Benzoyl Peroxide Initiated In Situ Functionalization, Processing, and

Jan 5, 2007 - of benzoyl peroxide during the high shear and high-temperature phase of ..... have lead points of high stress in the fibers at which fai...
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J. Phys. Chem. C 2007, 111, 1592-1600

Benzoyl Peroxide Initiated In Situ Functionalization, Processing, and Mechanical Properties of Single-Walled Carbon Nanotube-Polypropylene Composite Fibers Daneesh McIntosh,† Valery N. Khabashesku,*,‡ and Enrique V. Barrera*,† Department of Mechanical Engineering and Materials Science and Department of Chemistry and Richard E. Smalley Institute for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas, 77005-1892 ReceiVed: August 21, 2006; In Final Form: October 26, 2006

Benzoyl peroxide initiated in situ functionalization of single-walled carbon nanotubes (SWNT) represents a simple means of generating reactive sites on the surface of carbon nanotubes by exploiting free radical interactions with the surrounding polymer matrix. The resulting composite, when processed into fiber form, demonstrates improved mechanical properties. Functionalization of carbon nanotubes has in general been used as a means of increasing their solubility and dispersion in polymeric systems, allowing for the manufacture of composites with increased mechanical properties. Beyond the addition of chemical moieties to the surface of single-walled and multiwalled carbon nanotubes, researchers have used in situ reactions to link carbon nanotubes directly to the surrounding polymer matrix in hopes of taking better advantage of their strong reinforcing properties. In this work, we present a method to produce a composite of this nature by using a single processing step. An in situ reaction, initiated by the production of free radicals upon the decomposition of benzoyl peroxide during the high shear and high-temperature phase of processing, allows for the linkage of the single-walled carbon nanotube to the surrounding polypropylene matrix via a covalent bond. The resulting composite was spun into 2.5, 5, 7.5, and 10 wt. % SWNT/polypropylene fibers that demonstrated improved mechanical properties in tensile strength by 82.9, 89.8, 72.3, and 173.1%, and in the elastic modulus by 69.2, 99.7, 137.2, and 133.7%, respectively, over that of the neat polypropylene fibers.

1. Introduction The highly anisotropic mechanical properties of single-walled carbon nanotubes (SWNTs) can be more efficiently used by processing them into fully integrated SWNT composites and into nanotube continuous fibers (NCFs), which show very directional properties.1 In general, manipulation of these nanoscopic materials into an aligned configuration is more easily accomplished by processing the composites into fibers, which allows for better macroscopic handling of the nanosized components. This also aids in minimizing the effect of reduced mechanical properties resulting from the degree of misalignment of the SWNTs within the polymer matrix.2 In the configuration of an NCF, the nanotubes can be maneuvered and constrained into a more preferential orientation via the fiber processing techniques, which is further reinforced by their contact with the polymer chains that also align themselves during the extrusion phase of fiber processing. SWNTs are being increasingly used as nanosize reinforcements in polymer matrices3,4 to take advantage of their high elastic modulus approaching 1 TPa and tensile strengths in the range of 20-200 GPa for individual nanotubes.5 SWNTs, however, tend to form ropes or bundles as a result of their high aspect ratios and strong van der Waals forces that hold many entangled ropes together. These ropes or bundles are reported as having tensile strengths in the range of 15-52 GPa.6-8 In generating our composites, individual SWNTs, as well as the * To whom correspondence should be addressed. E-mail: (V.N.K.) [email protected]; (E.V.B.) [email protected]. † Department of Mechanical Engineering and Materials Science. ‡ Department of Chemistry and Richard E. Smalley Institute for Nanoscale Science and Technology.

