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Preparation and characterization of melt-processed polycarbonate/multiwalled carbon nanotube composites.
Abstract:
This study describes the preparation of polycarbonate (PC)/multiwalled carbon nanotube (MWCNT) composites by melt processing the PC and PC/MWCNT master batch at 260 C. The PC/ MWCNT master batch was prepared using ultrasonic mixing the carboxylic acid containing MWCNT and PC in a tetrahydrofuran (THF) solution. The HRTEM images of PC/MWCNT master batch and PC/MWCNT nanocomposites show that the MWCNT is well separated and uniformly distributed in the PC matrices. Mechanical properties of the fabricated nanocomposites measured by dynamic mechanical analysis indicate significant improvements in the storage modulus when compared with that of pure PC matrix. The conductivities of 2 and 5 wt% PC/MWCNT nanocomposites are more than four and seven orders in magnitude higher than that of PC without MWCNT, respectively. POLYM. ENG. SCI., 48: 1369-1375, 2008. (C) 2008 Society of Plastics Engineers

Article Type:
Technical report
Subject:
Nanotechnology (Research)
Polycarbonates (Chemical properties)
Nanotubes (Production processes)
Nanotubes (Chemical properties)
Tetrahydrofuran (Chemical properties)
Melting points (Methods)
Authors:
Wu, Tzong-Ming
Chen, Erh-Chiang
Lin, Yen-Wen
Chiang, Ming-Feng
Chang, Gwo-Yang
Pub Date:
07/01/2008
Publication:
Name: Polymer Engineering and Science Publisher: Society of Plastics Engineers, Inc. Audience: Academic Format: Magazine/Journal Subject: Engineering and manufacturing industries; Science and technology Copyright: COPYRIGHT 2008 Society of Plastics Engineers, Inc. ISSN: 0032-3888
Issue:
Date: July, 2008 Source Volume: 48 Source Issue: 7
Topic:
Event Code: 320 Manufacturing processes; 310 Science & research
Product:
Product Code: 2821950 Polycarbonate Resins; 2866975 Tetrahydrofuran; 8911282 Engineering for Polymers NAICS Code: 325211 Plastics Material and Resin Manufacturing; 325192 Cyclic Crude and Intermediate Manufacturing; 54133 Engineering Services SIC Code: 2821 Plastics materials and resins; 2860 Industrial Organic Chemicals; 8700 ENGINEERING & MANAGEMENT SERVICES
Geographic:
Geographic Scope: United States Geographic Code: 1USA United States
Accession Number:
181543652
Full Text:
INTRODUCTION

Multiwalled carbon nanotubes (MWCNTs), first reported by Iijima in 1991 (1), contain exclusive structural, mechanical, and electrical properties (2). Because of the exceptional properties of MWCNTs, many investigations have focused on exploiting these surprising characteristics for engineering applications. including polymer nanocomposites, field emitters, nanoelectronic devices, chemical sensors, and biomaterials [3-5]. For polymer/ MWCNT nanocomposites, the addition of MWCNT could extensively change the mechanical, thermal, and electronic properties of polymer matrix, which would remarkably extend their application regions. However, some reports indicate that the improvements of the mechanicalproperties of polymer/CNT composites are limited due to the phase separation between the polymer matrix and CNTs [6, 7]. To obtain uniform distribution of CNTs in the polymer matrix is still a very important subject until now.

