Synergistic Effects of Zirconia-Coated Carbon Nanotube on Crystalline

Nov 13, 2009 - Gyeongsang National University, Gyeongnam, Jinju, 660-701, South Korea. Received August 14, 2009. Revised Manuscript Received October ...
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Synergistic Effects of Zirconia-Coated Carbon Nanotube on Crystalline Structure of Polyvinylidene Fluoride Nanocomposites: Electrical Properties and Flame-Retardant Behavior Kaushik Pal,* Dong Jin Kang, Zhen Xiu Zhang, and Jin Kuk Kim Polymer Science and Engineering, School of Nano and Advanced Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju, 660-701, South Korea Received August 14, 2009. Revised Manuscript Received October 5, 2009 Pristine multiwalled carbon nanotubes (MWNTs) and zirconia-coated multiwalled carbon nanotubes (ZrO2/ MWNTs) by isothermal hydrolysis and the traditional chemical precipitation method have been dispersed into polyvinylidene fluoride (PVDF) copolymer by solution mixing in N,N-dimethylformamide (DMF). The effect of ZrO2coated MWNTs on morphological properties, electrical properties, and flame-retardant behavior has been studied in comparison with virgin PVDF and PVDF/MWNTs nanocomposites. Due to the improved dispersion of the coated nanotubes, the incorporation of 3 wt % of ZrO2-coated MWNTs leads to an increase of the thermal stability and dielectric properties and a decrease of the peak heat-release rate.

Introduction Since their discovery by Iijima,1 carbon nanotubes (CNTs) have been attractive materials for fundamental research studies and have become one of the most important materials in 21st century technology. Several applications have been proposed for CNTs, many of which are concerned with conductive or highstrength composites,2,3 in which the inclusion of CNTs in a ceramic matrix is expected to produce composites possessing high stiffness and improved mechanical properties compared to the single-phase ceramic material and have already been used as nano probes, gas storage containers, nano electronic devices, sensors, composite reinforcements, and integrated interconnection due to their extraordinary properties.4-7 Recently, it has been interesting to use carbon nanotubes at low loading content to obtain materials with enhanced mechanical properties and reduced *Corresponding author. Dr. Kaushik Pal Polymer Science and Engineering, School of Nano and Advanced Materials Engineering, Gyeongsang National University, Gyeongnam, Jinju, 660-701, South Korea. Tel: þ82-55751-5299; Fax: þ82-55-753-6311. E-mail: [email protected].

(1) Iijima, S. Nature 1991, 354, 56. (2) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (3) Dai, H. Surf. Sci. 2002, 500, 218. (4) Zhu, L.; Sun, Y.; Hess, D. W.; Wong, C. P. Nano Lett. 2006, 6, 243. (5) Chan, R. H. M.; Fung, C. K. M.; Li, W. J. Nanotechnology 2004, 1, 5. (6) Yang, S. B.; Song, H. H.; Chen, X. H.; Okotrub, A. V.; Bulusheva, L. G. Electrochim. Acta 2007, 52, 5286. (7) Ding, M. Q.; Shao, W. S.; Li, X. H.; Bai, G. D.; Zhang, F. Q.; Li, H. Y.; Feng, J. J. Appl. Phys. Lett. 2005, 87, 233118. (8) Peeterbroeck, S.; Laoutid, F.; Taulemesse, J.-M.; Monteverde, F.; LopezCuesta, J.-M.; Nagy, J. B.; Alexandre, M.; Dubois, P. Adv. Funct. Mater. 2007, 17, 2787. (9) Kashiwagi, T.; Du, F.; Douglas, J. F.; Winey, K.; Harris, R. H., Jr.; Shields, J. R. Nat. Mater. 2005, 4, 928. (10) Bourbigot, S.; Duquesne, S.; Jama, C. Macromol. Symp. 2006, 233, 180. (11) Peeterbroeck, S.; Laoutid, F.; Swoboda, B.; Lopez-Cuesta, J.-M.; Moreau, N.; Nagy, J. B.; Alexandre, M.; Dubois, P. Macromol. Rapid Commun. 2007, 28, 260. (12) Bredeau, S.; Peeterbroeck, S.; Bonduel, D.; Alexandre, M.; Dubois, P. Polym. Int. 2008, 57, 547. (13) Beyer, G. Fire Mater. 2002, 26, 291. (14) Kashiwagi, T.; Grulke, E.; Hilding, J.; Harris, R.; Awad, W.; Douglas, J. Macromol. Rapid Commun. 2002, 23, 761. (15) Beyer, G. Gummi Fasern. Kunstst. 2002, 55, 596. (16) Kashiwagi, T.; Grulke, E.; Hilding, J.; Groth, K.; Harris, R.; Butler, K.; Shields, J.; Kharchenko, S.; Douglas, J. Polymer 2004, 45, 4227.

