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carbon nanotube composites
Materials Science and Engineering C 26 (2006) 1198 – 1201 www.elsevier.com/locate/msec
Polyazomethine/carbon nanotube composites E. Lafuente a, M. Pin˜ol b, L. Oriol b, E. Mun˜oz a, A.M. Benito a, W.K. Maser a, A.B. Dalton c, J.L. Serrano b, M.T. Martı´nez a,* b
a Instituto de Carboquı´mica (CSIC), C/ Miguel Luesma Casta´n 4, E-50018 Zaragoza, Spain Quı´mica Orga´nica-ICMA, Facultad de Ciencias, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain c Department of Physics, University of Surrey, Guildford GU2 7XH, UK
Available online 8 November 2005
Abstract Composites were synthesized by ‘‘in-situ’’ polymerization of polyazomethine, a liquid crystal polymer (LCP), in presence of multi-walled carbon nanotubes (MWNTs) previously dispersed in one of the employed monomers. Fiber processing was carried out by extrusion from the composites containing 1 and 10 wt.% of MWNTs at the mesophase temperature. We have observed that the typical highly oriented internal fibrillar structure can be significantly disrupted by increasing the nanotube content in the composite fibers. Evidences of MWNT alignment were found in the studied LCP/MWNT composites. D 2005 Elsevier B.V. All rights reserved.
1. Introduction Because of their unique electronic, mechanical, an physical properties, carbon nanotubes (CNTs) have been envisioned as promising materials for potential applications in many fields [1,2]. Due to their high aspect ratio, combined with their high tensile strength and Young’s modulus, CNTs are particularly interesting as components in composite materials [3,4]. New composite materials providing novel mechanical, electrochemical, electronic, and optical capabilities may arise from the molecular-level interaction between CNTs and different matrices [5–22]. Thermotropic liquid-crystal polymers (LCPs) such as polyazomethines have attracted much attention because of the excellent thermal and mechanical properties of products obtained from their anisotropic melts [23–26]. Melt spinning of thermotropic polyazomethines is a common technique where the spontaneous alignment of the polymeric chains results in fibers having remarkable mechanical properties along the longitudinal direction [27,28]. Additionally, polyazomethines exhibit good thermal stability and environmental resistance, and are promising materials for optoelectronic and photonic devices [29]. We here report the preparation of new polyazomethine/CNT composite material by a step polymerization in presence of CNTs. * Corresponding author. Tel.: +34 976 733977; fax: +34 976 733318. E-mail address: [email protected] (M.T. Martı´nez). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.09.094
Fiber spinning processes were carried out by extrusion from the nematic melt. The produced composites are thermally stable and show the characteristic nematic transitions of the employed LCP. 2. Experimental 2.1. Materials The multi-walled carbon nanotubes (MWNTs) used in this work have been produced in an arc-discharge reactor by evaporation of pure graphite anodes (60 A, 25 V) under 66 kPa of helium [30]. The employed polyazomethine was prepared by polycondensation of a dialdehyde and a diamine in solution. The complex dialdehyde 1,10-bis[(4-formyl-3-hydroxyphenyl)oxy]decane was obtained by a previously reported procedure [31]. The diamine, 2-methyl-1,4-phenylenediamine, was obtained from its commercially available quaternary ammonium salt, 2methyl-1,4-phenylenediammonum sulfate, and purified prior to use by vacuum distillation. The solvent used in the polymerization processes, N,N-dimethylacetamide, was distilled under ˚ ). reduced pressure and collected over molecular sieves (type 4 A 2.2. Polymerization procedure Polyazomethine/MWNT composites were synthesized by ‘‘in situ’’ polymerization processes (i.e., in presence of the employed
E. Lafuente et al. / Materials Science and Engineering C 26 (2006) 1198 – 1201