Functionalization of MWCNTs is the strategy used to enhance dispersion in organic media. There are different methods used for the functionalization of MWCNTs through covalent and noncovalent reactions with organic-molecules including polymers. Recently, some investigations have used the functionalized MWCNTs as reinforced fillers to prepare the polymer/MWCNT nanocom-posites by using various methods such as solution processing, melt processing, and in situ polymerization [8-20]. They have demonstrated that the functionalized MWCNTs were well separated and randomly distributed in the polymer matrix. For example, Coleman et al. has effectively used CNTs as fillers to significantly enhance the mechanical and optical properties of polymers [8-11]. In our previous studies, conducting polymer/MWCNT composites were successfully prepared using the in situchemical oxidative polymerization (17), (18). The electrical conductivities at room temperature of conducting poly-mer/MWCNT composites are extensively enhanced compared to that of conducting polymer without MWCNT. But it is relatively difficult to homogeneously disperse MWCNT in a polymer matrix by melt processing. Potschke et al. has used commerically available polycarbonate (PC)/nanotube master batches as a starting material and diluted by adding an appropriate amount of pure polymer in the subsequent melt procesing (21-25). Their results suggested that the CNT is randomly distributed in the polymer matrix. The physical properties of fabricated PC/CNT nanocomposites significantly depend on the amount of CNTs and their distributions. Nevertheless, the preparation of PC/CNT master batch and their following melt processing of PC/CNT nanocomposites are rarely mentioned among these investigations.

In this study, the PC/MWCNT master batch has been fabricated by mixing the PC" and carbolic acids containing MWCNT in tetrahydrofuran (THF) solution. The PC/ MWCNT nanocomposites were then prepared by adding various amount of PC into PC/MWCNT master batch through the subsequent melt processing. The structure, morphology, and physical properties of PC/MWCNT nanocomposites were characterized by FESEM, HRTEM. thermogravimetric analyzer (TGA), and dynamic mechanical analysis (DMA).

EXPERIMENTAL

Preparation of Polycarbonatel Multiwalled Carbon Nanotube Composites Using Melt Processing

The MWCNTs fabricated by ethylene chemical vapor deposition using AUO, supported F[e.sub.2][O.sub.3] catalysts and polycarbonate (PC) with melt index (MI) of 12 g/10 minwere kindly supplied by Material and Chemical Research Laboratories (Hsinchu. Taiwan). The diameter of MWCNT is about 40 nm, and the purity of MWCNTs is higher than 90% after purification. The reagents. including nitric acid, THF. and cetyltrimethylammonium bromide (CTAB), were used without further purification. The as-received MWCNTs were treated using a solution of nitric acid under refluxing conditions at 160 C oil bath with various times, producing various ratio of carboxylic acid groups (designated as c-MWCNTs) at the defect sites. To ensure that the nitric acid was completely removed from the c-MWCNTs. a large amount of distilled water was used to neutralize the c-MWCNTs for 48 h. To avoid the strong interaction of the functional groups on the surface of c-MWCNTs during filtration, the 10 wt% c-MWCNT was first mixed with the CTAB cationic surfactant in THF solution and ultrasonicated over 3 h to form CTAB- coated c-MWCNT in solution. Then the filtered CTAB- coated c-MWCNTs were dried overnight inside a vacuum oven at 60 C to remove any remaining water vapor and nitric acid.

The PC/MWCNT master batch was produced using a various amount of the CTAB-coated c-MWCNTs and PC individually dissolved in THF by ultrasonication process for 3 h and 1 h, respectively. Then the THF-PC solution and THF-CTAC-coated c-MWCNTs solution were mixed together and ultrasonicated for 60 min. Finally, a lot of methanol solution was slowly poured into the solution to precipitate the mixture of PC/MWCNT. The precipitated PC/MWCNT master batch was filtered and rinsed several times with distilled water and methanol. The powder of PC/MWCNT master batch with 10 wt% MWCNT content thus obtained was vacuum dried at 60[degrees]C for 12 h. the 2. 5, and 7 wt% PC?MWCNT nanocomposites were then prepared using a melt-direct compounding process with PC/MWCNT master batch and PC pellet in a Haake mixer operated at 260 C and 60 rpm for 10 min.

Structural and Morphological Analysis

The Raman spectroscopy was used to characterize the structure of MWCNT. c-MWCNT, and CTAB-coated c-MWCNT. Raman spectra were performed on a Jobin Yvon TRIAX 550 system using a He-Nc laser operated at 632.8 nm with a CCD detector. The presented spectrum is an average of three spectra measured at different regions over the entire sample range. The morphology of the fabricated nanocomposite was characterized using Field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). FESEM was performed at 3KV using a JEOL JSM-6700f field-emission instrument, and high-resolution transmission electron microscope (HRTEM) was recorded on a Hitachi HF-2000 at 200 kV.

