- Email: [email protected]
multi-walled carbon nanotube composites produced by hot stretching
Composites Science and Technology 71 (2011) 1367–1372
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Structure and properties of highly oriented polyoxymethylene/multi-walled carbon nanotube composites produced by hot stretching Xiaowen Zhao, Lin Ye ⇑ State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 17 September 2010 Received in revised form 14 May 2011 Accepted 17 May 2011 Available online 20 May 2011 Keywords: A. Polymer–matrix composites (PMCs) A. Carbon nanotubes B. Mechanical properties B. Thermal properties
a b s t r a c t Highly-oriented polyoxymethylene (POM)/multi-walled carbon nanotube (MWCNT) composites were fabricated through solid hot stretching technology. With the draw ratio as high as 900%, the oriented composites exhibited much improved thermal conductivity and mechanical properties along the stretching direction compared with that of the isotropic samples before drawing. The thermal conductivity of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawing. Microstructure observation demonstrated that the POM matrix had an ordered fibrillar bundle structure and MWCNTs in the composite tended to align parallel to the stretching direction. Wide-angle X-ray diffraction results showed that the crystal axis of the POM matrix was preferentially oriented perpendicular to the draw direction, while MWCNTs were preferentially oriented parallel to the draw direction. The strong interaction between the POM matrix and the MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Since the landmark paper on multi-walled carbon nanotubes (MWCNTs) by Iijima [1] in 1991 and single-walled carbon nanotubes (SWCNTs) by Iijima and Ichihashi [2] and Bethunes et al. [3] in 1993, owing to the rapid development of synthesis methods for carbon nanotubes (CNTs), high quality, long and aligned CNT ropes are now available. These advances in synthesis methods enabled the mechanical properties of CNTs to be more easily assessed. On account of their novel, structural, mechanical, and electronic properties, there is a considerable interest in fabricating composite materials containing CNTs, both from the point of view of fundamental property determination and the potential applications in many fields. In common with conventional fiber composites, both mechanical properties, such as stiffness and strength, and functional properties, such as electrical, magnetic and optical properties, of polymer/CNT composites are linked directly to the alignment of CNTs in the matrix, which is a topic that has received much recent attention [4–7]. Kimura et al. used a high magnetic field to align MWCNTs in a polyester matrix and obtained electrically conductive and mechanically anisotropic composites [8]. Sen et al. fabricated SWNT-reinforced polystyrene (PS) nanofibers and polyurethane (PU) membranes through the electrospinning process [9]. Lynch and Patrick oriented the nematic low molar mass liquid crystals in ⇑ Corresponding author. Tel.: +862 885408802; fax: +862 885402465. E-mail address: [email protected] (L. Ye). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.05.011
an electric field and used these matrices to align the suspended MWCNTs [10]. Polyoxymethylene (POM), with [–CH2–O–] as the main chain, is an engineering plastic with high mechanical strength, excellent abrasion resistance, fatigue resistance and moldability, and has wide applications [11,12]. Although the use of dispersed CNT as electric conducting fillers in POM composites has been reported [13,14], the studies on POM/CNT thermal conductive composites were scarcely reported. The work in our group prepared POM/MWCNTs thermal conductive composites by solution–evaporation method [15]. Solid hot stretching technology presents the advantages of high production rates, high orientation, and significant enhancements in the properties without complex processing apparatus [16–18]. In this work, highly oriented POM/MWCNTs composites with good dispersion and alignment of MWCNTs were first successfully fabricated through solid hot stretching technology, and a force field-induced alignment of MWCNTs was developed by chemically functionalization of MWCNTs with polyethylene glycol (PEG) [15]. The structure and properties of the oriented composites were studied. 2. Experimental 2.1. Materials POM used in this study is a commercial grade powder supplied by Yuntianhua Co., Ltd. (Yunnan, China). It is a copolymer with a melt flow index of 9.0 g/10 min (M90). MWCNTs with about 1.63 wt% hydroxyl group and a small amount of carboxyl groups
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were supplied by Chengdu Institute of Organic Chemistry (Chengdu, China), and synthesized from natural gas via catalytic chemical vapor deposition, with average length of microns, diameter of 20–40 nm. Stabilizers and other processing aids are all commercial grade products and used without further purification. 2.3. Preparation of the oriented POM/MWCNT composites 2.3.1. Sample preparation Firstly MWCNTs were surface modified with PEG-substituted amine according to the method of our previous work [15] in order to improve the interfacial bonding between POM and MWCNTs. POM/MWCNT composites were prepared through a solution–evaporation method assisted by ultrasonic irradiation, and then the product was compounded with stabilizers in a high-speed mixer, and extruded by a HAAKE MiniLab twin-screw extruder. The extrudate was pelletized and dried, and then molded to tensile specimens by using a HAAKE MiniJet injection molding machine.
