Fabrication and properties of polycarbonate composites with polycarbonate grafted multi-walled carbon nanotubes by reactive extrusion

Polymer 60 (2015) 18e25

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Fabrication and properties of polycarbonate composites with polycarbonate grafted multi-walled carbon nanotubes by reactive extrusion Eun Yeob Choi, Ji Yeong Kim, C.K. Kim* School of Chemical Engineering and Materials Science, Chung-Ang University, 221 Huksuk-dong, Dongjak-gu, Seoul, 156-756, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 October 2014 Received in revised form 26 December 2014 Accepted 14 January 2015 Available online 22 January 2015

Bisphenol-A polycarbonate (PC) and multi-walled carbon nanotubes (MWCNTs) functionalized with hydroxyl groups were melt mixed in a twin extruder to produce PC composites containing PC-grafted MWCNTs (PC-g-MWCNTs) by reacting hydroxyl groups on MWCNTs with the carbonate groups in PC. Formation of PC-g-MWCNTs by reactive extrusion was explored as were their resulting properties including interfacial adhesion energies between PC and MWCNTs, dispersion of MWCNTs in the PC matrix and mechanical and electrical properties of PC/MWCNT composites. The interfacial adhesion energy of the PC/PC-g-MWCNT composite was higher than that of the PC/pristine MWCNT composite; as a result, the PC/PC-g-MWCNT composite exhibited a higher level of dispersion of MWCNTs in the PC matrix and better adhesion at the interface between PC and MWCNTs than the PC/pristine MWCNT composite. For a fixed MWCNT content in the composite, the mechanical strength of the PC/PC-gMWCNT composite was higher than those of the corresponding PC/pristine MWCNT composite. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Polycarbonate Polycarbonate grafted multi-walled carbon nanotubes Reactive extrusion

1. Introduction Carbon nanotubes (CNTs) are a one-dimensional material that possesses excellent mechanical strength and electrical and thermal conductivity [1e7]. Polymer/CNT composites have been the subject of many recent papers [7e17]. This interest is mainly driven by the promise of strongly enhanced physical properties. To achieve such properties in composites requires high levels of dispersion of the CNTs, which is possible when there is good affinity between the polymer and the CNT. Since CNTs are entangled due to the strong inter-tube van der Waals forces, which occur as they are synthesized, the inherent properties of CNTs cannot be exploited without eliminating this CNT entanglement in the polymer matrix. Much effort has been made to safely disentangle CNTs and disperse them in the polymer matrix [10e17]. Bisphenol-A polycarbonate (PC) has experienced substantial growth in its use since its introduction in the late 1950s [18]. The main reason for this growing market is its unique combination of properties including high heat distortion resistance and impact strength, fire retardant properties, transparency, and dimensional

* Corresponding author. Tel.: þ82 2 8205324; fax: þ82 2 8243495. E-mail address: [email protected] (C.K. Kim). http://dx.doi.org/10.1016/j.polymer.2015.01.031 0032-3861/© 2015 Elsevier Ltd. All rights reserved.

stability. Even though PC is especially suitable for the housings of electrical devices, its applications are often limited because of its low stiffness, strength, and electrical conductivity. In principle, such properties can be improved by adding small amounts of CNTs to the PC matrix [12e17,19e27]. It was shown that CNTs are very effective at increasing mechanical properties and electrical conductivity of PC composites at low loading levels without significantly increasing the melt viscosity [19e27]. The production of composites filled with CNTs requires the ability to effectively disperse the nanotubes throughout the polymer. Surface treatments of CNTs are commonly used to enhance dispersion of CNTs in a polymer matrix and improve interfacial adhesion between CNTs and polymer matrices [11e17,28]. One of the most widely used methods is the covalent introduction of functional groups on the CNT surface. PC composites with functionalized CNTs were produced to enhance the stiffness and strength of PC [12e17]. PC composites containing multi-walled carbon nanotubes (MWCNTs) functionalized with ozone were fabricated, and then their properties were examined [14]. Eitan et al. prepared MWCNTs functionalized with an organic molecule containing a terminal epoxide group, and then PC composite films with these MWCNTs were fabricated by solution casting to examine the load transfer mechanism at the interface [16]. PC composites with MWCNTs encapsulated with poly(methyl methacrylate) were