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flammability.8-16 Considering different ways of preparing CNT/ZrO2 nanocomposites, Rao et al. managed to fabricate hollow nanotubes of zirconia using CNTs as templates.17 Luo et al. obtained zirconia powder with an average particle size of 10 nm by adding CNTs in the hydrolytic process of ZrO(NO3)2.18 Lupo et al. prepared zirconium oxide/carbon nanotube composites by hydrothermal crystallization of zirconium hydroxide in the presence of carbon nanotubes at 200 °C.19 Shan et al. synthesized phase-controllable CNT/ZrO2 nanocomposites by hydrothermal treatment of MWNTs in ZrOCl2 3 8H2O aqueous solution at 150 °C, demonstrating homogeneous coverage of MWNTs with ZrO2 nanoparticles.20 Lu et al. showed a new and simple method to deposit ultrathin conformal ZrO2 coating on the surface of MWNTs.21 Also, a review article regarding the modification of CNT has been published recently.22 However, one of the major challenges is actually to easily and individually disperse these nanotubes in polymer matrices to obtain improved materials properties. The piezoelectricity and dielectric property of polyvinylidene fluoride (PVDF) and its polymeric composites have been extensively studied, since their electric properties were discovered in the past.23-26 However, polymers have low dielectric constants compared with ceramic and metal materials, generally used for high charge-storage capacitors, electrostriction systems for artificial muscles, and energy-storage devices. The frequencydependent dielectric constant, dielectric losses of PVDF, and (17) Rao, C. N. R.; Satishkumar, B. C.; Govindaraj, A. Chem. Commun. 1997, 1581. (18) Luo, T. Y.; Liang, T. X.; Li, C. S. Mater. Sci. Eng., A 2004, 366, 206. (19) Lupo, F.; Kamalakaran, R.; Scheu, C.; Grobert, N.; Ruhle, M. Carbon 2004, 42, 1995. (20) Shan, Y.; Gao, L. Nanotechnology 2005, 16, 625. (21) Lu, Y.; Zang, J. B.; Shan, S. X.; Wang, Y. H. Nano Lett. 2008, 8, 4070. (22) Rao, C. N. R.; Govindaraj, A. Adv. Mater. 2009, 21, 1. (23) Yao, S.-H.; Dang, Z.-M.; Jiang, M.-J.; Xu, H.-P.; Bai, J. Appl. Phys. Lett. 2007, 91, 212901. (24) Li, Q.; Xue, Q.; Hao, L.; Gao, X.; Zheng, Q. Compos. Sci. Technol. 2008, 68 (10-11), 2290. (25) Ho, C. H.; Liu, C. D.; Hsieh, C. H.; Hsieh, K. H.; Lee, S. N. Synth. Met. 2008, 158, 630. (26) Dang, Z.-M.; Nan, C. W. Ceram. Int. 2005, 31, 349.

Published on Web 11/13/2009

DOI: 10.1021/la903022j

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PVDF/MWNT have been reported by many investigators.27-29 A literature search shows a lack of studies about small amounts of ZrO2-coated MWNTs as flame-retardant systems using the PVDF matrix alone. With this work, we pretend to partially fill this gap. In this paper, PVDF with pristine MWNT and MWNT/ZrO2 nanocomposites were fabricated by a very simple and effective route for the compounding with excellent physical, thermal, electrical, and dielectric properties by the solution mixing process. Their homogeneous dispersion of Zr-coated MWNT in PVDF matrix and its effects on the dielectric and flame-retardant properties are studied. This work demonstrates, on one hand, the significant effect of the ZrO2 coating on the electrical behavior of the thus-obtained nanocomposites and explains, on the other hand, the flame-retardant efficiency of MWNTs in PVDF nanocomposites. An original mechanism related to the action of the MWNTs during the combustion process is proposed.

