Synthesis and Characterization of Thickness-Aligned Carbon

Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110, 8th Street,. Troy, New York 12180, Intel Corporation, CH5-159,...
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Chem. Mater. 2005, 17, 974-983

Synthesis and Characterization of Thickness-Aligned Carbon Nanotube-Polymer Composite Films Nachiket R. Raravikar,†,‡ Linda S. Schadler,*,† Aravind Vijayaraghavan,† Yiping Zhao,§,| Bingqing Wei,†,⊥ and Pulickel M. Ajayan*,† Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110, 8th Street, Troy, New York 12180, Intel Corporation, CH5-159, 5000, West Chandler BouleVard, Chandler, Arizona 85226, Department of Physics, Rensselaer Polytechnic Institute, 110, 8th Street, Troy, New York 12180, Department of Physics & Astronomy, UniVersity of Georgia, Athens, Georgia 30602, and Department of Electrical and Computer Engineering, Louisiana State UniVersity, Baton Rouge, Louisiana 70803 ReceiVed September 1, 2004. ReVised Manuscript ReceiVed December 5, 2004

To optimize the properties of carbon nanotube-polymer composites, it is important to control nanotube dispersion and alignment. One way to achieve such control is by growing homogeneous, well-aligned arrays of carbon nanotubes using chemical vapor deposition and infiltrating polymer or monomer into the arrays, followed by in situ polymerization. In this paper, pre-aligned multiwalled carbon nanotube arrays were infiltrated with methyl methacrylate (MMA) and the MMA was polymerized. The resulting composite films have well-dispersed, aligned nanotubes. Using the Washburn technique, it was found that the infiltration of monomers into aligned nanotube arrays is largely driven by the wetting of liquids against the nanotube walls and the low viscosity of liquids. Once polymerized, the PMMA had higher thermal stability. This synthesis process is adaptable to various polymers. It is possible to combine conventional micro-patterning techniques with infiltration process for achieving selective infiltration of polymer across nanotube arrays. Thus, the present synthesis strategy has tremendous implications toward building some of the novel architectures with nanotubes and polymers, having unique properties.

Introduction Carbon nanotubes are high strength, high modulus graphitic fibers with a diameter of a few nanometers (1-2 nm for single-walled and 10-40 nm for multiwalled nanotubes) and a length of several micrometers.1 Recently, there has been significant scientific and technological interest in developing nanotube-polymer composites for applications requiring unique combinations of properties.2-7 Nanotubepolymer composites have shown promise in applications such as ultrafast all-optical switches,2 EMI shelding,3 photovoltaic devices,4,5 gas sensors,6 and biocatalytic films.7 The potential of these nanocomposites, however, has not been fully * Authors to whom correspondence should be addressed. E-mail: [email protected] (L. S. Schadler); [email protected] (P. M. Ajayan). † Department of Materials Science and Engineering, Rensselaer Polytechnic Institute. ‡ Intel Corporation. § Department of Physics, Rensselaer Polytechnic Institute. | Department of Physics & Astronomy, University of Georgia. ⊥ Department of Electrical and Computer Engineering, Louisiana State University.

(1) Ajayan, P. M. Chem. ReV. 1999, 99 (7), 1787. (2) Chen, Y.-C.; Raravikar, N. R.; Schadler, L. S.; Ajayan, P. M.; Zao, Y.-P.; Lu, T.-M.; Wang, G.-C.; Zhang, X.-C. Appl. Phys. Lett. 2002, 81 (6), 975. (3) Kim, H. M.; Kim, K.; Lee, C. Y.; Joo, J.; Cho, S.; Yoon, H. S.; Pejakovic, D. A.; Yoo, J. W.; Epstein, A. J. Appl. Phys. Lett. 2004, 84 (4), 589. (4) Ago, H.; Petritsch, K.; Shaffer, M. S. P.; Windle, A. H.; Friend, R. H. AdV. Mater. 1999, 11 (15), 1281. (5) Kymakis, E.; Amartunga, G. A. Appl. Phys. Lett. 2002, 80 (1), 112. (6) Philip, B.; Abraham, J. K.; Chandrasekhar, A.; Varadan, V. K. Smart Mater. Struct. 2003, 12, 935. (7) Rege, K.; Raravikar, N. R.; Kim, D.-Y.; Schadler, L. S.; Ajayan, P. M.; Dordick, J. S. Nano Lett. 2003, 3 (6), 829.

exploited yet. Some of the main limitations are8 lack of control over the orientation and dispersion of nanotubes in a polymer matrix, as well as the tailoring of the nanotubepolymer interface.8-10 One approach to simultaneously control the nanotube alignment (especially in the third dimension) and the nanotube dispersion in a polymer is to infiltrate monomer into the pre-aligned arrays of nanotubes, followed by in situ polymerization. Composites, by this approach, are made in two steps. In the first step, pre-aligned arrays of multiwalled carbon nanotubes (MWNT) are grown on a large substrate by chemical vapor deposition (CVD). In the second step, a monomer or polymer is infiltrated into these arrays. The nanotubes grown by CVD are homogeneously distributed, and not bundled, which is an advantage over the bulk nonaligned nanotubes prepared by other techniques. The resulting composite films have good distribution, dispersion, and alignment of nanotubes in a polymer matrix, and they also provide reinforcement in the out-of-plane direction. A similar synthesis approach has been used to develop composite architectures such as intercalated networks of nanotubes and polymers,11-13 and aligned MWNT-based com(8) Ajayan, P. M.; Schadler, L. S.; Giannaris, C.; Rubio, A. AdV. Mater. 2000, 12 (10), 750. (9) Wagner, H. D. Chem. Phys. Lett. 2002, 361, 57. (10) Lau, K.-T. Chem. Phys. Lett. 2003, 370, 399. (11) Lahiff, E.; Ryu, C. Y.; Curran, S.; Minett, A. I.; Blau, W.; Ajayan, P. M. Nano Lett. 2003, 3 (10), 1333. (12) Vigolo, B.; Penicaud, A.; Coulon, C.; Sauder, C.; Pailler, R.; Journet, C.; Bernier. P.; Poulin, P. Science 2000, 299, 1331.