smaller ropes and/or bundles that remain after processing, are targeted as sites for attachment to the surrounding polypropylene chains promoting interfacial adhesion and summarily polymer reinforcement. Polypropylene is an established high-volume commodity plastic. It has found applications as a low-cost engineering plastic in household goods, the automotive industry, domestic appliances, packaging, furniture, pipes and fittings, and fibers.9 It is a thermoplastic material that has good fatigue resistance, good chemical resistance, and good mechanical properties with tensile strengths in the range of 30-38 MPa and tensile modulus ranging from 1.1 to 1.6 GPa for the bulk material.10 By processing these two materials into a fully integrated composite system, the chemically inert nature of each material must be overcome to facilitate good interfacial adhesion, which in turn allows for better load transfer when a tensile load is applied to the system. This allows for the manufacture of a composite that can share in some of the widespread uses already established for polypropylene, as well as open up new possibilities where this new polypropylene composite can be used. Issues that can affect composites containing single-walled nanotubes include ineffective interfacial bonding and sliding of individual nanotubes within nanotube ropes. This can hamper load transfer from the matrix to the fiber limiting the amount of mechanical reinforcement that can be achieved in the composite.11 In the present work, this problem is addressed by the introduction of benzoyl peroxide, a free radical initiator, into the processing stages of the composites to facilitate generation of radical sites along the polypropylene chain. These sites create an opportunity for the otherwise inert polymer to interact with the aromatic π-electron system of SWNT resulting in formation of a covalent bond between the polymer chains

10.1021/jp065399d CCC: $37.00 © 2007 American Chemical Society Published on Web 01/05/2007

SWNTs-Polypropylene Composite Fibers

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Figure 1. Highly ordered chains of isotactic polypropylene showing crystalinity and alignment when forced into an aligned configuration from fiber spinning.

the SWNT and the surrounding polymer matrix via in situ free radical functionalization reaction initiated by benzoyl peroxide during processing of polypropylene-SWNT composites. 2. Experimental Section Figure 2. An NCF showing nanotubes covalently bonded to the polypropylene chains and aligned in the axial direction.1

and the surface of the nanotube. Further processing via fiber spinning results in a polymer composite system in which the SWNTs are aligned in the axial direction. The high crystallinity of the isotactic polypropylene (Figure 1) is of great advantage since the SWNTs, which are covalently linked to the polymer chains, are forced to order themselves along with the chains into small diameter NCFs, as shown in Figure 2. It should be mentioned that prior to our present study other researchers, such as Kearns and Shambaugh,12 and Moore et al.,13 have studied fiber systems made from composites that incorporated SWNTs into polypropylene matrices. Kearns and Shambaugh used solvent processing to disperse SWNTs into polypropylene, melt spinning and post drawing the composite into fibers. For a 1-wt. % loading of nanotubes, they reported a 40% increase in fiber tensile strength (from 9 to 13.1 g/dernier) and a 55% increase in tensile modulus (from 60 to 93 g/denier). Moore et al. also made fiber systems from two separate composites of a highmelt-flow-rate (HMFR) polypropylene with SWNTs and a lowmelt-flow-rate (LMFR) polypropylene with SWNTs. For the LMFR post drawn fiber samples, Moore et al. reported a 44.4% increase in tensile strength and were depending on temperature increases in modulus for samples incorporating 1 wt. % SWNTs. These studies indicate that low weight percent incorporation of SWNTs in polypropylene increases the mechanical properties of the composite. However, in an effort to achieve composites that incorporate much higher weight percents of SWNTs while rendering even higher mechanical properties through efficient load transfer between the polymer matrix and the stronger reinforcing SWNTs, we have used an in situ initiated functionalization process. Substantial increases in mechanical properties of thermoplastic polymers, polyethylene, and polypropylene have been observed in recent studies14-16 because of the use of sidewall functionalized (fluorinated) nanotubes for incorporation into composite matrices. The present paper reports a novel approach to remedy the problem of a lack of interfacial adhesion between

