered crystal structures formed inside the fibers, for there are only two major diffraction peaks α (110) and α (131) that show up in both CNT/PP (Figure 4A) and CNT/PL/PP (Figure 4B) samples. The calculated results from Figure 3 and 4 are shown in Figure 5,

Figure 5. Crystallinity (Xc) of the electrospun fibrous membranes at varied CNT concentration obtained from DSC and WAXD.

which indicates that CNTs has a strong nucleating effect on polypropylene, and the nucleating effect is more obvious at low CNT concentrations because the upward trend of crystallinity slows down at high CNT concentrations. Furthermore, samples with PL have an increased crystallinity compared with samples without PL at identical CNT concentration, indicating PL 2312



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Figure 6. Tensile properties of the electrospun fibrous membranes at varied CNT concentration. Lines a, b, c, d, e, and f in panels A and B refer to samples with different CNT concentration: a, 0; b, 0.01; c, 0.025; d, 0.05; e, 0.10; f, 0.25 wt %.

Figure 7. Electrical conductivities of the electrospun fibrous membranes at different CNT concentration (A) without PL and (B) with PL.

for CNT/PL/PP fibrous membrane due to the lower fracture strain of CNTs.50 PP chains wrapped around the nanotubes and formed mechanical interlocking between the two phases, which brought enhanced deformation resistance to the hybrid fibrous membrane. In the CNT/PL/PP membrane, welldispersed CNTs favor the formation of the wrapping of PP chains, and the ductile to semiductile transitions occurs at higher filler concentration. Figure 6C illustrates that both the tensile strength and modulus first increase with increasing CNT concentration and then decrease when the concentration of CNTs exceed a certain value; the turning point occurs at higher CNT concentration in CNT/PL/PP membranes than in CNT/ PP membranes for the improved dispersion of CNTs in the former. Mechanical properties can be further explained by the morphologies of the fibrous membranes (Figure 1). Tensile strength of CNT/PP is higher than that of CNT/PL/PP when the content of CNTs is lower than 0.05%, which can be explained by when CNT concentration is low, the dispersion of CNTs are both good in CNT/PP and CNT/PL/PP samples and CNT/PP samples with thicker fibers have better mechanical property than CNT/PL/PP with thinner fibers. When CNT concentrations surpasses 0.05%, fibers in CNT/PP fibrous membrane are interrupted by CNTs aggregates and are no longer uniform in diameter for the unsteady melt flow, whereas fibers in CNT/PL/PP fibrous membrane are still reinforced by well-dispersed CNTs and good morphologies are maintained; therefore, tensile strength of the CNT/PL/PP fibrous membrane is higher than that of CNT/PP fibrous membrane. The results also indicate that the addition of PL improves the dispersion of CNTs in PP matrix and results in fibers with good morphologies, which enhance the mechanical

accelerated the chain diffusion of PP in the late crystallization stage and allowed more PP chain segments arranged into crystal lattices. 3.4. Mechanical Properties of Electrospun Fibrous Membranes. For CNT/PP nanocomposites, to achieve welldispersed nanotubes at high loading fraction is a great challenge. Proper distances between nanotubes are very important when the load transfer in reinforcement achievement or conductive paths formation in electrical properties is considered. Thus, the dispersion of CNTs is of great importance because the aspect ratio of a single CNT is higher than its bundles, and the average distance among well-dispersed CNTs are shorter than that among aggregated CNTs.45 Besides, when the load transfer from polymer matrix to CNTs is considered, a positive interfacial interaction of CNTs with polymer is also significant to a successful fabrication of composites.46−48 To disclose the wetting and adhesion phenomena of CNTs with polymer matrix, Tran49 measured the contact angle of a polymer nanodroplet on a single carbon tube and indicated that modified poly(vinylidene fluoride) (PVDF) showed intimate interaction with CNTs, whereas poor interaction was observed on the interface of original PVDF and CNTs. In this work, the dispersion of CNTs in PP matrix was improved by adding PL. The effect of CNTs on the mechanical property of CNT/PP electrospun fibrous membranes was investigated, and the results are shown in Figure 6. In Figure 6A, the stress−strain curve of pure PP fibrous membrane shows an obvious yield point, whereas the others that filled with CNTs show increased rigidity because of the reinforcement of CNTs. Besides, there is a ductile to semiductile transition at CNT content of 0.01% (Figure 6A− B) for CNT/PP fibrous membrane and 0.1% (Figure 6B−E) 2313