MWNTs [15]) as previous results did not provide any efficient CNT/polymer interaction when attempting to disperse CNTs in trifluoroacetic acid solutions of the polyazomethine due to the low solubility of this LCP. Based on the ability of aromatic amines to form CNTs dispersions [15,32], we decided to disperse the MWNTs in N,N-dimethylacetamide solutions of the employed diamine monomer prior to proceed with the polymerization process. This mixture was sonicated to promote good CNTs dispersion. Stable CNTs dispersions were achieved only for low CNTs loadings. 1,10-bis[4-formyl-3-hydroxyphenyl)oxy]decane was then added in presence of anhidrous LiCl to proceed with the synthesis of the LCP/MWNT composite. The polymerization mixture was kept away from light and stirred under inert atmosphere at room temperature for 16 h. Water was then added to the suspension, and the resulting precipitate was collected by filtration and thoroughly washed several times with water and methanol. The product was extracted with acetone for 48 h using a Soxhlet apparatus, and finally dried under vacuum over P2O5 for 24 h at 80 -C to yield the corresponding polyazomethine/MWNT composites. Using the polymerization procedure described above, composites with different MWNTs content were prepared: C1 (1 wt.% MWNT) and C-2 (10 wt.% MWNT). 2.3. Fiber spinning Fiber spinning was carried out by loading 200 mg of the MWNTs/polyazomethine composite material in a small-scale melt-spinning set-up. This set-up consists of a special steel (F155 Cr – Mo) cylindrical block (inner diameter: 5 mm; length: 90 mm) that is heated by a 400 W bracket resistor and thermally isolated by a Termalite jacket. The exit die was a commercial single-hole spinneret of 0.45 mm in diameter fixed at the lower end of the block. The fiber spinning process was performed at the melting temperature of the composite, as determined by differential scanning calorimetry (DSC). Pressure was applied by a driven piston using a weight that was varied depending on the viscosity of the extruded composite. The molten composite filament was spun into air at room temperature in the form of fibers containing 1 wt.% MWNT (C-1 fibers) or 10 wt.% MWNT (C-2 fibers).
FP-82 hot stage and a Mettler FP-80 control unit), and Raman spectroscopy (Jobin Yvon Horiba high-resolution LabRam spectrometer, k exc = 632.8 nm). 3. Results and discussion 3.1. Composites The composites were prepared in good yields by solution polycondensation following the experimental procedure described above. The thermal stability and phase transitions of the composites were studied by TGA, DSC, and optical microscopy. Both composites C-1 and C-2 showed good thermal stability with maximum decomposition temperatures of 380 -C under nitrogen, similar to the one corresponding to the unmodified polyazomethine [33]. The observed low-temperature weight loss is ascribed to evaporation of remaining solvent and of other volatile products. The typical poliazomethine thermal transitions were observed by DSC of the composite materials. Thus, both composites showed an endotherm associated to the nematic transition (T m) at about 167 -C, and isotropic transition (T i) at about 330 -C, which is characteristic for polyazomethines [33]. Polarized optical microscopy studies revealed the presence of a nematic mesophase, typical of polyazomethines, and MWNT aggregates, probably due to CNTs segregation and reagglomeration when melting the composite, or to poor MWNT dispersion in the monomer when employing high MWNT loadings (specially in the case of C-2, Fig. 1c,d). Topographic AFM images of films fabricated by melting the composites over glass slides show the presence of polymercoated aligned structures (Fig. 2). Since that the dimensions of the aligned structures correspond to those of MWNTs, and considering that they are only found in the composites, these
2.4. Techniques The thermal behaviour of the composite materials was studied by DSC and thermogravimetric analysis (TGA). A TA Instrument differential scanning calorimeter was used to determine the different thermal transitions that occur in the composite materials when heating at a scanning rate of 10 -C/ min. Glass transition temperatures was determined as the midpoint of the baseline jump. TGA was performed using a TA Instrument STD 2960 at a 10 -C/min rate in the 40– 600 -C range under nitrogen, and in the 600 – 750 -C range in air. Additional characterization studies were carried out by scanning electron microscopy (SEM, Jeol JSM 6400 microscope), atomic force microscopy (AFM), polarized optical microscopy (Nikon polarizing microscope fitted with a Mettler
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Fig. 1. Polarized optical microscopy of C-1 (a, b) and C-2 (c, d).
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E. Lafuente et al. / Materials Science and Engineering C 26 (2006) 1198 – 1201
Fig. 2. AFM images of C-2.