Experimental Methods Materials and Sample Preparation. The MWNTs prepared by chemical vapor deposition were purchased from Hanwha Nanotech Co. (Republic of Korea). Their outer diameter and length were 10-30 nm and 10-20 μm on average and purities of more than 95%. A commercially available polyvinylidene fluoride powder (SOLEF 1013, Tm = 172 °C, MI = 0.2) was supplied by Solvay, Inc. (Belgium). ZrOCl2 3 8H2O was employed as the precursor for the synthesis of ZrO2 coating from Sigma-Aldrich, USA. Solid ZrOCl2 3 8H2O was first dissolved in 100 mL of distilled water to produce 0.2 mol/L solution under magnetic stirring. Then, 30 mg of MWNTs without any pretreatment were added into the aqueous solution. After 30 min ultrasonic vibration, a black suspension with MWNTs homogeneously dispersed was obtained. The stable aqueous suspension was then reflux condensed in a thermostatic water bath at the temperature of 100 °C, ensuring the isothermal hydrolysis of ZrOCl2. During this hydrolytic process, the suspension was ultrasonicated for 10 min every 24 h to get a good dispersion of MWNTs in the aqueous solution. After approximately 72 h, the black suspension turned gray. For comparison, a conventional chemical precipitation method was also used to prepare MWNT/ZrO2 nanocomposites. By ultrasonic vibration, 30 mg of MWNTs were homogeneously dispersed into 0.2 mol/L of ZrOCl2 3 8H2O aqueous solution to acquire the same stable black suspension. Under vigorous stirring, an appropriate amount of NH4OH was added drop by drop into the above 100 mL suspension. After that, the whole mixture was further magnetically stirred for 60 min, a stable well-proportioned MWNT/ZrO2 composite suspension was obtained.30 After that, PVDF is mixed in N,N-dimethylformamide (DMF, density is 0.947 g/mL at 20 °C, Junsei Chemical Co. Ltd., P. R. China) solution under vigorous magnetic stirring for 2 h at 40 °C. Then, pristine MWNT and Zr-coated MWNT with 3 wt % loading is added with PVDF solution, and the solution is ultrasonicated for 30 min to get the homogeneous dispersion of pristine MWNT and Zr-coated MWNTs in PVDF solution. Afterward, the whole solution was dried at room temperature for approximately 72 h to obtain the PVDF/pristine-MWNTs and PVDF/Zrcoated MWNTs nanocomposites. The same process is followed for pristine PVDF without MWNT also. Also, film samples (with thickness of about 0.2 mm) were prepared using the doctor blade.

(27) Bai, Y.; Cheng, Z. Y.; Bharti, V.; Xu, H. S.; Zhang, Q. M. Appl. Phys. Lett. 2000, 76(25), 3804. (28) Wang, L.; Dang, Z.-M. Appl. Phys. Lett. 2005, 87, 042903. (29) Li, Q.; Xue, Q.; Zheng, Q.; Hao, L.; Gao, X. Mater. Lett. 2008, 62, 4229. (30) Schartel, B.; Potschke, P.; Knoll, U.; Abdel-Goad, M. Eur. Polym. J. 2005, 41, 1061.