Physical Properties

Thermal stabilities of these samples were obtained using PerkinElmer thermogravimetric analysis (TGA). The test was performed from room temperature until 800.sup.degrees]C at a scanning rate of 10 C/min. Dynamic mechanical analysis (DMA) experiments were obtained on a PerkinElmer instrument DMA 7e apparatus in three-point bending mode. A rectangular sample of PC matrix and nanoeomposites with dimensions. 18 mm X 10 mm X 2 mm was prepared. The test was carried out at the temperature range of 40-200[degrees] C using [2.degrees]C/min heating rate and 1 Hz constant frequency. Calibrations for force, mass, position, and temperature were made in accordance with PerkinElmer procedures. For the determination of conductivity, the samples of pure PC matrix, PC/MWCNT master batch, and PC/MWCNT nanoeomposites were pressed into pellet under 20 MPa. The conductivity at room temperature using the standard Van Der Pauw de four-probemethod was measured by a programmable DC voltage/ current detector. Each data shown here is the mean value of the measurement from at least three samples, and the experimental error using standard deviation of these data is obtained.

RESULTS AND DISCUSSION

Figure 1 shows the Raman spectra of the MWCNTs c-MWCNTs. and CTAB-coated c-MWCNTs. Both spectra of MWCNTs and c-MWCNTs have the same pattern, revealing that the surface modification using nitric acid solution under refluxing conditions at 160 C oil bath does not affect the graphite structure of the MWCNTs. Such chemical functionalization of the MWCNT can be used to produce carboxylic acid at local defects in the curved gra-phene sheets and tube ends. The Raman characteristic peaks at 1355 and 1580 [cm.sup.-1] are D and G hands of MWCNT corresponding to the s[p.sup.3]- and s[p.sup.2]-hybridized carbons of disordered graphite and the ordered state on the surface of MWCNT, respectively. The intensity ratios of G and D bands ([I.sub.G]/[I.sub.D]) are 0.88 for MWCNTs and 0.65 for c-MWCNTs. These results indicate that the chemical functionalization of nitric acid increases the degree of disorder. The Raman spectrum of CTAB-coated c-MWCNT has different characteristic peak compared to that of c-MWCNT, revealing that the cationic surfactant CTAB is successfully coated on the surface of c-MWCNTs.

[FIGURE 1 OMITTED]

The thermogravimetric analysis was conducted to identify the thermal stability of MWCNT, c-MWCNTs, and CTAB-coated c-MWCNTs. Figure 2 shows the TGA thermal curves of the MWCNT, c-MWCNTs, and CTAB-coated c-MWCNTs. As seen from the figure, the weight loss of MWCNT is mainly attributed to the loss of amorphous carbon. More weight loss of c-MWCNT compared to the pure MWCNT is due to the organic decomposition for the carboxylic acid groups after surface modification as suggested by earlier reports (26). Then the steepest weight loss of CTAB-coated c-MWCNT is observed in a temperature interval of 200-250 [degrees] C, which may be attributed to the thermal degradation of the CTAB cationic surfactant formed on the surface of the c-MWCNTs (27). Both Raman and TGA data indicate that the cationic surfactant is successfully coated on the surface of c-MWCNTs.

[FIGURE 2 OMITTED]

The PC/MWCNT master batch was fabricated using a various amount of the CTAB coated c-MWCNTs and PC individually dissolved in THF solution. Figure 3 shows the HRTEM images of the PC/MWCNT master batch. From this image, it can be clearly seen that the tubelike morphology representing the c-MWCNT in PC matrix is well separated and randomly distributed. This result suggests that the MWCNT in PC/MWCNT master batch was effectively separated via solution mixing method under ultrasonic treatment.

