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nylon-6 composite nanofibrous mats via electrospinning
Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 121–125
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Bimodal fiber diameter distributed graphene oxide/nylon-6 composite nanofibrous mats via electrospinning Hem Raj Pant a,b,∗ , Chan Hee Park a , Leonard D. Tijing c , Altangerel Amarjargal a , Do-Hee Lee a , Cheol Sang Kim a,c,∗ a
Department of Bionanosystem Engineering, Chonbuk National University, Jeonju 561-756, South Korea Institute of Engineering, Pulchowk Campus, Tribhuvan University, Kathmandu, Nepal c Division of Mechanical Design Engineering, Chonbuk National University, Jeonju, South Korea b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
Large scale fabrication of spiderwave-like nano-nets of nylon-6 was carried out by adding suitable amounts of GO. The mechanism of formation of bimodal fibers which differs by one order in their diameter was proposed. The suitable amounts of GO required to fabricate bimodal spider-wavelike nano-nets was investigated.
a r t i c l e
i n f o
Article history: Received 20 February 2012 Received in revised form 5 May 2012 Accepted 7 May 2012 Available online 26 May 2012 Keywords: Electrospinning Spider-wave Graphene oxide Nylon-6 Nanocomposite
a b s t r a c t In this work, nylon-6 spider-wave-like nano-nets are fabricated by regulating the amount of graphene oxide (GO) in polymer solution during electrospinning. The spider-wave-like nano-nets that comprise interlinked thin (≈14 nm diameter) and thick fibers (≈192 nm diameter) are widely distributed throughout the mat when suitable amount of GO is blended with nylon-6 solution. The heterogeneous composite mats were composed of bimodal nanofibers in which pore diameter was sufficiently decreased. The acceleration in ionization and degradation of nylon-6 (due to formic acid) solution caused by well-dispersed GO sheet as well as the formation of hydrogen bond between nylon-6 molecules and GO sheet during electrospinning are proposed as the possible mechanisms for the formation of these spider-wave-like nano-nets. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is regarded as a simple, versatile, and scalable technique to fabricate polymeric micro/nanofibers [1,2]. Due to the nano-sized diameter of the electrospun polymer fibers, the membranes collected from electrospun fibers possesses a large sur-
∗ Corresponding authors at: Department of Bionanosystem Engineering, Chonbuk National University, Jeonju 561-756, South Korea. Tel.: +82 632704284; fax: +82 632702460. E-mail addresses: [email protected] (H.R. Pant), [email protected] (C.S. Kim). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.05.018
face area per unit mass and small pore size (in m) [3,4]. These characteristics make the electrospun fibers have many potential applications such as filtration, protective clothing, tissue scaffold, wound dressing, drug delivery system, sensor, and optical materials [5–10]. For water/air filtration, nano-sized porous electrospun mat is essential for nanoparticle filtration. However, fabrication of nanosize porous electrospun membrane is very difficult from pure polymer solution. To get the nanoporous electrospun mats, we should decrease the fiber diameter below 50 nm (true nanofibers). Therefore, some techniques such as variation of material/process parameters should be applied to get nanoporous polymeric mats from electrospinning [11–13].
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H.R. Pant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 121–125
Fig. 1. Schematic illustration of fabrication of spider-wave-like nano-nets and interaction of GO sheets and nylon-6 molecules by hydrogen bonding.
