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Control of multiwall carbon nanotubes dispersion in polyamide6 matrix: An assessment through electrical conductivity
Chemical Physics Letters 432 (2006) 480–485 www.elsevier.com/locate/cplett
Control of multiwall carbon nanotubes dispersion in polyamide6 matrix: An assessment through electrical conductivity Pravin V. Kodgire a, Arup R. Bhattacharyya b,*, Suryasarathi Bose b, Nitin Gupta b, Ajit R. Kulkarni b, Ashok Misra a b
a Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Received 18 August 2006; in final form 19 October 2006 Available online 25 October 2006
Abstract The homogeneous dispersion of carbon nanotubes (CNT) remains a hindrance in exploiting the exceptional properties associated with CNT in polymer/CNT composites. Here we present for the first time the key role of sodium salt of 6-aminohexanoic acid (Na-AHA) in assisting debundling the multiwall carbon nanotubes (MWNT) through specific interactions leading to homogeneous dispersion within polyamide6 (PA6) matrix during melt-mixing. The composite fabricated via this route exhibits low electrical percolation threshold of 0.5 wt% at room temperature, the lowest reported value in this system so far. FTIR and Raman spectroscopy reveal the existence of ‘cation–p’ interaction between Na-AHA and MWNT. The phenomenon of reactive coupling between amine functionality of NaAHA and acid end group of PA6 during melt-mixing is also established. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Polymer composites containing carbon nanotubes (CNT) have emerged as advanced multifunctional materials in view of exceptional mechanical, thermal and electrical properties associated with CNT [1]. Apart from mechanical reinforcement [2,3], one of the key interests is to develop conductive polymer composites [4–6] preferably at low concentration of CNT utilizing their high aspect ratio (L/D) for numerous applications, which include antistatic devices, capacitors and materials for EMI shielding. However, with few exceptions [7–9] uniform dispersion of CNT in polymer matrix is restricted due to aggregate formation irrespective of the techniques employed for composite preparation. This is presumably due to (i) strong inter-tube van der Waals interaction and (ii) lack of interfacial interaction between the polymer and the CNT.
*
Corresponding author. Fax: +91 22 2576 3480. E-mail address: [email protected] (A.R. Bhattacharyya).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.10.088
In connection with the method of composites preparation, solvent casting of polymer/CNT suspensions and in situ polymerization lead to better dispersion of CNT as compared to melt-mixing, which is otherwise technologically more viable. It is also observed that the application of shear during melt-mixing promotes the breakdown of the ‘aggregate’ [10]. It is well established that ‘network-like structure’ [11,12] of CNT is predominantly responsible for electrical percolation in insulating polymer matrices. However, the electrical percolation threshold in polymer/CNT composites is found to be much higher in case of thermoplastic matrices [13,14] as compared to theoretically predicted percolation threshold considering L/D of CNT as one of the critical parameters which is otherwise found to be close to the predicted values in thermoset (epoxy, polyimide etc.) filled system [7–9]. Additional complexity arises in case of semi-crystalline matrices (polypropylene [14], polyethylene [15], polyamides [16]) where it is envisaged that the dispersion of CNT is significantly affected due to the crystallization induced phase separation and subsequent rejection of CNT by the advancing crystalline fronts.
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This is manifested in much higher electrical percolation threshold in multiwall carbon nanotubes (MWNT) filled polyamide6 (PA6) composites [16,17]. Even if conductivity measurements during melt-state indicated a very low electrical percolation threshold (0.0025 < / < 0.01) in MWNT filled polypropylene (PP) composites [11], the room temperature measurement of PP/MWNT composites [14] showed higher electrical percolation threshold (1–2 wt%). These observations necessitate the retention of ‘networklike structure’ of MWNT in polymer matrix even after solidification, which can otherwise be observed in meltstate at low volume fraction of MWNT. In this work a novel modifier (sodium salt of 6-aminohexanoic acid, Na-AHA) has been prepared to induce improved dispersion of MWNT in the melt-mixed PA6/ MWNT composites. The state of dispersion has been assessed through AC electrical conductivity measurements.
0 < n < 1; A is the constant dependent on the temperature, and n is an exponent dependent on both frequency and temperature). Thermo gravimetric analysis (TGA) was performed using powder samples of AHA and Na-AHA using Netzsch (STA 409 PC) thermo gravimetric analyzer in inert atmosphere with heating rate of 10 K min 1. FTIR spectroscopic analysis was carried out with Nicolet, MAGNA 550 for powder samples of AHA and Na-AHA using KBr pellets (thin films in case of composites) at room temperature in the scanning range of 400–4000 cm 1. Raman spectroscopy was performed using Jobin Yovon (HR 800 micro-Raman) on powder samples of purified and NaAHA modified MWNT over a scanning range of 200– 2000 cm 1 with incident laser excitation wavelength of 514 nm. Transmission electron microscopy (TEM) analysis was performed with Philips CM 200, operated at 200 kV.