2.3.2. Drawing The injection samples of the POM/MWCNT composites were drawn on the self-made hot stretching apparatus at 145 °C and the stretching rate was fixed at 25 mm/min. The draw ratio (DR) of the sample was determined with the following equation:
DR ¼ 100 ðL L0 Þ=L0 where L0 is the sample length prior to drawing and L is the final length after drawing. 2.4. Measurements The tensile properties of POM samples were measured with a 4302 material testing machine from Instron Co. (USA) according to ISO527/1-1993 (E). The thermal conductive properties of the POM samples were measured with a Hot Disk thermal analyser from Hot Disk AB Co. (Sweden). The surfaces morphology of the fractured samples was observed with a JEOL JSM-5900LV scanning
Fig. 1. The mechanical and thermal conductive properties of POM/MWCNT composites (1: before drawing, 2: after drawing).
X. Zhao, L. Ye / Composites Science and Technology 71 (2011) 1367–1372
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Fig. 2. SEM and TEM images of the oriented POM/MWCNT composites.
electron microscope (SEM) (Japan). The alignment of MWCNTs in the composite was analyzed with a JEM 1002CX transmission electron microscope (TEM) (Japan). Wide-angle X-ray diffraction (WAXD) analysis of the samples was conducted with Bruker D8 diffractometer with Cu Ka radiation generated at 40 kV and 30 mA.
3. Results and discussion 3.1. Mechanical and thermal conductive properties of drawn POM/ MWCNT composites The POM/MWCNT composite was drawn through the solid hot stretching technology, and the draw ratio can reach as high as 900%. The mechanical and thermal conductive properties of the isotropic and the drawn POM/MWCNT composites with 5.5% volume fraction of MWCNTs were shown in Fig. 1. Compared with the isotropic sample without drawing, the tensile strength and modulus of the drawn composites were enhanced by 620% and 870%, respectively, without remarkable drop of the elongation at break. In addition, the thermal conductive properties of the composites were also improved. The thermal conductivity increased from 0.44 W/m K for the undrawn sample to 0.81 W/m K for the drawn sample. The thermal conductivity of POM/MWCNT composites versus MWCNT content was shown in Fig. 1b. With increasing MWCNT volume fraction, the thermal conductivity of both the isotropic and the drawn POM/MWCNT composites increased remarkably. However, the composites after being drawn displayed higher thermal conductivity than the isotropic sample at varying MWCNT content, and with increasing MWCNT content, the difference of the thermal conductivity between these two samples became more and more large, which demonstrated that the thermal transfer was more effective in the aligned composites. The thermal conductivity
of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawn. Moreover, for pure POM, the thermal conductivity increased from 0.35 W/m K before drawing to 0.55 W/m K after drawing, which resulted from the formed mat texture crystals and fibrillar structure under stress [19]. It is known that thermal conduction of POM follows phonon conduction mechanism. The thermal conductivity is higher along the chain axis combined by covalent bonding than that perpendicular to the chain axis combined by van der Waals interaction in the crystal region, and the thermal conductivity in the crystal region is much higher than that in the amorphous region along the chain axis. Therefore, the deformation of POM from spherulites to high oriented micro-fibers and the stress-induced crystallization contributed to the significantly high thermal conductivity of the drawn sample. For POM/MWCNT composites, both the formation of high oriented micro-fibers of POM and orientation of MWCNTs contributed to the significantly high thermal conductivity of the composite.