E.Y. Choi et al. / Polymer 60 (2015) 18e25

studied to enhance the electrical conductivity of PC [17]. Choi et al. prepared poly(styrene-co-acrylonitrile) grafted MWCNTs by an atom transfer polymerization technique, and then PC/MWCNT composites were prepared by melt mixing [15]. Even though PC composites containing well-dispersed MWCNTs and exhibiting improved mechanical and electrical properties can be produced by modification of MWCNTs with functional groups, a new, simple route is still required to improve mechanical and electrical properties of PC composites to commercially produce PC/MWCNT composites. In this study, PC composites with PC grafted MWCNTs (PC-g-MWCNTs) were fabricated from PC and MWCNTs functionalized with hydroxyl groups (MWCNTs-OH) by reactive extrusion. It is expected that a grafting reaction will take place between the carbonate groups of PC and hydroxyl groups of MWCNTs-OH during melt extrusion. Formation of PC-g-MWCNTs was therefore conducted by melt extrusion, and the interfacial adhesion energies between PC and MWCNTs and the mechanical and electrical properties of PC/ MWCNT composites were explored. 2. Materials and procedure 2.1. Materials Commercially-available PC (grade: LUPOY GP-1000LP) was supplied by LG Chemicals (Seoul, Korea). According to the supplier, the molecular weight as determined by gel permeation chromatography (GPC) using polystyrene standards was M w ¼ 31; 000 g=mol (M n ¼ 14; 000 g=mol). MWCNTs (grade: CM250) grown by chemical vapor deposition were purchased from Hanwha Nanotech Co. (Seoul, Korea). The diameter of the MWCNTs was 10e15 nm, and the average bundle length and the average length of MWCNTs were 100 mm and 1.6 mm, respectively. Hydrogen peroxide (H2O2), used as a coupling agent to form hydroxyl groups on the MWCNT surface, was purchased from Aldrich Chemicals (Milwaukee, WI, USA). The tetrahydrofuran (THF) used as a PC solvent was also purchased form Aldrich chemicals. PC was dehydrated in a vacuum at 100  C for 24 h before use. 2.2. Preparation and characterization of PC-g-MWCNT The synthetic procedure for forming PC-g-MWCNTs is shown in Fig. 1. To produce MWCNTs functionalized with hydroxyl groups, pristine MWCNTs were reacted in a H2O2 solution, as shown in Fig. 1 [29e31]. The pristine MWCNTs (0.5 g) were dispersed in an

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aqueous solution of H2O2 (concentration: 30 vol.%, 500 ml) under sonication at 30  C. After reacting at 60  C for 24 h, the resulting mixture was diluted with deionized water (1000 ml) and then filtered through a nylon membrane (pore size: 450 nm). The resulting MWCNTs were washed with deionized water (5  200 ml) and dried for 24 h in a vacuum oven at room temperature (hereafter referred to as “MWCNT-OH”). PC/PC-g-MWCNT composites were prepared from PC and MWCNTs-OH by reactive extrusion in a twin extruder (BA-11, L/D ratio ¼ 40, Bau Technology, Seoul, Korea). The temperatures of the feeding zone, melting zone, mixing zone, and exit die of the twin extruder were 270, 280, 290, and 300  C, respectively. The material feed rate and the extrusion speed were held constant at 10 g/min and 300 rpm, respectively. Melt-mixed composites were immediately quenched in a water bath after extrusion. To obtain PC-gMWCNTs formed by a reaction between the hydroxyl groups on MWCNTs and the carbonate groups in PC during extrusion and to remove the unreacted PC, the composite (3 g) was dissolved in THF (200 ml), and then PC-g-MWCNTs were collected using a centrifuge. The collected PC-g-MWCNTs were again dispersed in THF (200 ml), and PC-g-MWCNTs were again collected using a centrifuge. This procedure was repeated five times to remove unreacted PC. The resulting product was dried for 12 h in a vacuum oven at 100  C. The molecular structure of PC-g-MWCNTs was confirmed by Fourier transform infrared (FT-IR, Magna 750, Nicolet, WI, USA) analyses and X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA2000, UK) using a spectrometer with a Mg Ka X-ray source (1253.6 eV) and a hemispherical analyzer. IR spectra were collected over 30 scans in the 4000e500 cm1 region using attenuated total reflection (ATR) mode at a resolution of 4 cm1. The XPS spectra were obtained in high-resolution mode with a 20 eV pass energy and a 0.1 eV step size. All binding energies were calibrated to carbon (C1s) at 284.5 eV. For curve fitting, the widths of the Gaussian peaks were kept constant in each spectrum. Field emission scanning electron microscopy (FE-SEM, model: Sigma, Carl Zeiss, Germany) and high resolution transmission electron microscopy (HRTEM, model: JEM 2000EXII, JEOL, Japan) were employed to investigate the morphologies of the pristine MWNTs, MWNCT-OH, PC-gMWCNT and composites. Thermogravimetric analysis (TGA, model: TGA-2050, TA Instruments, USA) of the samples was carried out to determine the amount of PC grafted on the MWCNTs. TGA analyses were performed under nitrogen at a heating rate of 10  C/min. The specimens for the TGA experiments were dried in a vacuum oven at 80  C for 1 day.

