Effect of modified carbon nanotube on physical properties of thermotropic liquid crystal polyester nanocomposites

European Polymer Journal 45 (2009) 316–324

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Effect of modified carbon nanotube on physical properties of thermotropic liquid crystal polyester nanocomposites Jun Young Kim a,b,*, Duk Ki Kim c, Seong Hun Kim c a

Material Laboratory, Corporate R&D Center, Samsung SDI Co., Ltd., 575 Shin-dong, Yeongtong-gu, Suwon-si, Gyeonggi-do 443-731, Republic of Korea Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA c Department of Fiber & Polymer Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea

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b

a r t i c l e

i n f o

Article history: Received 24 August 2008 Received in revised form 7 October 2008 Accepted 14 October 2008 Available online 7 November 2008

Keywords: Blending Carbon nanotube (CNT) Liquid-crystalline polymer (LCP) Polymer nanocomposites Reinforcement

a b s t r a c t Polymer nanocomposites based on a very small quantity of carbon nanotube (CNT) and thermotropic liquid crystal polymer (TLCP) were prepared by simple melt blending using a twin-screw extruder. Morphological observations revealed that modified CNT was uniformly dispersed in the TLCP matrix and increased interfacial adhesion between the nanotubes and the polymer matrix. The enhancement of the storage and loss moduli of the TLCP nanocomposites with the introduction of CNT was more pronounced at low frequency region, and non-terminal behavior observed in the TLCP nanocomposites resulted from the nanotube–nanotube and polymer–nanotubes interactions. There is significant dependence of the mechanical, rheological, and thermal properties of the TLCP nanocomposites on the uniform dispersion of CNT and the interfacial adhesion between CNT and TLCP matrix, and their synergistic effect was more effective at low CNT content than at high CNT content. The key to improve the overall properties of the TLCP nanocomposites depends on the optimization of the unique geometry and dispersion state of CNT and the interfacial interactions in the TLCP nanocomposites during melt processing. This study demonstrate that a very small quantity of CNT substantially improved thermal stability and mechanical properties of the TLCP nanocomposites, providing a design guide of CNT-filled TLCP composites with as great potential for industrial use. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Thermotropic liquid crystal polymer (TLCP) that is a class of the materials to produce the anisotropic melts of relatively low viscosity, has attracted a great deal of interest in fields ranging from the scientific to the industrial, because of their excellent mechanical properties, thermal endurance, and chemical stability [1–3]. TLCP that exhibit higher strength and stiffness due to its rigid-rod like molecules can be preferentially oriented to form fibrils under elongational or shear flow during melt processing, and the oriented fibrous structures are devel* Corresponding author. Tel.: +82 31 210 7035; fax: +82 31 210 7374. E-mail address: [email protected] (J.Y. Kim). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.10.043

oped in the extruded TLCP, resulting in self-reinforcing characteristics [4–6]. In this regard, much research have been extensively performed to date both to displace conventional thermoplastic polymers and to develop commercial applications of TLCP and its composites such a high performance polymer or industrial fiber [7–10]. Although promising, however, practical applications of TLCP are limited compared to conventional thermoplastic polymers, because TLCP is relatively expensive due to high cost of the monomer. Carbon nanotube (CNT) has attracted a great deal of scientific interest as advanced materials for next generation because of its excellent electronic properties, thermal conductivity, and mechanical strength, and created a high level of activity in materials research for its potential

J.Y. Kim et al. / European Polymer Journal 45 (2009) 316–324

2. Experimental 2.1. Materials The TLCP used was a flexible semi-aromatic copolyester synthesized from poly(p-hydroxybenzoate) (PHB) and poly(ethylene terephthalate) (PET) with a molar ratio of 80:20, purchased from Unitika Co. Ltd., Japan. The nanotubes used were multi-walled CNT (degree of purity > 95%) synthesized by a thermal chemical vapor deposition process, purchased from Iljin Nanotech Co., Korea. The diameter and length of CNT were in the range of 10–30 nm and 10– 50 lm, respectively, indicating that their aspect ratio reaches 1000. The concentrated nitric acid (HNO3, 68%) and sulfuric acid (H2SO4, 98%) were purchased from Sigma–Aldrich Co., and they used as-received without further purification. 2.2. Preparation of polymer nanocomposites The CNT was modified by the following steps: CNT was added to the mixture of concentrated HNO3 and H2SO4 with a volumetric ratio of 1:3 and this mixture was sonicated at 80 °C for 4 h to create the carboxylic acid groups on the CNT’s surface. This mixture was diluted with excess deionized water and then vacuum-filtered through 0.22 lm millipore PTFE membranes until the pH of the filtrate reached approximately 7. The filtrate solid was dried in vacuo at 100 °C for 24 h, yielding the modified CNT. The carboxylic acid groups on the nanotube surface were effectively induced via this chemical modification [16] to increase the chemical affinity of the CNT with the TLCP, yet to decrease the p–p stacking effect among the aromatic rings of the nanotubes, which often leads to the formation of their agglomeration [27]. All materials were dried at 120 °C in vacuo for at 24 h before use, to minimize the effects of moisture. The TLCP nanocomposites were prepared by a melt blending process in a Haake rheometer (Haake Technik GmbH, Germany) equipped with a twin-screw (intermeshing co-rotating type). The temperatures of the heating zone from the hopper to the die were set to 290, 300, 305, and 295 °C, and the screw speed was fixed at 40 rpm. Prior to melt blending, TLCP and CNT were physically premixed before being fed into hopper of the extruder to achieve better dispersion of CNT with TLCP matrix. For the fabrication of the TLCP nanocomposites, TLCP was melt blended with the addition of CNT content, specified as 0.5, 1.0, and 1.5 wt% in the polymer matrix, respectively. Upon completion of melt blending, the extruded strands were allowed to cool in the water-bath, and then cut into pellets with constant diameter and length using a rate-controlled PP1 pelletizer (Haake Technik GmbH, Germany). 2.3. Characterizations Chemical structure of CNT was characterized by means of FT-IR measurement using a Magna-IR 550 spectrometer (Nicolet Co.) in the range of 400–4000 cm1. The change in the structure of CNT by chemical modification was characterized by means of a JASCO NRS-3100 Raman spectrometer (JASCO Inc.) equipped with a 532 nm diode laser, and