3.2. Morphology of oriented POM/MWCNT composites The section morphology of POM/MWCNT composites after being drawn was shown in Fig. 2. It can be seen that the section of the sample exhibited orderly arranged fibrillar bundle structure, and these micro-fibers were mainly composed of highly oriented folded lamellar crystals of POM and noncrystal parts oriented along the drawing direction which alternately and periodically arranged. From the high magnified images (Fig. 2b), the alignment of MWCNTs along with the draw direction was observed. Moreover, MWCNTs were found to be dispersed well in POM matrix without significant aggregation. The alignment of MWCNTs in the composite was further demonstrated with TEM measurement, as shown in Fig. 2c and d. It can be seen that a large amount of MWCNTs were
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Fig. 3. 2D-WAXD patterns of POM and POM/MWCNT composites.
Fig. 4. WAXD curves of pure POM and POM/MWCNT composites (1: draw ratio 0; 2: draw ratio 200%; 3: draw ratio 500%; 4: draw ratio 900%).
Table 1 Crystallinity and orientation factor of POM/MWCNT composites. Draw ratio (%)
200 500 900
Crystallinity (%)
Orientation factor
POM
POM/MWCNT
POM
POM/MWCNT
76.02 77.81 79.74
77.10 76.04 79.04
0.8387 0.9751 0.9878
0.7857 0.9646 0.9776
aligned with their longitudinal axes parallel to the draw direction. The formation of these high oriented micro-fibers of POM and oriented MWCNTs contributed to the significantly high tensile strength and modulus as well as high thermal conductivity of the sample. As shown in Fig. 2b, it can also be seen that microvoid was produced in the process of drawing. In some areas, MWCNTs were found to bridge the microvoids in the matrix and presumably further enhanced the strength of the composites.
X. Zhao, L. Ye / Composites Science and Technology 71 (2011) 1367–1372
1371
Fig. 5. The schematic illustration of PEG-substituted amine functionalized MWCNTs and POM.
3.3. Orientation and crystallization structure The WAXD patterns of POM and the POM/MWCNT composites before and after drawing were shown in Fig. 3. POM has a hexagonal unit cell with dimensions of a = b = 4.45 Å and c = 17.3 Å. The molecular chains are arranged in a 9/5 helix where a and b axes are on the same plane and the chains are aligned parallel to the c axis of the crystal. The calculated positions of the peaks in the 2h scan are 22.9°, 34.6° and 48.4° for diffraction planes (1 0 0), (1 0 5) and (1 1 5) with Miller indices respectively. As shown in Fig. 3a, the isotropic sample of pure POM before drawing presented a series of clear and uniform Debye-Scherrer diffraction rings due to the random arrangement of grains. Compared to pure POM, in addition to the diffraction rings attributed to POM, the patterns of the POM/MWCNT composites (Fig. 3b) presented a new diffraction ring around 26.1° in 2h, corresponding to a d-spacing of 3.4 Å, which was the intershell spacing within the nanotubes. For the composites before drawing, when the MWCNTs were randomly dispersed inside the polymer matrix, a diffraction ring with uniform intensity distribution was expected. For the pure POM after drawing, as shown in Fig. 3c–e, the (1 0 0) reflection of POM appeared as two strong circular spots on the equator and (1 0 5) reflection of POM formed a four-point image. With increasing draw ratio, these arcs became narrower in spread and more prominent, suggesting that the crystal axis was preferentially oriented perpendicular to the draw direction. For the POM/MWCNT composites after drawing, with increasing draw ratio, the diffraction arcs of POM became narrower. Moreover, the Bragg intensities of the MWCNTs were also concentrated at two spots, suggesting that MWCNTs were preferentially oriented along the draw direction. One-dimensional X-ray diffraction curve corresponding to the two-dimensional diffraction pattern was shown in Fig. 4. The intensity of the diffraction peak represented the degree of the order in the material, including crystallization and orientation. It can be seen that the diffraction peak position of oriented POM matrix with different draw ratio had no change, however, there were significant differences in intensity, indicating that stretching did not affect the crystal type, but it can significantly affect the crystallinity and orientation factor of POM. According to Hermans orientation model (1–3), the orientation factor (f) of POM matrix at different draw ratio can be calculated, and the data were listed in Table 1.
f ¼
3hcos2 /i 1 2
2
cos / ¼
R 22 0
Ið/Þ sin /ðcos2 /Þd/ R 22 0 Ið/Þ sin /d/
cos / ¼ cos h cos u
ð1Þ
ð2Þ
ð3Þ
where h is Bragg diffraction angle; u is the angle between the normal of crystal surface and the tensile direction.