Fig. 1. (a) Synthetic route for PC-g-MWCNT by reactive extrusion and (b) reaction mechanism of MWCNT-OH and PC.

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To compare the structure disorder caused by amorphous carbon and defects between acid treated MWCNTs and MWCNTs-OH, carboxylic acid-terminated MWCNTs were also prepared. The pristine MWCNTs (1.0 g) were functionalized by heating (60  C) in a mixture of concentrated H2SO4 (concentration: 98 vol.%) and HNO3 (concentration: 70 vol.%, 1000 ml, mixing ratio of H2SO4/HNO3 ¼ 3/ 2 by volume) at reflux. After stirring at 60  C for 24 h, the resulting mixture was diluted with deionized water (1000 ml) and then filtered through a 450 nm nylon membrane (hereafter referred to as ‘‘MWCNTeCOOH’’). Raman spectroscopy (LabRAM HR800, Horiba Ltd., Japan) with a HeeNe laser (633 nm) was employed to study changes in the structures of the MWCNTs. 2.3. Characterization of composites PC/MWCNT composites prepared by extrusion were dried in an air-circulating oven at 100  C for one day before use. Specimens for tensile strength tests were prepared by compression molding. The mixture was poured into a mold and placed in a compression molding machine (model 25-12; Carver, Inc., USA) operating at a plate temperature of 300  C and a holding pressure of 12 MPa. The mixture was kept at 300  C for 5 min and was then cooled to room temperature for 2 h by natural convection. After being molded, the specimens were placed in a vacuum oven at 30  C prior to mechanical testing. Specimens for tensile testing were prepared in accordance with American Standards Testing Method (ASTM) specification D 638. Tensile tests were performed using a universal testing machine (UTM, UTM-301, R&B Corp, Daejon, Korea) at a cross-head speed of 5 mm/min. The reported tensile property values represent the averages of five specimens. Composite films were also prepared by solvent casting from THF to examine the effects of functional groups grafted to the MWCNT surface on the dispersion of MWCNTs in the PC matrix. PC (5 g) was dissolved in THF (100 ml), and MWCNTs (0.5 wt% of PC) were dispersed in the PC solution under sonication. Solutions cast onto a glass plate were dried in an oven at 30  C for 6 h until most of the solvent evaporated. The resulting films were finally dried for 24 h in a vacuum oven at 80  C. The electric conductivities of the composites were measured using an AC impedance analyzer (model: 1260A, Solarton, UK). A four-point probe method using a cell (Kisttech, Korea) consisting of two platinum plates carrying the current and two platinum wires monitoring the potential drop

was employed to measure the electric conductivities of the composites.

2.4. Characterization of interfacial adhesion energy The interfacial adhesion energy (or work of adhesion) was characterized by determining the contact angles between the MWCNTs and PC. Symmetric barrel-type PC droplets were formed on the MWCNT fiber, and then the generalized drop shape analysis method was used for an accurate evaluation of the contact angle [32e37]. To form the PC drop on MWCNTs, a dilute solution containing PC (20 mg), MWCNTs (20 mg), and THF (100 ml) was prepared. PC agglomerates were precipitated on MWCNTs from a THF solution using methanol as a nonsolvent. The MWNCTs coated with PC agglomerates were annealed at 300  C for 1 h to produce an equilibrium interfacial geometry. The morphologies of droplets formed on the MWCNTs were observed by FE-SEM.