10.1021/cm0485254 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/22/2005

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posite membranes.14 However, a detailed study of the composite synthesis process and the resulting properties has not been reported yet. This paper presents a detailed study of the process of infiltration of methyl methacrylate (MMA) into MWNT arrays, thereby showing the wettability of the arrays, and the improved thermal stability of the resulting PMMA. Experimental Section Synthesis. Chemical vapor deposition (CVD)15 was used to grow aligned multiwalled carbon nanotubes (MWNT) on a quartz substrate, 1 in. in diameter. A gaseous mixture of ferrocene (0.3 g) as a catalyst source and xylene (30 mL) as a carbon source, heated to above 150 °C, was passed for 10 min over the quartz substrate, which was heated to 800 °C in a quartz tube furnace. As shown in Figure 1, aligned MWNTs were 30-50 µm in length and ∼30 nm in diameter. The average distance of separation between the neighboring MWNT was ∼185 ((104) nm. The monomer, methyl methacrylate (MMA), the initiator, 2,2′azobisisobutyronitrile (AIBN), and the chain transfer agent, 1-decanethiol, were mixed together16 (60 mL of MMA:0.17 g of AIBN: 30 µL of 1-decanethiol). A portion of this solution was taken out in a glass vial in which the quartz substrate with aligned nanotubes was gently immersed, with the nanotube side facing the top. The remaining portion of the same solution was taken in a separate vial to make pure PMMA as a control sample. Both quartz vials were sealed with Argon balloons. The polymerization was carried out in a water bath at 55 °C, for 24 h. After polymerization, the glass vials were broken. PMMA-MWNT and pure PMMA disks were taken out. Schematics of the process are shown in Figure 2a. The resulting films have MWNT aligned in the thickness direction in a polymer matrix. The monomer, MMA, is always used in excess. This typically results in some extra polymer above the MWNT arrays, as shown in Figure 2b. It is possible to remove the extra layer of PMMA by gentle mechanical polishing or spin-coating.14 Spin-coating can be performed using a solvent in which PMMA dissolves.14 In the present case, the extra polymer layer was removed by mechanical polishing. To confirm the removal of the extra polymer layer, the polishing step was followed by SEM observation of the film cross section. These films were subsequently used for characterization tests such as the thermogravimetric analysis, gel permeation chromatography, and differential scanning calorimetry. Various characterization methods are discussed in the Appendix.

Results In this section, we discuss the results of various characterization techniques used to study the infiltration process, wettability of nanotube arrays, and the resulting properties of the composites. 1. Determination of Weight Fraction of MWNT in Composites. From the TGA plot shown in Figure 2c, the weight fraction of MWNT in composite is estimated to be ∼4%. It is obtained as the ratio of the step height of the (13) Coleman, J. N.; Blau, W. J.; Dalton, A. B.; Munoz, E.; Collins, S.; Kim, B. G.; Razal, J.; Selvidge, M.; Vieiro, G.; Baughman, R. H. Appl. Phys. Lett. 2003, 82 (11), 1682. (14) Hinds, B. J.; Chopra, N.; Rantell, T.; Andrews, R.; Gavalas, V.; Bachas, L. G. Science 2004, 303, 62. (15) Zhang, Z. J.; Wei, B. Q.; Ramanath, G.; Ajayan, P. M. Appl. Phys. Lett. 2000, 77 (23), 3764. (16) Balke, S. T.; Hamielec, A. E. J. Appl. Polym. Sci. 1973, 17, 905.

Figure 1. Aligned MWNT arrays grown by CVD. (a) and (b) SEM and TEM images of as-grown MWNT arrays, respectively; (c) MWNT diameter distribution based on the measurements performed using SEM and TEM images; (d) distribution of inter-nanotube distances based on measurements performed using SEM images.

TGA curve at MWNT thermal degradation temperature to the initial weight of the composite film. The density of MWNT17 ∼ 2.19 g/cm3, and thus the volume fraction of MWNT in these composite films ∼ 2%. This is consistent with the theoretically predicted volume fractions of aligned MWNT in these composites, for an average inter-nanotube distance of ∼200 nm (Figure 2d). To determine whether the nanotubes influence the polymerization process, the molecular weight and polydispersity (17) Safadi, B.; Andrews, R.; Grulke, E. A. J. Appl. Polym. Sci. 2002, 84, 2660.