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applications in a broad range of industry [11–13]. Fundamental research progressed to date suggests that CNT is regarded as promising reinforcements in the polymer composites due to the combination of their uniquely excellent properties with high aspect ratio and small size [11–13]. This feature has motivated a number of efforts to fabricate CNT/polymer nanocomposites, with the benefit of nanotechnology, in the development of advanced composite materials. However, high cost and limited availability, only a few practical applications in industrial field such as electronic and electric appliances have been realized to date. For the fabrication of CNT/polymer nanocomposites, major goals to realize the potential application of CNT as nanoreinforcing fillers are homogeneous dispersion of CNT in the polymer matrix and good interfacial adhesion between CNT and polymer matrix. In general, CNT tends to bundle together and to form some agglomeration due to intrinsic van der Waals attraction between the individual tubes [14]. Also, weak interfacial bonding between the nanotubes and the polymer matrix has limited the efficient load transfer to the polymer matrix, playing a limited reinforcement role in the polymer nanocomposites [15]. The functionalization of CNT, which can be considered as an effective method to achieve the uniform dispersion of CNT and their compatibility with the polymers, can lead to the enhancement of the interfacial adhesion between CNT and polymer matrix, thereby improving the overall properties of CNT/polymer nanocomposites [16–18]. In addition, one of major challenges for the fabrication of CNT/polymer nanocomposites is to optimize the processing operations at cheap manufacturing cost. Among various processing techniques for the fabrication of CNT/polymer nanocomposites, melt compounding has been accepted as the simplest and the most effective method from an industrial perspective, because this process makes it possible to fabricate high performance polymer nanocomposites at low process cost, and facilitates commercial scale-up [19–26]. Furthermore, the combination of a very small quantity of CNT with TLCP provides attractive possibilities for improving the physical properties of polymer nanocomposites using a cost-effective method. In this study, polymer nanocomposites based on TLCP and a very small quantity of CNT were prepared by direct melt compounding in a twin-screw extruder to create high performance composite materials with low cost for practical applications in various industrial fields. To our knowledge, an attempt to disperse CNT in the TLCP matrix and to fabricate TLCP/CNT nanocomposites by a simple melt compounding has been rarely investigated to date, and the effects of CNT on the mechanical, rheological, and thermal properties of TLCP/CNT nanocomposites have not yet been reported in the literature. Thus, we expect that this study will be helpful in providing the preliminary understanding on the physical properties of the TLCP nanocomposites reinforced with a very small quantity of CNT. Our study also suggests a simple and cost-effective method that will facilitate the industrial realization of CNT-filled TLCP nanocomposite with enhanced physical properties.

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the laser beam with a nominal power of 30 mW was focused to a spot size of 5 lm diameter. Morphologies of CNT and the TLCP nanocomposites were observed using a JEOL 2000-FX TEM and a JEOL JSM-6340F SEM. Rheological properties of the TLCP nanocomposites were performed on an ARES (Advanced Rheometric Expansion System) rheometer (Rheometric Scientific, Inc.) in oscillation mode with the parallel-plate geometry using the plate diameter of 25 mm and the plate gap setting of 1 mm at 300 °C. Frequency ranges were varied between 0.05 and 450 rad/s, and the strain amplitude was applied to be within the linear viscoelastic ranges. Mechanical properties of the TLCP nanocomposites were measured using an Instron 4465 testing machine, according to the procedures in the ASTM D 638 standard. The gauge length and the crosshead speed were set to 20 mm and 10 mm/min, respectively. TGA analysis of the TLCP nanocomposite were performed with a TA instrument SDF 2960 TGA over a temperature rage of 30–800 °C at a heating rate of 10 °C/min under N2.

