Polymeric Nanofibers - American Chemical Society

Chapter 17

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Coelectrospinning of Carbon Nanotube Reinforced Nanocomposite Fibrils Frank K . K o , Hoa L a m , Nick Titchenal, Haihui Ye, and Yury Gogotsi Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104

In order to assess the potential of carbon nanotube (CNT) for structural composite applications, a method was proposed to convert CNT to continuous fibrils and fibrous assemblies using the co-electrospinning process. Preliminary experiments demonstrated the feasibility for the formation of composite CNT fibrils. These fibrils provide a convenient material form to carry the CNT and facilitate the formation of macro composite structures. The level and the nature of CNT and fiber alignments, and the inclusion of CNT in the nanofiber were elucidated through SEM, TEM observations and Raman spectra analysis. Mechanical testing of the nanofibril, nanofibril spunbonded mats and composite yarns were carried out to assess the effect of fibril alignment and verify the nancomposite fibril concept.

© 2006 American Chemical Society

In Polymeric Nanofibers; Reneker, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

231


232 Carbon nanotubes (CNTs) (J) are seamless graphene tubule structures with nanometer-size diameters and high aspect ratios. This new class of onedimensional material is shown to have exceptional mechanical, thermal and novel electronic properties. The elastic moduli of the CNTs are in the range of 1-5 TPa (2-4) and fracture strains of 6 to 30%, both are about an order of magnitude better than those of the commercial carbon fibers, which typically have 0.1-0.5 TPa elastic moduli and 0.1-2% fracture strains (5). The factor of 10 enhancement in strength implies that, for the same performance, replacing the commercial carbon fibers with CNTs will lead to significant reduction in the volume and weight of the structural composites currently used in space applications. Based on a NASA study by Harris et al (6) using micromechanics computation as shown in Table I, it was predicted that an order of magnitude increase in specific modulus can be achieved with CNT composites. However, it was also recognized that it would be significantly more challenging in the conversion of CNT to useful structures.

Table I. Properties of SWNT and their Composites (60

Properties Tensile strength (GPa) Tensile Modulus (GPa) Rupture Elongation (%) Density (g/cc) Specific Strength Specific Modulus Thermal Conductivity (W/mK) Manufacturability"

IM7/8552 CNT/Polymer QuasiQuasiisotropic Aluminum isotropic 22I9-T87 Composite Composite' 2.5 1.3 0.46

CNT SWNT Crystal 180

73

58

240

1200

10

1.6

6

15

2.83 0.16 26 121

1.59 0.80 36 5

0.98 2.5 240 5

1.2 170 1000 5000

9

6-9

1

1

* based on 60% fiber volume fraction in a quasi-isotropic laminate, with strength at 1% strain **manufacturability rating, in the range of 0 to 10, with 10 being the best.

In addition to potential applications as high performance reinforcing fibers, carbon nanotubes are shown to have promising materials properties for applications as hydrogen storage materials (7, 8) high energy capacity battery electrodes (9), and cold-cathode electron emitters (ift //). Depending on their



233 chiralities and diameters, CNTs can be either semiconducting or metallic (12). The electrical conductivity of the metallic CNTs is (6000 S/cm) significantly higher than the best commercial carbon fibers. Because of their high degree of graphitization, CNTs are expected to have higher thermal conductivity (2000 W/m-K) than the best carbon fibers. Despite their promises, most of the current studies are limited to the physics and chemistry of individual CNTs. There is limited knowledge on the properties of macroscopic materials comprising CNTs as the basic building blocks for macroscopic structures. It is still not clear whether the superb properties observed at the individual molecular level can be translated to the macroscopic structures. For example, most of the studies on nanotube - polymer composites have been on electron microscopy investigation of deformation of the individual CNTs embedded in polymeric matrices (13-14). No significant enhancement in the mechanical strength has been achieved in nanotube-polymer composites (75), presumably due to the weak interface between CNTs and composites. In order to realize the exciting potential of CNT, there is a need for processing methodologies to convert the CNT to macroscopic structures. Accordingly, it is the objective of this paper to introduce a concept that converts CNT into nano scalefilamentouscomposites by the co-electrospinning process.