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Figure 2. Schematics of process for making thickness-aligned MWNT/PMMA films. (a) Schematic of composite synthesis. (b) Cross section of MWNT/ PMMA films shows thickness-aligned MWNT (∼30 µm long) in PMMA matrix. An extra polymer (∼20 µm) is observed over the nanotube arrays. (c) Determination of weight fraction of MWNT by thermogravimetry (TGA): The inset in the TGA curve in (c) is magnified to reveal details. (d) A theoretical relationship between MWNT volume fraction with inter-nanotube distance. MWNT volume % R 1/(inter-nanotube distance).2

of PMMA from composite films are compared with those of the pure PMMA films, using GPC. The GPC results in Table 1 indicate that the polydispersity of PMMA from composites is typically higher (>2) than that of bulk PMMA (∼1.5). The reported literature18,19 shows a consistent increase in Mw of PMMA upon addition of MWNT in a monomer solution. MWNT are known to scavenge the chain initiator (AIBN) and thereby increase the molecular weight of the resulting PMMA.19 However, in the present case, the addition of MWNT consistently shows a larger distribution of molecular weights (higher polydispersity) and a lower average molecular weight. High PDI may lower the glass transition temperature (Tg), owing to the plasticizing effect

of the lower molecular weight components.20 This is consistent with the DSC results, which show that the (Tg) of PMMA from composite films is lower than that of pure PMMA. It can be concluded from these results that the MWNT arrays influence the in situ polymerization of PMMA, by changing the polydispersity and molecular weight. 2. Wetting Studies Using the Washburn Technique. From the Washburn analysis, the overall surface energy of MWNT is estimated as ∼24 mJ/m2, which is close to that reported by Barber et al.21 Since the surface energies of most of the organic liquids are close to this value,21 these liquids are expected to wet the nanotubes. It is indeed the case for toluene and MMA (Table 2). In addition, we have observed that the CVD grown MWNT have a higher surface polarity (3.4%) than that of the carbon black. A nonzero value of surface polarity of MWNT is also reported by Barber et al.21 As a result, polar liquids such as MMA wet MWNT slightly better than nonpolar liquids, such as toluene. Because of the

(18) Park, S. J.; Cho, M. S.; Lim, S. T.; Choi, H. J.; Jhon, M. S. Macormol. Rapid Commun. 2003, 24 (18), 1070. (19) Jia, Z.; Wang, Z.; Xu, C.; Liang, J.; Wei, B.; Wu, D.; Zhu, S. Mater. Sci. Eng. 1999, A271, 395.

(20) Young, R. J.; Lovell, P. A. Introduction to Polymers, 2nd ed.; Chapman and Hall: New York, 1992. (21) Barber, A. H.; Cohen, S. R.; Wagner, H. D. Phys. ReV. Lett. 2004, 92 (18), 186 103.

Table 1. Comparison of Average Mw, Polydispersity, and Glass Transition Temperature (Tg) of PMMAa polymer

Mw

polydispersity

Tg (°C)

pure PMMA PMMA-MWNT

149000 86000

1.56 2.63

98.4 80.5

a

Data based on the average taken over two samples each.

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Chem. Mater., Vol. 17, No. 5, 2005 977 Table 2. Data from Washburn Analysisa

A. liquids

θ (deg)

γ (mN/m)

γP

γD

F (g/cm3)

η (cp)

toluene MMA waterb molten PMMAb n-hexaned

35.4 33.6 86.5 ∼57 ∼0

28.4 27 72.8 38 18.4

2.3 3.8 46.4 5.7 0

26.2 23.3 26.4 32.3 18.4

0.895 0.936 0.9978 1.2 0.661

0.593 0.644 1 3 × 106 c 0.33

B. solids

γs (mJ/m2)

γP

γD

surface polarity

MWNT carbon black (untreated)e

24 18

0.8 0.01

22.9 17.96

3.35% ∼0.1%

a Data based on the average taken over two samples each. b Contact angle of water and molten PMMA on MWNT was predicted by Fowke’s theory. For water θ ∼ 99.1° as predicted by theory. The observed contact angle of water was measured after an initial lag period of ∼30 s, when water did not rise into MWNT pellet. c Reference 22. d The n-hexane is one of the lowest surface tension liquids and shows complete wetting with most organic solids. It is typically used in Washburn analysis for determination of material constant (c). Average material constant of MWNT, as measured with hexane (c) ) 4.36 × 10-7(cm5). e Courtesy: C. Rulison, Augustine Scientific, Inc.

Figure 3. Wetting of PMMA with MWNT in a composite. (a) SEM images of the cross section of MWNT-PMMA film, (b) SEM image of pristine MWNT; an increase in the average diameter of MWNT is observed in (a) over pure MWNT in (b), due to the coating of PMMA on MWNT. (c) Statistical distribution of diameters of pristine and PMMA-coated MWNT indicate an average increase of ∼20 nm in the diameter of MWNT due to PMMA coating. This indicates that PMMA wets the nanotube surface. (d) An SEM image of PMMA-MWNT composite shows the wetting of PMMA with carbon nanotubes.