As shown in Table 1, with the increase of the draw ratio, the crystallinity and orientation factor increased. Compared with the pure POM at the same draw ratio, the orientation degree of POM in the composites was lower, indicating that there existed strong interaction between POM and MWCNTs, which hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs. As the experimental part described, since MWCNTs was functionalized with PEG-substituted amine, the amine group on one end of the PEG-substituted amine reacted with hydroxyl group and carboxyl groups on MWCNTs to form strong interactions and the PEG chain on the other end of the compound synchronously linked with POM molecules through physical entanglement due to their similar molecular structure as shown in Fig. 5 [15]. Hence it can bridge the MWCNTs and POM tightly together. 4. Conclusions Highly oriented POM/MWCNT composites with good dispersion and alignment of MWCNTs were fabricated through solid hot stretching technology. The mechanical and the thermal conductive properties of POM/MWCNT composite were studied and the results showed that compared with the isotropic POM/MWCNT composites, the tensile strength, modulus and thermal conductivity of the drawn composites were enhanced by 620%, 870% and 180% respectively. The SEM and TEM observation demonstrated that the POM matrix exhibited orderly arranged fibrillar bundle structure and MWCNTs were aligned parallel to the draw direction. The results of WAXD showed that the crystallinity and orientation factor of the POM increased with draw ratio, while the (0 0 2) diffraction of MWCNTs concentrated gradually onto the equator, indicating that MWCNTs were aligned and oriented simultaneously with the matrix. At the same draw ratio, the orientation degree of POM in the composites was lower than that of the pure POM, indicating that the strong interaction between POM and MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs. Acknowledgements This research was supported by New-Century Training Program Foundation for the Talents (NCET-07-0589) and Ph.D. Programs Foundation of Ministry of Education of China for new teacher (20100181120018). References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Iijima S, Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993;363:603–5. [3] Bethunes DS, Kiang CH, Devries MS, Gorman G, Savoy R, Vazquez J, et al. Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993;363:605–7. [4] Xie XL, Mai YW, Zhou XP. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Mater Sci Eng 2005;49:89–112. [5] Jin L, Bower C, Zhou O. Alignment of carbon nanotubes in a polymer matrix by mechanical stretching. Appl Phys Lett 1998;73:1197–202.
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[6] Haggenmuller R, Gommans HH, Rinzler AG, Fischer JE, Winey KI. Aligned single-wall carbon nanotubes in composites by melt processing methods. Chem Phys Lett 2003;330:219–25. [7] Bhattachacharyya AB, Sreekumar TV, Liu T, Kumar S, Ericson LM, Hauge H, et al. Crystallization and orientation studies in polypropylene/single wall carbon nanotube composite. Polymer 2003;44:2373–7. [8] Kimura T, Ago H, Tobita M. Polymer composites of carbon nanotubes aligned by a magnetic field. Adv Mater 2002;14:1380–3. [9] Sen R, Zhao B, Perea D, Itkis ME, Hu H, Love J, et al. Preparation of single-walled carbon nanotube reinforced polystyrene and polyurethane nanofibers and membranes by electrospinning. Nano Lett 2004;4:459–64. [10] Lynch MD, Patrick DL. Organizing carbon nanotubes with liquid crystals. Nano Lett 2002;2:1197–210. [11] Hasegawa S, Takeshita H, Yoshii F, Nishimoto S. Thermal degradation behavior of gamma-irradiated acetyloxy end-capped poly(oxymethylene). Polymer 2000;41:111–20. [12] Hisakatsu H, Kohji T. Structural changes in non-isothermal crystallization process of melt-cooled polyoxymethylene. [I] Detection of infrared bands characteristic of folded and extended chain crystal morphologies and extraction of a lamellar stacking model. Polymer 2003;44:3107–16.