AFM results might indicate that MWNT alignment would be induced by the employed LCP and/or during the film processing. Further work is needed to fully characterize this observed CNT alignment. 3.2. Composite fibers Composites C-1 and C-2 were processed into anisotropic fibers (C-1 fibers and C-2 fibers) by melt-spinning in the liquid crystalline state. As mentioned above, this fiber spinning technique implies the extrusion through a conical die of the molten composites at the mesophase temperature followed by a rapid cooling process to room temperature. The obtained fibers show a preferential alignment of the polymeric chains along their longitudinal axis. Both C-1 and C-2 composites were then extruded in the form of fibers at 190 -C. Smooth yellow filaments with metallic appearance were obtained from C-1, and creased filaments exhibiting a metallic black appearance were obtained from C-2. As-spun fibers show good thermal stability, with similar maximum decomposition temperatures than the starting composites and the unmodified polymer. In
this case, however, no weight loss at low temperature was observed due to the heating pretreatment prior to the extrusion process. On the other hand, the composite processing in the form of fibers results in increased T m and Ti temperatures (179 -C and 343 -C, respectively). These results might be related to significant changes in the polymer, probably due to an increase in molecular weight and crystallinity and/or as a result of an improved polymer/CNTs interaction [34] that might have occurred during the fiber spinning process. The morphology of the fibers was studied by SEM employing specimens prepared by the peel-back method, which consist of exposing the inner part of the fibers by making a cut on the fiber surface and pulling it back. SEM micrographs clearly show that the MWNT loading can significantly affect the morphology of the spun fibers (Fig. 3). C-1 fibers, which resulted of spinning a composite made with stable CNTs/diamine dispersions, presented the similar fibrilar microstructure observed in the poliazomethine fibers, with microfibers oriented along the longitudinal axis of the fiber (Fig. 3a,b). That degree of polymer structural organization is however eventually lost when employing high MWNTs contents (Fig. 3c,d). Additionally, preliminary Raman spectroscopy results indicate that the MWNTs have some degree of preferential orientation along the axis of the composite fiber. 4. Conclusions Polyazomethine/MWNT composites presented characteristic polyazomethine features, such as good thermal stability, and the formation of nematic mesophases. Recent reports indicate that liquid crystals can induce CNT alignment [35,36], and evidences of MWNTs alignment has also been detected in both the composites and the composite fibers. The observed CNT
Fig. 3. SEM images of C-1 fiber (a, b) and C-2 fiber (c, d).
E. Lafuente et al. / Materials Science and Engineering C 26 (2006) 1198 – 1201
alignment within our LCP/CNT composites may result in novel materials with improved mechanical, electronic, and optical properties, which are subject of current research. Acknowledgements Funded by the Gobierno de Arago´n (Spain). E.M. acknowledges support from MEC (Ramo´n y Cajal Program). References [1] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. [2] P.M. Ajayan, Chem. Rev. 99 (1999) 1787. [3] E.T. Thostenson, Z. Ren, T.-W. Chou, Compos. Sci. Technol. 61 (2001) 1899 (and references therein). [4] J.K.W. Sandler, J.E. Kirk, I.A. Kinloch, M.S.P. Shaffer, A.H. Windle, Polymer 44 (2003) 5893. [5] S.A. Curran, P.M. Ajayan, W.J. Blau, D.L. Carroll, J.N. Coleman, A.B. Dalton, A.P. Davey, A. Drury, B. McCarthy, S. Maier, A. Strevens, Adv. Mater. 10 (1998) 1091. [6] A.B. Dalton, S. Collins, E. Mun˜oz, J.M. Razal, V.H. Ebron, J.P. Ferraris, J.N. Coleman, B.G. Kim, R.H. Baughman, Nature 423 (2003) 703. [7] E. Mun˜oz, A.B. Dalton, S. Collins, M. Kozlov, J. Razal, J.N. Coleman, B.G. Kim, V.H. Ebron, M. Selvidge, J.P. Ferraris, R.H. Baughman, Adv. Eng. Mater. 6 (2004) 801. [8] E. Mun˜oz, D.-S. Suh, S. Collins, M. Selvidge, A.B. Dalton, B.G. Kim, J.M. Razal, G. Ussery, A.G. Rinzler, M.T. Martı´nez, R.H. Baughman, Adv. Mater. 17 (2005) 1064. [9] L. Jin, C. Bower, O. Zhou, Appl. Phys. Lett. 73 (1998) 1197. [10] Z. Jia, Z. Whang, C. Xu, J. Lang, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 271 (1999) 395. [11] C.A. Cooper, R.J. Young, M. Halsall, Composites: Part A, 32 (2001) 401. [12] I. Musa, M. Baxendale, G.A.J. Amaratunga, W. Eccleston, Synth. Met. 102 (1999) 1250. [13] J.N. Coleman, S. Curran, A.B. Dalton, A.P. Davey, B. McCarthy, W. Blau, R.C. Barklie, Synth. Met. 102 (1999) 1174.