3610 DOI: 10.1021/la903022j

Results and Discussion The morphology observation of ZrO2-coated MWCNT was performed on a JEM-2010(JEOL) high-resolution transmission electron microscope (HR-TEM) at 200 kV, and the images were recorded digitally with a charge-coupled device camera. HRTEM images of MWNT/ZrO2 nanocomposites prepared by isothermal hydrolysis and chemical precipitation method indicate that the MWNTs are coated with ZrO2 particles and they appear undamaged by the isothermal treatment. This is in agreement with studies on the stability of MWNTs under isothermal conditions. However, the ZrO2 coating is uniform throughout the MWNT in the isothermal process. The formation of ZrO2 coating is nonhomogeneous and nonuniform in the chemical precipitation process because the reaction is completed in less than 10 min when dropping NH4OH into the suspension. The nucleation and growth of the species tend to occur in the suspension rather than on the surface of MWNTs. Also, we observed that the zirconia particles tend to precipitate on and around the carbon nanotubes. We conjecture that, initially, small zirconia particles nucleate on the nanotube surface. These crystallites grow into larger clusters which may eventually detach, leaving only very small particles attached to the surface. However, it is not clear if the precipitation of zirconia occurs exclusively on the nanotube surfaces. Since hydrothermal conditions are conducive for the precipitation of zirconia from its hydroxide solution, it is not possible to rule out some random nucleation and growth of the zirconia clusters during the reaction.19 The process is terminated by the complete conversion of the hydroxide to oxide under the hydrothermal conditions. The end material consists of a homogeneous mixture of CNTs and ZrO2, with the ZrO2 particles attached to the CNTs. The hydrothermal crystallization of ZrO2 is known to result in particles with controlled properties, such as particle size, shape, and crystallinity. It is conjectured that the defects on the walls of the nanotubes might act as nucleating and growth centers for the ZrO2 particles. However, deeper investigations to establish the nature of the interface are required in the future. Field emission scanning electron microscopy (FE-SEM) is performed by Philips XL-20 SEM at an accelerating voltage of 10 kV. It is used to observe the morphology of the cryo-fractured surfaces (fractured by mechanical force) of PVDF/MWNT nanocomposites. Samples are sputtered with gold-palladium prior to testing. It is visible from the FE-SEM images that the outside surface of the coated MWNT by chemical precipitation are not as smooth and clean as MWNT coated by the isothermal hydrolysis method. It demonstrates that the MWNT is actually sheathed by an ultrathin ZrO2 layer. It is seen that some ZrO2 aggregates not only adhere to the sidewalls of MWNTs, but also distribute among them as clusters in the sample prepared by chemical precipitation. It also clearly illustrates the periodic multiwalls of the MWNT and the crystalline ZrO2 coating. The outer diameter of the MWNT after ZrO2 coating in the isothermal and chemical decomposition processes is 1.35 μm and 916 nm, respectively. The ZrO2 coating deposited by isothermal hydrolysis is not common hydrolysis-acquired amorphous zirconium hydroxide but monoclinic crystalline zirconia. Figure 1 shows the FE-SEM image of PVDF with MWNT/ ZrO2 nanocomposites and the corresponding HR-TEM images of the selected parts marked with circles. Further detailed structure of ZrO2 on MWNT in the PVDF matrix is realized in the FE-SEM images of two indicated parts of Figure 1b,c. In Figure 1, composites have 3 wt % of pristine MWNTs and ZrO2/MWNT nanocomposite loading show the good dispersion Langmuir 2010, 26(5), 3609–3614

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Figure 1. (a) FE-SEM images of PVDF with Pristine MWNT. (b) FE-SEM images of PVDF with MWNT/ZrO2 nanocomposites prepared by isothermal hydrolysis. (c) FE-SEM images of PVDF with MWNT/ZrO2 nanocomposites prepared by chemical precipitation.

and distribution in the matrix, despite some aggregation in the matrix. However, the composite with pristine MWNT loading shows some poorer dispersion and distribution properties than other composites and the MWNTs are observed to be 70-100 nm in diameter. That is mainly occurred due to the use of high MWNT concentrations and those are lumped together by van der Waals forces and interrupt the mobility of polymer molecules. Also, these retained and undistributed MWNTs in polymer matrix are a result of higher viscosity of PVDF. However, for isothermal and chemically treated ZrO2/MWNTs, adhesion is not confined to PVDF; in addition, the outer diameter of the Langmuir 2010, 26(5), 3609–3614