[13] Bao HD, Guo ZX, Yu J. Effect of electrically inert particulate filler on electrical resistivity of polymer/multi-walled carbon nanotube composites. Polymer 2008;49:3826–31. [14] Zeng Y, Liu PF, Du JH, Zhao L, Ajayan PM, Cheng HM. Increasing the electrical conductivity of carbon nanotube/polymer composites by using weak nanotube–polymer interactions. Carbon 2010;48:3551–8. [15] Zhao XW, Ye L. Preparation, structure, and property of polyoxymethylene/ carbon nanotubes thermal conductive composites. J Polym Sci Pol Phys 2010;48:905–12. [16] Galeski A. Strength and toughness of crystalline polymer systems. Prog Polym Sci 2003;28:1643–99. [17] Mohanraj J, Morawiec J, Pawlak A, Barton DC, Galeski A, Ward IM. Orientation of polyoxymethylene by rolling with side constraints. Polymer 2008;49:303–16. [18] Schaffner F, Vinson J, Jungnickel BJ. Hydrostatic extrusion rate relaxation in polyethylene, polyoxymethylene, and their blends: the relaxation of viscosity and compressibility in polymeric solids. Die Angewandte Makromolekulare Chemie 1991;185:137–45. [19] Zhao XW, Ye L. Structure and properties of highly oriented polyoxymethylene produced by hot stretching. Mater Sci Eng A 2011;528:4585–91.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Structure and properties of highly oriented polyoxymethylene/multi-walled carbon nanotube composites produced by hot stretching Xiaowen Zhao, Lin Ye ⇑ State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China
a r t i c l e
i n f o
Article history: Received 17 September 2010 Received in revised form 14 May 2011 Accepted 17 May 2011 Available online 20 May 2011 Keywords: A. Polymer–matrix composites (PMCs) A. Carbon nanotubes B. Mechanical properties B. Thermal properties
a b s t r a c t Highly-oriented polyoxymethylene (POM)/multi-walled carbon nanotube (MWCNT) composites were fabricated through solid hot stretching technology. With the draw ratio as high as 900%, the oriented composites exhibited much improved thermal conductivity and mechanical properties along the stretching direction compared with that of the isotropic samples before drawing. The thermal conductivity of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawing. Microstructure observation demonstrated that the POM matrix had an ordered fibrillar bundle structure and MWCNTs in the composite tended to align parallel to the stretching direction. Wide-angle X-ray diffraction results showed that the crystal axis of the POM matrix was preferentially oriented perpendicular to the draw direction, while MWCNTs were preferentially oriented parallel to the draw direction. The strong interaction between the POM matrix and the MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Since the landmark paper on multi-walled carbon nanotubes (MWCNTs) by Iijima [1] in 1991 and single-walled carbon nanotubes (SWCNTs) by Iijima and Ichihashi [2] and Bethunes et al. [3] in 1993, owing to the rapid development of synthesis methods for carbon nanotubes (CNTs), high quality, long and aligned CNT ropes are now available. These advances in synthesis methods enabled the mechanical properties of CNTs to be more easily assessed. On account of their novel, structural, mechanical, and electronic properties, there is a considerable interest in fabricating composite materials containing CNTs, both from the point of view of fundamental property determination and the potential applications in many fields. In common with conventional fiber composites, both mechanical properties, such as stiffness and strength, and functional properties, such as electrical, magnetic and optical properties, of polymer/CNT composites are linked directly to the alignment of CNTs in the matrix, which is a topic that has received much recent attention [4–7]. Kimura et al. used a high magnetic field to align MWCNTs in a polyester matrix and obtained electrically conductive and mechanically anisotropic composites [8]. Sen et al. fabricated SWNT-reinforced polystyrene (PS) nanofibers and polyurethane (PU) membranes through the electrospinning process [9]. Lynch and Patrick oriented the nematic low molar mass liquid crystals in ⇑ Corresponding author. Tel.: +862 885408802; fax: +862 885402465. E-mail address: [email protected] (L. Ye). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.05.011