2. Experimental
3. Results and discussion
2.1. Materials and composite preparation
The frequency dependence electrical conductivity at room temperature (20 °C) for PA6/MWNT composites with various weight content of MWNT is depicted in Fig. 1. As expected for insulating materials, the bulk conductivity of pure PA6 increases with increase in frequency with a value about 10 13 S cm 1 at 0.01 Hz. PA6/MWNT composites with 2 wt% MWNT content show a frequency independent plateau (DC conductivity) up to a critical frequency (xc) above which the conductivity dispersion is observed. This is consistent with ‘Johnscher Universal Power Law’ for frequency dependent conductivity of solids [18]. An abrupt increase in conductivity is observed for PA6/MWNT composites at 3 wt% MWNT where the electrical conductivity is independent over a wide range of frequency indicating the formation of percolating ‘network-like structure’ of MWNT. It is further observed that
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Conductivity, σ (S cm-1)
Polyamide6 (PA6, zero shear viscosity = 179.2 Pa s at 260 °C) was obtained from GSFC, Gujarat, India (Gujlon M28RC, relative viscosity 2.8, Mv is 38642 in 85% formic acid). Purified multiwall carbon nanotubes (MWNT) were obtained from Nanocyl CA Belgium (L/D: 10–1000, purity > 95%). 6-aminohexanoic acid (AHA) (Sigma Aldrich, Mw = 132.18; purity: 98%) was neutralized using sodium hydroxide (Sisco Research Laboratories, India, purity: 98%) to obtain Na-AHA. MWNT were initially sonicated in distilled water for 20 min. Required amount of NaAHA solution (to reach the desired weight ratio of x:y, where x = MWNT and y = Na-AHA) was then added to the MWNT and again sonicated for 10 min. The NaAHA modified MWNT solution was then subjected to evaporation. The obtained dry powder was then left in a vacuum oven at 80 °C for 3 h to ensure the complete removal of water. PA6/MWNT (either purified or Na-AHA modified) composites were prepared by melt-mixing in a conical twin-screw extruder (Micro 5, DSM Research, Netherlands) at 260 °C with a rotational speed of 150 rpm for 5 min except for 4:1 (Na-AHA/MWNT, 1 wt% MWNT content) and 15:1 (Na-AHA/MWNT, 0.5 wt% MWNT content) MWNT modified PA6 composites where the mixing time was 10 min. All the samples were vacuum dried over night at 80 °C. Compression moulded films of 0.5 mm thickness were used for electrical conductivity measurements.
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2.2. Characterization -14
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The AC conductivity measurements were performed in the frequency range 10 2 and 107 Hz using Alpha high resolution analyzer. The DC conductivity of the samples was determined by fitting power law (rAC = rDC + Axn,
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Frequency (Hz) Fig. 1. Frequency dependence AC conductivity of PA6/MWNT composites.
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MWNT / Na-AHA (1:8)
MWNT / Na-AHA (1:4)
% Transmitance
for these composites the value of critical frequency increases with increasing MWNT content. Above the percolation threshold (>3 wt%), the increase in conductivity is marginal. We observe electrical percolation threshold between 2 and 3 wt% of MWNT in melt-mixed PA6/ MWNT composites utilizing purified MWNT. Even if this value is much lower as compared to the reported values of 4–6 wt% in PA6/MWNT composites [16,17], this observation cannot be explained solely on the basis of L/D of MWNT. Considering the theoretically predicted percolation threshold, the observation of percolating network between 2 and 3 wt% in PA6 matrix indicates the existence of aggregated MWNT structure. In view of the existing challenges regarding the dispersion of MWNT in semi-crystalline polymer matrix, we have developed an ionic modifier containing reactive functionality to control the dispersion of MWNT in PA6 matrix. NaAHA has been prepared in order to establish ‘cation–p’ interaction with MWNT, and also to exploit the concept of reactive coupling between the amine group of NaAHA and acid end groups of PA6 during melt-mixing. It is expected that ‘cation–p’ interaction would lead to initial debundling of MWNT, which will be retained in the solidstate as well due to melt interfacial reaction. In this context, use of imidazolium salts in controlling the dispersion of either SWNT [19] or MWNT [20] was reported. In addition the use of ionic additive was also reported to induce uniform dispersion of carbon black in epoxy matrix in the past [21,22]. The use of styrene maleic anhydride copolymer (SMA) as a reactive modifier was also reported to induce improved