wetting of MMA with MWNT, the resulting PMMA is found to coat the MWNT. This is observed from an increase of ∼20 nm in the average diameter of pristine MWNT, as shown in Figures 3a-c. The wetting of PMMA with MWNT is also evident from another high-resolution SEM image of composites shown in Figure 3d. 3. Degree of Alignment by Polarized Raman Spectroscopy. Polarized Raman spectroscopy is performed in order to determine whether the degree of alignment of nanotubes remains the same during the infiltration process. Figure 4c shows an example of change in the Raman peak intensities of pristine MWNT (Figure 4a), for an angle of orientation of 0° and 50° with the polarization axis. The Raman peak intensities of E2g and D* modes of MWNT gradually

decrease as the angle between the nanotube axis and the plane of polarization of incident beam increases from zero to 90°. The same trend is observed for the Raman peak intensities of MWNT in composites. It is shown by the normalized intensities of the D* mode peak (Figure 4d and Table 3). The D* mode peak intensities are used to determine the degree of alignment of nanotubes.23,24 The data in Table 3 are obtained using Figure 4d and eq 8 from the Appendix. It shows that approximately 50% of the MWNT are aligned (22) Kozlowki, M.; Bucknall, C. B. Pure Appl. Chem.: IUPAC Tech. Rep. 2001, 73 (6), 913. (23) Frogley, M. D.; Zhao, Q.; Wagner, H. D. Phys. ReV. B 2002, 65, 113413. (24) Eitan, A. Doctoral Thesis, Rensselaer Polytechnic Institute, Troy, NY, 2003.

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Figure 5. Comparison of thermal stability of PMMA in composite films with pure PMMA, using TGA.

mass loss rate of PMMA from composite films is lower than that of the pure PMMA. An increase in TD and a decrease in maximum mass loss rate of PMMA from composites over pure PMMA imply that the kinetics of oxidative thermal degradation of PMMA from composites are delayed by MWNT, over those of pure PMMA.25-30 This behavior is consistent with the reported literature on the improvement in thermal stability of PMMA by fullerenes25-27 and carbon nanotubes.28-30 Discussion

Figure 4. Degree of nanotube alignment by polarized Raman spectroscopy. SEM images of aligned MWNT arrays, (a) before and (b) after the infiltration of MMA. (c) An example of change in Raman peak intensities of pristine MWNT with angle (0° and 50°) of orientation with the polarization axis. (d) Normalized intensity of D*-band peak versus the angle of orientation of MWNT with respect to the polarization axis of the incident laser beam. Table 3. Percentages of MWNT Aligned between the Intervals of Various Anglesa degree of alignment (%) angle (deg)

pure aligned MWNT

aligned MWNT/PMMA

0-22.5 22.5-45 45-67.5 67.5-90

49 18 18 15

49 14 14 23

a

Average taken over four data points each.

in the axis direction. The trends shown by Figure 4d and Table 3 indicate that the present composite synthesis process does not disturb the original alignment of the nanotubes. This implies that the synthesis process has the potential to create virtually any kind of architecture of nanotubes (involving various orientation patterns of nanotubes) in a polymer matrix. 4. Thermal Stability of PMMA in Composites. Based on the TGA curves in Figure 5 and the data in Table 4, it is observed that TD of PMMA from aligned MWNT/PMMA composite films is higher by ∼40 °C over that of pure PMMA. In addition, Table 4 indicates that the maximum

In this section, we discuss the relationship between the wettability of nanotube arrays and the infiltration of liquid, followed by a discussion on the applicability of the infiltration process. We also analyze the thermal stability improvement of PMMA in composites, with respect to the nanotube dispersion. 1. Understanding the Process of Infiltration of Liquids into MWNT Arrays. The driving force for infiltration of liquids in porous architectures is given by the difference in chemical potential across the liquid-vapor interface. Based on eq 3 from the Appendix, the driving force is decided by the shape of the infiltrating liquid meniscus. The shape of the liquid meniscus is influenced by the contact angle of liquid with nanotubes, and the inter-nanotube distance. Figure 6a shows the relationship of the chemical potential difference across liquid-vapor interface, with the contact angle of liquids, as well as with the inter-nanotub distance. For liquids such as toluene and MMA, the contact angle is 90° with MWNT. This is indeed observed during the initial “lag” period in the Washburn measurements, when water does not infiltrate the MWNT pellet. Such a lag period is not observed in the Washburn measurements of liquids having contact angles 0. For a given liquid, ∆µ varies inversely with the inter-nanotube distance (r). At r > 1 µm, the driving force becomes negligible (∆µ f 0), irrespective of the contact angle of liquid, owing to flattening of the liquid meniscus. (b) Dependence of flow rate on viscosity is very strong, showing almost a 6 orders of magnitude difference between the flow rates of molten PMMA and MMA for a given range of inter-nanotube distances.

with the contact angle of ∼86° with MWNT. Thus, ∆µ(water) > 0 before the lag period, and ∆µ(water) < 0 after the lag period, as shown by Figure 6a. Although it is not clear why the contact angle of water against MWNT reduces after a certain time, it is speculated that the surface polarity of MWNT is likely to change over the period, due to the vicinity of water. Change in surface polarity of MWNT may be responsible for change in contact angle of water, over time. Given these observations, many monomers, oligomers, and possibly polymers should easily infiltrate the MWNT arrays. However, two factors limit the infiltration process: the internanotube distance (the driving force is higher for smaller distances) and the viscosity of the polymer (high viscosity will require long infiltration times). The effect of viscosity on infiltration is shown in Figure 6b. To test the broad applicability of this method, parylene, a low-k dielectric material, was infiltrated into aligned MWNT arrays by physical vapor deposition (PVD) of its monomer,

Figure 7. Parylene/MWNT composite films. (a) The SEM images show a uniform coating of parylene around the CVD grown nanotubes, at lower doses of parylene. (b) At higher doses of parylene, thicker coatings of parylene form a continuous matrix between the MWNT in the aligned arrays.