Due to the polyelectrolytic and polymorphic characteristics, nylon-6 exhibits interesting modification in physical properties and fiber morphology by varying the process/material parameters during electrospinning [14–16]. Nylon-6 is an easily spinnable polymer and its crystalline phase change, crystal orientation development, polymer chain conformation, as well as fiber morphology modification have been widely studied [14,17]. The amide (CO-NH) group of nylon-6 molecule can easily interact with different materials by means of hydrogen bond formation or donor–acceptor complexes [6,18]. The authors’ previous reports shows that not only the addition of some foreign materials such as monomer, oligomer, inorganic salt, and TiO2 NPs but also successive electrospinning as well as solvent degradation of nylon-6 could cause the change in morphology and crystallization chain conformation of electrospun nylon-6 mat [4,6,11,13,16,18]. In these reports, the strategy was to get spider-wave-like nano-net in electrospun nylon-6 mat. Here a similar strategy was adopted in the fabrication of spider-wave-like structure (a bimodal fiber diameter distribution) by using graphene oxide (GO) which is potential nano-filler in composite science and technology. Graphene oxide (GO), dispersed in separate nano sheets in aqueous or polar organic solvent, offers some remarkable properties change of the polymer composite materials that are useful for various novel applications [19–21]. The GO with its combination of large specific surface area, two-dimensional high aspect ratio sheet geometry with functionality, and outstanding mechanical properties shows great promise as a nanofiller in polymeric composite materials. Due to the greater dispersion of the GO compared to other carbon materials (graphene, carbon nano tubes, etc.) in polymer solution, it does not sufficiently hinders the spinnability of polymer solution and provides the sufficient sites for polymeric molecules to interact with it by the formation of hydrogen bond as shown in Fig. 1. Therefore, the effect of different amounts of GO in nylon-6 solution for the morphology change of electrospun fibers was investigated in this work. The results showed that small amount of GO in nylon-6 solution allows large-scale fabrication of spider-wave-like nano-nets and such type of bimodal fiber diameter distributed electrospun mat may have great potentiality in air/water filter application. 2. Experimental procedure Graphene oxide (GO) was prepared using modified Hummer’s method [22] from synthetic graphite (Sigma–Aldrich). Here, 125 ml of concentrated sulfuric acid was poured into 500 ml three-necked round bottom flask filled with graphite powder (5 g) followed by the addition of solid potassium permanganate (17.5 g) slowly at 0 ◦ C (ice bath). The mixture was agitated by Teflon-coated stirrer for 3 h at 35 ◦ C then diluted by adding sufficient amount of distilled water at 0 ◦ C ice bath. H2 O2 (30 vol% in water from Aldrich) was added until the bubbling of the gas was completed. The washed
acid-free product was dried under vacuum (JEIO Tech, VO-20 vacuum oven, vacuum gauge 76 cm Hg) at 70 ◦ C for two days. Polymer blends were prepared by simple mixing of different amounts of “as-prepared GO” (0, 75, 125, 250, and 500 mg) into 25 g of 22 wt% nylon-6 solution (4:1 wt% ratio of CH3 COOH/HCOOH). Electrospinning was carried out at 18 kV and 16 cm distance between the collector and the tip of the syringe which was clamped at about 20◦ angle relative to the horizontal axis. Since GO sheets can block the flow of solution at 0◦ to the horizontal axis, the tip of the syringe was clamped at about 20◦ for self-flowing (without syringe pump) of solution. After vacuum drying for 24 h, the mats were further characterized using following techniques. The solutions/electrospun mats were named M1, M2, M3, M4, and M5 to indicate the 0, 75, 125, 250, and 500 mg GO containing nylon-6 solutions/mats, respectively. The schematic illustration of bimodal fiber distributed composite mat is given in Fig. 1. The morphology of the as-spun fibers was observed by using field-emission scanning electron microscopy (FE-SEM, Hitachi S-7400, Japan) and transmission electron microscopy (TEM, JEM2010, JEOL, Japan). The conductivity and viscosity of the solutions were measured by an EC meter CM 40G Ver. 1.09 (DKK, TOA, Japan) and Rheometer (DV-III Ultra, Brookfield, UK), respectively. Porosity was measured by using a porosimeter (autopore IV 9500, micromeritics, USA) by using mercury and the intrusion pressure range was within 0.1–60,000 psi. Raman spectra were acquired using FT-Raman spectroscopy (RFS-100S, Bruker, Germany). The thermal properties of the mat were measured using a thermogravimetric analyzer (TGA, Perkin-Elmer Co., USA). 3. Results and discussion During the electrospinning process, an electrified polymeric nylon-6 solution with GO sheets was extruded through a spinneret and collected as a mat on the rotating drum. The morphologies of the as-prepared electrospun nylon-6 mats with different amounts of GO are shown in Fig. 2 (FE-SEM images). For pristine nylon-6 electrospun mat (Fig. 2a), the fibers appear well-defined without any spider-wave-like nano-nets. The mats obtained from the nylon-6 solutions containing different amounts of GO showed some changes in fiber morphology (Fig. 2b–d). As GO was added into polymeric solution, a double-layered morphology (bimodal fiber diameter distributed mat) was noticeable, and became even more apparent at an amount of 125 mg of GO in polymer solution (M3 solution) (Fig. 2c). Further increase of GO in polymeric solution not only decreased the spider-wave-like structure but also decreased the spinnability and formed film-like structure (Fig. 2d) (M4 mat), and up to 500 mg of GO, the solution became difficult to electrospin due to the agglomeration of GO sheets. This trend in morphology can be understood by considering the fact that nylon-6 degraded by formic acid [4] and only certain amount of GO sheets should be well dispersed with nylon-6 solution (solution M3) to accelerate
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Fig. 2. FE-SEM images of (a) M1, (b) M2, (c) M3, and (d) M4 mats.