The Concept of CNT Nanocomposite Fibrils It is well known that the translation of reinforcement properties to the composite depends on the alignment or orientation 8, of the reinforcement for a given volume fraction of the reinforcement, V , with 0 and V functions of fiber architecture as illustrated in Figure 1 (16). Figure 1 shows the range of obtainable elastic moduli for various composites normalized by the fiber modulus E , versus the appropriate fiber volume for the fiber architecture indicated. It can be seen that the aligned fibers in discrete or continuous form have the most efficient translation of the material property of the reinforcing fibers to the composite. CNTs are known to have highly anisotropic mechanical, thermal, and electrical properties. To measure and utilize these anisotropic properties, many attempts have been made to fabricate materials with controllable degree of CNT alignment. These methods include: f

f

f

1.

Mechanical stretching - nanotubes can be aligned inside polymeric matrices by mechanical stretching and developed procedures to determine the direction and the degree of alignment (17).

2.

Roll-cast-membranes CNTs embedded in thermoplastic matrices by solution cast and produced composites with uni-axially aligned SWNTs by mechanical shearing (15, 17).

3.

Magnetic alignment - thick film of SWNT £nd ropes are aligned by filtration/deposition from suspension in strong magnetic fields (18).



234

Figure 1. Effect offiberarchitecture on material property translation in fiber reinforced composites (Reproduced with permission from reference 16. Copyright 1997 John Wiley & Sons.)

In this study the alignment of CNT and fibrils is demonstrated by coelectrospinning of mixtures of CNT and polymer solution to form aligned nanocomposite fibrils. In this electrostatic induced self-assembly process, ultrafine fibers down to the nanoscale are produced. In the electrospinning process, a high voltage electric field is generated between an oppositely charged polymer fluid contained in a glass syringe with a capillary tip and a metallic collection screen (79). Once the voltage reaches a critical value, the electric field overcomes the surface tension of the suspended polymer with cone formed on the capillary tip of the syringe (spinneret or glass pipette) and a jet of ultrafine fibers is produced. As the charged polymer jets are spun, the solvent quickly evaporates and the fibrils are accumulated on the surface of the collecting screen. This results in a nonwoven mesh of nano to micron scale fibers. A nanoscalefiberor nanofiber is also referred to as a fibril. Varying the electric field strength, polymer solution concentration and the duration of electrospinning can control thefiberdiameter and mesh thickness. A schematic


235


illustration and an example of a composite formed by the process are shown in Figure 2a. The concept of C N T nanocomposites (CNTNC) can be illustrated in Figure 2b, showing the orientation of the C N T in a polymer matrix through the electrospinning process by flow and charge induced orientation as well as confinement of the C N T in a nanocomposite filament (20, 21).

Figure 2. (a) Co-electrospinning of CNT; and (b) Concept of CNTNC.


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The nanofiber composite can also be subsequently deposited as a spunbonded nanofibril mat for subsequent processing into composites or for use as a nonwoven mat as illustrated in a previous study (22). Various techniques of electrospinning have successfully produced aligned nanofibers (23-25). Alternately, as shown in Figure 3, by proper manipulation, the CNTNC filaments can be aligned as a flat compositefilamentbundle or twisted to further enhance handling and/or tailoring of properties in higher order textile preforms for structural composites.

High voltage power supply Syringe containing polymer solution

Random fiber collection plate

Syringe pump assembly

Parallel electrode

Aligned fiber collection assembly Figure 3. Schematic setup of the electrospinning of random and aligned fiber assemblies.

To demonstrate the co-electrospun CNTNC concept and to study the reinforcement and alignment effects, 1 wt. % SWNT-loaded PAN aligned nanofibers were produced and characterized. The produced aligned nanofibers of pristine PAN and SWNT-loaded PAN were characterized by a field emission environmental electron scanning microscope (ESEM) for fiber morphology and diameter measurement. The incorporation of SWNT and their orientation in the nanofibers was verified by Raman microspectroscopy and HRTEM. The effectiveness of the reinforcement and fibrils alignment was investigated through tensile testing.