an electric field and used these matrices to align the suspended MWCNTs [10]. Polyoxymethylene (POM), with [–CH2–O–] as the main chain, is an engineering plastic with high mechanical strength, excellent abrasion resistance, fatigue resistance and moldability, and has wide applications [11,12]. Although the use of dispersed CNT as electric conducting fillers in POM composites has been reported [13,14], the studies on POM/CNT thermal conductive composites were scarcely reported. The work in our group prepared POM/MWCNTs thermal conductive composites by solution–evaporation method [15]. Solid hot stretching technology presents the advantages of high production rates, high orientation, and significant enhancements in the properties without complex processing apparatus [16–18]. In this work, highly oriented POM/MWCNTs composites with good dispersion and alignment of MWCNTs were first successfully fabricated through solid hot stretching technology, and a force field-induced alignment of MWCNTs was developed by chemically functionalization of MWCNTs with polyethylene glycol (PEG) [15]. The structure and properties of the oriented composites were studied. 2. Experimental 2.1. Materials POM used in this study is a commercial grade powder supplied by Yuntianhua Co., Ltd. (Yunnan, China). It is a copolymer with a melt flow index of 9.0 g/10 min (M90). MWCNTs with about 1.63 wt% hydroxyl group and a small amount of carboxyl groups
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were supplied by Chengdu Institute of Organic Chemistry (Chengdu, China), and synthesized from natural gas via catalytic chemical vapor deposition, with average length of microns, diameter of 20–40 nm. Stabilizers and other processing aids are all commercial grade products and used without further purification. 2.3. Preparation of the oriented POM/MWCNT composites 2.3.1. Sample preparation Firstly MWCNTs were surface modified with PEG-substituted amine according to the method of our previous work [15] in order to improve the interfacial bonding between POM and MWCNTs. POM/MWCNT composites were prepared through a solution–evaporation method assisted by ultrasonic irradiation, and then the product was compounded with stabilizers in a high-speed mixer, and extruded by a HAAKE MiniLab twin-screw extruder. The extrudate was pelletized and dried, and then molded to tensile specimens by using a HAAKE MiniJet injection molding machine.
2.3.2. Drawing The injection samples of the POM/MWCNT composites were drawn on the self-made hot stretching apparatus at 145 °C and the stretching rate was fixed at 25 mm/min. The draw ratio (DR) of the sample was determined with the following equation:
DR ¼ 100 ðL L0 Þ=L0 where L0 is the sample length prior to drawing and L is the final length after drawing. 2.4. Measurements The tensile properties of POM samples were measured with a 4302 material testing machine from Instron Co. (USA) according to ISO527/1-1993 (E). The thermal conductive properties of the POM samples were measured with a Hot Disk thermal analyser from Hot Disk AB Co. (Sweden). The surfaces morphology of the fractured samples was observed with a JEOL JSM-5900LV scanning
Fig. 1. The mechanical and thermal conductive properties of POM/MWCNT composites (1: before drawing, 2: after drawing).
X. Zhao, L. Ye / Composites Science and Technology 71 (2011) 1367–1372
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Fig. 2. SEM and TEM images of the oriented POM/MWCNT composites.
electron microscope (SEM) (Japan). The alignment of MWCNTs in the composite was analyzed with a JEM 1002CX transmission electron microscope (TEM) (Japan). Wide-angle X-ray diffraction (WAXD) analysis of the samples was conducted with Bruker D8 diffractometer with Cu Ka radiation generated at 40 kV and 30 mA.