the solvent degradation of nylon-6. This result is verified by the conductivity and viscosity measurement of the polymeric solution containing different amounts of GO (Fig. 3). It is clear from Fig. 3 that the conductivity of M3 solution is highest whereas the viscosity is increasing with the amounts of GO. This result revealed that up to M3 (125 mg) solution the GO sheets were well dispersed and accelerates the solvent degradation of nylon-6 which might help to form spider-wave-like structure during phase separation. Molecular weight and phase separation mechanism are important parameters for the surface morphology modification of electrospun mats. In the authors previous reports, porous PCL fibers were obtained not only from the water-bath electrospinning of methoxy poly(ethylene glycol) oligomer mixed with PCL [23] but also from pure PCL solution using same water-bath electrospinning set-up [24].
Fig. 3. Conductivity and viscosity of M1, M2, M3, M4, and M5 solutions.
Structure characterization of as-fabricated fibers was further carried out by transmission electron microscopy (TEM). The TEM observation reveals an exact structural morphology, as proved by FE-SEM analysis. Fig. 4a shows that pristine nylon-6 fiber has no any spider-wave-like structure whereas fiber obtained from M3 (125 mg) solution has sufficient thin fibers which are originated from the surface of main fibers. As shown in inset of Fig. 4b, the GO sheets were distributed throughout the fibers (M3 mat). However, Fig. 4c clearly shows that large amount of GO containing solution (M5) could neither form spider-wave-like structure nor smooth fibers due to the agglomeration of GO sheets. FESEM image (Fig. 2) and TEM images (Fig. 4) clearly shows that thin fibers are originated from the surface of thick fibers. From the authors’ previous experience and some other reported works, one may propose more convenient mechanism for the formation of spider-wave-like bimodal fiber diameter distributed nano-nets. It is well reported that nylon-6 solution (in formic acid) undergoes not only ionization but also degradation by formic acid [4]. The presence of foreign materials in liquid form or in nanoparticles may accelerate this degradation. Up to certain amounts (here M3 solution, 125 mg), the GO nano-sheets were well dispersed throughout the polymer solution and cause the increase in polymer degradation. Our conductivity measurements (Fig. 3) revealed that up to critical amount of GO (M3 solution), the conductivity is increased and further increasing of GO decreased the conductivity of solution (M4 and M5). Formation of low molecular weight ionic nylon-6 species might be the possible cause of increased conductivity and well dispersed GO sheets at this concentration (M3 solution) might be the cause of unchanged viscosity. The increased portion of solvent degraded nylon-6 due to the well dispersion of GO sheets might be responsible for the formation of spiderwave-like nano-nets because in the authors’ previous work, it was clearly showed that increasing portion of solvent degraded nylon6 in freshly prepared solution could produce sufficient amounts of spider-wave-like nano-nets [4]. When degraded portion of nylon-6
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Fig. 4. TEM images of (a) M1, (b) M3, and (c) M5 mats.
Fig. 5. Raman spectra of pristine nylon-6 and M3 composite mats.
