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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 242–245
Electrical and rheological properties of polycarbonate/multiwalled carbon nanotube nanocomposites Sung Hun Jin, Deok Kyu Choi, Dai Soo Lee ∗ Department of Chemical Engineering, Chonbuk University, Jeonju 561-756, Republic of Korea Received 1 November 2006; accepted 29 April 2007 Available online 2 June 2007
Abstract Nanocomposites of polycarbonate (PC) and multiwalled carbon nanotube (MWCNT) were prepared using a precipitation method, and the surface resistivities and rheological properties of the nanocomposites were investigated. The nanocomposites showed a threshold decrease in their surface resistivities with increasing MWCNT content at a relatively low concentration of 0.91 vol.%. The low concentration required for the threshold decrease in the surface resistivities was attributed to the pristine MWCNT, without modifications unfavorable for the electrically conductive fillers. Yield behavior was observed with the change in the steady shear viscosities with shear stresses. The viscosities of the nanocomposites were lower than that of the neat PC. It is postulated that the decreased viscosity had resulted from the increase in the mobilities of the PC molecules and free volumes in the PC/MWCNT nanocomposites, which were supported by the decreased glass transition temperatures of the nanocomposites compared to the neat PC. © 2007 Elsevier B.V. All rights reserved. Keywords: Nanocomposites; Multiwalled carbon nanotube; Polycarbonate; Rheology; Surface resistivity
1. Introduction Since the discovery of carbon nanotubes (CNTs) in 1991 , researchers have concentrated on their engineering applications, such as polymeric composites , electronic devices, sensor and field emission displays [3–5], due to their unique properties of electrical, thermal and mechanical properties [6–9]. Threshold percolation of CNT in the nanocomposites is observed at relatively low concentrations due to the large aspect ratios of CNTs. Generally, it is very difficult to disperse CNTs in a polymeric matrix as they have large van der Waals forces due to the large surface area, which result in aggregates. Various studies to modify CNT dispersions were undertaken, with functional groups introduced into the surfaces of CNTs by acid treatments or wrapping with polymers [10,11]. The treatment of CNT improves their dispersion in a polymeric matrix, but does not avoid damage to the CNT, which causes a drop in their unique properties . The introduction of functional groups onto the surface of
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MWCNTs is unfavorable, with respect to their use as electrically conductive fillers, as the sp2 hybrids partially change into sp3 hybrids. Recently, nanocomposites, prepared from polycarbonate (PC) and pristine multiwalled carbon nanotube (MWCNT), were investigated. Interesting electrical and rheological properties were observed with these PC/MWCNT nanocomposites, with the highlights of our observation discussed in this paper. 2. Experimental A MWCNT, CVD MWCNT 95, was supplied by Iljin Nanotech, Seoul, Korea. The polycarbonate used for the matrix of nanocomposites was a commercially available bis-phenol A type polycarbonate from Samyang Kasei, Trilex 3022IR. The characteristics of the MWCNT and PC are summarized in Table 1. The PC was dissolved in N-methylpyrrolidone (NMP) to make an 18 wt.% solution and mixed with various amounts of MWCNT dispersed in NMP which were sonicated for 12 h at 46 kHz and 350 W. Deionized water was used as a non-solvent for the precipitation of the PC and MWCNT from the dispersion. The sample code PC-xyz denotes a nanocomposite where the MWCNT content is xyz (vol.%). Specimens were made using hot press at
S.H. Jin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 242–245 Table 1 Charactersitics of raw materials Materials
Samyang Kasei, Korea
Multiwalled carbon nanotube
Iljin Nanotech, Korea
Trirex 3022PJ, injection molding grade, Mn = 22,000 CVD, purity: 95 wt.%, diameter: 6–9 nm, length: 10–20 m
240 ◦ C after drying, and the characteristics of the precipitates investigated. The surface resistivities of the nanocomposites were measured at room temperature using a resistivity meter (Trustat ST-3 from SIMCO Inc., Japan). The rheological properties of the nanocomposites were measured during steady shear and in the oscillation shear mode at 260 ◦ C employing a parallel plate rheometer (AR 2000 from TA Instruments). The thermal properties of the nanocomposites were studied using a differential scanning calorimeter (DSC, TA DSC2910), with a heating rate of 10 ◦ C/min. Both the rheological and thermal properties were investigated in nitrogen environments.