Materials. HiPcoSWNTs were purchased from Carbon Nanotechnology Inc. (batch number D0580). Purification was carried out by the supplier according to the published procedure.17 On the basis of thermogravimetric analysis (TGA) done in air, iron metal impurity content in the as supplied HiPco SWNT batch was calculated to be about 2 wt. %. Benzoyl peroxide was obtained from Fluka. Varying weight percent loads of SWNTs were incorporated into 20-g batches of isotactic polypropylene (Aldrich Chemicals) with Mn ) 67 000, Mw ) 250 000, and a melt-index of 12 g/10 min (230 °C/2.16 kg, ASTM D1238). Fabrication of Polypropylene-SWNT Composite Fibers. The composite materials were prepared from purified SWNTs (P-SWNTs) and polypropylene and also through benzoyl peroxide initiated functionalization of P-SWNT (BP-f-SWNT) with the polypropylene. During processing of each individual batch, the SWNTs first were sonicated in chloroform for several minutes. In the P-SWNT samples, the SWNT suspension in chloroform was combined with the polypropylene to overcoat the polymer pellets and to create an initial dispersion between the polymer and the SWNTs,1,14 initiating an incipient wetting phase. The polypropylene pellet/SWNT suspension mixture then was heated in an oil bath at temperatures between 70 and 80 °C and was dried in a vacuum oven at 80 °C to remove the solvent. The dried SWNT-overcoated powder/pellet mixture had a gray color. The preparation procedure for the BP-f-SWNT samples varied slightly. For each of the batch samples prepared, 0.5 g of benzoyl peroxide was added to the suspension of SWNTs and polymer in solvent. Thereafter, the solvent was gently removed in a rotary evaporator. The batches were then subject to high-temperature high shear mixing in a HAAKE Polylab System using a 30-cm3 mixing bowl. The P-SWNT/ polypropylene batches were mixed at a temperature of 165 °C for 12 min at a speed of 75 rpm. The BP-f-SWNT/polypropylene batches were heated by ramping the temperature by 10 °C per 3 min to 165 °C to initiate and sustain the in situ reaction, which leads to the covalent bonding of the SWNTs to the polymer chains (Scheme 1). After the high shear mixing step, the material was processed into pellets prior to introduction into a C.W. Brabender single-screw extruder to spin the fibers. The extruder

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SCHEME 1: Proposed Reaction Routes for Benzoyl Peroxide Initiated Functionalization of SWNT to Link the Polypropylene Chains Directly to SWNT Sidewall

had four independently heated zones through which the melting polymer flowed. The first zone, closest to the hopper, was heated to a temperature of 170 °C, the second zone to 187 °C, the third zone to 193 °C, and the fourth zone, located at the nozzle where the material was forced through a single 0.79 mm dye, was at a temperature of 216 °C. The extruder screw was programmed to rotate at 4 rpm. The extruded material was immediately pulled onto a fiber take-up reel, which took up the fiber at a constant rate (0.422 g/min of fiber). By using this procedure, the following fibers with varying compositions were prepared:

Sample Type 1 2 3 4 5 6 7 8 9

neat polypropylene 2.5 wt. % P -SWNT/polypropylene 5 wt. % P-SWNT/polypropylene 7.5 wt. % P-SWNT/ polypropylene, 10 wt. % P-SWNT/polypropylene 2.5 wt. % BP-f-SWNT/polypropylene, 5 wt. % BP-f -SWNT/polypropylene, 7.5 wt. % BP-f -SWNT/polypropylene, 10 wt. % BP-f -SWNT/polypropylene.

The final product resulting from these processing and fabrication steps were NCFs with diameters of approximately 130 µm. Characterization. Scanning electron microscopy (SEM) imaging, Raman spectroscopy, TGA, and mechanical testing then were used to evaluate the possibility of interaction between the SWNTs and polypropylene after the processing steps were completed. The SEM studies were performed with a Phillips Electroscan ESEM XL30 instrument. To prevent sample surface charging, all composite and neat polymer samples were sputter coated with 6-nm layer of chromium prior to analysis. The composite fiber diameters were estimated both by optical microscopy and SEM imaging and found to be on average 130 µm. The Raman spectra were taken with a Renishaw MicroRaman spectrometer using the 780 nm diode laser with a 1200 L/m grating, 10 s exposure, and a 50× objective. The TGA in air experiments were conducted using a TA Instruments SDT 2960 device. An Instron Model 5565 was used to perform the tensile tests on the fibers, following the mounting specification indicated by ASTM standard C1557-03. A 50 N load cell was used to test the samples in uniaxial tension. The gauge lengths were uniformly 25 mm, and the crosshead speed used was 254 mm/min. 3. Results and Discussion In Situ Functionalization of SWNTs. Benzoyl peroxide is widely used as a commercial free radical initiator for polymerization.18,19 The addition of free radicals generated from organic peroxides to SWNTs has been established previ-