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Figure 8. Dielectric constants of melt electrospun (A) CNT/PP and (B) CNT/PL/PP fibrous membranes with different CNT concentrations.

Figure 9. Contact angles of water droplets on (A) the melt electrospun fibrous membranes and (B) a statistic result. Panel A shows (a to f) CNT/PP fibrous membranes and (a′ to f′) CNT/PL/PP fibrous membranes. For both the samples, CNT concentration increases from 0 to 0.25 wt %.

composites fibrous membranes. Many efforts have been devoted to improve the dispersion of CNTs in polymer matrix, including surface functionalization of CNTs and addition of coupling agent.51−53 Some researchers found that surface functionalization improves the dispersion of CNTs in polymer matrix but decreases the electrical conductivity of CNT/ polymer nanocomposites compared with that filled with unmodified CNTs, whereas some other researchers found that functionalization of CNTs can increase the conductivity of the nanocomposites.24 In our work, PL was adopted to improve the dispersion of CNTs in PP matrix through physical interaction and a positive effect on the conductivity was observed. In view of the fluidity of PP melt during the electrospinning process, the maximal content of CNTs was limited to 0.25%, which is obviously far from the percolation amount for this material. Therefore, it is promising that conductive fibrous membranes that can be applied in electronics devices, sensors, and so forth could be fabricated by melt electrospinning process, yet more effort should be devoted to solve the contradiction of synchronously improving the PP melt fluidity and the conductivity of CNT/PP melt electrospun fibrous membranes.

property of CNT/PP fibrous membranes, especially at high CNTs percentage. 3.5. Electrical Properties of the Electrospun Fibrous Membranes. Electrical conductivity and dielectric properties of the CNT/PP hybrid fibrous membranes were investigated, and the results are shown in Figure 7 and 8. Figure 7 shows the electrical conductivity of CNT/PP (Figure 7A) and CNT/PL/PP (Figure 7B) melt electrospun fibrous membranes at different CNT concentration. The conductivity of pure PP fibrous membrane is about 10−8 S/ cm, and with the addition of CNTs in PP samples, the conductivity increases by 1 order of magnitude for CNT/PP membranes and by 2 orders of magnitude for CNT/PL/PP membranes because of the improved dispersion of CNTs inside PP fibers. For CNT-based functional materials, the dispersion and alignment of CNTs in polymer matrix have tremendous influence on the conductivity of the composites,51 and the effective aspect ratio and distance between tubes are also important for the formation of conductive paths. As for the samples with PL, CNTs are well dispersed in PP matrix; the high aspect ratio and short distance between the separated particles benefits the formation of conductive paths in the 2314