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MWNTs is also increased. Also, from Scheme 1, we can assume that PVDF forms β-phase crystals with ZrO2-coated MWNT, which is confirmed by the XRD results (Figure 5). It can be explained by the FESEM images that the outer diameter of ZrO2coated MWNT with PVDF composites increases because ZrO2 promotes PVDF for wrapping the outer surface of the MWNTs also. On the basis of the results of Figure 1, we can conclude that MWNTs can disperse well into the PVDF matrix, in which serious aggregation of MWNT bundles was absent. DSC is performed on PVDF/MWNT samples using a DSC 300 F3 (NETZSCH, Germany). Heat flow is monitored over the range of -60 to 230 °C with temperature modulation ((0.8 °C every 60 s) superimposed on a 10 °C/min heating and cooling rate under purge gas (nitrogen at 40 mL/min). The heating scan thermograms of PVDF and PVDF/ZrO2-MWNT nanocomposites are shown in Figure 2a. The pristine PVDF samples produce a main melting peak at 167 °C (Table 1). However, the addition of MWNTs produce the shoulder posterior to the main melting peak and an increasing end point of the peak. This phenomenon is attributed to the presence of two morphologically different crystallites. The β-phase crystal with all-trans conformation gives a higher endotherm than the R-phase crystal with TG-TG conformation. This is in good agreement with WAXD results. On the other hand, as shown in Figure 2b, the loading of MWNT has little effect on the crystallization temperature of PVDF. However, with the addition of pristine-MWNT loading, Tc values increase from 141.2 to 148.9 °C. This suggests that pristine MWNT acts as a nucleation agent for PVDF in this study. According to Nishi-Wang theory, the interaction parameter between the blend components can be quantitatively correlated to the slope of melting point versus the square of the MWNTs volume fraction curve. The more negative the slope is, the higher the miscibility. As is evident from Figure 2, the slope of melting point variation is most negative in PVDF/ZrO2-coated MWNTs, indicating higher miscibility between PVDF and ZrO2-coated MWNTs. Thermogravimetric analysis (TGA Q500, TA Instruments) is carried out to study the thermal stability of each PVDF/MWNT nanocomposite from room temperature to 650 °C at a heating rate of 10 °C/min under purge gas (nitrogen at 40 mL/min). Figure 3 shows the TGA thermograms of PVDF with ZrO2coated MWNT composites. The thermodegradation of PVDF and PVDF nanocomposites takes place in two steps. First, onset degradation is observed between 50 and 100 °C with the production of gaseous fluoride and the formation of carbon-carbon double bonds along the polymer backbone. The onset degradation temperature of PVDF is around 405 °C, but with the addition of MWNTs, the onset degradation temperature increases. In the second degradation step (between 400 and 500 °C), the unsaturated chains are volatilized through static chain cleavage. The presence of the carbon nanotubes modifies the thermal degradation of the polymer matrix (Figure 3), which presents the evolution of the mass loss with temperature of the two nanocomposites. The second step of the thermal degradation also takes place on the higher temperature side in the presence of MWNTs. This region is highly dependent on the types of MWNTs because the mass loss becomes higher with ZrO2/MWNT by isothermal hydrolysis. On the other hand, Figure 3 also suggests that the MWNT has good affinity to the PVDF region in the PVDF/MWNT nanocomposites, indicating that the MWNT is dominantly dispersed in the PVDF matrix. As the MWNT region increases, thermal properties of the PVDF/MWNT nanocomposites have been enhanced because the MWNT possesses good thermal properties. This behavior could be explained by the presence of the char DOI: 10.1021/la903022j

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Figure 2. (a) Endotherm peak of pristine PVDF and PVDF based nanocomposites. (b) Exotherm peak of pristine PVDF and PVDF based nanocomposites. Scheme 1. Schematic Diagram Shows the Assumed Mechanism for the Formation of β-Phase Crystal for PVDF/ZrO2-Coated MWNT Composites

Table 1. Summary of DSC Studies and Maximum Peak of Heat-Release Rate, as Measured by the Cone Calorimeter Test (50 kW m-2), for pristine PVDF and the Three Related Nanocomposites sample code

Tm (°C)

Hm (J/g)

crystallinity (%)

ΔH (J/g)

PHRR [kW m-2]

A - PVDF B - PVDFþ pristine-MWNT C - PVDFþ ZrO2 coated MWNT (Isothermal) D - PVDFþ ZrO2 coated MWNT (Chemical)

167.03 172.31 172.02 171.82

62.20 64.11 62.55 59.22

30.03 30.96 30.20 28.59

41.87, 141.22 38.99, 148.91 42.05, 145.71 38.02, 143.53

690 320 292 298

formed from the PVDF matrix during the first degradation step, which is further stabilized through π-π electronic interactions with the coated nanotubes.8 The degradation temperature of PVDF/MWNT composites is increased with pristine PVDF, but decreased with ZrO2-coated MWNT. In order to confirm this hypothesis, the flammability properties of the materials have been studied by cone calorimeter testing with ASTM E 1354/ISO 5660 under a heat flux of 50 kW m-2. The development of the heat-release rate (HRR), in particular, its maximum peak (PHRR), is discussed. The averaged cone calo3612 DOI: 10.1021/la903022j