Figure 3. Raman spectra of (A) neat P-SWNTs, (B) 10 wt. % P-SWNT-polypropylene composite, and (C) 10 wt. % BP-f-SWNTpolypropylene composite.

ously.19,20 In our experiments, benzoyl peroxide was added into SWNT/polypropylene batches to instigate an in situ functionalization reaction leading to the formation of covalent bonds between the polymer chains and the SWNTs. This is most likely accomplished through consecutive free radical reactions taking place during the high temperature, high shear mixing processing of the composite. Under combined thermal and mechanochemical conditions, thermal decomposition of benzoyl peroxide generates phenyl radicals along with the liberation of carbon dioxide. Phenyl radicals in turn scavenge a proton from the C-H unit on the polypropylene chain creating a polymer radical site that bonds directly to the SWNT surface thereafter (Scheme 1).22 Benzene, formed as a byproduct in this reaction, is volatile and completely evaporates during the processing. Raman Spectroscopy. The Raman spectroscopy analyses of the polymer composites processed with the corresponding

SWNTs-Polypropylene Composite Fibers

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Figure 4. Comparative Raman spectra for BP-f-SWNT and P-SWNT in polypropylene, normalizing the G-peaks to show relative intensity to the D-peak indicating covalent modification.

weight percent loads of P-SWNTs and BP-f-SWNTs support the proposed idea of covalent bonding of the polypropylene to the SWNTs, as shown in Scheme 1. In Raman spectra, the disorder mode (D band) is indicative of the presence of sidewall defects on nanotubes arising from the sp3 C-C bonds formed in the course of surface modification. The tangential mode (G band) is due to the sp2 CdC bond stretching vibrations.23 The G band frequencies can serve as a useful probe for the charge transfer arising from doping the SWNT24 and for the evaluation of the interaction between the nanotubes and the polymer.14-16 Nevertheless, as in the case of P-SWNT-polyethylene composites studied earlier,15 the Raman spectra of P-SWNT-polypropylene composites show no significant changes in the G and D-peak location when compared with the spectrum of P-SWNT (Figure 3). The observed shifts of breathing modes frequencies are too small to indicate a significant interaction between the P-SWNT and polypropylene matrix. However, comparison of the Raman spectra of polypropylenenanotube composites processed with 10 wt. % BP-f-SWNT and 10 wt. % P-SWNT, shown in Figure 3, indicate that the sidewall functionalization of the SWNT has been accomplished in the former in comparison with the latter. The increase in intensity of D-band (1319 cm-1) with respect to the intensity of the G-band (1588 cm-1) observed for BP-f-SWNT-polypropylene composite sample indicates that the symmetry along the nanotube length has been changed because of disruption in the hexagonal framework of the SWNTs present. The observed simultaneous decrease in the intensity of the diameter-dependent radial breathing mode at 264 cm-1 is due to predominant functionalization of smaller diameter SWNTs, which are known to be more reactive toward free radical addition.20,21 Similar trends were observed in the Raman spectra for the series of samples containing four different weight percentages of BP-f-SWNTs relatively to polypropylene composites pro-

TABLE 1: Actual Raman G/D Integrated Peak Area Ratios for BP-f-SWNT and P-SWNT Polypropylene Composites Illustrated in the Raman spectra in Figure 4 weight percent