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4. CONCLUSIONS CNT/PP fibrous membrane with fiber diameters of 1−3 μm were prepared through melt electrospinning. PL was employed as an additive to improve the dispersion of CNTs in PP matrix and the spinnability of CNT/PP compounds. The addition of PL increased the crystallinity of polypropylene fibers due to the fact that PL diluted the PP matrix and allowed the polymer chains restrained in the amorphous area to arrange into a crystal lattice. CNTs acted as a nucleating agent in PP fibers and the crystallinity of PP increased with increasing the concentration of CNTs. Besides, the tensile strength and tensile modulus of CNT/PP fibrous membrane first increased with the CNT concentration and then decreased with further increasing of CNT concentration because of the unsteady melt flow at high CNTs loading amounts resulted in nonuniformity of fibers. For the CNT/PP and CNT/PL/PP fibrous membranes, when the CNT concentration is low, CNT/PP melt showed a relatively good fluidity during the electrospinning, the tensile strength of CNT/PP membranes is higher than that of CNT/PL/PP membranes because of thicker fibers and better reinforcing effect of CNT in PP than that in CNT/PL/PP fibrous membranes. At high CNT concentrations, the increased viscosity of CNT/PP melt resulted in the aggregation of CNTs inside PP fibers and the discontinuity of fiber surfaces, whereas the fibers of CNT/PL/PP membranes are uniform and continuous, ensuring high tensile strength of CNT/PL/PP membranes. Electrical properties of the fibrous membranes were also investigated. It was found that the conductivity of the fibrous membrane increased with increasing CNT concentration. At the same time, compared with CNT/PP membranes, CNT/ PL/PP membranes showed higher conductivity for the improved dispersion of CNTs in PP fibers. Besides, dielectric constant increased with increasing the concentration of CNTs, and the CNT/PL/PP membrane showed a higher dielectric constant than the CNT/PP membrane did at identical CNT concentration. The challenge in this system is pursuing of good conductivity, which requires high CNT concentration, whereas the increased viscosity of polymer melt at high CNT concentrations brings processing difficulties. In our laboratory, some efforts have been made to improve our melt electrospinning device to prepare PP melt electrospun fibers with high CNT concentration. The fibrous membranes with improved conductivity and dielectric constant prepared by melt electrospinning are promising for various future applications. Furthermore, the excellent hydrophobicity of the electrospun PP fibrous membrane provides PP fibrous membranes with good air and water permeation performance. The hydrophobic fibrous membrane may also be applied in protein adsorption and desorption, electronic devices, electrochemical sensors, filters, tissue engineering, protective clothing, and coatings on solar cells.