3. Results and discussion 3.1. CNT modification FT-IR spectra of pristine and modified CNTs are shown in Fig. 1a. The characteristic peaks observed at approximately 1570 cm1 was attributed to the IR-phonon mode of multi-walled CNT [28]. For modified CNT, the characteristic peaks observed at 1090, 1189, 1399, and 1723 cm1 corresponded to the stretching vibrations of carboxylic acid groups [29]. This result demonstrates that carboxylic acid groups on the nanotube surface were effectively induced via chemical modification. Raman spectra of pristine and modified CNTs are shown in Fig. 1b. Similar pattern of the Raman spectra indicated that the graphite structure of CNT was not affected by this chemical modification. Two characteristic peaks observed at approximately 1350 and 1570 cm1, respectively, can be termed as D-band and G-band, respectively. The G-band was related to the structural intensity of the sp2-hybridized carbon atoms of the CNT [30]. The D-band reflected the disorder-induced carbon atoms, resulting from the defects in the nanotube and their ends, and its intensity decreases with the degree of the graphitization of CNT [30]. The relative intensity of D-band was slightly increased and the relative ratio of G-band to D-band (IG/ID) was slightly increased as compared to pristine CNT. This result indicated that the sp2-hybrid decreased and the sp3 hybrid increased after the chemical modification and that the degree of disorder and the defects on the nanotube’s surface increased by chemical modification. Typically, pristine CNT often tends to bundle together and to form some agglomeration because of intrinsic van der Waals attractions between the individual tubes in combination with high aspect ratio and large surface area, thus leading to the obstruction of efficient load transfer to the polymer matrix [14]. As shown in Fig. 1, modified CNT exhibits less entangled organization after chemical modification instead of showing the agglomerated structures observed in pristine CNT. Thus, it is expected that the functional groups induced on

the CNT surface are helpful for enhancing the dispersion state of CNT and the interfacial interaction in the TLCP nanocomposites through hydrogen bonding formation, as shown in Scheme 1. However, Grossiord et al. [31] reported that there was a possibility to occur the slight damage and the decreased length in the nanotube structure induced by strong acid treatment, may be having a negative effect on the physical properties of polymer nanocomposites. The elemental compositions of the TLCP nanocomposites were characterized by XPS. In the XPS survey spectra of the TLCP nanocomposites, it can be seen that two characteristic peaks, corresponding to C1s and O1s at approximately 285 and 523 eV, respectively, were observed, which were assigned to the sp2 carbon and the ester-type groups with oxygen bonded to carbon. In the high resolution C1s XPS spectra of TLCP and the TLCP/CNT 1.5 nanocomposites (see Supplementary Information), the C1s spectra of pure TLCP exhibited four characteristic peaks at 285.0, 286.5, 287.5, and 289.0 eV, corresponding to the CAC, CAO, C@O, and O@CAO groups, respectively. Compared with that of pure TLCP, the C1s spectra of the TLCP nanocomposites shift to higher binding energy, and also revealed the presence of four peaks, corresponding to the CAC (285.3 eV), CAO (286.8 eV), C@O (287.8 eV), and O@CAO (289.6 eV) groups, respectively. This slight shifting of the characteristic peaks to higher bonding energy for the TLCP nanocomposites was attributed to the interfacial interactions of the functional groups effectively induced on the CNT surface via chemical modification with TLCP macromolecular chains as well as good dispersion of CNT in TLCP matrix. 3.2. Rheological properties The complex viscosities (|g*|) of the TLCP nanocomposites as a function of frequency are shown in Fig. 2. The |g*| of the TLCP nanocomposites were decreased with increasing frequency, indicating a non-Newtonian behavior over the frequency range measured. The shear thinning behavior observed in the TLCP nanocomposites may be attributed to the random orientation of rigid molecular chains in polymer nanocomposites during shearing force. The TLCP nanocomposites exhibited higher |g*| value than that of pure TLCP at low frequency, indicating that the interconnected or network-like structures were formed in the TLCP nanocomposites because of the nanotube–nanotube or nanotube–polymer interactions. In addition, the TLCP nanocomposites exhibited strong shear thinning behavior, resulting from the break-down of these structures with increasing frequency. The |g*| of the TLCP nanocomposites increased with increasing CNT content over the whole frequency range measured, and this effect was more pronounced at low frequency than that at high frequency. The increase in the |g*| of the TLCP nanocomposites with increasing CNT content was attributed to the increase in the nanotube–nanotube and nanotube–polymer interactions. Furthermore, the TLCP nanocomposites exhibited higher |g*| and more distinct shear thinning behavior as compared to pure TLCP, suggesting either better dispersion of CNT or stronger nanotube–polymer interactions. Similar

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Fig. 1. (a) FT-IR and (b) Raman spectra of CNT (solid line represent modified CNT and the dotted line represent pristine CNT). The insect shows the representative TEM image of modified CNT. The relative intensity ratio of G-band to D-band (IG/ID) of modified CNT was slightly decreased as compared to pristine, indicating the increased in the degree of disorder and the presence of the defects on the surface of modified CNT by chemical modification.

Scheme 1. Schematic of possible interactions of hydrogen bonding between CNT and TLCP matrix.

observation has been reported by Abdalla et al. [32] that higher viscosity and more distinct shear thinning behavior were indicative of stronger interfacial interactions in CNT/ epoxy nanocomposites with more uniform dispersion of CNT. The increase in the |g*| of the TLCP nanocomposites was closely related to the large increase in the storage modulus, which will be described in the following section. The shear thinning exponent (n) of the TLCP nanocomposites can be estimated from the relationship of |g*|  xn [33], and their results are summarized in Table 1. There

is a significant dependence of the shear thinning behavior of the TLCP nanocomposites on the presence of CNT. The n values of the TLCP nanocomposites decreased with the introduction of CNT, which may be related to the increase in the interfacial interactions between CNT and TLCP matrix as well as the uniform dispersion of CNT. In addition, the TLCP nanocomposites exhibited the relationship between shear thinning behavior and mechanical properties: the more the shear thinning behavior, the better reinforcing effect on the mechanical properties of the TLCP nano-

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Fig. 2. Variations of complex viscosities of the TLCP nanocomposites with CNT content measured at 300 °C as a function of frequency.