is increased, the solvent-degraded polymer solution (low molecular weight) with low viscosity and high charge density forms a surface layer whereas the non-degraded one (high molecular weight) forms the core of the jet by means of complex phase separation phenomenon during jet whipping. The formation of core-shell structure nanofibers from the mixture of low and high molecular weight polymers through single-nozzle electrospinning is also reported [25,26] and further supports our hypothesis of spiderwave-like nano-nets formation mechanism. Since M3 mat had sufficient spider-wave-like structure and smooth fiber surface, we took pristine nylon-6 and M3 mats for compression. Table 1 shows the porosity of pristine nylon-6 (M1) and composite (M3) mats. It was observed from FE-SEM images (Fig. 2c) that both porosity and pore diameter in M3 mat were decreased when compared to the pristine nylon-6 (M1) mat. The decreased porosity is due to the presence of well distributed thin fiber (as separate layer) in the form of spider-wave-like structure throughout the mat. From Fig. 2c, it is clear that the thinner fibers were densely arranged as double layer, therefore, porosity is
decreased. It can be observed from Fig. 2c that the pore diameter is decreased by the factor of 2 in M3 mat compared to the pristine nylon-6 (M1) mat. Therefore, the obtained composite mat can be used for effective nanoparticles filtration from contaminated water and air. The proper loading of the GO sheets into/on nylon-6 fibers was demonstrated by Raman spectroscopy and TGA analysis. Fig. 5 shows the Raman spectra of pristine nylon-6 and M3 (having highest spider-wave-like structure) composite mats. Comparing to the band of pristine nylon-6, M3 composite mat clearly shows the presence of D and G-band of GO sheets and indicate the formation of nanocomposite between GO and nylon-6. The interaction of GO sheets with nylon-6 should be mainly hydrogen bond as shown in Fig. 1. Since nylon-6 and GO sheets have different functionalities, they can easily interact with each other through intermolecular hydrogen bonding. The functional groups such as carboxylic, phenolic, and epoxide on the surface of GO sheets can interact with C O or N H groups of nylon-6 molecules to form strong intermolecular hydrogen bond between them. This interaction was further supported by TGA and DTG analysis of pristine nylon-6 (M1) and composite (M3) mats as shown in Fig. 6. A temperature correlation with maximum thermal degradation rates of pristine nylon-6 mat at 463.6 ◦ C was shifted toward lower temperature (at 450 ◦ C) revealed that there is proper interaction of the GO and nylon-6 at molecular level. The decreased weight loss percent of M3 mat with compared to pristine nylon-6 mat further indicated the sufficient loading of GO sheet on polymer matrix.
Table 1 Average fiber diameters and porosity of different mats. Mat
Average diameter of thick fibers (nm)
Average diameter of thin fibers (nm)
Porosity (%)
Pure nylon-6 (M1) Composite mat (M3)
192 212
– 14
96.2 87
Fig. 6. TGA and DTG graphs of pristine nylon-6 and M3 composite mats.
H.R. Pant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 121–125
4. Conclusion We successfully fabricated GO sheets loaded spider-wave-like nano-nets of nylon-6 by electrospinning. This work is anticipated to open a new possibility in the formation of spider-wave-like structure in different polyelectrolytes, especially in protein-based polymer because the backbone structure of nylon-6 is similar to the amino acid sequences found in polypeptides. The simple blending of GO could sufficiently decrease the pore size of electrospun nylon6 mat and thus formed nanoporous mat may have great potentiality for nanoparticles filtration. The large scale fabrication of ultrafine spider-wave-like nano-nets using GO, is relatively inexpensive, and it paves the way to build up of new dimension not only for air/water filtration but also for nano device applications. The optimal GO concentration was found to be 125 mg at which the conductivity and viscosity are suitable for the fabrication of bimodal fibrous mat. Acknowledgments This research was supported by a grant from the Korean Ministry of Education, Science and Technology through the National Research Foundation (Water Treatment Project no. 20110011807). References [1] N. Bhardwaj, S.C. Kundu, Electrospinning: a fascinating fiber fabrication technique, Biotechnol. Adv. 28 (2010) 325–347. [2] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [3] S. Megelski, J.S. Stephens, D.B. Chase, J.F. Rabolt, Micro- and nanostructured surface morphology on electrospun polymer fibers, Macromolecules 35 (2002) 8456–8466. [4] H.R. Pant, K. Nam, H. Oh, G. Panthi, H. Kim, B. Kim, H.Y. Kim, Effect of polymer molecular weight on the fiber morphology of electrospun mats, J. Colloid Interface Sci. 364 (2011) 107–111. [5] R.A. Damodar, S.J. You, H.H. Chou, Study the self cleaning, antibacterial and photocatalytic properties of TiO2 entrapped PVDF membranes, J. Hazard. Mater. 172 (2009) 1321–1328. [6] H.R. Pant, M.P. Bajgai, K.T. Nam, Y.A. Seo, D.R. Pandeya, S.T. Hong, H.Y. Kim, Electrospun nylon-6 spider-net like nanofiber mat containing TiO2 nanoparticles: a multifunctional nanocomposite textile material, J. Hazard. Mater. 185 (2011) 124–130.