Fig. 1 shows the changes in the surface resistivities of the nanocomposites with respect to the MWCNT contents. The surface resistivities showed a threshold decrease with increasing MWCNT content. According to the percolation theory , the threshold decrease in the electrical resistivity (σ) of the composites containing MWCNT should obey the scaling law, as follows: σ = k(p − pc )−t
where p is the CNT volume fraction and pc is the critical volume fraction corresponding to the percolation threshold. The percolation threshold value of the MWCNT content for the nanocomposites, pc , was 0.91 vol.%. The scaling index in Eq. (1) was 1.57 for all the nanocomposites investigated. The critical percolation threshold value was very low compared with those previously reported, where pristine CNTs were frequently treated with strong acids to introduce functional groups onto the surface of the MWCNT and improve their dispersion in polymer matrices . Such treatments result in partial changes of the sp2 hybrids into sp3 hybrids on the surface of the MWCNTs. This modification is necessary for the fine dispersion of the MWCNT, but undesirable as fillers for lowering the electrical resistivities of the polymers. The relatively low critical threshold value found in this study was attributable to the pristine MWCNT, which remained chemically unmodified. Fig. 2 shows the shear viscosities of the PC/MWCNT nanocomposites at 260 ◦ C. From the plot of the steady shear viscosity versus shear stress in Fig. 2, yield behaviors are clearly observed in the nanocomposites. In order to calculate the yield stress (τ y ) of the systems, Casson’s equation was employed, as follows: τ (1/2) = τy(1/2) + (μγ)(1/2)
3. Results and discussion
Fig. 1. Surface resistivity vs. MWCNT content of the PC/MWCNT nanocomposites.
where τ is the shear stress, τ y the yield stress, μ the shear viscosity and γ is the shear rate. The yield stresses of the nanocomposites obtained from Casson’s equations are given in Fig. 3. A parabolic increase in the yield stress was observed with increasing MWCNT content. It is postulated that the yield behavior results from the entanglement of MWCNTs and polycarbonate molecules. From Fig. 2, it is worth noting that the shear viscosities of the PC/MWCNT nanocomposites were lower than that of the neat PC above the yield stresses. The lower viscosities of the PC/MWCNT nanocomposites compared to the neat PC imply larger free volumes of the nanocomposites in the melts. In order to confirm the change in the free volume of the nanocom-
Fig. 2. Steady shear viscosity vs. shear stress for the PC/MWCNT nanocomposites at 260 ◦ C.
S.H. Jin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 242–245
Fig. 3. Yield stress vs. MWCNT content of PC/MWCNT nanocomposites at 260 ◦ C.
posites, DSC thermograms were obtained, which are shown in Fig. 4. The glass transition temperatures (Tg s) of the nanocomposites were lower than that of the neat PC. No harsh conditions were used in the preparation of the nanocomposites, to avoid a decrease in the molecular weight of the PC. There must be certain parameters that increase the free volumes of nanocomposites. According to Ash et al. , the Tg s of poly(methyl methacrylate)/alumina nanocomposites decreased by 25 ◦ C compared to the neat polymer. They suggested that a significant fraction of polymer had higher mobility in nanocomposites, which brought about the decrease in the Tg s. Similar phenomena have been reported with various nanocomposites [16,17]. The decrease in the Tg s of PC/MWCNT nanocomposites may be attributable to the higher mobility of the PC molecules in the nanocomposites, where pristine MWCNT are included, without chemical modifications. It is speculated that the decrease in the steady shear viscosities of the nanocomposites compared to the neat PC above the yield stresses resulted from the decrease in the Tg s of nanocomposites and the increased free volume at 260 ◦ C.
Fig. 5. Storage modulus (G ) vs. frequency (a) and loss modulus (G ) vs. frequency (b) for the PC/MWCNT nanocomposites at 260 ◦ C.
Fig. 5 shows the storage modulus and loss modulus of the PC/MWCNT nanocomposites. From this figure it can be seen that the slope of the storage moduli at the terminal region decrease with increasing MWCNT content of the nanocomposites. The decreased slope and solid like behaviors are attributable to the entanglements of PC molecules and MWCNTs. It was found that the G values were higher than the G values within the frequency ranges investigated when the MWCNT content was above 1.0 vol.%. The fact the G value was higher than the G value means the system becomes gel like, with percolation occurring in the nanocomposites. 4. Conclusion
Fig. 4. DSC thermograms of the PC/MWCNT nanocomposites.
The PC/MWCNT nanocomposites prepared using a precipitation method showed a threshold decrease in the surface resistivities at a relatively low critical value of 0.91 vol.%. The low critical value was attributed to the pristine MWCNT, without chemical modifications. It was found that the nanocomposites in melts show yield behavior and a parabolic increase in the
S.H. Jin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 242–245
yield stress with increasing MWCNT contents. The shear viscosities of the nanocomposites were lower than that of the neat PC. The decreased viscosities resulted from the decrease in the Tg s of the PC/MWCNT nanocomposites, where greater mobility of the PC molecules resulted in increased free volumes in the nanocomposites. Acknowledgements This research was financially supported by the MOCIE and KOTEF through the Human Resource Training Project for Regional Innovation. References  S. Iijima, Nature (London) 354 (1991) 56.  P.M. Ajayan, O. Stephan, C. Colliex, D. Trauth, Science 265 (1994) 1212.
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