3. Results and discussion 3.1. Mechanical and thermal conductive properties of drawn POM/ MWCNT composites The POM/MWCNT composite was drawn through the solid hot stretching technology, and the draw ratio can reach as high as 900%. The mechanical and thermal conductive properties of the isotropic and the drawn POM/MWCNT composites with 5.5% volume fraction of MWCNTs were shown in Fig. 1. Compared with the isotropic sample without drawing, the tensile strength and modulus of the drawn composites were enhanced by 620% and 870%, respectively, without remarkable drop of the elongation at break. In addition, the thermal conductive properties of the composites were also improved. The thermal conductivity increased from 0.44 W/m K for the undrawn sample to 0.81 W/m K for the drawn sample. The thermal conductivity of POM/MWCNT composites versus MWCNT content was shown in Fig. 1b. With increasing MWCNT volume fraction, the thermal conductivity of both the isotropic and the drawn POM/MWCNT composites increased remarkably. However, the composites after being drawn displayed higher thermal conductivity than the isotropic sample at varying MWCNT content, and with increasing MWCNT content, the difference of the thermal conductivity between these two samples became more and more large, which demonstrated that the thermal transfer was more effective in the aligned composites. The thermal conductivity
of the composite with 11.6 vol.% MWCNTs can reach as high as 1.2 W/m K after drawn. Moreover, for pure POM, the thermal conductivity increased from 0.35 W/m K before drawing to 0.55 W/m K after drawing, which resulted from the formed mat texture crystals and fibrillar structure under stress [19]. It is known that thermal conduction of POM follows phonon conduction mechanism. The thermal conductivity is higher along the chain axis combined by covalent bonding than that perpendicular to the chain axis combined by van der Waals interaction in the crystal region, and the thermal conductivity in the crystal region is much higher than that in the amorphous region along the chain axis. Therefore, the deformation of POM from spherulites to high oriented micro-fibers and the stress-induced crystallization contributed to the significantly high thermal conductivity of the drawn sample. For POM/MWCNT composites, both the formation of high oriented micro-fibers of POM and orientation of MWCNTs contributed to the significantly high thermal conductivity of the composite.
3.2. Morphology of oriented POM/MWCNT composites The section morphology of POM/MWCNT composites after being drawn was shown in Fig. 2. It can be seen that the section of the sample exhibited orderly arranged fibrillar bundle structure, and these micro-fibers were mainly composed of highly oriented folded lamellar crystals of POM and noncrystal parts oriented along the drawing direction which alternately and periodically arranged. From the high magnified images (Fig. 2b), the alignment of MWCNTs along with the draw direction was observed. Moreover, MWCNTs were found to be dispersed well in POM matrix without significant aggregation. The alignment of MWCNTs in the composite was further demonstrated with TEM measurement, as shown in Fig. 2c and d. It can be seen that a large amount of MWCNTs were
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Fig. 3. 2D-WAXD patterns of POM and POM/MWCNT composites.
Fig. 4. WAXD curves of pure POM and POM/MWCNT composites (1: draw ratio 0; 2: draw ratio 200%; 3: draw ratio 500%; 4: draw ratio 900%).
Table 1 Crystallinity and orientation factor of POM/MWCNT composites. Draw ratio (%)
200 500 900
Crystallinity (%)
Orientation factor
POM
POM/MWCNT
POM
POM/MWCNT
76.02 77.81 79.74
77.10 76.04 79.04
0.8387 0.9751 0.9878
0.7857 0.9646 0.9776
aligned with their longitudinal axes parallel to the draw direction. The formation of these high oriented micro-fibers of POM and oriented MWCNTs contributed to the significantly high tensile strength and modulus as well as high thermal conductivity of the sample. As shown in Fig. 2b, it can also be seen that microvoid was produced in the process of drawing. In some areas, MWCNTs were found to bridge the microvoids in the matrix and presumably further enhanced the strength of the composites.
X. Zhao, L. Ye / Composites Science and Technology 71 (2011) 1367–1372
1371
Fig. 5. The schematic illustration of PEG-substituted amine functionalized MWCNTs and POM.