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Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Bimodal fiber diameter distributed graphene oxide/nylon-6 composite nanofibrous mats via electrospinning Hem Raj Pant a,b,∗ , Chan Hee Park a , Leonard D. Tijing c , Altangerel Amarjargal a , Do-Hee Lee a , Cheol Sang Kim a,c,∗ a
Department of Bionanosystem Engineering, Chonbuk National University, Jeonju 561-756, South Korea Institute of Engineering, Pulchowk Campus, Tribhuvan University, Kathmandu, Nepal c Division of Mechanical Design Engineering, Chonbuk National University, Jeonju, South Korea b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
Large scale fabrication of spiderwave-like nano-nets of nylon-6 was carried out by adding suitable amounts of GO. The mechanism of formation of bimodal fibers which differs by one order in their diameter was proposed. The suitable amounts of GO required to fabricate bimodal spider-wavelike nano-nets was investigated.
a r t i c l e
i n f o
Article history: Received 20 February 2012 Received in revised form 5 May 2012 Accepted 7 May 2012 Available online 26 May 2012 Keywords: Electrospinning Spider-wave Graphene oxide Nylon-6 Nanocomposite
a b s t r a c t In this work, nylon-6 spider-wave-like nano-nets are fabricated by regulating the amount of graphene oxide (GO) in polymer solution during electrospinning. The spider-wave-like nano-nets that comprise interlinked thin (≈14 nm diameter) and thick fibers (≈192 nm diameter) are widely distributed throughout the mat when suitable amount of GO is blended with nylon-6 solution. The heterogeneous composite mats were composed of bimodal nanofibers in which pore diameter was sufficiently decreased. The acceleration in ionization and degradation of nylon-6 (due to formic acid) solution caused by well-dispersed GO sheet as well as the formation of hydrogen bond between nylon-6 molecules and GO sheet during electrospinning are proposed as the possible mechanisms for the formation of these spider-wave-like nano-nets. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is regarded as a simple, versatile, and scalable technique to fabricate polymeric micro/nanofibers [1,2]. Due to the nano-sized diameter of the electrospun polymer fibers, the membranes collected from electrospun fibers possesses a large sur-
∗ Corresponding authors at: Department of Bionanosystem Engineering, Chonbuk National University, Jeonju 561-756, South Korea. Tel.: +82 632704284; fax: +82 632702460. E-mail addresses: [email protected] (H.R. Pant), [email protected] (C.S. Kim). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.05.018
face area per unit mass and small pore size (in m) [3,4]. These characteristics make the electrospun fibers have many potential applications such as filtration, protective clothing, tissue scaffold, wound dressing, drug delivery system, sensor, and optical materials [5–10]. For water/air filtration, nano-sized porous electrospun mat is essential for nanoparticle filtration. However, fabrication of nanosize porous electrospun membrane is very difficult from pure polymer solution. To get the nanoporous electrospun mats, we should decrease the fiber diameter below 50 nm (true nanofibers). Therefore, some techniques such as variation of material/process parameters should be applied to get nanoporous polymeric mats from electrospinning [11–13].
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H.R. Pant et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 407 (2012) 121–125
Fig. 1. Schematic illustration of fabrication of spider-wave-like nano-nets and interaction of GO sheets and nylon-6 molecules by hydrogen bonding.