dispersion of single wall carbon nanotubes (SWNT) in PA6 matrix [23], however encapsulating SMA layer around SWNT restricted the increase in conductivity. In order to check the thermal stability of Na-AHA, thermogravimetric analysis is carried out for AHA and NaAHA. The onset of thermal degradation is found to be above 375 °C for Na-AHA, which is much higher as compared to pure AHA (210 °C) and much above the processing temperature of PA6/MWNT composites (260 °C). Fig. 2a, shows the FTIR spectra of Na-AHA and different mixtures of MWNT and Na-AHA which is carried out to investigate any specific interaction existing between the MWNT and Na-AHA. One can clearly observe the presence of carboxylate ions corresponding to a strong peak at 1571 cm 1and a weak band at 1400 cm 1 in the FTIR spectrum of Na-AHA. Interestingly we observe a strong peak shift corresponding to carboxylate ion stretch for the solid mixtures of MWNT and Na-AHA with increasing concentration of Na-AHA, which is presumably due to the existence of ‘cation–p’ interaction between the Na+ ion of Na-AHA and p electron clouds of MWNT. Raman spectroscopic analysis (Fig. 2b) further supports this interaction, which manifests in strong up-shifts in both G-band and D-band of MWNT in the mixtures of MWNT and Na-AHA. It is worthy to mention that with increase in Na-AHA concentration in the MWNT/Na-AHA mixture the peak intensity of G-band increases. It has been reported
Na-AHA
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Wavelength (cm ) Fig. 2a. FTIR spectra in transmittance mode for AHA, Na-salt of AHA, solid mixture (1:4 and 1:8) of MWNT and Na-salt of AHA using KBr pellet. AHA shows the N–H stretch 3372 cm 1, C–O stretch 1688 cm 1. Na-AHA shows strong carboxylate ion stretch 1571 cm 1, and a weak band at 1400 cm 1. 1:4 and 1:8 mixtures of MWNT and Na-salt of AHA show a strong carboxylate ion stretch at 1576 cm 1 and 1413 cm 1.
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Raman Shift (cm-1) Fig. 2b. Raman spectra for purified MWNT and 1:1 and 1:4 solid mixtures of purified MWNT and Na-salt of AHA. Raman bands at 1574 and 1341 cm 1 are a result of tangential G and D bands, respectively.
that increase in peak intensity related to G-band corresponds to the debundling of MWNT aggregates [24], which suggests that Na-AHA aids in exfoliating MWNT by over-
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Fig. 2c–2f. TEM image of (c) ‘aggregated’ MWNT in tetrahydrofuran (THF), (d) 1:1 mixture of MWNT and Na-AHA in THF, (e) 1:4 mixture of MWNT and Na-AHA in THF, (f) 1:4 mixture of MWNT and Na-AHA in distilled water.
modified MWNT even at 0.5 wt%, where the effective content of Na-AHA is 7.5 wt%. In this context, it is worthy to mention that PA6/Na-AHA composite (7.5 wt% of -2
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Conductivity, σ (S cm )
coming strong inter-tube van der Waals interaction. Qualitative evidence in this context can be found from the transmission electron micrographs (Fig. 2c–2f) of Na-AHA modified MWNT. As compared to purified MWNT, modified MWNT show exfoliated MWNT networks in smaller bundles and even as individual tubes. In order to further assess the state of dispersion of NaAHA modified MWNT in PA6 matrix we perform AC electrical conductivity measurements. It may be noted in Fig. 3a, that AC electrical conductivity increases by several order of magnitude (DC conductivity of 10 5 S cm 1) on incorporation of 1:1 Na-AHA modified 2 wt% MWNT in PA6 matrix indicating the formation of percolating network at 2 wt% MWNT in presence of Na-AHA, which is associated with increase in critical frequency as compared to PA6/MWNT composites of 2 wt% purified MWNT. The formation of percolating network (DC conductivity of 10 7 S cm 1) is also evident at 4:1 Na-AHA modified MWNT with 1 wt% effective MWNT content. With further increase in Na-AHA concentration (15:1, NaAHA:MWNT), conductivity value (DC conductivity of 10 7 S cm 1) manifests the ‘network-like structure’ formation of MWNT even at 0.5 wt%. Preliminary investigations (dielectric constant measurements) suggest metallic-like conduction in the PA6 composites with Na-AHA
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Frequency (Hz) Fig. 3a. Variation of AC conductivity of PA6/MWNT composites where purified and Na-salt of AHA modified MWNT are used. The ratio of MWNT and Na-AHA has been varied from 1:1 to 1:15 in the respective composites.
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MWNT (wt%) Fig. 3b. Variations in DC electrical conductivity of PA6/MWNT composites with purified MWNT and dependence with various ratio of MWNT/Na-AHA mixture.