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[14] H.S. Woo, R. Czerw, S. Webster, D.L. Carroll, J.W. Park, J.H. Lee, Synth. Met. 116 (2001) 369. [15] M. Cochet, W.K. Maser, A.M. Benito, M.A. Callejas, M.T. Martinez, J.M. Benoit, J. Schreiber, O. Chauvet, Chem. Commun. (2001) 1450. [16] W.K. Maser, A.M. Benito, M.A. Callejas, T. Seeger, M.T. Martinez, J. Schreiber, J. Muszynski, O. Chauvet, Z. Osva´th, A.A. Koo´s, L.P. Biro´, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 23 (2003) 87. [17] T.V. Sreekumar, T. Liu, B.G. Min, H. Guo, S. Kumar, R.H. Hauge, R.E. Smalley, Adv. Mater. 16 (2004) 58. [18] B. Zhao, H. Hu, R.C. Haddon, Adv. Funct. Mater. 14 (2004) 71. [19] I.-C. Liu, H.-M. Huang, C.-Y. Chang, H.-C. Tsai, C.-H. Hsu, R.C.-C. Tsiang, Macromolecules 37 (2004) 283. [20] O. Meincke, D. Kaempfer, H. Weickmann, C. Friedrich, M. Vathauer, H. Warth, Polymer 45 (2004) 739. [21] J.K.W. Sandler, S. Pegel, M. Cadek, F. Gojny, M. van Es, J. Lohmar, W.J. Blau, K. Schulte, A.H. Windle, M.S.P. Shaffer, Polymer 45 (2004) 2001. [22] H. Guo, T.V. Sreekumar, T. Liu, M. Minus, S. Kumar, Polymer 46 (2005) 3001. [23] G. Sawnhney, S.K. Gupta, A. Misra, J. Appl. Polym. Sci. 62 (1996) 1395. [24] G. Wegner, Macromol. Symp. 101 (1996) 257. [25] A.A. Hanloss, D.G. Baird, Int. Polym. Process. 11 (1996) 82. [26] W. Chinsirikul, T.C. Hsu, I.R. Harrison, Polym. Eng. Sci. 36 (1996) 2708. [27] M.G. Dobb, M.J. McIntirey, in: M. Gordon (Ed.), Liquid Crystal Polymers II/III: Advances in Polymer Science, Springer-Verlag, Berlin, 1984, p. 89. [28] P.W. Morgan, T.C. Pletcher, S.L. Kwolek, Polym. Prepr. 24 (1983) 470. [29] C.J. Yang, S.A. Jenekhe, Macromolecules 28 (1995) 1130. [30] S. Iijima, Nature 354 (1991) 56. [31] M. Marcos, L. Oriol, B. Ros, J.L. Serrano, P. Keller, Mol. Cryst. Liq. Cryst. 155 (1988) 103. [32] Y. Sun, S.R. Wilson, D.I. Schuster, J. Am. Chem. Soc. 123 (2001) 5348. [33] P. Cerrada, L. Oriol, M. Pin˜ol, J.L. Serrano, I. Iribarren, S. Mun˜oz Guerra, Macromolecules 29 (1996) 2515. [34] M. Cadek, J.N. Coleman, V. Barron, K. Hedicke, W.J. Blau, Appl. Phys. Lett. 81 (2002) 5123. [35] I. Dierking, G. Scalia, P. Morales, D. LeClere, Adv. Mater. 16 (2004) 865. [36] I. Dierking, G. Scalia, P. Morales, J. Appl. Phys. 97 (2005) 044309.