Table 1 Variations of low frequency slopes of |g*|, G0 , and G0 0 versus x of the TLCP nanocomposites. Materials

Slope of |g*| versus x

Slope of G0 versus x

Slope of G0 0 versus x

TLCP TLCP/CNT 0.5 TLCP/CNT 1.0 TLCP/CNT 1.5

0.43 0.64 0.65 0.72

0.74 0.45 0.41 0.31

0.62 0.35 0.30 0.27

composites. Wagner and Reisinger [34] reported that the tensile modulus of poly(butylene terephthalate) nanocomposites containing modified clay prepared via melt compounding was significantly enhanced with lower values of the shear thinning exponent. The storage modulus (G0 ) and loss modulus (G00 ) of the TLCP nanocomposites as a function of frequency are shown in Fig. 3. The values of G0 and G00 of the TLCP nanocomposites significantly increased with increasing frequency and CNT content, and this enhancing effect was more pronounced at low frequency. This phenomenon is similar to the relaxation behavior of typical filled-polymer composite system [35]: if polymer chains are fully relaxed and exhibit a characteristic homopolymer-like terminal behavior, the flow curves of polymers can be expressed by a power law of G0 1 x2 and G0 0 1 x. Krisnamoorti et al. [36] reported that the slopes of G0 (x) and G00 (x) for layered silicate and polymer nanocomposite system were much smaller than 2 and 1, respectively, and they suggested that large deviations in the presence of a small quantity of layered silicate resulted from the formation of a network structure in the polymer molten state. As shown in Table 1, the variations of the terminal zone slopes of the TLCP nanocomposites indicated the non-terminal behavior with the power law dependence for G0 (x) and G00 (x) of the TLCP nanocomposites. Similar non-terminal low-frequency behaviors have been also observed in the ordered block copolymers and the smectic liquid-crystalline small molecules [37]. The decrease in the slope for the TLCP nanocomposites with the introduction of CNT was explained by the fact that the nanotube–nanotube or the nanotube–polymer interactions

Fig. 3. Variations of (a) storage modulus (G0 ) and (b) loss modulus (G0 0 ) of the TLCP nanocomposites as a function of frequency.

can induce the formation of the interconnected or network-like structures in the TLCP nanocomposites, leading to the pseudo solid-like behavior of the TLCP nanocomposites. Similar observation has been reported in that the difference of the slope of G0 and G00 for CNT/epoxy nanocomposites at terminal regime was closely related to the internal structure of the resulting nanocomposites, depending on the particle–particle interaction of CNT in the epoxy matrix [38]. As shown in Fig. 2, the extent of the increase in the G0 value of the TLCP nanocomposites was higher than that of G0 0 value over the frequency range measured. Higher values of G0 and G00 for the TLCP nanocomposites at low frequency demonstrated the formation of the interconnected or network-like structures via the nanotube–nanotube and nanotube–polymer with the introduction of CNT, resulting in more elasticity than pure TLCP. As the applied frequency increased the interconnected or network-like structures formed by the nanotube–nanotube and the polymer–nanotube interactions were collapsed by high levels of shear force and the TLCP nanocomposites exhibited almost similar or slightly higher G0 and G00 values than that of pure TLCP at high frequency. In addition, the values of G0 and G00 of the TLCP nanocomposites were higher than that of TLCP over the whole frequency range measured suggests the increase in the interactions between CNT and TLCP matrix.

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3.3. Morphology

Fig. 5. Plots of ln[ln(1  a)1] versus h as shown for the TLCP nanocomposites. By Eq. (3), the slope provides an estimate of the activation energy (Ea) for the thermal decomposition of the TLCP nanocomposites.

has been reported that the presence of the fractured nanotubes, along with the polymer matrix still adhered to the fractured tubes matrix in terms of a crack interacting with the nanotube reinforcement significantly increased the mechanical properties of CNT/polystyrene nanocomposites [40]. In the TLCP nanocomposites, the presence of the functional groups on the surfaces of CNT resulted in the interfacial interaction between CNT and TLCP matrix: the hydrogen atoms at the ACOOH groups of the CNT may form hydrogen boding with the C@O groups of TLCP molecular chains [26], as shown in Scheme 1. This result can be considered as the evidence for efficient load transfer from the TLCP matrix to the nanotubes during the tensile testing, leading to the reinforcing effect on the overall mechanical properties of the TLCP nanocomposites. 3.4. Mechanical properties The effects of CNT on the mechanical properties of the TLCP nanocomposites are shown in Tables 2 and 3. There is a significant dependence of the mechanical properties of the TLCP nanocomposites on the presence of CNT. The incorporation of a very small quantity of CNT significantly improved the tensile strength and tensile modulus of the TLCP nanocomposites due to an effective nanoreinforcing effect of CNT with high aspect ratio, and this enhancing effect was more pronounced with lower CNT content. In comparison with pure TLCP, higher tensile strength and tensile modulus of the TLCP nanocomposites was attributed to the better interfacial adhesion between CNT and

Table 2 Mechanical properties of the TLCP nanocomposites with CNT content and the theoretically predicted values based on Eqs. (1) and (2). Fig. 4. Representative SEM image of the fracture surfaces for the TLCP nanocomposites containing 0.5 wt% of CNT. The arrows indicates that the nanotubes were to be broken with their ends still embedded in the TLCP matrix or they were bridging the local microcracks in the nanocomposites, suggesting good wetting with the matrix and enhanced adhesion between the nanotubes and the matrix, thus being favorable to efficient load transfer form the polymer matrix to the nanotubes.