3. Results and discussion 3.1. Characteristics of MWCNTs-OH and PC-g-MWCNTs Fig. 2 shows FT-IR spectra of pristine MWCNTs, MWCNTs-OH and PC-g-MWCNTs. As shown in Fig. 2, the FT-IR spectrum of MWCNTs-OH is nearly the same as that of pristine MWCNTs. The stretching peak at about 3400 cm1, corresponding to the hydroxyl groups in MWCNTs-OH, is also observed in the FT-IR spectrum of pristine MWCNTs. Stretching peaks corresponding to methyl groups (2850e3000 cm1) and to the carbonate groups (1720 cm1) in PC were observed in the FT-IR spectrum of PC-gMWCNTs. XPS analyses were also performed to confirm the formation of the MWCNTs-OH and PC-g-MWCNTs. Fig. 3 shows XPS wide scan spectra of pristine MWCNTs, MWCNTs-OH and PC-g-MWCNTs, the atomic ratios calculated from Fig. 3 are listed in Table 1. The C1s and O1s peaks were observed in the wide scan spectra of pristine MWCNTs, MWCNTs-OH and PC-g-MWCNTs. The C1s/O1s ratio of pristine MWCNTs (96.9/3.1) was larger than that of MWCNTs-OH (93.9/6.1). A decrease observed in the C1s/O1s ratio in the MWCNT-OH spectrum is due to the formation of hydroxyl groups on MWCNTs by H2O2 treatment.

Fig. 2. FT-IR spectra of pristine MWCNTs, MWCNTs-OH, and PC-g-MWCNTs.

E.Y. Choi et al. / Polymer 60 (2015) 18e25

Fig. 3. XPS wide scan spectra of pristine MWCNTs, MWCNTs-OH, and PC-g-MWCNTs.

The fits of C1s and O1s bands are provided in Fig. 4. C1s peaks of pristine MWCNTs (Fig. 4a) and MWCNTs-OH (Fig. 4b) were deconvoluted into four binding energies at 284.6 (eC]Ce or eC-C), 286.0 (-C-O-), 287.2 (>C]O), and 289 (eCOOe). The C1s region of PC-g-MWCNT (Fig. 4c) could be deconvoluted into five binding energies at 284.6 (eC]Ce or eC-C-), 286.0 (-C-O-), 287.2 (>C]O), 289 (eCOOe), and 290.5 (-OeCOOe). O1s peaks of pristine MWCNTs (Fig. 4d) and MWCNTs-OH (Fig. 4e) were deconvoluted into two binding energies at 532.3 (-C-O-) and 533.8 (>C]O), while that of PC-g-MWCNTs was deconvoluted into three binding energies at 532.3 (-C-O-), 533.8 (>C]O), and 534.8 (-OeCOOe). The deconvoluted peak at 290.5 eV in the C1s of PC-g-MWCNTs and that at 534.8 eV in the O1s of PC-g-MWCNTs, which were not observed in the spectra of pristine MWCNTs and MWCNTs-OH, originated from carbonate groups in the repeat unit of PC. The changes in the C1s and O1s spectra reflected the formation of PC-g-MWCNTs. Fig. 5 shows FE-SEM and TEM images of pristine MWCNTs, MWCNTs-OH and PC-g-MWCNTs obtained after extrusion and removal of unreacted PC. Fig. 5a depicts pristine MWCNTs, the diameters of which ranged from 10 to 15 nm. There were no remarkable changes in the image of MWCNTs-OH compared to that of pristine MWCNTs. In contrast, the diameters of the PC-gMWCNTs were greater than those of the pristine MWCNTs, and their surfaces became rough because of the PC that had reacted with hydroxyl groups on the MWCNTs. TGA analysis was performed to measure the quantity of introduced PC in PC-g-MWCNTs. Fig. 6 shows TGA thermograms of PC, pristine MWCNTs, MWCNTs-OH and PC-g-MWCNTs. Pristine MWCNTs first experienced thermal degradation at 640  C due to the degradation of disordered or amorphous carbon and other metal impurities, as reported in previous investigations [38]. A slight mass loss (2.5 wt%) was observed for MWCNTs-OH in the temperature range of 190e320  C. Reactions of MWCNTs with hydrogen peroxide provided organic characteristics on MWCNTs;

Table 1 Chemical compositions of pristine MWCNTs, MWCNTs-OH and PC-g-MWCNTs.