followed by polymerization around the nanotube arrays. An advantage of such an infiltration process includes excellent step coverage. In fact, at lower doses of dimer, we observed the nanotubes to be uniformly coated with parylene (Figure 7a), and at higher doses, composite films of parylene-MWNT were formed (Figure 7b). Low molecular weight (Mw ∼ 1000) polyethylene (PE wax)-MWNT films were prepared by infiltrating molten PE wax at ∼120 °C into aligned MWNT arrays, followed by solidification. PDMS-MWNT films were prepared by the infiltration of a mixture of silane resin and a curing agent into the aligned MWNT arrays, followed by thermal cure at 60 °C for 12 h and then at 80 °C for 2 h. In another experiment, polyimide-MWNT films were prepared by the infiltration of a solution of uncured resin into the aligned MWNT arrays, followed by removal of the solvent and subsequent UV cure of the resin. An interesting application of this method is for use in patterning. To test the same, an uncured photoresist was infiltrated into MWNT arrays, followed by exposure to UV under a patterned mask and subsequent thermal cure. The portion of photoresist exposed to UV was not polymerized, whereas the portion exposed to UV was polymerized upon thermal cure. The composite was then dissolved in a developer solution. The uncured photoresist dissolved in the developer solution and bare MWNT were exposed in that region. The resulting architecture had alternate regions in

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Figure 8. Selective infiltration of photoresist in MWNT arrays using conventional micro-patterning techniques along with polymer infiltration. (a) An array of MWNT filled with the photoresist in alternate parallel bands or stripes. The brighter portions of the stripes are MWNT arrays without the photoresist and darker portions are MWNT arrays with the photoresist. (b) A magnified image of one of these bands, showing the central bright band consisting of MWNT without the photoresist, surrounded by the two sides having MWNT with the photoresist. (c) Higher magnification image of MWNT arrays in the brighter portion, after the removal of the photoresist. (d) Top view of the bright/dark interface of a band showing porous MWNT arrays in bright region, where photoresist is removed, and photoresist-filled MWNT arrays in dark region.

the MWNT arrays with and without the photoresist, as shown in Figure 8. Thus, “selectivity” can be achieved in the infiltration polymer, by combining infiltration with conventional micropatterning methods. 2. Understanding the Improvement in Thermal Stability of PMMA. The thermal degradation of pure PMMA in air is called oxidative thermal degradation, and the reason it is studied in the present paper is because of its practical relevance.31-33 Fullerenes (C60) are known to increase the oxidative thermal degradation temperature of PMMA through the formation and stabilization of the fullerene-bonded macroradicals.25-27 Carbon nanotubes, being structurally similar to fullerenes, have also shown similar behavior.28-30 Moreover, when MWNT are well-dispersed in a composite, such as in the present case, the thermal degradation temperature of PMMA in such composites is expected to be higher than that of the pure PMMA, and the PMMA in poorly dispersed MWNT-PMMA films having the same weight fraction of MWNT.26 (31) Kashiwagi, T.; Hirata, T.; Brown, J. E. Macromolecules 1985, 18, 131. (32) Song, J.; Fischer, Ch.-H.; Schnabel, W. Polym. Degrad. Stab. 1992, 36, 261. (33) Hirata, T.; Kashiwagi, T.; Brown, J. E. Macromolecules 1985, 18, 1410.

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Figure 9. MWNT arrays intercalated with PMMA. (a) DTG curve shows the effect of nanotube dispersion on thermal stability of PMMA; better dispersions improve the thermal stability. (b) SEM micrographs of intercalated PMMA-MWNT arrays show that PMMA is mostly near the MWNT surface. (c) DTG curve of intercalated MWNT-PMMA, superimposed onto those of the pure PMMA and the thickness-aligned MWNTPMMA films.

As further proof of this hypothesis, the thermal degradation behavior of composite samples having different degrees of nanotube dispersion was compared. MWNT-PMMA films were processed using a solution method,7 resulting in a poor dispersion of pristine MWNT. The thermal stability of PMMA in these films was compared to that of the pure PMMA and PMMA from the thickness-aligned MWNTPMMA films. As shown by Figure 9a and Table 4, pure PMMA and PMMA from poorly dispersed MWNT-PMMA films both show a lower thermal stability than PMMA from aligned MWNT-PMMA films. The large surface areas offered by well-dispersed MWNT, for interaction with PMMA, may be the reason behind the observed improvement in thermal stability of PMMA. The relationship between the surface areas offered by welldispersed nanotubes and the PMMA thermal stability is further supported from a comparison of activation energies of PMMA from various composite films (Table 4 and Figures 9 and 10). For this purpose, a control sample was prepared in which the thermal stability of PMMA would be largely influenced by the nanotube surface area. In a separate experiment, MWNT arrays were partly infiltrated (or inter-

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absolute value of the ratio of slopes of tangents to the DTG curve at the inflection point, Tpeak. In the present case, the reaction is approximately of the first order.35 [n(pure PMMA) ∼ 0.7, n(MWNT-PMMA film) ∼ 1.4, and n(MWNT-intercalated array) ∼ 0.6]. Figure 9c, Figure 10, and Table 4 clearly indicate that PMMA in the intercalated PMMA-MWNT arrays requires the highest activation energy for degradation. As the influence of the nanotube surface area is reduced, such as in the case of the pure PMMA, the activation energy of degradation of PMMA is also lowered. Thus, the dispersion of nanotubes helps improve the thermal stability of PMMA because a higher nanotube surface area is available for interaction with PMMA macroradicals. Conclusion