[FIGURE 3 OMITTED]

Figure 4 shows the FESEM data of 2, 5, and 7 wt% PC/MWCNT nanocomposites fabricated by mixing the PC/MWCNT master batch and PC pellet through the melt-direct-compounding process. It is clear that the tubelike structure of MWCNT is randomly distributed in the PC matrix. Closer inspection of HRTEM images of PC/MWCNT nanocomposites shown in Fig. 5 reveals that the distribution of MWCNT for the resulting nanocomposites is well uniform. Therefore, the fabrication of PC/MWCNT nanocomposites is successfully prepared through melt processing.

[FIGURE 4 OMITTED]

To study the effect of MWCNT on the thermal behaviors of PC matrix, TGA analysis was used to identify the thermal degradation behavior of PC and PC/MWCNT nanocomposites. Figure 6 shows the TGA curves of PC and PC/MWCNT nanocomposites at a heating rate of 10 [degrees] C/min. The TGA profiles of PC/MWCNT nanocomposites show similar tendency, and the onset temperature of degradation ([T.sub.onset]) shown in Fig. 6b can be determined from these curves by extrapolating from the curve of maximum degradation back to the initial weight of the polymer. Detailed [T.sub.onset] of PC matrix and PC/MWCNT nanocomposites are summarized in Table 1. The [T.sub.onset] of PC is 488 [degrees] C and extensively increases to 495, 497, and 498 [degrees] C as the loading of 2, 5, and 7 wt% MWCNT content, respectively. From these experimental results, it can be seen that the presence of MWCNT in PC induced better thermal stability and therefore the degradation starting temperatures clearly shifted to higher temperatures.

[FIGURE 6 OMITTED]

Figure 7 shows the dynamic storage modulus G' of neat PC matrix and PC/MWCNT nanocomposites at a temperature range of 40-180 [degrees] C. The data of storage modulus at 40 and 160 [degrees] C are listed in Table 1. It is clear that the storage modulus of PC starts to decrease at about 150 [degrees]C, which is probably as a result of the glass transition temperature ([T.sub.g]) of PC. The [T.sub.g] of PC/MWCNT nanocomposites is slightly slower than that of pure PC matrix, which may be due to the presence of cationic surfactant CTAB coated on the surface of MWCNT to induce a decrease of glass transition temperature of PC (shown in Fig. 8). At 40 [degrees] C, the storage modulus of PC is 3.48 X [10.sub.8] Pa, which decreases with the increasing temperature; at 160 [degrees] C it drops to 6.36 X [10.sub.6] Pa. This is attributed to insufficient thermal energy to overcome the potential barrier for transitional and rotational motions of segments of the polymer molecules in the glassy region, whereas above the [T.sub.g], the thermal energy becomes comparable to the potential energy barriers for the segmental motions. For PC/MWCNT nanocomposite, significant enhancement of G' can be seen in the lower temperature range, indicating that the experimental introduction of MWCNT into PC matrix have strong influence on the elastic properties of the PC matrix. Below [T.sub.g], the enhancement of G' of PC/MWCNT nanocomposites when compared with virgin PC matrix is 20.7%, 58.3%, and 102% for 2, 5, and 7 wt% PC/MWCNT nanocomposites, respectively. These results indicate that the reinforcement effects of PC/MWCNT nanocomposite are predominated by the presence of MWCNT and possible interaction zone between the CTAB-coated c-MWCNT and PC induced an annular interphase region of immobilized polymer surrounding the embedded MWCNTs (28). It is necessary to point out that the enhancement of storage modulus above [T.sub.g] for nanocomposites (rubbery plateau region) is remarkably increased with introducing the MWCNT into PCmatrix. Above [T.sub.g], the storage modulus of pure PC matrix decreases about two order in magnitude, but the storage modulus of nanocomposites remains roughly 25-30% in magnitude compared to those data below [T.sub.g], Therefore, the improvement of storage modulus for PC/MWCNT nanocomposites is more than one order in magnitude higher than those of PC without MWCNT. These results reveal that the presence of MWCNT in PC/MWCNT nanocomposites is possibly confined and retarded the segmental chain motion of PC at the interface. Uniform distribution of the MWCNT at the nanoscale level is possible to induce a phenomenal increase in the storage modulus for the PC/MWCNT nanocomposites. Figure 8 illustrates the tan [delta] for PC/MWCNT nanocomposites. The peak positions are allocated at 152[degrees]C for pure PC matrix and peak profile of PC/MWCNT is clearly shifted into lower temperature region with less intense and broader compared to pure PC samples. A decrease of tan [delta] for fabricated nanocomposites is probably due to the presence of cationic surfactant CTAB coated on the surface of MWCNT. The broader profiles of nanocomposites indicate that a segmental chain motion of the PC/MWCNT nanocomposites is retarded due to the confinement of the PC by MWCNT (29).