Due to the polyelectrolytic and polymorphic characteristics, nylon-6 exhibits interesting modification in physical properties and fiber morphology by varying the process/material parameters during electrospinning [14–16]. Nylon-6 is an easily spinnable polymer and its crystalline phase change, crystal orientation development, polymer chain conformation, as well as fiber morphology modification have been widely studied [14,17]. The amide (CO-NH) group of nylon-6 molecule can easily interact with different materials by means of hydrogen bond formation or donor–acceptor complexes [6,18]. The authors’ previous reports shows that not only the addition of some foreign materials such as monomer, oligomer, inorganic salt, and TiO2 NPs but also successive electrospinning as well as solvent degradation of nylon-6 could cause the change in morphology and crystallization chain conformation of electrospun nylon-6 mat [4,6,11,13,16,18]. In these reports, the strategy was to get spider-wave-like nano-net in electrospun nylon-6 mat. Here a similar strategy was adopted in the fabrication of spider-wave-like structure (a bimodal fiber diameter distribution) by using graphene oxide (GO) which is potential nano-filler in composite science and technology. Graphene oxide (GO), dispersed in separate nano sheets in aqueous or polar organic solvent, offers some remarkable properties change of the polymer composite materials that are useful for various novel applications [19–21]. The GO with its combination of large specific surface area, two-dimensional high aspect ratio sheet geometry with functionality, and outstanding mechanical properties shows great promise as a nanofiller in polymeric composite materials. Due to the greater dispersion of the GO compared to other carbon materials (graphene, carbon nano tubes, etc.) in polymer solution, it does not sufficiently hinders the spinnability of polymer solution and provides the sufficient sites for polymeric molecules to interact with it by the formation of hydrogen bond as shown in Fig. 1. Therefore, the effect of different amounts of GO in nylon-6 solution for the morphology change of electrospun fibers was investigated in this work. The results showed that small amount of GO in nylon-6 solution allows large-scale fabrication of spider-wave-like nano-nets and such type of bimodal fiber diameter distributed electrospun mat may have great potentiality in air/water filter application. 2. Experimental procedure Graphene oxide (GO) was prepared using modified Hummer’s method [22] from synthetic graphite (Sigma–Aldrich). Here, 125 ml of concentrated sulfuric acid was poured into 500 ml three-necked round bottom flask filled with graphite powder (5 g) followed by the addition of solid potassium permanganate (17.5 g) slowly at 0 ◦ C (ice bath). The mixture was agitated by Teflon-coated stirrer for 3 h at 35 ◦ C then diluted by adding sufficient amount of distilled water at 0 ◦ C ice bath. H2 O2 (30 vol% in water from Aldrich) was added until the bubbling of the gas was completed. The washed
acid-free product was dried under vacuum (JEIO Tech, VO-20 vacuum oven, vacuum gauge 76 cm Hg) at 70 ◦ C for two days. Polymer blends were prepared by simple mixing of different amounts of “as-prepared GO” (0, 75, 125, 250, and 500 mg) into 25 g of 22 wt% nylon-6 solution (4:1 wt% ratio of CH3 COOH/HCOOH). Electrospinning was carried out at 18 kV and 16 cm distance between the collector and the tip of the syringe which was clamped at about 20◦ angle relative to the horizontal axis. Since GO sheets can block the flow of solution at 0◦ to the horizontal axis, the tip of the syringe was clamped at about 20◦ for self-flowing (without syringe pump) of solution. After vacuum drying for 24 h, the mats were further characterized using following techniques. The solutions/electrospun mats were named M1, M2, M3, M4, and M5 to indicate the 0, 75, 125, 250, and 500 mg GO containing nylon-6 solutions/mats, respectively. The schematic illustration of bimodal fiber distributed composite mat is given in Fig. 1. The morphology of the as-spun fibers was observed by using field-emission scanning electron microscopy (FE-SEM, Hitachi S-7400, Japan) and transmission electron microscopy (TEM, JEM2010, JEOL, Japan). The conductivity and viscosity of the solutions were measured by an EC meter CM 40G Ver. 1.09 (DKK, TOA, Japan) and Rheometer (DV-III Ultra, Brookfield, UK), respectively. Porosity was measured by using a porosimeter (autopore IV 9500, micromeritics, USA) by using mercury and the intrusion pressure range was within 0.1–60,000 psi. Raman spectra were acquired using FT-Raman spectroscopy (RFS-100S, Bruker, Germany). The thermal properties of the mat were measured using a thermogravimetric analyzer (TGA, Perkin-Elmer Co., USA). 3. Results and discussion During the electrospinning process, an electrified polymeric nylon-6 solution with GO sheets was extruded through a spinneret and collected as a mat on the rotating drum. The morphologies of the as-prepared electrospun nylon-6 mats with different amounts of GO are shown in Fig. 2 (FE-SEM images). For pristine nylon-6 electrospun mat (Fig. 2a), the fibers appear well-defined without any spider-wave-like nano-nets. The mats obtained from the nylon-6 solutions containing different amounts of GO showed some changes in fiber morphology (Fig. 2b–d). As GO was added into polymeric solution, a double-layered morphology (bimodal fiber diameter distributed mat) was noticeable, and became even more apparent at an amount of 125 mg of GO in polymer solution (M3 solution) (Fig. 2c). Further increase of GO in polymeric solution not only decreased the spider-wave-like structure but also decreased the spinnability and formed film-like structure (Fig. 2d) (M4 mat), and up to 500 mg of GO, the solution became difficult to electrospin due to the agglomeration of GO sheets. This trend in morphology can be understood by considering the fact that nylon-6 degraded by formic acid [4] and only certain amount of GO sheets should be well dispersed with nylon-6 solution (solution M3) to accelerate
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Fig. 2. FE-SEM images of (a) M1, (b) M2, (c) M3, and (d) M4 mats.