Figure 8 shows the dielectric constants of melt electrospun CNT/PP (Figure 8A) and CNT/PL/PP (Figure 8B) fibrous membranes with different CNT concentrations. It is obvious that dielectric constant of the CNT-filled PP fibrous membrane first increases with the CNT concentration, and when the concentration of CNTs reaches a certain value, the dielectric constant no longer increases for the CNT/PP samples and the upward trend slows down for the CNT/PL/PP samples because the aggregation of CNTs is unfavorable for the formation of conductive paths in the fibrous membranes. Besides, the dielectric constant of the CNT/PL/PP fibrous membrane is higher than that of CNT/PP fibrous membrane at identical CNT concentration, indicating that the addition of PL improved the dispersion of CNTs in PP matrix and favors the formation of conductive paths. As can be seen in Figure 8B, when the CNT concentration reaches 0.25%, the dielectric constant of CNT/PL/PP fibrous membrane keeps increasing, whereas that of CNT/PP fibrous membrane no longer changes. 3.6. Hydrophobicity of the Electrospun Fibrous Membranes. As we mentioned above, melt electrospun fibrous product can be applied in protective clothing, tissue engineering, medical and pharmaceutical applications, filtration and separation, and protective coatings for solar cells.13 For all these applications, hydrophobicity of the fibrous membrane or coating is of great importance to the successful application. A hydrophobic surface can protect solar cells from contamination and increase longevity of solar cells. For separation-related applications, a hydrophobic surface ensured easy handling of the deposited wastes on the filter. As for protective clothing, hydrophobicity of the fibrous product ensures good air and water permeation. Cho et al.19 has prepared polypropylene ultrafiber webs both from solution and melt electrospinning. It was found that the fiber webs exhibit super hydrophobicity, and the protection performance of PP nonwoven webs with electrospun PP fibers are better than that of bare PP nonwoven webs. Via a fluorescence protein adsorption process, an increased chemical retention of the nonwoven webs with PP electrospun fibers was observed. All the results indicated that the super hydrophobicity imparted the PP webs excellent protection performance. Therefore, in our work, hydrophobicity of the fibrous membranes was investigated, and the results are shown in Figure 9. As Figure 9 shows, water droplets on the fibrous membrane exhibit large contact angles, contact angles for CNT/PP fibrous membranes (Figure 9A, a to f) are as large as 128°, whereas that of CNT/PL/PP(Figure 9A, a′ to f′) as large as 132°, indicating excellent hydrophobicity of the melt electrospun PP membranes. A statistic result of the contact angles is shown in Figure 9B. It illustrates that contact angle on both CNT/PP and CNT/PL/PP fibrous membrane changes little with the increase of CNT concentration. The contact angles on the fibrous membrane with PL are all a little larger than those without PL. The reason may be ascribed to the fact fibers in the CNT/PL/ PP fibrous membranes are thinner, which provided enhanced roughness on the fibrous membrane surface compared with that in CNT/PP membranes. Besides, PL on the fiber surface also contributes to improved hydrophobicity. The excellent hydrophobicity ensures that the melt electrospun fibrous membrane can be a good candidate material for electronic devices, electrochemical sensors, filters, tissue engineering, protective clothing, and coatings.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Z. Su) *E-mail: [email protected]. (X. Chen) Notes