Figure 10. A plot of ln[(-dW/dt)/W] versus [-1/RT]. The activation energy (Ea) is evaluated from the slope of the dashed lines of the portion of the curve (a) shown in the rectangular inset, which is magnified in (b). A steady increase in slope from pure PMMA to intercalated MWNT/PMMA corresponds to an increase in activation energy.

calated) with only a small amount of MMA, followed by the in situ polymerization to PMMA. These arrays are shown in Figure 9b. In the intercalated PMMA-MWNT arrays, most of the PMMA is observed to be closer to the nanotube surface, owing to wetting with the latter. The internanotube gaps are not completely filled by the PMMA. The activation energy (Ea)34-36 for the thermal degradation of PMMA from various films was calculated by applying the analysis by Seo et al.,34 as an approximation. In this analysis, for a firstorder reaction, the activation energy (Ea) is related to the weight fraction of the polymer remaining in the TGA run, W, the rate of weight loss, -dW/dt, and the temperature, T, by the following equation: ln[(-dW/dt)/W] ) Ea(-1/RT) + ln(A)

(1)

Therefore, for a first-order degradation process, a plot of ln[(-dW/dt)/W] versus [-1/RT] is a straight line, with the slope equal to the activation energy (Ea). Such a plot is shown in Figure 10. The reaction order of the nonisothermal degradation process, n, is determined by the Kissinger method, as follows,34,36 n ) 1.26xs

(2)

where s is the shape index of the DTG curve for nonisothermal dynamic degradation. The shape index is also the (34) Seo, M.-K.; Park, S.-J. Macromol. Mater. Eng. 2004, 289, 368. (35) Chan, J. H.; Balke, S. T. Polym. Degrad. Stab. 1997, 57, 135. (36) Kissinger, H. E. Anal. Chem. 1957, 29, 1702.

We propose a two-step strategy toward the development of thickness-aligned MWNT-polymer films made by infiltration of monomer (or polymer) and subsequent in situ polymerization into the aligned arrays of MWNT grown by CVD. Thus, using CVD, the distribution, dispersion, and alignment of MWNT can be controlled prior to being added to the polymer matrix. It is observed that the infiltration of liquids is driven by the large surface areas offered by the nanotubes, the wetting of liquids against the nanotube surfaces, the internanotube distance, and the low viscosity of liquid. The MWNT retard the oxidative thermal degradation of PMMA by stabilization of macroradicals formed during the degradation process. The thermal stability of PMMA from thickness-aligned MWNT-PMMA films is improved due to the large surface area offered by welldispersed MWNT for interactions with PMMA macroradicals. This novel, two-step synthesis strategy has tremendous implications toward building some of the novel architectures with nanotubes and polymers, having unique properties. Acknowledgment. The authors thank the NSF funded Nanoscale Science and Engineering Center for directed assembly of nanostructures, for financial support. NR acknowledges Dr. Christopher Rulison from Augustine Scientific Inc. for contact angle measurements; Prof. Chang Ryu, RPI, and Mettler-Toledo Thermal Analysis grant for DSC and TGA measurements; Dr. Ami Eitan and Dr. Amitabh Bansal, RPI, for their help with the polarized Raman intensity analysis; Dr. Brian Benicewicz and Chunzhao Li, RPI, for the GPC measurements; Dr. Benjamin Ash, RPI, for the initial help with the polymerization experiments, and Shripad J. Gokhale, RPI, for useful discussions on the thermodynamics and kinetics of infiltration process. The authors thank Prof. D. Wu, Tsinghua University, China, for providing the 1.5-mm-long, aligned MWNT arrays, on which the polarized Raman spectroscopy was performed. The authors also thank Prof. T.-M. Lu and Prof. G.-C. Wang, Department of Physics, RPI, for the availability of the physical vapor deposition setup, which was used to grow parylene on MWNT arrays.

Appendix Characterization Techniques. 1. Wetting Studies by Washburn Technique. It is assumed that infiltration of liquid into MWNT arrays occurs via capillarity owing to the narrow

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internanotube gaps.37,38 Similar assumptions were used by Neimark et al.38 to calculate the effective pore size of poly(vinyl alcohol)-intercalated SWNT arrays by absorption of liquid drops into these networks. Thermodynamically, the driving force for infiltration of liquid into a capillary is given by the difference in chemical potential (∆µ) across the liquid-vapor interface. The latter can be derived from the Kelvin-Clapeyron equation39-41 for the change in liquidvapor interfacial profile (meniscus) of a single-component liquid on a solid surface, as a function of capillarity,39 ∆µ ) -κγ ) -(2γ cos θ)/r ) -∆Pc

(3)

where κ is the curvature of the liquid meniscus and γ is the surface tension of the liquid as obtained from the literature.39,42 Curvature, κ, is given as ∼2/(R), where R is the radius of curvature of the liquid meniscus. R can be expressed in terms of the radius of capillary, r, as R ) r/cos θ. The radius of capillary, “r” is equal to half of the internanotube distance (Figure 1). It is further related to the capillary pressure, ∆Pc, via the Young-Laplace equation.43-47 The contact angle of a liquid against a porous solid was obtained using the Washburn technique. The Washburn theory45 indicates that if the porous solid is kept such that it is not submerged in the liquid but is just touching the liquid’s surface, then the rise of liquid into the pores of the solid due to capillary action can be measured by measuring the change in mass of the solid sample with time, as it absorbs the liquid. It is given by the Washburn equation (also known as Lucas-Washburn equation) as follows: 38,42-44,48 t ) Am2