sample

G-peak intensity

D-peak intensity

G/D ratio

2.5 wt %

P-SWNT BP-f-SWNT P-SWNT BP-f-SWNT P-SWNT BP-f-SWNT P-SWNT BP-f-SWNT

2804 3344 5006 3344 6721 2745 9170 3975

534 1044 777 1002 1314 950 1049 1168

5.25 3.2 6.44 3.3 5.11 2.88 8.68 3.4

5 wt % 7.5 wt % 10 wt %

cessed with the corresponding weight percent P-SWNTs, as shown in Figure 4. The ratios of the G to D integrated peak area shown in Table 1 reflect the change in the structure of the pristine nanotube caused by sidewall functionalization. This indicates that the reaction initiated during the high shear mixing stage of the processing has been successful in functionalizing the nanotubes in all composite samples processed with benzoyl peroxide. As shown in Table 1, changes in G/D ratios reflect the significant decrease in the crystal symmetry of the hexagonally arrayed graphene sheets of which the nanotubes is made.23 The Raman data show a general trend of increase in the intensities of the D peak relative to the G peak for all 2.5, 5.0, 7.5, and 10 wt. % BP-f-SWNT composite samples as compared to the corresponding peak intensities for the P-SWNT composite samples (Figure 4). This is the result of the functionalization process in which the long chains of the polymer bond covalently, presumably at random intervals, to the nanotube thereby yielding the lower G/D ratios for the BP-f-SWNT/polypropylene composite systems (Table 1). The mechanical strength attainable by the final composite system can be significantly affected by degree of alignment of the nanotubes in the polymer matrix.2,15 Raman spectra of the

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Figure 5. Polarized Raman spectra for a 10 wt. % P-SWNT-polypropylene composite fiber sample. Note the differences in the intensities of the D and G peaks.

TABLE 2: Integrated G/D Raman Peak Ratios for 0° and 90° Oriented Fibers of P-SWNT in Polypropylene Composite sample 10 wt % P-SWNT (4r) fiber ratio of 0°/90°

fiber-axis rotation

G-peak intensity

D-peak intensity

G/D ratio

0°

9220

1049

8.79

90°

1910 G/G 4.83

466 D/D 2.25

4.09

individual fibers with their fiber-axis oriented either parallel (0°) or perpendicular (90°) to the incident laser polarization have been acquired using 780 nm laser to probe the nanotube alignment.16,23 Carbon nanotubes behave as antennas, with the absorption/emission of light being highly suppressed for light polarized perpendicular to the nanotube axis.24,25 Therefore, in aligned nanotubes, whether individual or roped, the highest Raman intensity will be observed for light polarized along the tube axis and a much lower signal intensity will be detected for cross-polarized light. The Raman spectra (Figure 5) for an

NCF sample containing 10 wt. % P-SWNT in a polypropylene matrix show this expected polarization effect. According to the data presented in Table 2, the intensity of tangential G-mode of the SWNTs in the configuration in which light is polarized along their tube axis (0° direction) is being enhanced more than the intensity of the G-mode of the fibers that contain SWNTs that are oriented perpendicular to polarized light (90° direction). Alignment in the 0° direction correlates with the alignment in the axial direction previously illustrated in Figure 2. The effects of nanotube alignment on the polarized Raman spectra also were observed for the composite fibers processed with benzoyl peroxide functionalized SWNTs, illustrated in Figure 6. Short-range scans were used to more closely monitor the D and G peaks in the Raman spectra. The decreases in the ratio of the G to D peaks (Table 3) in the 0° direction, as compared to that observed in the P-SWNT fibers, supports the proposed idea of covalent bonding of polypropylene chain to SWNT sidewalls, taking place according to mechanism shown

Figure 6. Polarized Raman spectra for a 10 wt. % BP-f-SWNT-polypropylene composite fiber sample. Note variance in the intensities attained with the degree of alignment.

SWNTs-Polypropylene Composite Fibers

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TABLE 3: Integrated G/D Raman Peak Ratios for 0° and 90° Oriented Fibers of BP-f-SWNT in Polypropylene sample 10 wt % BP-f-SWNT fiber Ratio of 0°/90°