237

Materials and Methods

Spinning Dope Preparation SWNTs (produced by HiPco method) and MWNTs used for the study were produced at Rice University and Duke University, respectively. Polyacrylonitrile Dimethylformamide (DMF) and polyvinylpyrrolidone (PVP, M ~ 10,000) were purchased from Aldrich Chemical Co. For the PAN/SWNT spinning dope, 1 wt. % SWNT (weight percent relative to polymer) was added to DMF and magnetically stirred at ambient temperature for 24 hours. The SWNT/DMF suspension was then sonicated for 3 hours. Upon completion, 1 wt. % PVP (relative to PAN) was added to the dispersion to wrap the SWNT. Sonication continued for an additional hour before the mixture was transferred to a heating/stirring plate. PAN powder (11 wt. % relative to DMF) was incrementally added to the mixture and allowed to stir at 90° C for 3 hours.


w

Co-electrospinning of CNT/PAN Nanofibers The homogenously mixed solution was transferred to a 3 ml capacity syringe with a 16G stainless steel needle attached. A syringe pump (KD Scientific, KDS-220) was used to maintain a consistent flow rate of the solution during spinning. The syringe pump assembly was mounted horizontally with the needle perpendicular to the collection plate, approximately 17 cm apart. A positive electrode was connected to the needle while the collection plate was grounded. Thefibercollection plate consisted of a 12 cm x 12 cm copper plate covered with aluminum foil for ease of removal of the fiber mat. Modification was made to the traditional electrospinning process to allow simultaneous fabrication of random and aligned fiber configurations. A set of parallel electrodes was mounted adjacent to the polymer jet to collect aligned fibers as shown in Figure 3. A flow rate of 5 jil per minute and a potential of 10 kV were used in the electrospinning process. During the electrospinning process, some of the polymer jets traveled toward the front target and form a random mat, while the rest bent 90° and deposited between the parallel electrodes forming aligned fibers. The deposited fibers were collected in regular intervals of 2 - 3 minutes and carefully transferred to a holder.

Characterization The collected fiber aligned fiber bundles were characterized by various techniques including scanning electron microscopy, Raman microspectroscopy,



238 transmission electron microscopy, and tensile testing. A field emission environmental scanning electron microscope (ESEM, Phillips XL30) was used to measure the fiber diameter. To avoid over heating of the fiber sample, an accelerating voltage of 22 kV and a spot size of 3 were used to analyze the fiber samples. Thirty readings were taken at various locations within a 1cm x 1cm fibers mat. In the case of the aligned fiber bundles, same number of readings was taken along a 1 cm length specimen. The inclusion of SWNT in the electrospun nanofibers was verified by Raman microspectroscopy (Renishaw 1000). For the random fiber sample, a small and thin sample was removed from the nonwoven mat and laid flat on a clean glass slide. Similarly, a small sample was removed from the aligned fiber bundle and laid flat on a glass slide. A low energy excitation wavelength (diode laser, X = 780 nm) operating at 10 % power was used to analyze the pristine PAN and SWNT-loaded PAN in both random and aligned fibers. Mechanical properties of the random and aligned fiber assemblies were characterized using a micro-tensile tester (Kawabata KES-G1). Strips of Smm x 40mm were cut from the randomfibermat and the aluminum foil backing was carefully removed. Thefiberspecimen was then mounted on a paper frame with a gauge length of 30mm. Double-sided adhesive tape was used to mount the fiber strips to the paperframefor testing. For the aligned fiber assembly, yarns of approximately 300 denier and 50 mm in length were slightly twisted at 1 twist per centimeter. A gauge length of 30 mm was used for the aligned fiber bundles. Super glue was used to fix the yarn ends between the paper tabs. Silicon rubber was also used at the inner edges of the tabs to prevent stress concentration imposes during gripping. Tensile testing was carried out at an extension rate of 0.2 mm/sec. Five specimens were tested from each fiber samples. In order to gain an understanding of the failure mechanism of the nanofibers, ESEM and HRTEM analysis were performed on the fracture surfaces of the tested specimens. The samples were prepared by drawing a small amount of thefibersat the failure surface using a pair of tweezers and then placed on the lacey carbon coated copper grids. HRTEM was performed using JEOL 2010F T E M with a relatively low accelerating voltage of 100 kV to minimize SWNT damage by the electron beam. Further verification of the inclusion of SWNT and their alignment in the fibers were also carried out by HRTEM.