(4)

where t ) time after the solid and the liquid are brought into contact, m ) mass of liquid sucked into the solid, and A ) a constant which is dependent on the properties of the liquid and the solid in question. Specifically, A ) η/[cF2γ cos θ]

(5)

where η ) viscosity of liquid, F ) density of liquid, γ ) surface tension of the liquid, θ ) contact angle between the solid and the liquid, and c ) material constant which depends on the porous architecture of the solid. Experimentally, A is (37) Hildings, J.; Grulke, E. A.; Sinnott, S. B.; Qian, D.; Andrews, R.; Jagtoyen, M. Langmuir 2001, 17, 7540. (38) Neimark, A. V.; Ruetsch, S.; Kornev, K. G.; Ravikovich, P. I. Nano Lett. 2003, 3 (3), 419. (39) Gokhale, S. J.; Plawsky, J. L.; Wayner Jr., P. C.; DasGupta, S. Phys. Fluids 2004, 16 (6), 1942. (40) DasGupta, S.; Kim, I. Y.; Wayner, P. C., Jr. J. Heat Trans. 1994, 116, 1007. (41) Potash, M.; Wayner, P. C., Jr. Brit. J. Heat Mass Trans. 1972, 15, 1851. (42) Rulison, C. Augustine Scientific, Inc.: Newbury, OH (Private Communication). (43) Wiryana, S.; Berg, J. C. Wood Fib. Sci. 1991, 23 (3), 457. (44) Mokhtar, A.-A.; Amiri, O.; Dumargue, P.; Bouguerra, A. Mater. Struct. 2004, 37, 107. (45) Washburn, E. W. Phys. ReV. 1921, XVII (3), 273. (46) Dujardin, E.; Ebbesen, T. W.; Hiura H.; Tanigaki, K. Science 1994, 265, 1850. (47) Ebbesen, T. W. J. Phys. Chem. Solids 1996, 57 (6-8), 951.

obtained as the slope of the m2 versus t plot. Contact angle, θ, is then calculated using eq 5, knowing all other parameters. CVD grown MWNT were gently pressed into pellets. Each pellet was hung from a tared microbalance, via a thin wire, and was slowly brought down to touch the surface of each liquid. Hexane was used to determine the material constant, c, because it is known to be one of the lowest surface tension liquids and is known to fully wet most solid substrates. Contact angles of three different liquids were measured against the nanotube surface: toluene, MMA, and water. Toluene is a good solvent for organic polymers and is often used in the processing of nanotube-polymer composites by the solvent route.7 MMA has been used in the present analysis for infiltration into aligned nanotube arrays and subsequent polymerization. All of these measurements were carried out in duplicate and each individual experiment was performed on a fresh MWNT pellet. Based on the contact angle data of various liquids against the nanotube surface, the overall surface energy and surface polarity of nanotubes were experimentally measured, using the primary equation of Fowkes’ surface energy theory:42,49 (γLD)1/2(γSD)1/2 + (γLP)1/2(γSP)1/2 ) γL(cos θ + 1)/2 (6) where γLD is the dispersive component of the surface energy of liquid, γSD is the dispersive component of the surface energy of solid, γLP is the polar component of the surface energy of liquid, γSP is the polar component of the surface energy of solid, γL is the overall surface energy of the liquid, and θ is the contact angle of liquid against the solid surface. The dispersive and polar components of surface energies of MWNT, γSD and γSP, were obtained by solving two simultaneous equations, by using the values of γLP and γLD for MMA and toluene, each, in eq 6. Surface polarity (expressed in %) was obtained by comparing the polar component of surface energy of solid with its overall surface energy ∼(γSP/γS) × 100. The overall surface energy, γS ) γSP + γSD. Further, using eq 6, the contact angle of liquids such as molten PMMA and uncured PDMS resin against MWNT could be predicted (Table 1). The kinetics of infiltration is estimated in terms of the rate of flow of liquid through a capillary tube. The flow rate of liquid flowing through a small tube can be obtained using Darcy’s equation.50,51 Washburn45 modified Darcy’s equation,50,51 specifically for the flow of liquid through a capillary tube, as follows, u ) -(k/η)(δP/l) ) (r2/8η)(∆Pc)/l

(7)

where u is the velocity of liquid flowing through a tube, η is the viscosity of liquid, k is the permeability of the tube, and δP/l is the pressure gradient in the liquid across the tube length (l). It is assumed that the liquid permeates through a capillary, only under the capillary pressure.45 The pressure gradient, (δP/l), can be then expressed as (∆Pc)/l, where ∆Pc (48) Schuchardt, D. R.; Berg, J. C. Wood Fiber Sci. 1991, 23 (3), 342. (49) Du, F.; Fischer, J. E.; Winey, K. I. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 3333. (50) Nutting, P. G. Am. Assoc. Pet. Geo. Bull. 1930, 14, 1337. (51) Darcy, H. Les Fountaines des Ville de Dijon; Paris, Victor Dalmont, 1856; (original paper in French).