fiber-axis rotation

G-peak intensity

D-peak intensity

G/D ratio

0°

2643

598

4.42

90°

1497 G/G 1.76

429 D/D 1.39

3.49

in Scheme 1. The ratio of the tangential G-modes, however, indicated smaller changes in the degree of alignment in the fibers tested. In these BP-f-SWNT fiber samples, the nanotubes become attached at intervals to the polymer chains during the processing stage. These attached nanotubes may be able to orient themselves with the long polymer chains that should spontaneously align in the direction of pull during fiber take-up; however these covalently attached nanotubes may also limit the overall mobility of the polymer chains. This may occur if for example two different polymer chains are linked to the same nanotube. The nanotubes present in the P-SWNT fibers, however, may have less restriction in motion during flow because no chemical linkages were initiated during processing. This allows for better alignment of the nanotubes and their ropes during the drawing phase of fiber spinning. Mechanical Properties. Mechanical tests conducted on the NCFs show a consistent improvement in the properties of those composite fibers into which in situ functionalized nanotubes (BP-f-SWNT) have been incorporated. The data on fiber tensile strengths, elastic modulus, and % elongation are compared by the bar graphs in Figures 7-9. The test results for the mechanical properties of the neat polypropylene fibers and composites processed with the varying weight percent loads of P-SWNTs as well as BP-f-SWNTs are presented in Table 4. The testing data shows an increase in tensile strength by 48.4, 39.7, 64.2, and 61.0% for the 2.5, 5.0, 7.5, and 10 wt. % BPf-SWNT processed NCF samples, respectively, over the corresponding weight percent loaded P-SWNT NCFs (Figure 7). The enhancement of the elastic modulus by 42.1, 61.0, 76.0, and 61.0% for the BP-f-SWNT NCFs over the corresponding weight percent P-SWNT loaded NCFs was observed (Figure 8). All samples show an increase in brittleness with the increase in concentration of nanotube content for both the P-SWNT and BP-f-SWNTs NCFs; however, the samples are capable of achieving significant elongations, as shown in Figure 9, in spite of the addition of carbon nanotubes, which previously were shown to contribute to the embrittlement of thermoplastic

Figure 7. Comparison of tensile strengths for 130 µm fibers.

Figure 8. Comparison of elastic modulus for 130 µm fibers.

Figure 9. Comparison of % elongation for 130 µm fibers.

composites. For this grade of polypropylene, in the case of BPf-SWNT processed NCF samples, we still were able to achieve 900-1000% elongations to failure, which compared to the neat polymer are decreased only by less than 1/3. The data from the mechanical testing reinforces previous spectroscopic observations that suggested that the benzoyl peroxide initiated in situ functionalization of the carbon nanotubes has taken place. This is because the in situ initiated functionalization of the carbon nanotubes aids in securing the fiber-matrix interface, as proposed in Scheme 1, to allow for greater load transfer from the matrix to the nanotube resulting in improved mechanical properties in the BP-f-SWNT fiber samples. The result of this improved interface is that not only are the mechanical properties of the BP-f-SWNT processed NCFs superior to those loaded with the P-SWNTs, but there is an even larger increase in the tensile strength by 82.9, 89.8, 72.3, and 173.1%, and in the elastic modulus by 69.2, 99.7, 137.2, and 133.7% for the respective weight percent of loads of BP-f-SWNT NCFs in comparison with that of the neat polypropylene fiber (Table 4). The data, however, does indicate a decrease in the expected tensile strength and modulus of the neat polypropylene fiber. This may be as a result of the high molecular weight of the polymer used. Mechanical testing of high molecular weight polymers often leads to a reduction in tensile strength, stiffness, hardness, and brittle point.9 The high test rate also has an impact on the mechanical properties of the neat fiber. Improvement in the mechanical properties of the composite has been achieved using the benzoyl peroxide initiated in situ functionalization of carbon nanotubes in polypropylene. En-

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TABLE 4: Improvement of Mechanical Properties of Varying Weight Percentages of BP-f-SWNT NCFs and P-SWNTs NCFs over Neat Polypropylene Fiber

sample (4r) neat polypropylene 2.5 wt. % BP-f-SWNT 5 wt. % BP-f-SWNT 7.5 wt. % BP-f-SWNT 10 wt. % BP-f-SWNT 2.5 wt. % P-SWNT 5 wt. % P-SWNT 7.5 wt. % P-SWNT 10 wt. % P-SWNT a

average diameter (mm)

extension at failure [mm]

maximum stress [MPa]

increase maximum stress [%]

tensile modulus [MPa]

increase tensile modulus [%]