Results and Discussion Shown in Figure 4 are ESEM images of the electrospun aligned (Fig. 4a-c) and random (Fig. 4d) PAN nanofibers with 1 wt. % SWNT-loading. Although some misalignment of fibers are present but the degree of fibrils alignment in



239 the yarn axis is considerably higher than that of the random fiber mats. Fiber diameters were measured by ESEM and the average diameter was determined to be approximately 370nm and 430nm for the pristine PAN and 1 wt. % SWNT/PAN, respectively. A large distribution of fiber diameter is observed in all cases. The fibers are relatively uniform in cross section and are free of beads and surface defects. A strong dependency of fiber diameter on polymer concentration was observed in the SWNT/PAN fibers. Incorporation of SWNT increases the solution viscosity resulting in thicker fibers. The presence of SWNT also changes the spinning dope properties leading to a narrower window for processing. Therefore, the polymer concentration must be adjusted accordingly in order to maintain spinnablility of the spinning dope. The degree of difficulty in spinning became greater as the SWNT loading increases, partly due to the significant change in solution viscosity resulting in higher concentration of SWNT aggregates. The inclusion of SWNT in the aligned composite nanofibrils was verified by Raman microspectroscopy. Similar characterization was performed on the aligned fiber assembly to ensure the inclusion of SWNT in the fibrils. Characteristic peaks (1561 and 1589 cm") in the tangential and the radial breathing (RBM, 166 - 268 cm ) modes confirmed the incorporation of SWNT in the aligned fibers as well. As can be seen in 1

1

Figure 4. ESEM images of electrospun nanofibers. (a-c) aligned 1 wt. % SWNT/PANfibersat various magnifications; and (d) random 1 wt. % SWNT/PAN fibers.


240 Figure 5, the Raman spectrum of pristine PAN fiber (top curve) contains no distinctive peak. The lower spectrum is of purified SWNT with characteristic peaks in the tangential mode (1561 cm" and 1589 cm") and the radial breathing mode (140 - 256 cm' ). The spectrum of the composite aligned nanofibril containing 1 wt. % SWNT (middle spectrum) appears similar to that of purified SWNT except that the peaks in the RBM are up shifted by 20 - 30 cm" . This up shift in wave number indicates the interaction between the SWNT and the PAN matrix. The electrospun SWNT/PAN random fibrils [13] possess similar peaks as those seen in the aligned fibers. Thus, confirming the success in incorporating the SWNT in the both the random and aligned electrospun composite nanofibers. 1

1

1


1

2000

1800

1600

1400

1200

1000

800

Raman Shift (cm ) 1

600

400

200

A = 780 nm

Figure 5. Raman spectra ofpurified SWNT, pristine PAN and SWNT-loaded PAN composite fibrils.

Results from Raman microspectroscopy confirmed the presence of SWNT in the samples. However, whether the SWNT are included inside or resided on the surface of the fibers could not be determined by Raman spectroscopy. Further confirmation by HRTEM is necessary in order to conclude that the SWNT are indeed incorporated in the fibers. Shown in Figure 6 are HRTEM images evidencing the incorporation of SWNT in the PAN fibers. Figure 6a is a featureless image of an unfilled PAN fiber whereas, Figure 6b shows the HRTEM image of the nanocomposite fibril that contains 1 wt. % of SWNT. The


241


alignment of SWNT along the fiber axis is clearly visible. The alignment of SWNT along the fiber axis could be attributed to the following three mechanisms: (1) shear flow induced alignment; (2) charge induced alignment; and (3)fiberdiameter confinement induced orientation of SWNT along the fiber direction.

Figure 6. HRTEM of electrospun nanofibers. a) Pristine PAN; b) 1 wt. % SWNT/PAN.

In order to assess the translation efficiency of properties, tensile test of the aligned fiber bundles were performed. For comparison purposes, the random mat specimens were also tested under the same conditions. Figure 7 shows the stress-strain properties of electrospun pristine PAN and 1 wt. % SWNT/PAN nanofibers. The CNT reinforcement effect can be observed by comparing the stress-strain properties of the pristine PAN samples with that of the random fiber mat with lwt% CNT. It can be seen that over two folds increase in tensile strength and elastic modulus was obtained with the addition of 1 wt. % SWNT. The increase of strength and modulus was off-set by an over 50% reduction of breaking elongation. By aligning the CNT compositefibrilsa significant increase in strength and toughness were realized. Specifically, the aligned 1 wt. % SWNT/PAN has a tensile strength of 30 MPa and over 15% elongation at break. The nearly twofold increase in failure stress and strain compared to that of the randomfibermat indicates that the reinforcement effect of CNT can be significantly enhanced by proper alignment of the composite fibrils. Of particular interest is the large increase in the area under the stress-strain curve of the aligned SWNT/PAN specimen, indicating the effectiveness of SWNT in toughening and strengthening the fibers. In order to understand the deformation and failure behaviors of the nanocomposite fibrils, ESEM and HRTEM examination of the failure surfaces