the solvent degradation of nylon-6. This result is verified by the conductivity and viscosity measurement of the polymeric solution containing different amounts of GO (Fig. 3). It is clear from Fig. 3 that the conductivity of M3 solution is highest whereas the viscosity is increasing with the amounts of GO. This result revealed that up to M3 (125 mg) solution the GO sheets were well dispersed and accelerates the solvent degradation of nylon-6 which might help to form spider-wave-like structure during phase separation. Molecular weight and phase separation mechanism are important parameters for the surface morphology modification of electrospun mats. In the authors previous reports, porous PCL fibers were obtained not only from the water-bath electrospinning of methoxy poly(ethylene glycol) oligomer mixed with PCL [23] but also from pure PCL solution using same water-bath electrospinning set-up [24].
Fig. 3. Conductivity and viscosity of M1, M2, M3, M4, and M5 solutions.
Structure characterization of as-fabricated fibers was further carried out by transmission electron microscopy (TEM). The TEM observation reveals an exact structural morphology, as proved by FE-SEM analysis. Fig. 4a shows that pristine nylon-6 fiber has no any spider-wave-like structure whereas fiber obtained from M3 (125 mg) solution has sufficient thin fibers which are originated from the surface of main fibers. As shown in inset of Fig. 4b, the GO sheets were distributed throughout the fibers (M3 mat). However, Fig. 4c clearly shows that large amount of GO containing solution (M5) could neither form spider-wave-like structure nor smooth fibers due to the agglomeration of GO sheets. FESEM image (Fig. 2) and TEM images (Fig. 4) clearly shows that thin fibers are originated from the surface of thick fibers. From the authors’ previous experience and some other reported works, one may propose more convenient mechanism for the formation of spider-wave-like bimodal fiber diameter distributed nano-nets. It is well reported that nylon-6 solution (in formic acid) undergoes not only ionization but also degradation by formic acid [4]. The presence of foreign materials in liquid form or in nanoparticles may accelerate this degradation. Up to certain amounts (here M3 solution, 125 mg), the GO nano-sheets were well dispersed throughout the polymer solution and cause the increase in polymer degradation. Our conductivity measurements (Fig. 3) revealed that up to critical amount of GO (M3 solution), the conductivity is increased and further increasing of GO decreased the conductivity of solution (M4 and M5). Formation of low molecular weight ionic nylon-6 species might be the possible cause of increased conductivity and well dispersed GO sheets at this concentration (M3 solution) might be the cause of unchanged viscosity. The increased portion of solvent degraded nylon-6 due to the well dispersion of GO sheets might be responsible for the formation of spiderwave-like nano-nets because in the authors’ previous work, it was clearly showed that increasing portion of solvent degraded nylon6 in freshly prepared solution could produce sufficient amounts of spider-wave-like nano-nets [4]. When degraded portion of nylon-6
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Fig. 4. TEM images of (a) M1, (b) M3, and (c) M5 mats.
Fig. 5. Raman spectra of pristine nylon-6 and M3 composite mats.