PA6 + MWNT (1 wt%) (1:4)
% Transmitance
PA6 + MWNT (1 wt%) (1:1)
PA6 + MWNT(1 wt%)
3 wt% purified MWNT clearly manifests the significance of refinement in percolating network in achieving higher conductivity. We report here for the first time low electrical percolation threshold in semi-crystalline polymer matrix specifically in PA6/MWNT composites. This result clearly emphasizes the efficiency of this novel modifier (Na-AHA) in controlling the state of dispersion of MWNT in PA6 matrix. This concept can further be extended to other polymer matrices containing functional end groups in dispersing MWNT. FTIR spectroscopic analysis (Fig. 3c) also reveals the formation of amide bonds due to the reactive coupling between acid end groups of PA6 and the amine functionality of NaAHA during melt-mixing which manifests in increasing the peak intensity corresponding to amide I (1635 cm 1) and amide II (1537 cm 1) [25] in the PA6/Na-AHA modified MWNT composites. The phenomenon of reactive coupling is also evident in PA6/Na-AHA composites. To provide more insight related to the nature of the modifier (Na-AHA) in controlling the dispersion of MWNT in PA6/MWNT composites, we further selected Na-salt of mono and di substituted adipic acid and prepared composites with 2 wt% MWNT modified with the respective Na-salt of adipic acid keeping the ratio 1:1. We observed a significant change in the DC conductivity values in case of modifying MWNT with mono-substituted adipic acid, which is higher (10 4 S cm 1) as compared to di-substituted adipic acid modified MWNT (10 5 S cm 1). This observation eventually supports the fact that the phenomenon of reactive coupling is a dominant factor besides ‘cation–p’ interaction in controlling the dispersion of MWNT in PA6 matrix. 4. Conclusions
PA6 + Na-AHA(1 wt%)
PA6
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Wavenumbers (cm )
Fig. 3c. FTIR spectra in transmittance mode for thin films of PA6, PA6/ Na-AHA (1 wt%), PA6/MWNT (1 wt%) and PA6/Na-AHA modified MWNT (1 wt%) composites.
Na-AHA) exhibits DC conductivity of 10 9 S cm 1, the conductivity of the composite is found unaltered even at 12 wt% of Na-AHA. Fig. 3b, shows that the percolation threshold exists between 2 and 3 wt% in case of purified MWNT, whereas the percolation threshold shifts towards lower MWNT content (0–0.5 wt%) in presence of NaAHA. The observation of higher DC conductivity (10 4 S cm 1) of 1:1 Na-AHA modified MWNT at 3 wt% as compared to PA6/MWNT composites with
A novel modifier has been prepared to facilitate uniform dispersion of MWNT in melt-mixed PA6/MWNT composites. Retention of ‘network-like structure’ in solid state with significant refinement was observed even at lower MWNT concentration in presence Na-AHA, which was assessed through AC electrical conductivity measurements. Reactive coupling was found to be a dominant factor besides ‘cation–p’ interaction in achieving low electrical percolation in PA6/MWNT composites. FTIR and Raman spectroscopic analysis further revealed the existence of the specific interactions involving Na-AHA and MWNT. These nanocomposite materials can therefore be tailored depending on the end use as antistatic devices, capacitors and EMI shielding materials. Acknowledgments The authors duly acknowledge the financial support from the Department of Science and Technology (DST), India (SERC Fast Track Scheme, Project No. 04DS047). We would also like to acknowledge Microcompounder Central Facility at IIT Bombay, Centre for Research in
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Nanotechnology and Science (CRNTS) and Sophisticated Analytical Instrumentation Facility (SAIF), IIT Bombay for Raman Spectroscopy and TEM experiments. References [1] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. [2] S. Kumar et al., Macromolecules 35 (2002) 9039. [3] T. Liu, I.Y. Phang, L. Shen, S.Y. Chow, W.D. Zhang, Macromolecules 37 (2004) 7214. [4] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A.H. Windle, Polymer 40 (1999) 5967. [5] P. Po¨tschke, T.D. Fornes, D.R. Paul, Polymer 43 (2002) 3247. [6] M. Terrones, Ann. Rev. Mater. Res. 33 (2003) 419. [7] S. Barrau, P. Demont, A. Peigney, C. Laurent, C. Lacabanne, Macromolecules 36 (2003) 5187. [8] C.A. Martin, J.K.W. Sandler, M.S.P. Shaffer, M.-K. Schwarz, W. Bauhofer, K. Schulte, A.H. Windle, Comp. Sci. Tech. 64 (2004) 2309. [9] M.B. Bryning, M.F. Islam, J.M. Kikkawa, A.G. Yodh, Adv. Mater. 17 (2005) 1186. [10] D.W. Ferguson, E.W.S. Bryant, H.C. Fowler, ESD Thermoplastic Product Offers Advantage for Demanding Electronic Applications, SPE ANTEC, Atlanta, USA, 1998.