were performed (Figure 8a). As shown in Figure 8, a substantial deformation in the form of necking prior to failure was observed. Such extensive multiple necking (Figure 8b-c) are characteristic behavior of materials with high toughness and ductility. Figure 8d-e show ESEM and HRTEM images of a bundle of SWNT protrudes from the end of a rupture fiber. It appears that the fibril bundle was pulled out during the necking process. SWNT pulled out suggested poor bonding between SWNT and PAN matrix. However, pull out mechanism consumes extra energy which is responsible for the increase in toughness. Proper tailoring of interfacial bonding will allow better load transferring therefore would further increase the toughness and strength of the fiber. As previously reported (26), further increase in SWNT content did not increase the mechanical properties of the nanofiber mat. This was due to the non uniform dispersion of SWNT in the solution since aggregates act as inclusion that weakened the fiber. The level of dispersion is a function of the dispersing medium, the dispersion method, the type of polymer, and the CNT and polymer concentrations. Increase SWNT content creates more challenge in obtaining a high degree of dispersion. As the SWNT concentration increase the fibers become more brittle resulting in inconsistency in sample preparation. A combination of defect inclusion due to poor SWNT dispersion and defect introduced to the fiber during sample preparation could be the contributing


243


factors. An improved dispersion method has been developed and preliminary results indicate significant improvement in the degree of CNT dispersion.

Figure 8. ESEM images of rupturedfiberspecimens, (a-d) rupture surface of randomfiberstrip; (b, c) rupture ends of 1 and 10 wt. % SWNT/PAN nanofibers showing multiple necking prior to failure; (d) Pullout of SWNT from tensile testing; and (e) HRTEM image showing ductile failure behavior of SWNT/PAN nanofiber.

Conclusions In order to assess the potential of CNT for structural composite applications, a method was proposed to convert CNT to continuous fibrils and fibrous assemblies using the electrospinning process. Preliminary experiments demonstrated the feasibility for the formation of composite CNT fibrils. These fibrils provide a convenient material form to carry the CNT and facilitate the formation of macro composite structures. The level and the nature of CNT alignment have been elucidated through TEM/AFM observations and spectra analysis. Mechanical testing of the nanofibril, nanofibril spunbonded mats and



244 composite yarns were carried out to assess the level of alignment and verify the nanocomposite fibril concept. Our preliminary results have successfully demonstrated that CNTs reinforced polymer nanofibers can be produced by the co-electrospinning process. Up to 10 wt. % SWNT reinforcing PAN can be electrospun in a range of diameters ranging from 50 - 400 nm by controlling the SWNT/PAN concentration in the solution. It is demonstrated in this study that orientation of SWNTs in the fiber direction as well as fibrillar alignment in the yarn assembly can greatly enhance the properties of the fibers. Our continuing effort in improving the dispersion of CNT, interfacial bonding between CNT and matrix through post-treatment of the fiber promises to maximize the translation of the superior properties of CNT to larger scale composite structures. The feasibility in producing continuous aligned composite nanofibers that allows them to be integrated into textile structure for advanced composite applications has been demonstrated. Furthermore, the ability to co-electrospin with various polymer/reinforcement systems and reinforcement concentrations allow a wide range of multifunctional composite nanofibers to be produced thus suitable for many applications. It should be further noted that none of the electrospun specimen has undergone any post treatment such as drawing, annealing and further heat treatment at various temperature regimes. Future study will also explore higher level of CNT loading and optimization of dispersion and spinning processes. Accordingly, with proper tailoring of CNT-matrix interface and post treatment, it is anticipated that further improvement of the mechanical properties and translation efficiency of the CNT properties to the nanocomposite assemblies is expected.

Acknowledgements This work was supported by the National Science Foundation through the Integrative Graduate Education and Research Traineeship (NSF-IGERT) program. Support from NASA and the Pennsylvania Nanotechnology Institute are also gratefully appreciated.

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Polymeric Nanofibers - American Chemical Society

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