Materials

TLCP TLCP/CNT 0.5 TLCP/CNT 1.0 TLCP/CNT 1.5

Experimental results

Theoretically predicted values

rC (MPa)

EC (GPa)

rC (MPa)

EC (GPa)

83.10 110.23 114.41 117.54

1.96 2.43 2.55 2.63

– 118.79 154.61 190.55

– 2.46 2.97 3.48

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The morphologies of the TLCP nanocomposites are shown in Fig. 5. In general, the drawbacks related to the uniform dispersion of CNT in the polymer matrix were caused by intrinsic van der Waals attractions between the individual nanotubes in combination with high aspect ratio and large surface area. This feature made it difficult for CNT to disperse in the polymer matrix and could lead to more aggregated bundles of CNT, causing severe stress concentration phenomenon and preventing efficient load transfer to the polymer matrix, thus resulting in some deterioration of the overall performance of CNT/polymer nanocomposites. In this study, chemical modification was performed in order to achieve both uniform dispersion of CNT in the TLCP matrix and strong interfacial adhesion between CNT and TLCP matrix. After chemical modification, modified CNT exhibited less entangled organization due to the functional groups formed on the nanotube surfaces instead of showing more agglomerated structures or bundles observed in pristine CNT. The CNT could stabilize its dispersion by good interactions with TLCP matrix, resulting from the interfacial interactions of the COOH groups of CNT with the C@O groups of TLCP matrix, and could lead to the well-dispersion of CNT in the TLCP nanocomposites. This interfacial adhesion between CNT and TLCP plays a critical role in improving the mechanical properties of the TLCP nanocomposites. The fracture surfaces of the TLCP nanocomposites are shown in Fig. 4. It can be seen that two ends of CNT were covered by the TLCP matrix despite a few of CNT pulled out from the TLCP matrix. Interestingly, some of CNT was broken with their two ends still strongly embedded in the TLCP matrix, and was bridging the local microcracks, which may delay the failure of the polymer nanocomposites [39]. This result indicates that CNT has a good wetting with TLCP matrix, suggesting the existence of strong interactions between them. Similar observation

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Table 3 Reinforcing efficiency of pristine and modified CNTs on the mechanical properties of the TLCP nanocomposites containing 0.5 wt% of CNT. Materials

TLCP/pristine CNT TLCP/modified CNT

Reinforcing efficiency (%)a Tensile strength

Tensile modulus

16.46 32.64

13.83 23.98

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a Reinforcing efficiency (%) = [(Mc  Mm)/Mm]  100, where Mc and Mm represent the mechanical properties, such as tensile strength and tensile modulus, of the TLCP nanocomposites and pure TLCP, respectively).

TLCP matrix as well as the better dispersion of CNT in the TLCP matrix. For characterizing the effect of CNT on the mechanical properties of the TLCP nanocomposites, it is very instructive to compare the reinforcing efficiency of CNT for a given content in the TLCP nanocomposites. In this study, the reinforcing efficiency of CNT is defined as normalized mechanical properties of the TLCP nanocomposites with respect to those of pure TLCP. As shown in Table 3, the enhancing effect of the mechanical properties by incorporating CNT was more significant in the TLCP nanocomposites containing modified CNT than in the case of pristine CNT. This result indicated that the incorporation of modified CNT into the TLCP matrix was more effective in improving the mechanical properties of the TLCP nanocomposites. The incorporation of modified CNT into the TLCP matrix resulted in the good interfacial adhesion between CNT and TLCP matrix: the ACOOH groups of CNT were helpful for enhancing the interactions with the C@O groups in the TLCP matrix, being favorable to more efficient load transfer form the polymer matrix to the nanotubes. Thus, the enhanced interfacial adhesion between CNT and TLCP matrix as well as uniform dispersion of CNT in TLCP matrix can lead to significant improvement in the overall mechanical properties of the TLCP nanocomposites. A comparative study of the experimental results with the values predicted from the theoretical models is also very instructive to characterize the effect of CNT on the mechanical properties of the TLCP nanocomposites. It is well-known that modified Halpin-Tsai equation has been used to predict the modulus of CNT-filled polymer nanocomposites [41]. By assuming the random-oriented discontinuous distribution of CNT in the matrix, modified HalpinTsai equation can be expressed as follows [41–43]:        3 1 þ 2ðlCNT =dCNT ÞgL V CNT 5 1 þ 2gT V CNT EC ¼ þ ETLCP 8 8 1  gL V CNT 1  gT V CNT ðECNT =ETLCP Þ  1 gL ¼ ðECNT =ETLCP Þ þ 2ðlCNT =dCNT Þ ðE =E Þ  1 gT ¼ CNT TLCP ðECNT =ETLCP Þ þ 2 ð1Þ where EC, ETLCP, and ECNT are the tensile modulus of the nanocomposites, TLCP, and CNT, respectively; lCNT/dCNT is the ratio of length to diameter for CNT, and VCNT is the volume fraction of CNT in the nanocomposites. To fit the Eq. (1) to the experimental results for the TLCP nanocomposites, the weight fraction was transformed to the volume fraction, taking the densities of TLCP (1.41 g/cm3) [44] and

