Electrical properties of low-density polyethylene/multiwalled carbon nanotube nanocomposites

Abstract

Low-density polyethylene (LDPE)/multiwalled carbon nanotube (CNT) nanocomposites were prepared via melt compounding. The electrical properties of LDPE/CNT nanocomposites as a function of CNT volume content, frequency and temperature were investigated. The results showed that dielectric constant of LDPE/CNT nanocomposites increases slightly with increasing CNT content up to 1.9 vol.%. Thereafter, the dielectric constant of the nanocomposites increases sharply. The dielectric constant of LDPE/3.6 vol.% CNT nanocomposite is more than two orders of magnitude larger than that of pure LDPE. The frequency dependence of electrical properties of LDPE/3.6 vol.% CNT nanocomposite can be well described by the percolation theory.

Introduction

Electrical properties of composites based on conducting fillers such as metal particle [1], [2], [3], [4], carbon black [5], [6], [7], [8], [9], [10] and carbon nanotube [11], [12], [13], [14], [15], [16], [17], [18], dispersed in insulating polymer matrix have found widespread application in industrial sectors. The electrical properties of the composites may vary from those of an insulating material to those of conducting filler network depending on the concentration, property of the conducting fillers and dispersion of conducting fillers in polymeric matrix [19]. Electrical properties of the conducting fillers/insulating polymer composites are often analyzed in term of the statistical percolation theory. At low concentration, conducting fillers are dispersed within polymeric matrix as insolated clusters. Beyond a critical concentration of conducting filler, known as percolation threshold, filler clusters begin to connect each other to form a filler network throughout the entire composite, resulting in several orders of magnitude increase in the conductivity and dielectric properties of the composite. The transition from isolated cluster to connected network of conducting filler is referred to as the percolation transition [20].

Since carbon nanotube (CNT) was synthesized successfully in 1991 by Ijima, the properties and potential applications of carbon nanotube have attracted increasing attention of materials scientist [21]. Its excellent conductivity makes CNT an ideal material for the production of conductive polymer composites, capable of dissipating the electrostatic charge or shielding devices from the electromagnetic radiation. More interestingly, the electrical properties of CNT is dependent on the nanotube diameter, number of concentric shell, and chirality, which allows tuning electrical or magnetic properties conveniently by selecting the proper parameters [21]. Nogales et al. [22] prepared the poly(butylenes terephthalate)/CNT nanocomposites via in situ polycondensation, and they reported low CNT percolation threshold content of 0.2 wt.%. Sandler et al. [23], [24] found an ultra-low percolation threshold of 0.0025 wt.% in the epoxy/CNT composites. Velasco-Santos et al. [25] prepared the PMMA/CNT nanocomposites by incorporating chemically functionalized CNT into PMMA matrix via in situ polymerization. They reported that the storage modulus of PMMA is increased by ∼10-folds and the glass transition temperature is increased by ∼40 °C associated with the CNT addition. Further enhancement in the physical and mechanical properties of the polymer/CNT composites can be achieved by using aligned CNTs that are induced by the electric field method [26], [27].

The temperature dependence of resistivity is a typical characteristic of the CNT or other conducting particles filled polymer composites. The resistivity of conducting particles filled polymer composites sharply increases with temperature, known as positive temperature coefficient (PTC) effect in the vicinity of melting temperature or glass temperature. Several reports have focused on the temperature dependence of resistivity of conducting particle such as carbon black [8], [28], [29], [30], [31], [32] or CNT [33] filled polymers near the melting temperature of polymeric matrix. In this paper, LDPE/CNT nanocomposites are prepared by melting compounding. The dependence of electrical properties on CNT content, frequency and temperature is examined.