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Zhou et al. – eXPRESS Polymer Letters Vol.2, No.1 (2008) 40–48Figure 12. Fracture surface of 0.4% CNT/epoxyfractographic features. For example, the failure surfacesof the nanocomposite are rougher with theCNTs added into the epoxy matrix (Figures 11band 11d). The higher magnification SEM picture inFigure 11e shows that the size of the cleavage planedecreased to 10 μm after the infusion of the CNTs.The decreased cleavage plane and the increasedsurface roughness imply that the path of the cracktip is distorted because of the carbon nanotubes,making crack propagation more difficult. Figure11d also shows that CNTs were uniformly dispersedin the epoxy. The flat cleavage planes wereformed by the network of cleavage steps and eachplane contains at least one carbon nanotube. Duringthe failure process, the crack propagation changeddirection as it crossed CNTs. The bridge effect,which prevents crack opening, increased strength inthe CNT/epoxy matrix.When the CNT content increased to 0.4 wt%, alarge particle, an agglomeration of several carbonnanotubes (Figure 12), was observed in the fracturesurface. At a low stress level, the agglomerated particleincreased the stiffness of the material, but at ahigh stress level, the stress concentration caused bythe agglomerated particle initiated a crack, whichmade the sample fail quickly.4. ConclusionsMulti-walled carbon nanotubes have been infusedin epoxy by ultrasonic method. Based on electrical,thermal and mechanical experiment results, wereached the following conclusions.1. Ultrasonic cavitation is an efficient method ofinfusing carbon nanotubes into epoxy resinwhen CNT weight fractions are lower than0.3 wt%. Above the 0.3 wt%, CNTs agglomerated.2. The frequency dependent behavior of CNT/epoxy can be described by using as a R-C circuitmodel. The resistivity of epoxy decreased withCNT content, and permittivity increased withincreasing of CNT.3. Compared to neat epoxy, DMA results indicateda 93% improvement in storage modulus in0.4 wt% CNT/epoxy at room temperature and a17°C increase in T g . However, the decompositiontemperature decreased with higher CNTcontents.4. Flexural modulus steadily increases with ahigher CNT weight percent. Modulus improvedby 11.7% with an addition of a 0.4 wt% ofCNTs. Flexural strength and fracture toughnesspeaked in a 0.3 wt% CNT/epoxy system. Thedecrease in strength and fracture toughness in0.4% CNT/epoxy was attributed to poor dispersionsof nanotubes in the composite.AcknowledgementsThe authors would like to gratefully acknowledge the supportof the Air Force Minority Leaders NanocompositesResearch and Education Program and National ScienceFundation.References[1] Imanaka M., Nakamura Y., Nishimura A., Iida T.:Fracture toughness of rubber-modified epoxy adhesives:Effect of plastic deformability of the matrixphase. Composites Science and Technology, 63, 41–51 (2003).[2] Chikhi N., Fellahi S., Bakar M.: Modification ofepoxy resin using reactive liquid (ATBN) rubber.European Polymer Journal, 38, 251–264 (2002).[3] Xian G., Walter R., Haupert F.: Friction and wear ofepoxy/TiO 2 nanocomposites: Influence of additionalshort carbon fibers, Aramid and PTFE particles. CompositesScience and Technology, 66, 3199–3209(2006).[4] Vasconcelos P. V., Lino F. J., Magalhaes A., Neto R.J. L.: Impact fracture study of epoxy-based compositeswith aluminium particles and milled fibres. Journalof Materials Processing Technology, 170, 277–283 (2005).47
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