perfectly graphitized CNT (2.16 g/cm3) [41]. Theoretical values of the TLCP nanocomposite can be estimated assuming the aspect ratio of 1000 and ECNT of 450 GPa. The ECNT values used in this study represents a mid-range value in the modulus ranges of CNT previously measured [45]. The tensile strength of the TLCP nanocomposites can be estimated from the following equation [46]:

rC ¼ rCNT V CNT þ rTLCP V TLCP

ð2Þ

where rC, rCNT, and rTLCP are the tensile strength of the nanocomposites, the CNT, and the TLCP, respectively. Theoretically predicted values of the nanocomposite strength can be estimated assuming the rCNT of 11 GPa based on the previous literature [47]. Theoretically predicted values and the experimental data for the modulus and strength of the TLCP nanocomposites are compared in Table 2. At lower CNT content, the experimental results for the mechanical properties of the TLCP nanocomposites were fitted well with the theoretical predicted values, while the large deviations were observed at higher CNT content. Nanoreinforcing effect of CNT on the mechanical properties of the TLCP nanocomposites appeared to be more pronounced at lower CNT content than at higher CNT content. This result suggested that the interfacial interaction was more effective in the enhancement of the mechanical properties of the TLCP nanocomposites at lower CNT content than at higher CNT content [26]. Similar effect has been reported that for CNT-grafted polyhedral oligomeric silsequioxane (CNT-gPOSS) and poly(L-lactide) (PLLA) nanocomposite, the interfacial interactions were more effective in improving the mechanical properties of CNT-g-POSS/PLLA nanocomposites at lower CNT loading than at higher CNT loading [48]. Thus, the synergistic effect of the increased interfacial interactions between CNT and TLCP matrix in combination with the uniform dispersion of CNT was more effective at lower CNT content for enhancing the mechanical properties of the TLCP nanocomposites (see Supporting Information). However, the experimental results for the mechanical properties of the TLCP nanocomposites were lower than those of theoretically predicted values, as shown in Table 2. This result may be explained by the curvature of the introduced CNT in the TLCP nanocomposites: some CNT embedded in the TLCP matrix often exhibit curvature morphology, not straight one (Fig. 4) and this feature of CNT may reduce the nanoreinforcing effect of CNT in the TLCP nanocomposites, in comparison with the theoretical reinforcement provided by straight inclusions. Fisher et al. [49] have developed a model combining finite element results and micromechanical methods to determine the effective reinforcing modulus of wavy embedded CNT, and they found that even slight curvature of CNT significantly reduced the effective reinforcement as compared to straight nanotubes. This feature may be an additional mechanism to limit the reinforcing effect induced by CNT on the mechanical properties of CNT/polymer nanocomposites, leading to the nanocomposite modulus and strength that were less than predicted by traditional theoretical models [49]. In addition, the orientation of CNT can be also important factor in determining the mechanical

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3.5. Thermal stability Thermal stability of polymer composites plays a crucial role in determining the limit of their working temperature and the environmental conditions for use. The incorporation of CNT into the TLCP matrix increased the thermal decomposition temperatures and the residual yields of the TLCP nanocomposites, indicating that the presence of CNT could lead to the stabilization of TLCP matrix, resulting in the enhancement of the thermal stability of the TLCP nanocomposites (see Supporting Information). In the TLCP nanocomposites, CNT as protective barriers against thermal decomposition may retard thermal decomposition of the TLCP nanocomposites, resulting from the effective function of CNT acting as physical barriers to hinder the transport of volatile decomposed products out of the TLCP nanocomposites during thermal decomposition [51]. In addition, good interfacial adhesion between CNT and TLCP matrix may restrict the thermal motion of the TLCP macromolecules, resulting in the enhancement of the thermal stability of the TLCP nanocomposites. TGA kinetic analysis was conducted on the TLCP nanocomposites to clarify the effect of CNT on thermal stability of the TLCP nanocomposites. The decomposition temperatures and decomposition kinetic parameters such as the initial decomposition temperatures, integral procedure decomposition temperature (IPDT), temperature at maximum rate of the weight loss (Tdm), and activation energy for thermal decomposition (Ea) are in common use to determine the thermal stability of polymers or polymer composites. As shown in Table 4, the thermal stability factors such as Tdm and IPDT of the TLCP nanocomposites were higher than those of pure TLCP. This result indicated that the incorporation of a small quantity of CNT into the TLCP matrix improved thermal stability of the TLCP nanocomposites, and that the thermal volatilization of TLCP was retarded by the presence of CNT during thermal decomposition. The activation energy for thermal decomposition (Ea) of the TLCP nanocomposites can be calculated from the TGA thermograms by the Horowits-Metzger integral kinetic method [52] as follows:

Table 4 Effect of CNT on the thermal stability of the TLCP nanocomposites. Materials

T5a (°C)