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[11] S.B. Kharchenko, J.F. Douglas, J. Obrzut, E.A. Grulke, K.B. Migler, Nat. Mater. 3 (2004) 564. [12] T. Kashiwagi, F. Du, J.F. Douglas, K.I. Winey, R.H. Harris, J.R. Shields, Nat. Mater. 4 (2005) 928. [13] P. Po¨tschke, A.R. Bhattacharyya, A. Janke, H. Goering, Comp. Interf. 10 (2003) 389. [14] M.-K. Seo, S.-J. Park, Chem. Phys. Lett. 395 (2004) 44. [15] T. McNally et al., Polymer 46 (2005) 8222. [16] O. Meincke, D. Kaempfer, H. Weickmann, C. Friedrich, M. Vathauer, H. Warth, Polymer 43 (2004) 739. [17] B. Schartel, P. Po¨tschke, U. Knoll, M. Abdel-Goad, Eur. Polym. J. 41 (2005) 1061. [18] A.K. Jonscher, Nature 267 (1977) 673. [19] T. Fukushima, A. Kosaka, Y. Ishimura, T. Yamamoto, T. Takigawa, N. Ishii, T. Aida, Science 300 (2003) 2072. [20] S. Bellayer et al., Adv. Fun. Mater. 15 (2005) 910. [21] R. Schueler, J. Petermann, K. Schulte, H.-P. Wentzel, J. Appl. Polym. Sci. 63 (1997) 1741. [22] R. Schueler, J. Petermann, K. Schulte, H.-P. Wentzel, Macromol. Symp. 104 (1996) 261. [23] A.R. Bhattacharyya, P. Po¨tschke, M. Abdel-Goad, D. Fischer, Chem. Phys. Lett. 392 (2004) 28. [24] S. Lefrant, J.P. Buisson, J. Schreiber, O. Chauvet, M. Baibarac, I. Baltoget, Syn. Met. 39 (2003) 783. [25] A.R. Bhattacharyya, A.K. Ghosh, A. Misra, K.-J. Eichhorn, Polymer 46 (2005) 1661.
Control of multiwall carbon nanotubes dispersion in polyamide6 matrix: An assessment through electrical conductivity Pravin V. Kodgire a, Arup R. Bhattacharyya b,*, Suryasarathi Bose b, Nitin Gupta b, Ajit R. Kulkarni b, Ashok Misra a b
a Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Received 18 August 2006; in final form 19 October 2006 Available online 25 October 2006
Abstract The homogeneous dispersion of carbon nanotubes (CNT) remains a hindrance in exploiting the exceptional properties associated with CNT in polymer/CNT composites. Here we present for the first time the key role of sodium salt of 6-aminohexanoic acid (Na-AHA) in assisting debundling the multiwall carbon nanotubes (MWNT) through specific interactions leading to homogeneous dispersion within polyamide6 (PA6) matrix during melt-mixing. The composite fabricated via this route exhibits low electrical percolation threshold of 0.5 wt% at room temperature, the lowest reported value in this system so far. FTIR and Raman spectroscopy reveal the existence of ‘cation–p’ interaction between Na-AHA and MWNT. The phenomenon of reactive coupling between amine functionality of NaAHA and acid end group of PA6 during melt-mixing is also established. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Polymer composites containing carbon nanotubes (CNT) have emerged as advanced multifunctional materials in view of exceptional mechanical, thermal and electrical properties associated with CNT [1]. Apart from mechanical reinforcement [2,3], one of the key interests is to develop conductive polymer composites [4–6] preferably at low concentration of CNT utilizing their high aspect ratio (L/D) for numerous applications, which include antistatic devices, capacitors and materials for EMI shielding. However, with few exceptions [7–9] uniform dispersion of CNT in polymer matrix is restricted due to aggregate formation irrespective of the techniques employed for composite preparation. This is presumably due to (i) strong inter-tube van der Waals interaction and (ii) lack of interfacial interaction between the polymer and the CNT.
*
Corresponding author. Fax: +91 22 2576 3480. E-mail address: [email protected] (A.R. Bhattacharyya).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.10.088
In connection with the method of composites preparation, solvent casting of polymer/CNT suspensions and in situ polymerization lead to better dispersion of CNT as compared to melt-mixing, which is otherwise technologically more viable. It is also observed that the application of shear during melt-mixing promotes the breakdown of the ‘aggregate’ [10]. It is well established that ‘network-like structure’ [11,12] of CNT is predominantly responsible for electrical percolation in insulating polymer matrices. However, the electrical percolation threshold in polymer/CNT composites is found to be much higher in case of thermoplastic matrices [13,14] as compared to theoretically predicted percolation threshold considering L/D of CNT as one of the critical parameters which is otherwise found to be close to the predicted values in thermoset (epoxy, polyimide etc.) filled system [7–9]. Additional complexity arises in case of semi-crystalline matrices (polypropylene [14], polyethylene [15], polyamides [16]) where it is envisaged that the dispersion of CNT is significantly affected due to the crystallization induced phase separation and subsequent rejection of CNT by the advancing crystalline fronts.
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This is manifested in much higher electrical percolation threshold in multiwall carbon nanotubes (MWNT) filled polyamide6 (PA6) composites [16,17]. Even if conductivity measurements during melt-state indicated a very low electrical percolation threshold (0.0025 < / < 0.01) in MWNT filled polypropylene (PP) composites [11], the room temperature measurement of PP/MWNT composites [14] showed higher electrical percolation threshold (1–2 wt%). These observations necessitate the retention of ‘networklike structure’ of MWNT in polymer matrix even after solidification, which can otherwise be observed in meltstate at low volume fraction of MWNT. In this work a novel modifier (sodium salt of 6-aminohexanoic acid, Na-AHA) has been prepared to induce improved dispersion of MWNT in the melt-mixed PA6/ MWNT composites. The state of dispersion has been assessed through AC electrical conductivity measurements.