3.3. Orientation and crystallization structure The WAXD patterns of POM and the POM/MWCNT composites before and after drawing were shown in Fig. 3. POM has a hexagonal unit cell with dimensions of a = b = 4.45 Å and c = 17.3 Å. The molecular chains are arranged in a 9/5 helix where a and b axes are on the same plane and the chains are aligned parallel to the c axis of the crystal. The calculated positions of the peaks in the 2h scan are 22.9°, 34.6° and 48.4° for diffraction planes (1 0 0), (1 0 5) and (1 1 5) with Miller indices respectively. As shown in Fig. 3a, the isotropic sample of pure POM before drawing presented a series of clear and uniform Debye-Scherrer diffraction rings due to the random arrangement of grains. Compared to pure POM, in addition to the diffraction rings attributed to POM, the patterns of the POM/MWCNT composites (Fig. 3b) presented a new diffraction ring around 26.1° in 2h, corresponding to a d-spacing of 3.4 Å, which was the intershell spacing within the nanotubes. For the composites before drawing, when the MWCNTs were randomly dispersed inside the polymer matrix, a diffraction ring with uniform intensity distribution was expected. For the pure POM after drawing, as shown in Fig. 3c–e, the (1 0 0) reflection of POM appeared as two strong circular spots on the equator and (1 0 5) reflection of POM formed a four-point image. With increasing draw ratio, these arcs became narrower in spread and more prominent, suggesting that the crystal axis was preferentially oriented perpendicular to the draw direction. For the POM/MWCNT composites after drawing, with increasing draw ratio, the diffraction arcs of POM became narrower. Moreover, the Bragg intensities of the MWCNTs were also concentrated at two spots, suggesting that MWCNTs were preferentially oriented along the draw direction. One-dimensional X-ray diffraction curve corresponding to the two-dimensional diffraction pattern was shown in Fig. 4. The intensity of the diffraction peak represented the degree of the order in the material, including crystallization and orientation. It can be seen that the diffraction peak position of oriented POM matrix with different draw ratio had no change, however, there were significant differences in intensity, indicating that stretching did not affect the crystal type, but it can significantly affect the crystallinity and orientation factor of POM. According to Hermans orientation model (1–3), the orientation factor (f) of POM matrix at different draw ratio can be calculated, and the data were listed in Table 1.
f ¼
3hcos2 /i 1 2
2
cos / ¼
R 22 0
Ið/Þ sin /ðcos2 /Þd/ R 22 0 Ið/Þ sin /d/
cos / ¼ cos h cos u
ð1Þ
ð2Þ
ð3Þ
where h is Bragg diffraction angle; u is the angle between the normal of crystal surface and the tensile direction.
As shown in Table 1, with the increase of the draw ratio, the crystallinity and orientation factor increased. Compared with the pure POM at the same draw ratio, the orientation degree of POM in the composites was lower, indicating that there existed strong interaction between POM and MWCNTs, which hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs. As the experimental part described, since MWCNTs was functionalized with PEG-substituted amine, the amine group on one end of the PEG-substituted amine reacted with hydroxyl group and carboxyl groups on MWCNTs to form strong interactions and the PEG chain on the other end of the compound synchronously linked with POM molecules through physical entanglement due to their similar molecular structure as shown in Fig. 5 [15]. Hence it can bridge the MWCNTs and POM tightly together. 4. Conclusions Highly oriented POM/MWCNT composites with good dispersion and alignment of MWCNTs were fabricated through solid hot stretching technology. The mechanical and the thermal conductive properties of POM/MWCNT composite were studied and the results showed that compared with the isotropic POM/MWCNT composites, the tensile strength, modulus and thermal conductivity of the drawn composites were enhanced by 620%, 870% and 180% respectively. The SEM and TEM observation demonstrated that the POM matrix exhibited orderly arranged fibrillar bundle structure and MWCNTs were aligned parallel to the draw direction. The results of WAXD showed that the crystallinity and orientation factor of the POM increased with draw ratio, while the (0 0 2) diffraction of MWCNTs concentrated gradually onto the equator, indicating that MWCNTs were aligned and oriented simultaneously with the matrix. At the same draw ratio, the orientation degree of POM in the composites was lower than that of the pure POM, indicating that the strong interaction between POM and MWCNTs hindered the orientation movement of molecules of POM, but induced the orientation movement of MWCNTs. Acknowledgements This research was supported by New-Century Training Program Foundation for the Talents (NCET-07-0589) and Ph.D. Programs Foundation of Ministry of Education of China for new teacher (20100181120018). References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [2] Iijima S, Ichihashi T. Single-shell carbon nanotubes of 1-nm diameter. Nature 1993;363:603–5. [3] Bethunes DS, Kiang CH, Devries MS, Gorman G, Savoy R, Vazquez J, et al. Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls. Nature 1993;363:605–7. [4] Xie XL, Mai YW, Zhou XP. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Mater Sci Eng 2005;49:89–112. [5] Jin L, Bower C, Zhou O. Alignment of carbon nanotubes in a polymer matrix by mechanical stretching. Appl Phys Lett 1998;73:1197–202.
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