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Zhou et al. – eXPRESS Polymer Letters Vol.2, No.1 (2008) 40–48attracted great interest because they frequentlyinclude superior mechanical, electronic, and flameretardantproperties. Different polymer/CNT nanocompositeshave been synthesized by incorporatingCNTs into various polymer matrices, such aspolyamides [12], polyimides [13–15], epoxy [16],polyurethane [17, 18] and polypropylene [19–21].Previous results indicated that the addition of smallamounts of CNT ( 95%)used in this study. The tube diameter ranges from30 to 50 nm, the tube length ranges from 3 to μm.The weight fraction of the carbon nanotubes rangedfrom 0 to 0.4 wt%, to help identify the loading withthe best thermal and mechanical properties.Figure 1 shows the pictures of received carbon nanotubesat different magnifications. High specificsurface area and cotton-like entanglements causedthe formation of agglomerates [16]. Agglomerates ofCNTs, called nanoropes, are difficult to separateand infiltrate with the matrix. For polymer matrixnanocomposites, high power dispersion methods,such as ultrasound and high-speed shearing, are thesimplest and most convenient to improve the dispersionof nanosized fillers in a polymer matrix[15, 16]. In this study, the components were mixedusing a high-intensity ultrasonic processor. In thiscase, the application of alternating acoustic pressureabove the cavitation threshold creates numerouscavities in the liquid. Some of these cavitiesoscillate at a frequency of the applied field (usually20 kHz) whereas the gas content inside these cavitiesremains constant. However, some other cavitiesgrow intensely under tensile stresses whereasyet another portion of these cavities that are notcompletely filled with gas start to collapse underthe compression stresses of the sound wave. In thelatter case, the collapsing cavity generates tiny par-Figure 1. SEM pictures of as received carbon nanotube at different magnification41
Zhou et al. – eXPRESS Polymer Letters Vol.2, No.1 (2008) 40–48ticles of debris and the energy of the collapsed oneis transformed into pressure pulses. It is noteworthythat the formation of the debris further facilitatesthe development of cavitation. It is assumed thatacoustic cavitation in liquids develops according toa chain reaction. Therefore, individual cavities onreal nuclei develop so rapidly that within a fewmicroseconds an active cavitation region is createdclose to the source of the ultrasound probe. Thedevelopment of cavitation processes in the ultrasonicallyprocessed melt creates favorable conditionsfor the intensification of various physicochemicalprocesses. Acoustic cavitation acceleratesheat and mass transfer processes such as diffusion,wetting, dissolution, dispersion, and emulsification.The uniform dispersion of CNF was observed insonication process [8].Pre-calculated amounts of carbon nanotubes andEpon 862 resin were carefully weighed and mixedtogether in a beaker. A high-intensity, ultrasonicirradiation mixed the CNTs and resin for an hour onpulse mode, 50 sec on/25 sec off (Ti-horn, 20 kHzSonics Vibra Cell, Sonics & Materials, Inc). Thebeaker containing the mixture was submerged in anice bath to keep it cool during the sonicationprocess. Once the irradiation was complete, Epi-Cure curing agent W was added to the modifiedresin and mixed using a high-speed mechanicalstirrer for about 10 minutes. The mix ratio ofEpon 862 and W agent was 100:26. The mixing ofepoxy and curing agent initially produced highlyreactive, volatile vapor bubbles, which could createvoids and detrimentally affect the properties of thefinal product. To reduce the chance of voids, theliquid was preheated to 80°C to reduce its viscosityand a high vacuum system was used for about30 minutes. After the bubbles were completelyremoved, the mixture was transferred to plasticandTeflon-coated metal rectangular molds andcured for 4 hours at 120°C. The cured material wasthen trimmed. Finally, test samples were machinedfor thermal and mechanical characterization and allpanels were post-cured at 170°C for 4 hours in aLindberg/Blue Mechanical Convection Oven.3. Experimental results and discussions3.1. Electric propertiesThe electrical properties of the neat and nanophasedepoxy were measured by using an Agilent4294-A Precision Impedance analyzer. The compositeswere cut into rectangular bars with dimensionof 8 mm × 4mm× 1 mm (length, width andthickness) by a diamond saw. The two end surfaceswere fully coated with gold as electrodes, which isabout 4 mm 2 . The measurement was carried at frequenciesfrom 100 Hz to 10 MHz at room temperature.In the measurement, the impedance of thesample at each frequency was measured andrecorded. The resistivity of the composites was calculatedusing the measured impedance and thegeometry of the sample.The application of conductive nano-particles to aninsulating polymer matrix is supposed to induce anelectrical conductivity, when the volume fractionexceeds the percolation threshold [23]. Generally,the percolation threshold is considered to be lowerfor fiber-shaped fillers (high aspect ratio) than forspherical particles. The results shown in Figure 2are the resistivity of the neat and nanophased epoxyat frequency 100 Hz. In this figure, the impedanceresults of each material are consistent, even somesamples are not very uniform. A dramatic decreasein resistivity has been found in nanophased epoxy.With only 0.2% CNT, resistivity of epoxydecreased from 10 14 Ω·m of neat epoxy to 10 Ω·m.Figure 2. Effect of CNT weight fraction on resistivity ofepoxy42
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