130

398 ( 19.9

30.65 ( 1.86

453.26 ( 14

130

250 ( 32

56.06 ( 5.54

82.9

766.7 ( 19.1

69.2

130

275 ( 38

58.18 ( 13.2

89.8

905.5 ( 20

99.7

130

187 ( 24

52.8 ( 7.38

72.3

1075 ( 25.2

130

147 ( 86

83.7 ( 49

130

133.13 ( 27

28.9 ( 5.39

a

440 ( 8.7

a

130

197.56 ( 21

35.1 ( 4.83

a

349.5 ( 6.4

a

130

153.2 ( 15

18.9 ( 4.337

a

255 ( 5.4

a

130

151.5 ( 24

32.6 ( 8.54

a

366.5 ( 7.8

a

173.1

1059.3 ( 23

137.2 133.7

Data showing either a small increase or decrease in mechanical properties indicated.

Figure 10. SEM image showing a failure initiation in a BP-f-SWNT/ polypropylene composite fiber.

hancement in the mechanical properties of the fibers have been affected significantly by more efficient load transfer in contrast to improved alignment of the nanotubes and polymer chains. The extent of attachment of the nanotubes to the surrounding polymer resulting in this more efficient load transfer cannot at this time be defined in terms of regularity of attachment. There is a general decrease in the mechanical properties of the fibers containing P-SWNT. This may be a result of poor overall dispersion resulting from insufficient initial dispersion of the nanotubes in the polymer matrix or from reaggregation of nanotube ropes and/or bundles during processing. This may have lead points of high stress in the fibers at which failure could be initiated. This in conjunction with poor load transfer of stress from the polymer matrix to the stronger reinforcing SWNT probably led to premature failure in the fibers tested. SEM Studies. Analysis by SEM was performed for imaging the fracture surfaces of the fibers tested for tensile properties. Fracture occurs in the fibers at regions of high-accumulated stress, as is the case when the polymer flows to a point where the diameter of the original fiber has decreased significantly under tensile loading. However, other regions within the composite in which ropes of nanotubes have not been completely disrupted and dispersed can result in high stress regions that lead to fiber failure. These regions can thereby act as initiators for failure before maximum elongation of the polymer matrix

Figure 11. The low magnification image of the fracture surface of a BP-f-SWNT continuous fiber (A). A higher magnification image of ropes of nanotubes coated with the polymer at the fractured surface of a BP-f-SWNT fiber (B).

is achieved. The initiation of a premature failure in a BP-fSWNT processed NCF is shown in Figure 10. To maximize the effectiveness of the in situ functionalization scheme proposed (Scheme 1), better dispersion of the nanotubes in the polymer is still necessary. This will minimize high stress regions within the fibers allowing for the formation of better quality composites. If individual nanotubes or at very least smaller ropes of nanotubes can be accomplished before the incorporation of the benzoyl peroxide, there will be more surface area for interaction with the free radical site on the polymer chains promoting a better and more complete interfacial adhesion between the components of the composite. Figure 11 shows the fracture surface of a BP-f-SWNT processed NCF. The polymer coated ropes of nanotubes

SWNTs-Polypropylene Composite Fibers

J. Phys. Chem. C, Vol. 111, No. 4, 2007 1599 4. Conclusions

Figure 12. The low magnification image of the fracture surface of a P-SWNT continuous fiber (A). A higher magnification image of ropes of nanotubes at the fractured surface of a P-SWNT fiber, indicating fiber pull out (B).

demonstrate points at which these bundles have undergone breakage without separation from the polymer coating. The images show ropes of nanotubes that remain coated with polymer even after composite fiber fracture, indicating interface between the materials has remained intact even up to the point failure. This is in contrast to the normally observed pullout of nanotubes and their ropes from the polymer matrix when no interfacial adhesion between the two components has been achieved. This type of failure is shown in Figure 12, where the fracture surface of a P-SWNT fiber has been imaged. Instead of the coated ropes of nanotubes as illustrated in Figure 11B, we see the uncoated ropes of nanotubes remaining. The image in Figure 11B also shows broken ends of preferentially aligned ropes of nanotubes still coated with the polymer, as previously identified with Raman spectroscopy. The observed breaking of the aligned nanotubes and their ropes, instead of their pull-out from the matrix, provides strong evidence that effective load transfer has been accomplished. This, in addition to the formation of a strong interface resulting from the covalent bonding of the polymer to the ropes of nanotubes, contributes to enhanced mechanical properties previously demonstrated. The BP-f-SWNT being processed with polypropylene matrix into composite fibers is the first step in the designing and manufacturing a composite material whose strength can be tailored during processing by regulating the addition of one of its components (the benzoyl peroxide) during the processing stage. The processing stage now becomes more fundamental in defining the final properties of the composite system thereby, perhaps in the future, allowing the material processor more control to tailor the system for desired standards.