0 < n < 1; A is the constant dependent on the temperature, and n is an exponent dependent on both frequency and temperature). Thermo gravimetric analysis (TGA) was performed using powder samples of AHA and Na-AHA using Netzsch (STA 409 PC) thermo gravimetric analyzer in inert atmosphere with heating rate of 10 K min 1. FTIR spectroscopic analysis was carried out with Nicolet, MAGNA 550 for powder samples of AHA and Na-AHA using KBr pellets (thin films in case of composites) at room temperature in the scanning range of 400–4000 cm 1. Raman spectroscopy was performed using Jobin Yovon (HR 800 micro-Raman) on powder samples of purified and NaAHA modified MWNT over a scanning range of 200– 2000 cm 1 with incident laser excitation wavelength of 514 nm. Transmission electron microscopy (TEM) analysis was performed with Philips CM 200, operated at 200 kV.
2. Experimental
3. Results and discussion
2.1. Materials and composite preparation
The frequency dependence electrical conductivity at room temperature (20 °C) for PA6/MWNT composites with various weight content of MWNT is depicted in Fig. 1. As expected for insulating materials, the bulk conductivity of pure PA6 increases with increase in frequency with a value about 10 13 S cm 1 at 0.01 Hz. PA6/MWNT composites with 2 wt% MWNT content show a frequency independent plateau (DC conductivity) up to a critical frequency (xc) above which the conductivity dispersion is observed. This is consistent with ‘Johnscher Universal Power Law’ for frequency dependent conductivity of solids [18]. An abrupt increase in conductivity is observed for PA6/MWNT composites at 3 wt% MWNT where the electrical conductivity is independent over a wide range of frequency indicating the formation of percolating ‘network-like structure’ of MWNT. It is further observed that
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Conductivity, σ (S cm-1)
Polyamide6 (PA6, zero shear viscosity = 179.2 Pa s at 260 °C) was obtained from GSFC, Gujarat, India (Gujlon M28RC, relative viscosity 2.8, Mv is 38642 in 85% formic acid). Purified multiwall carbon nanotubes (MWNT) were obtained from Nanocyl CA Belgium (L/D: 10–1000, purity > 95%). 6-aminohexanoic acid (AHA) (Sigma Aldrich, Mw = 132.18; purity: 98%) was neutralized using sodium hydroxide (Sisco Research Laboratories, India, purity: 98%) to obtain Na-AHA. MWNT were initially sonicated in distilled water for 20 min. Required amount of NaAHA solution (to reach the desired weight ratio of x:y, where x = MWNT and y = Na-AHA) was then added to the MWNT and again sonicated for 10 min. The NaAHA modified MWNT solution was then subjected to evaporation. The obtained dry powder was then left in a vacuum oven at 80 °C for 3 h to ensure the complete removal of water. PA6/MWNT (either purified or Na-AHA modified) composites were prepared by melt-mixing in a conical twin-screw extruder (Micro 5, DSM Research, Netherlands) at 260 °C with a rotational speed of 150 rpm for 5 min except for 4:1 (Na-AHA/MWNT, 1 wt% MWNT content) and 15:1 (Na-AHA/MWNT, 0.5 wt% MWNT content) MWNT modified PA6 composites where the mixing time was 10 min. All the samples were vacuum dried over night at 80 °C. Compression moulded films of 0.5 mm thickness were used for electrical conductivity measurements.
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The AC conductivity measurements were performed in the frequency range 10 2 and 107 Hz using Alpha high resolution analyzer. The DC conductivity of the samples was determined by fitting power law (rAC = rDC + Axn,
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Frequency (Hz) Fig. 1. Frequency dependence AC conductivity of PA6/MWNT composites.