Polyazomethine/carbon nanotube composites E. Lafuente a, M. Pin˜ol b, L. Oriol b, E. Mun˜oz a, A.M. Benito a, W.K. Maser a, A.B. Dalton c, J.L. Serrano b, M.T. Martı´nez a,* b
a Instituto de Carboquı´mica (CSIC), C/ Miguel Luesma Casta´n 4, E-50018 Zaragoza, Spain Quı´mica Orga´nica-ICMA, Facultad de Ciencias, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain c Department of Physics, University of Surrey, Guildford GU2 7XH, UK
Available online 8 November 2005
Abstract Composites were synthesized by ‘‘in-situ’’ polymerization of polyazomethine, a liquid crystal polymer (LCP), in presence of multi-walled carbon nanotubes (MWNTs) previously dispersed in one of the employed monomers. Fiber processing was carried out by extrusion from the composites containing 1 and 10 wt.% of MWNTs at the mesophase temperature. We have observed that the typical highly oriented internal fibrillar structure can be significantly disrupted by increasing the nanotube content in the composite fibers. Evidences of MWNT alignment were found in the studied LCP/MWNT composites. D 2005 Elsevier B.V. All rights reserved.
1. Introduction Because of their unique electronic, mechanical, an physical properties, carbon nanotubes (CNTs) have been envisioned as promising materials for potential applications in many fields [1,2]. Due to their high aspect ratio, combined with their high tensile strength and Young’s modulus, CNTs are particularly interesting as components in composite materials [3,4]. New composite materials providing novel mechanical, electrochemical, electronic, and optical capabilities may arise from the molecular-level interaction between CNTs and different matrices [5–22]. Thermotropic liquid-crystal polymers (LCPs) such as polyazomethines have attracted much attention because of the excellent thermal and mechanical properties of products obtained from their anisotropic melts [23–26]. Melt spinning of thermotropic polyazomethines is a common technique where the spontaneous alignment of the polymeric chains results in fibers having remarkable mechanical properties along the longitudinal direction [27,28]. Additionally, polyazomethines exhibit good thermal stability and environmental resistance, and are promising materials for optoelectronic and photonic devices [29]. We here report the preparation of new polyazomethine/CNT composite material by a step polymerization in presence of CNTs. * Corresponding author. Tel.: +34 976 733977; fax: +34 976 733318. E-mail address: [email protected] (M.T. Martı´nez). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.09.094
Fiber spinning processes were carried out by extrusion from the nematic melt. The produced composites are thermally stable and show the characteristic nematic transitions of the employed LCP. 2. Experimental 2.1. Materials The multi-walled carbon nanotubes (MWNTs) used in this work have been produced in an arc-discharge reactor by evaporation of pure graphite anodes (60 A, 25 V) under 66 kPa of helium [30]. The employed polyazomethine was prepared by polycondensation of a dialdehyde and a diamine in solution. The complex dialdehyde 1,10-bis[(4-formyl-3-hydroxyphenyl)oxy]decane was obtained by a previously reported procedure [31]. The diamine, 2-methyl-1,4-phenylenediamine, was obtained from its commercially available quaternary ammonium salt, 2methyl-1,4-phenylenediammonum sulfate, and purified prior to use by vacuum distillation. The solvent used in the polymerization processes, N,N-dimethylacetamide, was distilled under ˚ ). reduced pressure and collected over molecular sieves (type 4 A 2.2. Polymerization procedure Polyazomethine/MWNT composites were synthesized by ‘‘in situ’’ polymerization processes (i.e., in presence of the employed
E. Lafuente et al. / Materials Science and Engineering C 26 (2006) 1198 – 1201
MWNTs [15]) as previous results did not provide any efficient CNT/polymer interaction when attempting to disperse CNTs in trifluoroacetic acid solutions of the polyazomethine due to the low solubility of this LCP. Based on the ability of aromatic amines to form CNTs dispersions [15,32], we decided to disperse the MWNTs in N,N-dimethylacetamide solutions of the employed diamine monomer prior to proceed with the polymerization process. This mixture was sonicated to promote good CNTs dispersion. Stable CNTs dispersions were achieved only for low CNTs loadings. 1,10-bis[4-formyl-3-hydroxyphenyl)oxy]decane was then added in presence of anhidrous LiCl to proceed with the synthesis of the LCP/MWNT composite. The polymerization mixture was kept away from light and stirred under inert atmosphere at room temperature for 16 h. Water was then added to the suspension, and the resulting precipitate was collected by filtration and thoroughly washed several times with water and methanol. The product was extracted with acetone for 48 h using a Soxhlet apparatus, and finally dried under vacuum over P2O5 for 24 h at 80 -C to yield the corresponding polyazomethine/MWNT composites. Using the polymerization procedure described above, composites with different MWNTs content were prepared: C1 (1 wt.% MWNT) and C-2 (10 wt.% MWNT). 2.3. Fiber spinning Fiber spinning was carried out by loading 200 mg of the MWNTs/polyazomethine composite material in a small-scale melt-spinning set-up. This set-up consists of a special steel (F155 Cr – Mo) cylindrical block (inner diameter: 5 mm; length: 90 mm) that is heated by a 400 W bracket resistor and thermally isolated by a Termalite jacket. The exit die was a commercial single-hole spinneret of 0.45 mm in diameter fixed at the lower end of the block. The fiber spinning process was performed at the melting temperature of the composite, as determined by differential scanning calorimetry (DSC). Pressure was applied by a driven piston using a weight that was varied depending on the viscosity of the extruded composite. The molten composite filament was spun into air at room temperature in the form of fibers containing 1 wt.% MWNT (C-1 fibers) or 10 wt.% MWNT (C-2 fibers).