Pristine MWCNTs MWCNTs-OH PC-g-MWCNTs

C1s

O1s

96.9 93.9 94.5

3.1 6.1 5.5

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as a result, the thermal stability of MWCNTs-OH decreased [40]. Thermal degradation of PC started at about 480  C, while that of PC grafted onto MWCNTs started at about 430  C. The difference in the thermal stability might stem from the residual hydroxyl groups on the surface of MWCNTs. These residual hydroxyl groups could react again with grafted PC on the MWCNTs; as a result, PC grafted to the MWCNTs could decompose into small molecules [39,40]. The mass loss observed for PC-g-MWCNTs due to thermal degradation of the PC was about 13.5 wt%. Raman spectroscopy is well known as a valuable tool for the characterization of carbon-based nanostructures [41,42]. Raman spectra of MWCNTs usually exhibit two characteristic bands: the tangential stretching G band (1580 cm1), the D band (~1350 cm1), which can be understood as a measurement of structural disorder coming from amorphous carbon and any defects. The ratio of the intensities of the D and G bands, R ¼ ID/IG, can be used to evaluate the disorder density of the MWCNT walls [43]. Fig. 7 presents a series of Raman spectra of pristine MWCNTs, MWCNTs-OH and MWCNTs-COOH. The R value of MWCNTs-OH (ID/ IG ¼ 0.990) was slightly higher than that of pristine MWCNTs (ID/ IG ¼ 0.984), which indicates the chemical treatment with H2O2 caused a subtle increase in the structural disorder on the MWCNT surfaces. In contrast, the R value of MWCNTs-COOH (ID/IG ¼ 1.254) was increased compared to that pristine MWCNTs and MWCNTsOH as a result of an increase in defects caused by the acid treatment. 3.2. Changes in interfacial adhesion energies with surface treatment of MWCNT The interfacial adhesion energy between MWCNTs and PC was estimated from the contact angle between PC and MWCNTs. To determine the contact angle of the PC on the MWCNT, the drop-onfiber method was used because it is suitable for cylindrical nanofibers with a small diameter, and gravity can be neglected [32e37]. Fig. 8a shows PC agglomerates precipitated on MWCNTs from the THF solution using methanol as a nonsolvent. MWCNT-OH and PCg-MWCNT coated with PC agglomerates by nonsolvent precipitation exhibited a similar morphology with pristine MWCNTs. Fig. 8b shows FE-SEM microphotographs of axisymmetric PC nanodrops formed on pristine MWCNTs after annealing at 300  C for 1 h. The contact angle between PC and pristine MWCNTs determined from the geometries of the droplets using a generalized drop lengthheight method [28,32,33] was 15.2 . The interfacial adhesion energy (or work of adhesion) between the solid and the liquid phases is given by Young's equation [44].

Wa ¼ gL ð1 þ cosqÞ;

(1)

where gL and q represent the surface energy of PC (45 mJ/m2 [28]) and the contact angle between the MWCNT and PC, respectively. The adhesion energy of PC with pristine MWNCTs calculated from equation (1) was 88.4 mJ/m2. When MWCNTs-OH and PC-gMWCNTs coated with PC agglomerates were annealed, MWCNTs were encapsulated with PC, as shown in Fig. 8c and d. This indicates that the contact angle between PC and PC-g-MWNCT is 0 . Note that the contact angle between PC and MWCNTs-OH observed after annealing was also 0 . This result indicates that the reactions between MWCNTs-OH and PC agglomerates occurred during annealing, and as a result, PC-g-MWCNTs were formed. The interfacial adhesion energy between PC and PC-g-MWNCTs is the same as the cohesion energy of PC (90 mJ/m2). This value is the highest interfacial adhesion energy that can be achieved with PC composites and modified (or pristine) MWCNTs. In summary, PC/MWCNT