Fibers made from BP-f-SWNT processed with polypropylene were shown to have improved interfacial adhesion and some alignment, resulting in increased mechanical properties. The BPf-SWNTs are generated during the processing stage in which benzoyl peroxide is added to molten polypropylene serving as a free radical initiator, which is completely consumed during processing. The free radials scavenge protons from the polypropylene matrix providing sites for direct covalent bonding of the SWNTs and their ropes. The composites made through this high temperature in situ functionalization scheme proposed in this paper can be thought of as a material composed of macromolecules instead of the conventional matrix and reinforcing filler type of composite. This is supported by the strong interface that is formed from the reinforcing filler (SWNTs), covalently linked to the surrounding polymer matrix. This interface is exploited when the material is subjected to tensile loading yielding the increased mechanical properties demonstrated. The goal of this type of composite is to move toward a fully integrated composite system, where even greater mechanical reinforcement can be achieved. In this type of fully integrated composite, every carbon nanotube will have a linkage at consistent intervals along its outer surface. This will result in far more efficient load transfer and render far better mechanical reinforcement by the stronger SWNT component. The final outcome of this work is that as a consequence of the developed in situ functionalization processing method we are now able to create a stronger material that can be used for other applications in which light weight robust polymer composites are in high demand. Acknowledgment. This research was funded by the Robert Welch Foundation of Texas (Grant C-1494), NASA Cooperative Agreement (NCC-1-02036), and the Texas ATP (Grant 0035990010). The authors also thank Dr. Felipe Chibante of NanoTex Corporation for providing access to his facilities for the preparation of fiber samples. References and Notes (1) Barrera, E. V. JOM 2000, 52, 38. (2) Khatibzadeh, M.; Piggott, M. R. Comp. Sci. Tech. 1996, 56, 1435. (3) Coleman, J. N.; Khan, U.; Gun’ko, Yu. K. AdV. Mater. 2006, 18, 689. (4) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Yu. K. Carbon 2006, 44, 1624. (5) Krishnan, A.; Dujardin, E.; Ebbesen, T. W.; Yianilos, P. N.; Treacy, M. M. J. Phys. ReV. B 1998, 58, 14013. (6) Shenderova, O. A.; Zhirnov, V. V.; Brenner, D. W. Crit. ReV. Solid State Mater. Sci. 2002, 27, 227. (7) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (8) Lourie, O.; Cox, D. M.; Wagner, H. D. Phys. ReV. Lett. 1998, 81, 1638. (9) Tripathi, D. Practical Guide to Polypropylene; Rapra Technology Ltd., 2002. (10) Hertzberg, R. W. Deformation and Fracture Mechanics of Engineering Material, 4th Ed.; John Wiley and Sons, 1996. (11) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. AdV. Mater 2000, 12, 750. (12) Kearns, J. C.; Shambaugh, R. L. J. Appl. Polym. Sci. 2002, 86, 2079. (13) Moore, E. E.; Ortiz, D. L.; Marla, V. T.; Shambaugh, R. L.; Grady, B. P. J. Appl. Polym. Sci. 2004, 93, 2926. (14) Shofner, M. Nanotube Reinforced Thermoplastic Polymer Matrix Composites. Ph.D. Thesis, Rice University, 2003. (15) Shofner, M. L.; Khabashesku, V. N.; Barrera, E. V. Chem. Mater. 2006, 18, 906. (16) McIntosh, D.; Khabashesku, V. N.; Barrera, E. V. Chem. Mater. 2006, 18, 4561.

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