The authors declare no competing financial interest. 2315



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Amide Derivative as β Form Nucleating Agent. J. Polym. Sci., Part B: Polym. Phys. 2008, 46, 1725. (19) Cho, D.; Zhou, H.; Cho, Y.; Audus, D.; Joo, Y. L. Structural Properties and Super Hydrophobicity of Electrospun Polypropylene Fibers from Solution and Melt. Polymer 2010, 51, 6005. (20) Lee, S.; Obendorf, S. K. Developing Protective Textile Materials as Barriers to Liquid Penetration Using Melt-Electrospinning. J. Appl. Polym. Sci. 2006, 102, 3430. (21) Lee, S.; Obendorf, S. K. Use of Electrospun Nanofiber Web for Protective Textile Materials as Barriers to Liquid Penetration. Text. Res. J. 2007, 77 (9), 696. (22) Fang, J.; Zhang, L.; Sutton, D.; Wang, X.; Lin, T. Needleless Melt-Electrospinning of Polypropylene Nanofibers. J. Nanomater. 2012, http://dx.doi.org/10.1155/2012/382639 (23) Blond, D.; Walshe, W.; Young, K.; Blighe, F. M.; Khan, U.; Almecija, D.; Carpenter, L.; McCauley, J.; Blau, W. J.; Coleman, J. N. Strong, Tough, Electrospun Polymer−Nanotube Composite Membranes with Extremely Low Density. Adv. Funct. Mater. 2008, 18, 2618. (24) Ra, E. J.; An, K. H.; Kim, K. K.; Jeong, S. Y.; Lee, Y. H. Anisotropic Electrical Conductivity of CNT/PAN Nanofiber Paper. Chem. Phys. Lett. 2005, 413, 188. (25) Miltner, H. E.; Grossiord, N.; Lu, K.; Loos, J.; Koning, C. E.; Van Mele, B. Macromolecules 2008, 41, 5753. (26) Sahoo, N. G.; Rana, S.; Cho, J. W.; Li, L.; Chan, S. H. Polymer Nanocomposites Based on Functionalized Carbon Nanotubes. Prog. Polym. Sci. 2010, 35, 837. (27) Spitalsky, Z.; Dimitrios, T.; Papagelis, K.; Galiotis, C. Carbon Nanotube−Polymer Composites: Chemistry, Processing, Mechanical and Electrical Properties. Prog. Polym. Sci. 2010, 35, 357. (28) Müller, M. T.; Krause, B.; Pötschke, P. A Successful Approach To Disperse CNTS in Polyethylene by Melt Mixing Using Polyethylene Glycol As Additive. Polymer 2012, 53, 3079. (29) Li, Z. M.; Li, S. N.; Yang, M. B.; Huang, R. A Novel Approach to Preparing Carbon Nanotube Reinforced Thermoplastic Polymer Composites. Carbon 2005, 43, 2397. (30) Cao, L.; Dong, M.; Zhang, A.; Liu, Y.; Yang, W.; Su, Z.; Chen, X. Morphologies and Crystal Structures of Styrene−Acrylonitrile/ Isotactic Polypropylene Ultrafine Fibers Fabricated by Melt Electrospinning. Polym. Eng. Sci. 2013, 53 (12), 2674. (31) Deng, R.; Liu, Y.; Ding, Y.; Xie, P.; Luo, L.; Yang, W. Melt Electrospinning of Low-Density Polyethylene Having A Low-Melt Flow Index. J. Appl. Polym. Sci. 2009, 114, 166. (32) Service instructions, TA-4000 thermal analysis, measurement and evaluation system, Version 6.7, Mettler Instrumente AG, GH-8606 Greifensee, Switzerland, 1990. (33) Su, Z.; Dong, M.; Guo, Z.; Yu, J. Study of Polystyrene and Acrylonitrile-styrene Copolymer as Special β-nucleating Agents to Induce the Crystallization of Isotactic Polypropylene. Macromolecules 2007, 40, 4217. (34) Yoon, Y. I.; Park, K. E.; Lee, S. J.; Park, W. H. Fabrication of Microfibrous and Nano-/Microfibrous Scaffolds: Melt and Hybrid Electrospinning and Surface Modification of Poly(L-lactic acid) with Plasticizer. BioMed Res Int. 2013, DOI: 10.1155/2013/309048. (35) Hunley, M. T.; Pötschke, P.; Long, T. E. Melt Dispersion and Electrospinning of Non-Functionalized Multiwalled Carbon Nanotubes in Thermoplastic Polyurethane. Macromol. Rapid Commun. 2009, 30, 2102. (36) Huang, S.; Yee, W. A.; Tjiu, W. C.; Liu, Y.; Kotaki, M.; Boey, Y. C. F.; Ma, J.; Liu, T.; Lu, X. Electrospinning of Polyvinylidene Difluoride with Carbon Nanotubes: Synergistic Effects of Extensional Force and Interfacial Interaction on Crystalline Structures. Langmuir 2008, 24, 13621. (37) Su, Z.; Li, J.; Li, Q.; Ni, T.; Wei, G. Chain Conformation, Crystallization Behavior, Electrical and Mechanical Properties of Electrospun Polymer-Carbonnanotube Hybrid Nanofibers with Different Orientations. Carbon 2012, 50, 5605. (38) Jose, M. V.; Dean, D.; Tyner, J.; Price, G.; Nyairo, E. Polypropylene/Carbon Nanotube Nanocomposite Fibers: Process −