rimetry experimental results for five samples of PVDF and the corresponding nanocomposites filled with 3 wt % of either MWNTs or ZrO2-coated MWNTs by isothermal hydrolysis and traditional chemical precipitation methods are presented in Figure 4 and Table 1. The flame-retardant behavior for virgin PVDF, the PHRR, reaches a value of around 690 kWm-2, and combustion is complete after 400 s and shows a rather abrupt flame-out, which was indicated by a quick decrease of the heat release rate to zero. The sample shows very strong bubbling during the combustion. A slightly different behavior is observed Langmuir 2010, 26(5), 3609–3614

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Figure 5. XRD study of pristine PVDF and PVDF-based nanocomposites. Figure 3. TGA studies of pristine PVDF and PVDF-based nanocomposites.

Figure 4. Cone calorimeter heat-release rate versus time of pristine PVDF and PVDF-based nanocomposites.

when pristine MWNTs are used in PVDF matrix. The flame-out of the PVDF/pristine-MWNT was not as abrupt and was followed by thermo-oxidative decomposition of the MWNT, accompanied by a slight heat release rate for some minutes. Hence, the fire behavior of PVDF with pristine MWNT composites was influenced more by the consolidation of an interconnected network structure than by a growing layer of closed surface.30 The maximum HRR decreases (320 kW m-2) and the combustion time increases (>800 s). With the incorporation of 3 wt % MWNTs (by isothermal hydrolysis and chemical precipitation) in PVDF, the result is totally different. The heat release rate increases up to a plateau value, which is nearly constant until flame-out. The formation of an intensive carbonaceous protective layer during the combustion test performed on the PVDF/ MWNT nanocomposites is most likely responsible for a decrease in the ultimate HRR value. Another interesting effect of the coated MWNTs by ZrO2 has been observed on the cohesion of the combustion residue. The same feature has already been discussed previously.8 The residue from the coated MWNTs has shown a uniform and cohesive type of residue with small cracks, whereas the PVDF with pristine MWNT has shown a vast crust type of carbonaceous residue with deep cracks. A more detailed characterization regarding FESEM and XRD is needed to understand the char formation of the Langmuir 2010, 26(5), 3609–3614

nanocomposites. In spite of excellent dispersion of the nanotube in the PVDF matrix as demonstrated by FESEM and XRD analyses, this leads to a strong decrease of PHRR, which reaches a value of ca. 292 and 298 kW m-2 (Table 1). The combustion time is also much longer (>900 s). It is also noteworthy that the diameter of the coating is increased in the isothermal hydrolysis process rather than the traditional chemical precipitation method. For this, the formation of an intensive ZrO2 protective layer during the combustion test performed on the PVDF/ZrO2-coated MWNT nanocomposites by isothermal hydrolysis is most likely responsible for the observed reduction of PHRR than PVDF/ZrO2-coated MWNT nanocomposites by traditional chemical precipitation. The absolute difference between polymer and MWNT nanocomposites in time to heat release rate decreases. Since the thermal analysis did not carry any convincing argument as to why the heat release rate decreases, e.g., a temperature shift or a change in the mass loss rate, it is concluded that a change in physical properties such as improved thermal conductivity may cause an improvement in terms of heat release rate. XRD study is performed on the samples using WAXD machines. The change of composite crystallinity is measured on a Bruker AXS X-ray diffractrometer (Germany). WAXD is used to observe the effect of pristine and ZrO2-coated MWNT content on the microstructure of pristine PVDF. Figure 5 describes the WAXD patterns for pristine PVDF and PVDF/MWNT nanocomposites. Within a given range of scattering angles, three characteristic diffraction peaks appear at 2θ values of 17.7°, 18.4°, and 19.9°, respectively, which correspond to (100), (020), and (110) reflections, respectively. This is assigned to the R-phase crystal, which has a nonpolar trans-gauche-trans-gauche (TG-TG) conformation.31 PVDF with pristine MWNT nanocomposite exhibits a decrease in peaks for the R-phase crystal. In addition to the features associated with β-phase crystal, the introduction of MWNTs produces a shoulder at a 2θ value of 20.7°, and it is clear with ZrO2-coated MWNTs also. This is attributed to the formation of a β-phase crystal, which has alltrans conformation (Scheme 1). This may be due to better interaction between the MWNTs and matrix surfaces resulting in improved adhesion between them at the interface, which in turn favors the crystal growth mechanism. As one can observe, the incorporation of the filler causes a reduction in the intensity of the R-phase characteristic peaks, especially for the peak at 26.5°. (31) Nam, Y. W.; Kim, W. N.; Cho, Y. H.; Chae, D. W.; Kim, G. H.; Hong, S. P.; Hwang, S. S.; Hong, S. M. Macromol. Symp. 2007, 249, 478.