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Fig. 4. XPS scan spectra of (a) C1s curve fitting for pristine MWCNTs, (b) C1s curve fitting for MWCNTs-OH, (c) C1s curve fitting for PC-g-MWCNTs, (d) O1s curve fitting for pristine MWCNTs, (e) O1s curve fitting for MWCNTs-OH, and (f) O1s curve fitting for PC-g-MWNCTs.

composites having the best interfacial adhesion could be produced from PC and MWCNTs-OH by melt extrusion. To examine the MWCNT dispersion in the PC matrix with surface treatment, composite films containing PC and various MWCNTs were prepared by solvent casting. Fig. 9 shows MWCNT

dispersion in the composite films. The dispersity of MWCNTs was enhanced in the order of PC/pristine MWCNTs < PC/MWCNTsOH < PC/PC-g-MWCNTs. The reaction between the hydroxyl groups on MWCNTs-OH and carbonate groups in PC could not occur when composite films are prepared at 30  C by solvent casting. The better

Fig. 5. FE-SEM and HR-TEM photomicrographs of (a) pristine MWCNTs, (b) MWCNTs-OH and (c) PC-g-MWCNTs after melt extrusion.

E.Y. Choi et al. / Polymer 60 (2015) 18e25

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3.3. Characteristics of PC/MWCNT composites 100

Weight (%) W

80

60

40 PC MWCNT MWCNT OH MWCNT-OH PC-g-MWCNT

20

0

200

400

600

800

Temperature (¡É) É Fig. 6. TGA thermograms of PC, pristine MWCNT, MWCNT-OH and PC-g-MWCNT.

Raman n Intensity

MWCNT-COOH MWCNT-OH pristine MWCNT

1000

1200

1400

1600

1800

2000

Raman shift (cm-1) Fig. 7. Raman spectra of pristine MWCNTs, MWCNTs-OH and MWCNTs-COOH.

dispersion of MWCNTs-OH relative to pristine MWCNTs might originate from polar interactions between hydroxyl groups on MWCNTs and carbonate groups in PC. The PC/PC-g-MWCNT composite exhibited the best dispersion of MWCNTs in the PC matrix because it had the highest interfacial adhesion among the composites.

Fig. 10 shows changes in the tensile strength and modulus of the PC composites with MWCNT content and the surface treatment of the MWCNTs. The tensile strength and modulus of the composite increased with increasing MWCNT content over the MWCNT content range examined. For composites containing the same amount of MWCNTs, the tensile strength and modulus of the PC/PC-gMWCNT composite were better than those of the PC/pristine MWCNTs composite. The fractured cross-sectional morphology of the composites, including the dispersion of the MWCNTs and the interface state between the MWCNTs and the PC matrix, was observed with FESEM. Fig. 11 shows that MWCNTs are uniformly dispersed in the PC matrix regardless of their surface modification. Interfacial debonding between the PC matrix and the MWCNTs was not observed in the composites containing pristine MWCNTs or PC-gMWCNTs. This result indicates that the interfacial adhesion between the PC matrix and pristine MWCNTs (or PC-g-MWCNTs) is strong enough to prevent interfacial debonding between the PC matrix and the MWCNTs. It is known that the interfacial adhesion between the PC and pristine MWCNTs is stronger than that between the PC and MWCNTs functionalized with carboxylic acid or acyl chloride [28]. High-magnification SEM photomicrographs of PC/pristine MWNCTs and PC/PC-g-MWCNT composites showed that a significant difference in the detailed structure exists. Adhesion between PC and PC-g-MWCTs is better than that between PC and pristine MWCNTs. Further, PC-g-MWCNTs existing at the fracture surface are still encapsulated with PC. The high level of reinforcement observed in the PC-g-MWCNT composites compared to pristine MWCNT composites is likely caused by the high interfacial adhesion, as indicated by the interfacial adhesion energy data and chain entanglement between PC grafted on MWCNTs and that in the matrix. To compare electrical conductivities of PC/pristine MWCNT composites with those of PC/PC-g-MWCNT composites, composite films were prepared from melt extruded pellets by compression molding. Fig. 12 shows the conductivities of composite films as a function of the MWCNT content. As expected, the electrical conductivity of the composite increased with increasing MWCNT content. No significant changes in the conductivity were observed when the composites contained less than about 0.6 wt% MWCNTs, while an abrupt increase in the conductivity was observed at higher concentrations. These results indicate that the channels through which electron are able to move (i.e., the percolation threshold), occurred when the composites contained about 0.7 wt% MWCNTs. When composites contain the same amount of MWCNTs, the electrical conductivity of PC/PC-g-MWCNT composite did not actually increase a lot compared to that of PC/pristine MWCNT

Fig. 8. FE-SEM photomicrographs of (a) pristine MWCNTs coated with PC agglomerates by nonsolvent precipitation, PC droplets formed on (b) pristine MWCNT, (c) MWCNT-OH and (d) PC-g-MWCNT surfaces after 300  C annealing.