Carbon Nanotube-Polymer Composite Films

Chem. Mater., Vol. 17, No. 5, 2005 983

is the capillary pressure. The permeability of the tube is given by k ) r2/8, where r is the radius of the capillary tube. This equation can be used only as an approximation, assuming the liquid meniscus is moving at a certain distance into the capillary. Equation 4 can be further derived from eq 7 and, hence, eq 7 is called the Lucas-Washburn equation for flow of liquids through capillaries. Equation 4 is more useful for experimental measurements, where the change in weight of the sample can be measured with time. 2. Degree of Alignment of MWNT by Polarized Raman Spectroscopy. The degree of alignment of nanotubes before and after the infiltration of liquid was measured using polarized Raman spectroscopy.23,24,52-55 The measurements were performed with a Renishaw S2000 Raman spectroscope, using an argon-ion incident laser beam (514 nm) with a laser power of 10 mW and a detector data acquisition time of 30 s. The high-frequency in-plane graphitic stretching mode (E2g mode or G-band) peak, the disorder-induced mode (D-band) peak, and its second-order peak (D* mode) were monitored, with the focus on the intensities of D′ mode peak, centered at ∼2700 cm-1. The aligned nanotube array was placed at different angles, θ, relative to the polarization axis, from 0° to 90°, at the intervals of 10°, and the polarizer was kept parallel to the incident polarization (X-X). Four different points were measured for each sample direction. The peak intensities were determined by fitting the peaks with a mixed Gausian-Lorentzian function. The normalized Intensities of the D* mode peak were plotted against the angle of the nanotube axis with the polarization of the incident beam. The percentage of MWNT oriented in four different directions (φ) was estimated using eq 8: 23 M

R(θ) )

∑ NK[∫(-θ+φ K)1

(-θ+φK) K-1)

I(θ - O) d(θ - φ) + (-θ-φ ∫(-θ-φ )

K-1)

I(θ - O) d(θ - φ)] (8)

K

where I(θ - O) is the Raman intensity for a specific orientation and can be expressed as I(θ - O) ) ×a6[cos4(θ - φ)]

(9)

NK is the relative population of nanotubes aligned between (52) Rao, A. M.; Jorio, A.; Pimenta, M. A.; Dantas, M. S. S.; Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. Phys. ReV. Lett. 2000, 84 (8), 1820. (53) Gommans, H. H.; Alldredge, J. W.; Tashiro, H.; Par, J.; Magnuson, J.; Rinzler, A. J. Appl. Phys. 2000, 88 (5), 2509. (54) Duesberg, G. S.; Loa, I.; Burghard, M.; Syassen, K.; Roth, S. Phys. ReV. Lett. 2000, 85 (25), 5436. (55) Liu, T.; Kumar, S. Chem. Phys. Lett. 2003, 378, 257.

angles φK-1 and φK, M is the number regions of angles that are decided between 0° and 90°, and R(θ) is the overall measured intensity of the sample at a given position angle (θ). To find the degree of alignment of MWNT, four alignment angle regions were defined (M ) 4). The angle regions were given by (φ): 0°-22.5°, 22.5°-45°, 45°-67.5°, and 67.5°90°. The curve fitting of the measured R(θ) for the different sample positions, θ, was done by finding out four parameters NK (where K ) 1-4). For further information about this equation, the reader is referred to refs 23 and 24. For the present experiment, 1.5 mm long CVD grown aligned MWNT arrays were used.56 3. ThermograVimetry,31 Gel Permeation Chromatography and Differential Scanning Calorimetry. Thermogravimetric analysis (TGA) was used to estimate the weight fraction of MWNT in composite films prepared by the present method. The thermal stability of PMMA was also studied using TGA. Thermal degradation temperature, TD, was measured at the maximum mass loss rate from differential thermo-gravimetry (DTG) curve, which was obtained as the first derivative of the TGA curve. The TGA was performed using the MettlerToledo SDTA851 differential thermal analyzer. The sample was heated from 25 to 700 °C at 10 °C/min in air, with the flow rate of ∼50 mL/min. The samples were 3-5 mg in weight. The (TD) can be obtained from the peak of the differential thermogravimetry curve (DTG), which was obtained as a first derivative of the TGA curve. To determine the molecular weight of PMMA, gel permeation chromatography (GPC) was performedusing a Waters 515 HPLC pump coupled with the Waters Refractive Index detector. The solutions of PMMA from bulk control samples as well as from aligned MWNT/PMMA films were made by dissolving the respective samples in HPLC-grade tetrahydrofuran (THF) to make 0.1 wt % concentrations of PMMA in THF. Then these solutions were filtered twice through 0.2 µm PTFE filters, prior to analysis in the GPC. Differential scanning calorimetry (DSC) (Mettler-Toledo DSC822) was used to determine the glass transition temperature (Tg) of PMMA and the composites. In the first cycle, the sample was heated from 25 to 180 °C at the rate of 10 °C/min, soaked at 180 °C for 2 min, and then cooled to 25 °C at -10 °C/min. In the second cycle, the sample was first soaked at 25 °C for 2 min and then heated to 200 °C at 10 °C/min followed by cooling to 25 °C at -40 °C/min. All of the above tests were performed on two samples each. CM0485254 (56) Zhang, X.; Cao, A.; Wei, B.; Li, Y.; Wei, J.; Xu, C.; Wu, D. Chem. Phys. Lett. 2002, 362 (3-4), 285.