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permittivity. According to Debey’s theory, the decrease of viscosity between dipole and neighboring medium in accordance with increase of temperature affected a reduction in the relaxation time of the permanent dipole, so that dielectric absorption moves to a high frequency.33 The dielectric property of the PVDF with ZrO2coated MWNTs is better than that of pristine MWNT, which is confirmed by the XRD results, because the crystal form of PVDF with ZrO2 coating transfers from R-form to β-form, which in turn increases the dielectric constant. Also, it is remarkable that ZrO2 is a metallic material, which gives the free electron in the β-form crystalline formation.

Conclusions

Figure 6. Dielectric properties of pristine PVDF and PVDF-based nanocomposites.

Also, this effect is more remarkable for MWNTs.32 In the extreme case of the PVDF with ZrO2-coated MWNT composite, such an effect is very clear. Figure 5 shows that drawing of a PVDF film causes orientation of the chain molecules and the change of the crystal structure from the R-phase crystal form to β-phase crystal conformation. It is also noteworthy that interplanar distance corresponding to every peak position increases in the case of modified PVDF with MWNTs, which again supports the nucleating ability of nanotube into PVDF systems and supports the results obtained from DSC study. The dielectric measurements are carried out using a programmable automatic impedance analyzer (Agilent E 4294A) in the frequency range from 1 kHz to 10 MHz at room temperature. The cell is calibrated using air and Teflon sheet as standard references. The dielectric properties of the PVDF/MWNTs are taken at off voltages. Dielectric constant is measured as a function at different frequencies and as shown in Figure 6. Dielectric constants (ε0 ) are tremendously improved with the incorporation of both the ZrO2-coated MWNTs. Ionic conduction phenomena are slightly observed with ZrO2-coated MWNTs. This result implies that the PVDF resin filled with ZrO2-coated MWNTs could be a prospective actuator, supercapacitor material since it can enhance the conductivity and (32) Hong, S. M., Nam, Y. W., Sang, S. H., Chae, D. W. Mol. Cryst. Liq. Cryst. 2007, 464, 195 [777]. (33) Li, Q.; Xue, Q.; Zheng, Q.; Hao, L.; Gao, X. Mater. Lett. 2008, 62, 4229.

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In summary, we present a new approach to ZrO2-coated nanotubes in PVDF matrix, leading to material with enhanced thermal properties, electrical properties, and thermo-oxidative stability. Interestingly, a homogeneous dispersion of the ZrO2coated nanotubes has been observed throughout the polymer matrix. The presence of the carbon nanotubes modifies the thermal stability, electrical properties, and flame retardant properties of the polymer matrix: the onset degradation steps of the PVDF matrix are shifted to higher temperature, as observed by TGA in nitrogen. As determined by cone calorimeter testing, two important parameters are influenced by adding coated MWNTs to PVDF: heat release rate, which is decreased, and dielectric properties, which are largely improved. It is also significant that the nanotube surface coating by isothermal hydrolysis slightly improved the reduction of the heat-release rate with respect to the value recorded for nanotube surface coating by the traditional chemical precipitation method. This phenomenon can be attributed to the high quality of dispersion of the ZrO2-coated nanotubes in the polymer matrix. Acknowledgment. K. Pal acknowledges I-Cube Centre, Gyeongsang National University, South Korea, for BK-21 fellowship award for support of this research. Supporting Information Available: HR-TEM images of overall view of ZrO2-coated MWNTs by isothermal hydrolysis and chemical precipitation method, FE-SEM images of PVDF with MWNT/ZrO2 nanocomposites prepared by isothermal hydrolysis and chemical precipitation in PVDF matrix, FE-SEM images of pristine PVDF. This material is available free of charge via the Internet at http:// pubs.acs.org.

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