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MWNT / Na-AHA (1:8)
MWNT / Na-AHA (1:4)
% Transmitance
for these composites the value of critical frequency increases with increasing MWNT content. Above the percolation threshold (>3 wt%), the increase in conductivity is marginal. We observe electrical percolation threshold between 2 and 3 wt% of MWNT in melt-mixed PA6/ MWNT composites utilizing purified MWNT. Even if this value is much lower as compared to the reported values of 4–6 wt% in PA6/MWNT composites [16,17], this observation cannot be explained solely on the basis of L/D of MWNT. Considering the theoretically predicted percolation threshold, the observation of percolating network between 2 and 3 wt% in PA6 matrix indicates the existence of aggregated MWNT structure. In view of the existing challenges regarding the dispersion of MWNT in semi-crystalline polymer matrix, we have developed an ionic modifier containing reactive functionality to control the dispersion of MWNT in PA6 matrix. NaAHA has been prepared in order to establish ‘cation–p’ interaction with MWNT, and also to exploit the concept of reactive coupling between the amine group of NaAHA and acid end groups of PA6 during melt-mixing. It is expected that ‘cation–p’ interaction would lead to initial debundling of MWNT, which will be retained in the solidstate as well due to melt interfacial reaction. In this context, use of imidazolium salts in controlling the dispersion of either SWNT [19] or MWNT [20] was reported. In addition the use of ionic additive was also reported to induce uniform dispersion of carbon black in epoxy matrix in the past [21,22]. The use of styrene maleic anhydride copolymer (SMA) as a reactive modifier was also reported to induce improved dispersion of single wall carbon nanotubes (SWNT) in PA6 matrix [23], however encapsulating SMA layer around SWNT restricted the increase in conductivity. In order to check the thermal stability of Na-AHA, thermogravimetric analysis is carried out for AHA and NaAHA. The onset of thermal degradation is found to be above 375 °C for Na-AHA, which is much higher as compared to pure AHA (210 °C) and much above the processing temperature of PA6/MWNT composites (260 °C). Fig. 2a, shows the FTIR spectra of Na-AHA and different mixtures of MWNT and Na-AHA which is carried out to investigate any specific interaction existing between the MWNT and Na-AHA. One can clearly observe the presence of carboxylate ions corresponding to a strong peak at 1571 cm 1and a weak band at 1400 cm 1 in the FTIR spectrum of Na-AHA. Interestingly we observe a strong peak shift corresponding to carboxylate ion stretch for the solid mixtures of MWNT and Na-AHA with increasing concentration of Na-AHA, which is presumably due to the existence of ‘cation–p’ interaction between the Na+ ion of Na-AHA and p electron clouds of MWNT. Raman spectroscopic analysis (Fig. 2b) further supports this interaction, which manifests in strong up-shifts in both G-band and D-band of MWNT in the mixtures of MWNT and Na-AHA. It is worthy to mention that with increase in Na-AHA concentration in the MWNT/Na-AHA mixture the peak intensity of G-band increases. It has been reported
Na-AHA
AHA
1000 1200 1400 1600 2800 3000 3200 3400 3600 -1
Wavelength (cm ) Fig. 2a. FTIR spectra in transmittance mode for AHA, Na-salt of AHA, solid mixture (1:4 and 1:8) of MWNT and Na-salt of AHA using KBr pellet. AHA shows the N–H stretch 3372 cm 1, C–O stretch 1688 cm 1. Na-AHA shows strong carboxylate ion stretch 1571 cm 1, and a weak band at 1400 cm 1. 1:4 and 1:8 mixtures of MWNT and Na-salt of AHA show a strong carboxylate ion stretch at 1576 cm 1 and 1413 cm 1.
2500
MWNT MWNT / Na-AHA (1:1) MWNT / Na-AHA (1:4)
D-band 2000
G-band
Intensity (a.u.)
482
1588
1500
1585 1574
1000
500
0 1000
1200
1400
1600
1800
2000
Raman Shift (cm-1) Fig. 2b. Raman spectra for purified MWNT and 1:1 and 1:4 solid mixtures of purified MWNT and Na-salt of AHA. Raman bands at 1574 and 1341 cm 1 are a result of tangential G and D bands, respectively.
that increase in peak intensity related to G-band corresponds to the debundling of MWNT aggregates [24], which suggests that Na-AHA aids in exfoliating MWNT by over-
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Fig. 2c–2f. TEM image of (c) ‘aggregated’ MWNT in tetrahydrofuran (THF), (d) 1:1 mixture of MWNT and Na-AHA in THF, (e) 1:4 mixture of MWNT and Na-AHA in THF, (f) 1:4 mixture of MWNT and Na-AHA in distilled water.
modified MWNT even at 0.5 wt%, where the effective content of Na-AHA is 7.5 wt%. In this context, it is worthy to mention that PA6/Na-AHA composite (7.5 wt% of -2
10
-4
10 -1
Conductivity, σ (S cm )
coming strong inter-tube van der Waals interaction. Qualitative evidence in this context can be found from the transmission electron micrographs (Fig. 2c–2f) of Na-AHA modified MWNT. As compared to purified MWNT, modified MWNT show exfoliated MWNT networks in smaller bundles and even as individual tubes. In order to further assess the state of dispersion of NaAHA modified MWNT in PA6 matrix we perform AC electrical conductivity measurements. It may be noted in Fig. 3a, that AC electrical conductivity increases by several order of magnitude (DC conductivity of 10 5 S cm 1) on incorporation of 1:1 Na-AHA modified 2 wt% MWNT in PA6 matrix indicating the formation of percolating network at 2 wt% MWNT in presence of Na-AHA, which is associated with increase in critical frequency as compared to PA6/MWNT composites of 2 wt% purified MWNT. The formation of percolating network (DC conductivity of 10 7 S cm 1) is also evident at 4:1 Na-AHA modified MWNT with 1 wt% effective MWNT content. With further increase in Na-AHA concentration (15:1, NaAHA:MWNT), conductivity value (DC conductivity of 10 7 S cm 1) manifests the ‘network-like structure’ formation of MWNT even at 0.5 wt%. Preliminary investigations (dielectric constant measurements) suggest metallic-like conduction in the PA6 composites with Na-AHA
-6
10
-8
10
-10
10
PA6 PA6 + MWNT (2 wt%) PA6 + MWNT (2 wt%) (1:1) PA6 + MWNT (1 wt%) (1:4) PA6 + MWNT (0.5 wt%) (1:15)
-12
10
-14
10
10
-3
-1
10
1
10
10
3
10
5
7
10
Frequency (Hz) Fig. 3a. Variation of AC conductivity of PA6/MWNT composites where purified and Na-salt of AHA modified MWNT are used. The ratio of MWNT and Na-AHA has been varied from 1:1 to 1:15 in the respective composites.