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 5 OMITTED]

The electrical conductivities of PC matrix. PC/MWCNT master batch, and PC/MWCNT nanocomposites were measured using the standard Van Der Pauw dc four-probe method (30). The conductivity for PC matrix at room temperature was [10.sup.-13] S/cm. As the addition of 2 wt% MWCNT into PC, the conductivities at room temperature dramatically increase from [10.sup.-13] S/cm to 2 X [10.sup.-8] S/cm. With the continuous increase in the content of MWCNT, the conductivities at room temperature gradually increase from 1.9 X [10.sup.-8] S/cm for 2 wt% PC/MWCNT nanocomposites to 1.8 X [10.sup.-6] S/cm for 5 wt% PC/MWCNT nanocomposites. Further increasing content of MWCNT, the conductivity at room temperature approached the conductivity of PC/MWCNT master batch (4.9 X [10.sup.-5] S/cm). Detail electrical conductivities of PC matrix and PC/MWCNT nanocomposites are also summarized in Table 1. Nevertheless, the conductivities of fabricated nanocomposites with very low MWCNT con-tent at room temperature are more than four orders in magnitude higher than those of PC without MWCNT. This result is perhaps due to the presence of MWCNTs containing a large aspect ratio and surface area, so serving as a conducting path between the insulated PC domains and increasing the conductivity with increasing MWCNT content.

CONCLUSIONS

The PC/MWCNT nanocomposites have successfully prepared by mixing the PC/MWCNT master batch and PC pellet through melt processing. The HRTEM micrograph of nanocomposites shows that the MWCNT is well separated and uniformly distributed in PC matrices. The storage modulus of fabricated PC/MWCNT nanocomposites shows significant enhancement compared to that of pure PC matrix. These results indicate that the reinforcement effects of PC/MWCNT nanocomposite are predominated by the presence of MWCNT and better interaction between the MWCNT and PC. The conductivities of 2 and 5 wt% PC/MWCNT nanocomposites are more than four and seven orders in magnitude higher than that of PC without MWCNT, respectively. That is because conductive MWCNT serves as a conducting path between insulated PC, increasing the conductivity with increasing MWCNT content.

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Correspondence to: T.-M. Wu; e-mail: tmwu@dragon. nchu.edu.tw

Contract grant sponsor: Material and Chemical Research Laboratories.

Industrial Technology Research Institute (ITRI).

DOI 10.1002/pen.21094

Published online in Wiley InterScience (www.interscience.wiley.com).

[C] 2008 Society of Plastics Engineers

Tzong-Ming Wu, (1) Erh-Chiang Chen, (1) Yen-Wen Lin, (1) Ming-Feng Chiang, (1) Gwo-Yang Chang (2)

(1) Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan 402

(2) Material and Chemical Research Laboratories, Industrial Technology Research Laboratories, Industrial Technology Research Institute, Chu,Tung, Hsinchu, Taiwan 310
TABLE 1. Onset temperature of degradation (Tonset), dynamic storage
modulus (G'). and electrical conductivity of PC matrix and PC/MWCNT
nanocomposites.

                                G' (MPa)
Sample           Tonsei (C)   40 C  160 C   Conductivity (S/cm)

PC matrix          488         348    6.36   1.0 X [ 10.sup.-13]
2 wt% PC/MWCNT     495         420  108      1.9 X [10.sup.-8]
5 wt% PC7MWCNT     497         551  145      1.8 X [10.sup.6]
7 wt% PC/MWCNT     498         702  210      2.5 X [10.syp.-5]
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