T10a (°C)

Tdmb (°C)

IPDTc (°C)

WRd (%)

TLCP TLCP/CNT 0.5 TLCP/CNT 1.0 TLCP/CNT 1.5

430.7 434.2 439.4 444.6

441.7 446.7 450.7 454.6

448.8 456.7 460.4 464.7

1046.2 1062.7 1081.2 1098.2

32.8 32.9 33.8 34.4

a The decomposition temperatures at the weight loss of 5% and 10%, respectively. b The decomposition temperature at the maximum rate of the weight loss. c The integral procedure decomposition temperature, IPDT = AK (Tf  Ti) + Ti (where A is the area ratio of total experimental curve divided by total TGA thermograms; K is the coefficient of A; Ti is the initial experimental temperature, and Tf is the final experimental temperature). d The residual weight of the samples at 800 °C.

ln½lnð1  aÞ1  ¼

Ea h RT 2dm

ð3Þ

where a is the weigh loss; Ea is the activation energy for thermal decomposition; Tdm is the temperature at the maximum rate of weight loss; h is the variable auxiliary temperature defined as h = T – Tdm, and R is the universal gas constant. The Ea value can be estimated from the slope of the plot of ln[ln(1  a)1] versus a as shown in Fig. 5. The Ea values of pure TLCP and the TLCP nanocomposites were estimated to be 290.8, 293.7, 303.8, and 322.6 kJ/ mol, respectively. In comparison with pure TLCP, higher Ea value of the TLCP nanocomposites suggested that the TLCP nanocomposites were more thermally stable than pure TLCP. This feature was also attributed to the interactions between CNT and TLCP matrix, which increase the Ea value of the TLCP nanocomposites. Due to excellent thermal conductivity of CNT [53], the enhanced interfacial interactions between CNT and TLCP matrix resulted in the increased thermal conductivity of the TLCP nanocomposites, leading to the improvement in the thermal stability of the TLCP nanocomposites. 4. Conclusion TLCP nanocomposites reinforced with a very small quantity of CNT were prepared by a simple melt blending in a twin-screw extruder. TLCP nanocomposites exhibited higher complex viscosity and more distinct shear thinning behavior than pure TLCP, resulting from strong interactions between CNT and TLCP matrix, which significantly influenced the relaxation behavior of polymer chains in the TLCP nanocomposites. The extent of the increase in the complex viscosity, storage modulus (G0 ), and loss modulus (G00 ) of the TLCP nanocomposites with CNT content was more pronounced at low frequency than at high frequency. The decrease in the slopes of G0 and G0 0 for the TLCP nanocomposites with the introduction of CNT was attributed to the formation of the interconnected or network-like structures via the nanotube–nanotube and polymer–nanotube interactions, resulting in the pseudo solid-like behavior of the TLCP nanocomposites. The incorporated CNT plays a crucial role in improving the thermal stability by acting as effective physical barriers against

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properties of polymer nanocomposites, and the curvature morphology of CNT was closely related to the orientation of CNT in the polymer nanocomposites. Gorga and Cohen [50] reported that aligned non-entangled nanotubes should disperse and orient more readily in the polymer nanocomposites, resulting in the enhancement of the mechanical properties more significantly with optimal processing conditions. The nanoreinforcing effect of CNT will be more effective in enhancing the mechanical properties of polymer nanocomposites when the introduced nanotubes exhibit straight morphology within polymer matrix and preferentially aligned along their axial direction. Thus, the overall mechanical properties of the TLCP nanocomposites is expect to be further improved by optimizing the unique geometric feature and alignment of CNT in the TLCP matrix as well as the combination of the enhanced interfacial adhesion between CNT and TLCP matrix with the good dispersion of CNT in the TLCP matrix during melt processing.

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the thermal decomposition in the TLCP nanocomposites. The improvement in the mechanical properties of the TLCP nanocomposites resulted from the enhancement of the interfacial adhesion between CNT and TLCP as well as the uniform dispersion of CNT in TLCP matrix, and their synergistic effect combining good interfacial interaction and uniform dispersion was more effective at lower CNT content than at higher CNT content. This study demonstrates that the optimization of straight morphology and preferential alignment of CNT during melt processing as well as the enhanced interfacial adhesion between CNT and TLCP and uniform dispersion of CNT in the TLCP matrix plays a key role in further improving the mechanical properties of the TLCP nanocomposites.

[22] [23] [24] [25] [26] [27]

References

[37]

