Low percolation behavior of HDPE/CNT nanocomposites for EMI shielding application: Random distribution to segregated structure

Abstract

In this study, three different types of nanocomposites with random (r-CPC), segregated (s-CPC) and semi-segregated (ss-CPC) distribution of carbon nanotubes in High Density Polyethylene matrix were fabricated using hot compaction method. Microstructure of nanocomposite samples were observed by Scanning Electron Microscopy, which revealed the formation of carbon nanotubes conductive network at the polymer granule interfaces in segregated structure. While, partial formation of segregated structure was shown in semi-segregated samples. A significant reduction of Young modulus and strength of s-CPC6 and ss-CPC6 compared to r-CPC6 nanocomposite has been correlated to weak interface and voids between HDPE granules in segregated structure. DSC results proved that the crystallinity index of HDPE decreases when the distribution of CNTs in polymer matrix changes from segregated to random structure. Electrical conductivity was measured using four-point probe. Segregated structure samples showed electrical conductivity up to 9 and 5 order of magnitude larger than semi-segregated and random structure nanocomposites, respectively. Nanocomposites follow percolation behavior and applying percolation theory showed a decrease in percolation threshold from 7.1 vol% in r-CPC to 0.099 vol% in s-CPC. Finally, Electromagnetic Interference shielding properties were studied and the highest Shielding Effectiveness (SE) was obtained 21.8 dB in segregated sample at 6 wt% CNTs. While, in random structure the highest EMI SE was 9 dB at 20 wt% CNTs. Absorption and reflection were the dominant shielding mechanisms in segregated and random structure nanocomposites, respectively.