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Fig. 9. Photographs of PC/MWCNT composites prepared by solvent casting: (a) PC/pristine MWCNT composite, (b) PC/MWCNT-OH composite, and (c) PC/PC-g-MWCNT composite.

(a)

(b) 1500

62 60

pristine MWCNT PC-g-MWCNT

pristine MWCNT PC-g-MWCNT

Tensile e modulus(MPa a)

Tensile e strength(MPa)

58 56 54 52 50

1400

1300

1200

48 46 0 wt%

0.5 wt%

1 wt%

2 wt%

wt% of MWCNT

1100 0 wt%

0.5 wt%

1 wt%

2 wt%

wt% of MWCNT

Fig. 10. Changes in the (a) tensile strength and (b) tensile modulus as a function of MWCNT content in the PC composite.

composite. The electrical conductivity of composite is closely related to the dispersion of MWCNTs in polymer matrix and the remnant lengths of the MWCNTs after application of a melt mixing process [21,27]. A high contact resistance among PC-g-MWCNTs caused by grafted PC might reduce the electrical conductivity of PC/PC-g-MWCNT composite comparing to PC/pristine MWCNT composite. The average length of the MWCNTs as received (1.6 mm) is compared with those of the MWCNTs after extrusion. Reduction in the average length was approximately 33% for the pristine MWCNTs and 42% for the PC-g-MWCNTs. A great reduction in the length of MWCNTs during melt processing and a contact resistance among MWCNTs give adverse effects on increasing electric conductivity of the composite. Because of these, PC/PC-g-MWCNT

composite having a high level of MWCNT dispersion exhibited a similar level of electrical conductivity compared to PC/pristine MWCNT composite. 4. Conclusion PC composites containing PC-g-MWCNT were fabricated from PC and MWCNTs-OH by reactive extrusion. Formation of PC-gMWCNTs by the reactions between hydroxyl groups of MWCNTsOH and carbonate groups in PC was confirmed by FT-IR, XPS, TGA, HR-TEM, and FE-SEM. The interfacial adhesion energies of PC and pristine MWCNTs (or PC-g-MWCNTs) were quantified from the contact angles of cylindrical drop-on-fiber systems determined

Fig. 11. Cross-sectional photomicrographs of PC composites containing 1 wt% of (a) pristine MWCNTs and (b) PC-g-MWCNTs.

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References

10-4 pristine MWCNT PC-g-MWCNT

10-5

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10-6

Cond ductivity (S/cm m)

10-7 10-8 10-9 10-10

[14] [15] [16]

10-11 10-12

[17] [18]

10-13 10-14 10-15 0.0

25

[19]

0.5

1.0

1.5

2.0

2.5

Contents of MWCNT (wt%)

[20] [21] [22] [23]

Fig. 12. Changes in the electrical conductivities of PC/pristine MWCNT and PC/PC-gMWNCT composites as a function of MWCNT content in the PC composite.

[24]

using a generalized droplet shape analysis. The interfacial adhesion energy of the PC/PC-g-MWCNT composite was greater than that of PC/pristine MWCNT composites. PC-g-MWCNTs exhibited excellent dispersion in the composite, whereas pristine MWCNTs exhibited poor dispersion when composite films were prepared by solvent casting. When MWCNT content in the composite was fixed, PC/PCg-MWCNT composites exhibited a high level of interfacial adhesion, MWCNT dispersion and reinforcement compared to PC/pristine MWCNT composites. These beneficial properties originated from polar interactions and chain entanglement between the PC in the matrix and the PC grafted on the MWCNTs.

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This research was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea. This research was also supported by the Chung-Ang University Excellent Student Scholarship.

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