FP-82 hot stage and a Mettler FP-80 control unit), and Raman spectroscopy (Jobin Yvon Horiba high-resolution LabRam spectrometer, k exc = 632.8 nm). 3. Results and discussion 3.1. Composites The composites were prepared in good yields by solution polycondensation following the experimental procedure described above. The thermal stability and phase transitions of the composites were studied by TGA, DSC, and optical microscopy. Both composites C-1 and C-2 showed good thermal stability with maximum decomposition temperatures of 380 -C under nitrogen, similar to the one corresponding to the unmodified polyazomethine [33]. The observed low-temperature weight loss is ascribed to evaporation of remaining solvent and of other volatile products. The typical poliazomethine thermal transitions were observed by DSC of the composite materials. Thus, both composites showed an endotherm associated to the nematic transition (T m) at about 167 -C, and isotropic transition (T i) at about 330 -C, which is characteristic for polyazomethines [33]. Polarized optical microscopy studies revealed the presence of a nematic mesophase, typical of polyazomethines, and MWNT aggregates, probably due to CNTs segregation and reagglomeration when melting the composite, or to poor MWNT dispersion in the monomer when employing high MWNT loadings (specially in the case of C-2, Fig. 1c,d). Topographic AFM images of films fabricated by melting the composites over glass slides show the presence of polymercoated aligned structures (Fig. 2). Since that the dimensions of the aligned structures correspond to those of MWNTs, and considering that they are only found in the composites, these
2.4. Techniques The thermal behaviour of the composite materials was studied by DSC and thermogravimetric analysis (TGA). A TA Instrument differential scanning calorimeter was used to determine the different thermal transitions that occur in the composite materials when heating at a scanning rate of 10 -C/ min. Glass transition temperatures was determined as the midpoint of the baseline jump. TGA was performed using a TA Instrument STD 2960 at a 10 -C/min rate in the 40– 600 -C range under nitrogen, and in the 600 – 750 -C range in air. Additional characterization studies were carried out by scanning electron microscopy (SEM, Jeol JSM 6400 microscope), atomic force microscopy (AFM), polarized optical microscopy (Nikon polarizing microscope fitted with a Mettler
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Fig. 1. Polarized optical microscopy of C-1 (a, b) and C-2 (c, d).
1200
E. Lafuente et al. / Materials Science and Engineering C 26 (2006) 1198 – 1201
Fig. 2. AFM images of C-2.