[1] Gray GW. Thermotropic liquid crystals. New York: John Wiley & Sons; 1987. [2] Chung TS. Thermotropic liquid crystal polymers: thin film polymerization, characterization, blends, and applications. London: CRC Press; 2001. [3] Demus D, Gray GW, Speiss HW, Goodby JW, Vill V. Handbook of liquid crystals. Weinheim: Wiley-VCH; 1998. [4] Kiss G. Polym Eng Sci 1987;27:410. [5] Dutta D, Fruitwala H, Kohli A, Weiss RA. Polym Eng Sci 1990;30:1005. [6] He J, Bu W. Polymer 1994;35:5061. [7] Kim JY, Kim SH. Polym Int 2006;55:449. [8] Kim JY, Kim SH. J Appl Polym Sci 2006;99:2211. [9] Kim JY, Kim SH. J Polym Sci Part B: Polym Phys 2005;43:3600. [10] Kim JY, Kim SH, Kikutani T. J Polym Sci Part B: Polym Phys 2004;42:395. [11] Wong EW, Sheehan PE, Lieber CM. Science 1997;277:1971. [12] Schadler LS, Giannaris SC, Ajayan PM. Appl Phys Lett 1998;73:3842. [13] Ebbesen TW. Annu Rev Mater Sci 1994;24:235. [14] Dresselhaus MS, Dresselhaus G, Avouris PH. Carbon nanotubes: synthesis, structure, properties, and applications. Berlin: Springer; 2001. [15] Lourie O, Cox DM, Wagner HD. Phys Rev Lett 1998;81:1638. [16] Liu J, Rinzler AG, Dai H, Hafner JH, Bradley RK, Boul PJ, et al. Science 1998;280:1253. [17] Sun YP, Fu K, Lin Y, Huang W. Acc Chem Res 2002;35:1096. [18] Bellayer S, Gilman JW, Eidelman N, Bourbigot S, Flambard X, Fox DM, et al. Adv Funct Mater 2005;15:910. [19] Pötschke P, Fornes TD, Paul DR. Polymer 2002;43:3247. [20] Mu M, Walker AM, Torkelson JM, Winey KI. Polymer 2008;49:1332. [21] Kim JY, Kim SH. Multiwall Carbon Nanotube Reinforced Polyester Nanocomposites. In: Thomas S, Zaikov GE. editors. New York: Nova Science Publishers, 2008.

[28] [29] [30] [31] [32] [33] [34] [35] [36]

[38] [39] [40] [41]

[42] [43] [44] [45] [46] [47] [48] [49]

[50] [51] [52] [53]

Kim JY, Park HS, Kim SH. Polymer 2006;47:1379. Kim JY, Kim SH. J Polym Sci Part B: Polym Phys 2006;44:1062. Kim JY, Park HS, Kim SH. J Appl Polym Sci 2007;103:1450. Kim JY, Han SI, Kim SH. Polym Eng Sci 2007;47:1715. Kim JY, Han SI, Hong S. Polymer 2008;49:3335. Tzavalas S, Drakonakis V, Mouzakis DE, Fisher D, Gregoriou G. Macromolecules 2006;39:9150. Liu L, Qin Y, Guo ZX, Zhu D. Carbon 2003;41:331. Wu CS, Liao HT. Polymer 2007;48:4449. Ruan SL, Gao P, Yang XG, Yu TX. Polymer 2003;44:5643. Grossiord N, Loss J, Regev O, Koning CE. Chem Mater 2006;18:1089. Abdalla M, Derrick D, Adibempe D, Nyario E, Robinson P, Thompson G. Polymer 2007;48:5662. Abdel-Goad M, Pötschke P. J Non-Newtonian Fluid Mech 2005;128:2. Wagener R, Reisinger TJG. Polymer 2003;44:7513. Shenoy AV. Rheology of filled polymer systems. Dordrecht: Kluwer Academic Publishers; 1999. [a] Krishnamoorti R, Vaia RA, Giannelis EP. Chem Mater 1996;8:1728; [b] Krisnamoorti R, Giannelis EP. Macromolecules 1997;30:4097. [a] Rosedalev JH, Bates FS. Macromolecules 1990;23:2329; [b] Larson RG, Winey KI, Patel SS, Watanabe H, Bruinsma R. Rheol Acta 1993;32:245. Song YS, Youn JR. Carbon 2005;43:1378. Cho J, Daniel IM. Scripta Mater 2008;58:536. Thostenson ET, Chou TW. J Phys D: Appl Phys 2002;35:L77. [a] Qian D, Dickey EC, Andrew R, Rantall T. Appl Phys Lett 2000;76:2868; [b] Cadek M, Coleman JN, Barron V, Hedicke K, Blau WJ. Appl Phys Lett 2002;81:5123. Mallick PK. Fiber-reinforced composites. New York: Marcel Dekker; 1993. Zhang X, Liu T, Sreekumar TV, Kumar S, Moore VC, Hauge RH, et al. Nano Lett 2003;3:1285. Saengsuwan S, Bualek-Limcharoen S, Mitchell GR, Olley RH. Polymer 2003;44:3407. Pan ZW, Xie SS, Lu L, Chang BH, Sun LF, Zhou WY, et al. Appl Phys Lett 1999;74:3152. Agarwal BD, Broutman LG. Analysis and performance of fiber composites. New York: Wiley; 1980. Yu MF, Lourie O, Dyer MJ, Moloni K, Kelly TF, Ruoff RS. Science 2000;287:637. Chen GX, Shimizu H. Polymer 2008;49:943. [a] Fisher FT, Bradshow RD, Brinson LC. Appl Phys Lett 2002;90:4647; [b] Fisher FT, Bradshow RD, Brinson LC. Compos Sci Technol 2003;63:1689. Gorga RE, Cohen RE. J Polym Sci Part B: Polym Phys 2004;42:2690. Kashiwagi T, Grulke E, Hilding J, Harris R, Awad W, Douglas JF. Macromol Rapid Commun 2002;23:761. Horowitz HH, Metzger G. Anal Chem 1963;35:1464. Berber S, Kwon YK, Tomanek D. Phys Rev Lett 2000;84:4614.