is increased, the solvent-degraded polymer solution (low molecular weight) with low viscosity and high charge density forms a surface layer whereas the non-degraded one (high molecular weight) forms the core of the jet by means of complex phase separation phenomenon during jet whipping. The formation of core-shell structure nanofibers from the mixture of low and high molecular weight polymers through single-nozzle electrospinning is also reported [25,26] and further supports our hypothesis of spiderwave-like nano-nets formation mechanism. Since M3 mat had sufficient spider-wave-like structure and smooth fiber surface, we took pristine nylon-6 and M3 mats for compression. Table 1 shows the porosity of pristine nylon-6 (M1) and composite (M3) mats. It was observed from FE-SEM images (Fig. 2c) that both porosity and pore diameter in M3 mat were decreased when compared to the pristine nylon-6 (M1) mat. The decreased porosity is due to the presence of well distributed thin fiber (as separate layer) in the form of spider-wave-like structure throughout the mat. From Fig. 2c, it is clear that the thinner fibers were densely arranged as double layer, therefore, porosity is
decreased. It can be observed from Fig. 2c that the pore diameter is decreased by the factor of 2 in M3 mat compared to the pristine nylon-6 (M1) mat. Therefore, the obtained composite mat can be used for effective nanoparticles filtration from contaminated water and air. The proper loading of the GO sheets into/on nylon-6 fibers was demonstrated by Raman spectroscopy and TGA analysis. Fig. 5 shows the Raman spectra of pristine nylon-6 and M3 (having highest spider-wave-like structure) composite mats. Comparing to the band of pristine nylon-6, M3 composite mat clearly shows the presence of D and G-band of GO sheets and indicate the formation of nanocomposite between GO and nylon-6. The interaction of GO sheets with nylon-6 should be mainly hydrogen bond as shown in Fig. 1. Since nylon-6 and GO sheets have different functionalities, they can easily interact with each other through intermolecular hydrogen bonding. The functional groups such as carboxylic, phenolic, and epoxide on the surface of GO sheets can interact with C O or N H groups of nylon-6 molecules to form strong intermolecular hydrogen bond between them. This interaction was further supported by TGA and DTG analysis of pristine nylon-6 (M1) and composite (M3) mats as shown in Fig. 6. A temperature correlation with maximum thermal degradation rates of pristine nylon-6 mat at 463.6 ◦ C was shifted toward lower temperature (at 450 ◦ C) revealed that there is proper interaction of the GO and nylon-6 at molecular level. The decreased weight loss percent of M3 mat with compared to pristine nylon-6 mat further indicated the sufficient loading of GO sheet on polymer matrix.
Table 1 Average fiber diameters and porosity of different mats. Mat
Average diameter of thick fibers (nm)
Average diameter of thin fibers (nm)
Porosity (%)
Pure nylon-6 (M1) Composite mat (M3)
192 212
– 14
96.2 87
Fig. 6. TGA and DTG graphs of pristine nylon-6 and M3 composite mats.
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4. Conclusion We successfully fabricated GO sheets loaded spider-wave-like nano-nets of nylon-6 by electrospinning. This work is anticipated to open a new possibility in the formation of spider-wave-like structure in different polyelectrolytes, especially in protein-based polymer because the backbone structure of nylon-6 is similar to the amino acid sequences found in polypeptides. The simple blending of GO could sufficiently decrease the pore size of electrospun nylon6 mat and thus formed nanoporous mat may have great potentiality for nanoparticles filtration. The large scale fabrication of ultrafine spider-wave-like nano-nets using GO, is relatively inexpensive, and it paves the way to build up of new dimension not only for air/water filtration but also for nano device applications. The optimal GO concentration was found to be 125 mg at which the conductivity and viscosity are suitable for the fabrication of bimodal fibrous mat. Acknowledgments This research was supported by a grant from the Korean Ministry of Education, Science and Technology through the National Research Foundation (Water Treatment Project no. 20110011807). References [1] N. Bhardwaj, S.C. Kundu, Electrospinning: a fascinating fiber fabrication technique, Biotechnol. Adv. 28 (2010) 325–347. [2] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [3] S. Megelski, J.S. Stephens, D.B. Chase, J.F. Rabolt, Micro- and nanostructured surface morphology on electrospun polymer fibers, Macromolecules 35 (2002) 8456–8466. [4] H.R. Pant, K. Nam, H. Oh, G. Panthi, H. Kim, B. Kim, H.Y. Kim, Effect of polymer molecular weight on the fiber morphology of electrospun mats, J. Colloid Interface Sci. 364 (2011) 107–111. [5] R.A. Damodar, S.J. You, H.H. Chou, Study the self cleaning, antibacterial and photocatalytic properties of TiO2 entrapped PVDF membranes, J. Hazard. Mater. 172 (2009) 1321–1328. [6] H.R. Pant, M.P. Bajgai, K.T. Nam, Y.A. Seo, D.R. Pandeya, S.T. Hong, H.Y. Kim, Electrospun nylon-6 spider-net like nanofiber mat containing TiO2 nanoparticles: a multifunctional nanocomposite textile material, J. Hazard. Mater. 185 (2011) 124–130.
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