AFM results might indicate that MWNT alignment would be induced by the employed LCP and/or during the film processing. Further work is needed to fully characterize this observed CNT alignment. 3.2. Composite fibers Composites C-1 and C-2 were processed into anisotropic fibers (C-1 fibers and C-2 fibers) by melt-spinning in the liquid crystalline state. As mentioned above, this fiber spinning technique implies the extrusion through a conical die of the molten composites at the mesophase temperature followed by a rapid cooling process to room temperature. The obtained fibers show a preferential alignment of the polymeric chains along their longitudinal axis. Both C-1 and C-2 composites were then extruded in the form of fibers at 190 -C. Smooth yellow filaments with metallic appearance were obtained from C-1, and creased filaments exhibiting a metallic black appearance were obtained from C-2. As-spun fibers show good thermal stability, with similar maximum decomposition temperatures than the starting composites and the unmodified polymer. In
this case, however, no weight loss at low temperature was observed due to the heating pretreatment prior to the extrusion process. On the other hand, the composite processing in the form of fibers results in increased T m and Ti temperatures (179 -C and 343 -C, respectively). These results might be related to significant changes in the polymer, probably due to an increase in molecular weight and crystallinity and/or as a result of an improved polymer/CNTs interaction [34] that might have occurred during the fiber spinning process. The morphology of the fibers was studied by SEM employing specimens prepared by the peel-back method, which consist of exposing the inner part of the fibers by making a cut on the fiber surface and pulling it back. SEM micrographs clearly show that the MWNT loading can significantly affect the morphology of the spun fibers (Fig. 3). C-1 fibers, which resulted of spinning a composite made with stable CNTs/diamine dispersions, presented the similar fibrilar microstructure observed in the poliazomethine fibers, with microfibers oriented along the longitudinal axis of the fiber (Fig. 3a,b). That degree of polymer structural organization is however eventually lost when employing high MWNTs contents (Fig. 3c,d). Additionally, preliminary Raman spectroscopy results indicate that the MWNTs have some degree of preferential orientation along the axis of the composite fiber. 4. Conclusions Polyazomethine/MWNT composites presented characteristic polyazomethine features, such as good thermal stability, and the formation of nematic mesophases. Recent reports indicate that liquid crystals can induce CNT alignment [35,36], and evidences of MWNTs alignment has also been detected in both the composites and the composite fibers. The observed CNT
Fig. 3. SEM images of C-1 fiber (a, b) and C-2 fiber (c, d).
E. Lafuente et al. / Materials Science and Engineering C 26 (2006) 1198 – 1201
alignment within our LCP/CNT composites may result in novel materials with improved mechanical, electronic, and optical properties, which are subject of current research. Acknowledgements Funded by the Gobierno de Arago´n (Spain). E.M. acknowledges support from MEC (Ramo´n y Cajal Program). References [1] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. [2] P.M. Ajayan, Chem. Rev. 99 (1999) 1787. [3] E.T. Thostenson, Z. Ren, T.-W. Chou, Compos. Sci. Technol. 61 (2001) 1899 (and references therein). [4] J.K.W. Sandler, J.E. Kirk, I.A. Kinloch, M.S.P. Shaffer, A.H. Windle, Polymer 44 (2003) 5893. [5] S.A. Curran, P.M. Ajayan, W.J. Blau, D.L. Carroll, J.N. Coleman, A.B. Dalton, A.P. Davey, A. Drury, B. McCarthy, S. Maier, A. Strevens, Adv. Mater. 10 (1998) 1091. [6] A.B. Dalton, S. Collins, E. Mun˜oz, J.M. Razal, V.H. Ebron, J.P. Ferraris, J.N. Coleman, B.G. Kim, R.H. Baughman, Nature 423 (2003) 703. [7] E. Mun˜oz, A.B. Dalton, S. Collins, M. Kozlov, J. Razal, J.N. Coleman, B.G. Kim, V.H. Ebron, M. Selvidge, J.P. Ferraris, R.H. Baughman, Adv. Eng. Mater. 6 (2004) 801. [8] E. Mun˜oz, D.-S. Suh, S. Collins, M. Selvidge, A.B. Dalton, B.G. Kim, J.M. Razal, G. Ussery, A.G. Rinzler, M.T. Martı´nez, R.H. Baughman, Adv. Mater. 17 (2005) 1064. [9] L. Jin, C. Bower, O. Zhou, Appl. Phys. Lett. 73 (1998) 1197. [10] Z. Jia, Z. Whang, C. Xu, J. Lang, Mater. Sci. Eng., A Struct. Mater.: Prop. Microstruct. Process. 271 (1999) 395. [11] C.A. Cooper, R.J. Young, M. Halsall, Composites: Part A, 32 (2001) 401. [12] I. Musa, M. Baxendale, G.A.J. Amaratunga, W. Eccleston, Synth. Met. 102 (1999) 1250. [13] J.N. Coleman, S. Curran, A.B. Dalton, A.P. Davey, B. McCarthy, W. Blau, R.C. Barklie, Synth. Met. 102 (1999) 1174.
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