ACKNOWLEDGMENTS The authors wish to acknowledge the support from the National Natural Science Foundation of China (NSFC, grant no. 20974010), Fundamental Research Funds for the Central Universities (project no. ZZ1307), and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, grant no. IRT0807).

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REFERENCES

(1) Edwards, S. L.; Church, J. S.; Werkmeister, J. A.; Ramshaw, J. A. M. Tubular Micro-scale Multiwalled Carbon Nanotube-based Scaffolds for Tissue Engineering. Biomaterials 2009, 30, 1725. (2) Anka, F. H.; Balkus, K. J., Jr. Novel Nanofiltration Hollow Fiber Membrane Produced via Electrospinning. Ind. Eng. Chem. Res. 2013, 52, 3473. (3) Wu, H.; Hu, L.; Rowell, M. W.; Kong, D.; Cha, J. J.; McDonough, J. R.; Zhu, J.; Yang, Y.; McGehee, M. D.; Cui, Y. Electrospun Metal Nanofiber Webs as High-performance Transparent Electrode. Nano Lett. 2010, 10, 4242. (4) Bedford, N. M.; Dickerson, M. B.; Drummy, L. F.; Koerner, H.; Singh, K. M.; Vasudev, M. C.; Durstock, M. F.; Naik, R. R.; Steckl, A. J. Nanofiber-based Bulk-heterojunction Organic Solar Cells Using Coaxial Electrospinning. Adv. Energy Mater. 2012, 2, 1136. (5) Kim, C.; Yang, K. S.; Kojima, M.; Yoshida, K.; Kim, Y. J.; Kim, Y. Ahm.; Endo, M. Fabrication of Electrospinning-derived Carbon Nanofiber Webs for the Anode Material of Lithium-ion Secondary Batteries. Adv. Funct. Mater. 2006, 16, 2393. (6) Laforgue, A. All-textile Flexible Supercapacitors Using Electrospun Poly(3,4-ethylenedioxythiophene) Nanofibers. J. Power Sources 2011, 196, 559. (7) Ouyang, Z.; Li, J.; Wang, J.; Li, Q.; Ni, T.; Zhang, X.; Wang, H.; Li, Q.; Su, Z.; Wei, G. Fabrication, Characterization and Sensor Application of Electrospun Polyurethane Nanofibers Filled with Carbon Nanotubes and Silver Nanoparticles. J. Mater. Chem. B 2013, 1, 2415. (8) Hutmacher, D. W.; Dalton, P. D. Melt Electrospinning. Chem. Asian. J. 2011, 6, 44. (9) Megelski, S.; Stephens, J. S.; Chase, D. B.; Rabolt, J. F. Micro- and Nanostructured Surface Morphology on Electrospun Polymer Fibers. Macromolecules 2002, 35, 8456. (10) Brown, T. D.; Dalton, P. D.; Hutmacher, D. W. Direct Writing by Way of Melt Electrospinning. Adv. Mater. 2011, 23, 5651. (11) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. A Review on Polymer Nanofibers by Electrospinning and Their Applications in Nanocomposites. Compos. Sci. Technol. 2003, 63, 2223. (12) Larrondo, L.; John Manley, R. S. Electrostatic Fiber Spinning from Polymer Melts. I. Experimental Observations on Fiber Formation and Properties. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 909. (13) Góra, A.; Sahay, R.; Thavasi, V.; Ramakrishna, S. MeltElectrospun Fibers for Advances in Biomedical Engineering, Clean Energy, Filtration, and Separation. Polym. Rev. 2011, 51, 265. (14) Farrugia, B. L.; Brown, T. D.; Upton, Z.; Hutmacher, D. W.; Dalton, P. D.; Dargaville, T. R. Dermal Fibroblast Infiltration of Poly (E-Caprolactone) Scaffolds Fabricated by Melt Electrospinning in a Direct Writing Mode. Biofabrication 2013, 5 (2), 025001. (15) Brown, T. D.; Slotosch, A.; Thibaudeau, L.; Taubenberger, A.; Loessner, D.; Vaquette, C.; Dalton, P. D.; Hutmacher, D. W. Design and Fabrication of Tubular Scaffolds via Direct Writing in a Melt Electrospinning Mode. Biointerphases 2012, 7, 13. (16) Sun, Y.; Zeng, Y.; Wang, X. Three-Dimensional Model of Whipping Motion in the Processing of Microfibers. Ind. Eng. Chem. Res. 2011, 50, 1099. (17) Li, Q.; Zhang, X.; Li, J.; Ouyang, Z.; Wang, H.; Wei, G.; Su, Z. A Highly Effective Reactive Liquid Crystal for the Improved βNucleation of Isotactic Polypropylene. Polym. Eng. Sci. 2013, DOI: 10.1002/pen.23760. (18) Dong, M.; Guo, Z.; Yu, J.; Su, Z. Crystallization Behavior and Morphological Development of Isotactic Polypropylene with an Aryl 2316