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10
-4
-1
DC Conductivity (S cm )
10
-6
10
-8
10
-10
10
MWNT MWNT / Na-AHA (1:1) MWNT / Na-AHA (1:4) MWNT / Na-AHA (1:15)
-12
10
-14
10
0
1
2
3
4
MWNT (wt%) Fig. 3b. Variations in DC electrical conductivity of PA6/MWNT composites with purified MWNT and dependence with various ratio of MWNT/Na-AHA mixture.
PA6 + MWNT (1 wt%) (1:4)
% Transmitance
PA6 + MWNT (1 wt%) (1:1)
PA6 + MWNT(1 wt%)
3 wt% purified MWNT clearly manifests the significance of refinement in percolating network in achieving higher conductivity. We report here for the first time low electrical percolation threshold in semi-crystalline polymer matrix specifically in PA6/MWNT composites. This result clearly emphasizes the efficiency of this novel modifier (Na-AHA) in controlling the state of dispersion of MWNT in PA6 matrix. This concept can further be extended to other polymer matrices containing functional end groups in dispersing MWNT. FTIR spectroscopic analysis (Fig. 3c) also reveals the formation of amide bonds due to the reactive coupling between acid end groups of PA6 and the amine functionality of NaAHA during melt-mixing which manifests in increasing the peak intensity corresponding to amide I (1635 cm 1) and amide II (1537 cm 1) [25] in the PA6/Na-AHA modified MWNT composites. The phenomenon of reactive coupling is also evident in PA6/Na-AHA composites. To provide more insight related to the nature of the modifier (Na-AHA) in controlling the dispersion of MWNT in PA6/MWNT composites, we further selected Na-salt of mono and di substituted adipic acid and prepared composites with 2 wt% MWNT modified with the respective Na-salt of adipic acid keeping the ratio 1:1. We observed a significant change in the DC conductivity values in case of modifying MWNT with mono-substituted adipic acid, which is higher (10 4 S cm 1) as compared to di-substituted adipic acid modified MWNT (10 5 S cm 1). This observation eventually supports the fact that the phenomenon of reactive coupling is a dominant factor besides ‘cation–p’ interaction in controlling the dispersion of MWNT in PA6 matrix. 4. Conclusions
PA6 + Na-AHA(1 wt%)
PA6
1000 1200 1400 1600 1800 2600 2800 3000 3200 3400 3600 -1
Wavenumbers (cm )
Fig. 3c. FTIR spectra in transmittance mode for thin films of PA6, PA6/ Na-AHA (1 wt%), PA6/MWNT (1 wt%) and PA6/Na-AHA modified MWNT (1 wt%) composites.
Na-AHA) exhibits DC conductivity of 10 9 S cm 1, the conductivity of the composite is found unaltered even at 12 wt% of Na-AHA. Fig. 3b, shows that the percolation threshold exists between 2 and 3 wt% in case of purified MWNT, whereas the percolation threshold shifts towards lower MWNT content (0–0.5 wt%) in presence of NaAHA. The observation of higher DC conductivity (10 4 S cm 1) of 1:1 Na-AHA modified MWNT at 3 wt% as compared to PA6/MWNT composites with
A novel modifier has been prepared to facilitate uniform dispersion of MWNT in melt-mixed PA6/MWNT composites. Retention of ‘network-like structure’ in solid state with significant refinement was observed even at lower MWNT concentration in presence Na-AHA, which was assessed through AC electrical conductivity measurements. Reactive coupling was found to be a dominant factor besides ‘cation–p’ interaction in achieving low electrical percolation in PA6/MWNT composites. FTIR and Raman spectroscopic analysis further revealed the existence of the specific interactions involving Na-AHA and MWNT. These nanocomposite materials can therefore be tailored depending on the end use as antistatic devices, capacitors and EMI shielding materials. Acknowledgments The authors duly acknowledge the financial support from the Department of Science and Technology (DST), India (SERC Fast Track Scheme, Project No. 04DS047). We would also like to acknowledge Microcompounder Central Facility at IIT Bombay, Centre for Research in
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