Article

Morphology − Property Relationships. J. Appl. Polym. Sci. 2007, 103, 3844. (39) Okada, T.; Saito, H.; Inoue, T. Nonlinear Crystal Growth in the Mixture of Isotactic Polypropylene and Liquid Paraffin. Macromolecules 1990, 23, 3865. (40) Hu, W. Chain-Folding via Intramolecular Crystal Nucleation: Theory and Simulations. Polym. Mater. Sci. Eng. 2007, 97, 811. (41) Chen, Y. H.; Zhong, G. J.; Lei, J.; Li, Z. M.; Hsiao, B. S. In Situ Synchrotron X-Ray Scattering Study on Isotactic Polypropylene Crystallization under the Coexistence of Shear Flow and Carbon Nanotubes. Macromolecules 2011, 44, 8080. (42) Assouline, E.; Lustiger, A.; Barber, A. H.; Cooper, C. A.; Klein, E.; Wachtel, E.; Wagner, H. D. Nucleation Ability of Multiwall Carbon Nanotubes in Polypropylene Composites. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 520. (43) D’Haese, M.; Puyvelde, P. V.; Langouche, F. Effect of Particles on the Flow-Induced Crystallization of Polypropylene at Processing Speeds. Macromolecules 2010, 43, 2933. (44) Li, L.; Li, C. Y.; Ni, C. Polymer Crystallization-Driven, Periodic Patterning on Carbon Nanotubes. J. Am. Chem. Soc. 2006, 128, 1692. (45) Esawi, A. M. K.; Salem, H. G.; Hussein, H. M.; Ramadan, A. R. Effect of Processing Technique on the Dispersion of Carbon Nanotubes within Polypropylene Carbon Nanotube-Composites and Its Effect on Their Mechanical Properties. Polym. Compos. 2010, 31 (5), 772. (46) Wang, Y.; Wen, X.; Wan, D.; Zhang, Z.; Tang, T. Promoting the Responsive Ability of Carbon Nanotubes to An External Stress Field in A Polypropylene Matrix: A Synergistic Effect of the Physical Interaction and Chemical Linking. J. Mater.Chem. 2012, 22, 3930. (47) Desai, A. V.; Haque, M. A. Mechanics of the Interface for Carbon Nanotube−Polymer Composites. Thin-Walled Struct. 2005, 43, 1787. (48) Gao, J.; Zhao, B.; Itkis, M. E.; Bekyarova, E.; Hu, H.; Kranak, V.; Yu, A.; Haddon, R. C. Chemical Engineering of the Single-Walled Carbon Nanotube-Nylon 6 Interface. J. Am. Chem. Soc. 2006, 128, 7492. (49) Tran, M. Q.; Cabral, J. T.; Shaffer, M. S. P.; Bismarck, A. Direct Measurement of the Wetting Behavior of Individual Carbon Nanotubes by Polymer Melts: the Key to Carbon Nanotube-Polymer Composites. Nano Lett. 2008, 8 (9), 2744. (50) Satapathy, B. K.; Gan, M.; Weidisch, Roland.; Pötschke, P.; Jehnichen, D.; Keller, T.; Jandt, K. D. Ductile-to-Semiductile Transition in PP-MWNT Nanocomposites. Macromol. Rapid Commun. 2007, 28, 834. (51) Xie, N.; Jiao, Q.; Zang, C.; Wang, C.; Liu, Y. Study on Dispersion and Electrical Property of Multi-Walled Carbon Nanotubes/Low-Density Polyethylene Nanocomposites. Mater. Des. 2010, 31, 1676. (52) Prashantha, K.; Soulestin, J.; Lacrampe, M. F.; Claes, M.; Dupin, G.; Krawczak, P. Multi-walled Carbon Nanotube Filled Polypropylene Nanocomposites Based on Masterbatch Route: Improvement of Dispersion and Mechanical Properties through PP-g-MA Addition. Polym. Lett. 2008, 2 (10), 735. (53) Szentes, A.; Varga, C.; Horváth, G.; Bartha, L.; Kónya, Z.; Haspel, H.; Szél, J.; Kukovecz, Á . Electrical Resistivity and Thermal Properties of Compatibilized Multi-Walled Carbon Nanotube/ Polypropylene Composites. Polym. Lett. 2012, 6 (6), 494.

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