Mechanical and rheological properties of carbon nanotube-reinforced polyethylene composites
Introduction
Both theoretical and experimental studies have shown that the elastic modulus of a carbon nanotube (CNT) is in the range of 1–5 TPa [1], [2], [3], [4], which is significantly higher than that of a carbon fiber of 0.1–0.8 TPa [5]. Such a superior property makes CNTs a promising reinforcing material. However, recent results showed that CNTs, when incorporated into polymer matrix, do not necessarily warrant enhanced mechanical properties. Bhattacharyya et al. [6] investigated a melt-blended single-walled carbon nanotube (SWNT)/PP composite and observed a slight drop in tensile strength, elastic modulus and fracture strain with an addition of 0.8 wt% CNT. In a CNT/PMMA composite system, Jia et al. [7] found a decrease in tensile strength, toughness and hardness when untreated CNTs were used. Some other studies showed a moderate increase in mechanical properties, for example, 24% increase in elastic modulus but slightly lower fracture load in CNT/epoxy film with 0.1 wt% MWNTs [8]; about 10% increase in tensile stiffness and a slight increase in tensile strength of MWNT/PS rod samples prepared by extrusion [9]; 20% increase in storage modulus of CNT/epoxy at room temperature with 0.30 wt% of fluorinated SWNT [10]; about 20% increase in tensile and compressive moduli of a CNT/epoxy composite with 5 wt% CNT [11]; etc. On the other hand, a significant improvement of the mechanical properties was also reported by a few investigators. Ganguli et al. [12] showed that the ultimate strength and fracture strain of a bifunctional epoxy were increased by 139% and 158%, respectively, after adding 1 wt% MWNTs. Allaoui et al. [13] reported that the Young’s modulus and yield strength of a MWNT/epoxy composite have been doubled and quadrupled with an addition of 1 or 4 wt% CNTs. The elastic modulus and yield strength of nylon-6 polymer were found to increase by 214% and 162%, respectively, when 2 wt% MWNTs were used [14]. It was also concluded that the interface chemical bonding between CNTs and matrix plays a critical role in determining the properties and performance of a CNT composite [15].
Some common methods for the preparation of CNT/polymer composites include in-situ filling-polymerization [7], solution mix [16], [17] and melt blending [18], [19], [20]. The composites prepared by the first two methods may result in contaminations because of the residual monomer or solvent. However, those by melt blending are essentially free of such contaminations. In addition, the tendency of CNTs to form aggregates may be minimized by appropriate application of shear during melt mixing [20], [21]. These advances make the melt blending method a promising technique to produce CNT/polymer composites. Therefore, in recent years, there has been an increasing interest in the investigation of the behavior of CNT composites by melt blending [22], [23], [24], [25].
A few investigations on dynamic frequency sweeps [26], [27], [28], [29], [30] have been reported using melt processed thermoplastic polymer/CNT composites, whose matrices include polycarbonate [26], [27], [28], polypropylene [29], and polyamide-6 and its blends with acrylonitrile/butadiene/styrene [30]. These studies found that the complex viscosity continuously decreases with increasing frequency while storage modulus G′ and loss modulus G″ increase. A characteristic change in the rheological behavior with increasing the nanotube content was also observed at temperatures well above the glass transition or melting temperature, referred to as the percolation threshold or gelation point in relation to fluid-to-solid, fluid-to-gel transition or a combined nanotube-polymer network.
The mechanical properties of low density polyethylene (LDPE) composites have been widely investigated in the past decades with varying reinforcements from carbon, glass and natural fibers to metal and non-metal particles [31], [32], [33], [34], [35], [36], however, it seems that results on the MWNT reinforced LDPE have not been available . The present paper will investigate some key properties of a low density polyethylene (LDPE) polymer reinforced by MWNTs with varying nanotube contents.
Section snippets
Experiment
MWNTs used in the present experiments were prepared by chemical vapor deposition (provided by Nanolab) with diameters ranging from 10 nm to 20 nm and lengths varying from 1 μm to 5 μm. The raw granules of LDPE were provided by Qenos Pty Ltd, Australia. LDPE and various amounts of MWNTs were mixed at 140 °C for 20 min using a mechanical mixer at the speed of 120 rpm. The mixed samples were then compressed in a steel mold under a pressure of about 8 MPa at 140 °C for 5 min using a hydraulic press.The
Tensile properties
The stress/strain curves of the MWNT-LDPE composites with varying tube contents are shown in Fig. 1 and the maximum tensile strength σf, Young’s modulus Ec and failure strain εf of the composites are summarized in Table 1, where εf is defined as the strain at which the tensile stress starts to drop sharply. The increases in Young’s modulus (89%) and tensile strength (56%), when nanotube loading reaches 10 wt%, indicate that an effective reinforcement of the MWNTs. The rapid decrease of εf with
Conclusions
Some important properties of MWNT-reinforced LDPE composites have been studied. The investigation concludes that the mixing and the Cox-Merz rules cannot be applied directly to MWNT composites and a further development is necessary. The study found that the Young’s modulus and tensile strength of the composites increase by 85% and 46%, respectively, when 10 wt% MWNTs was added. However, the superior property of high tensile modulus of MWNTs has not been fully utilized in the composites because
Acknowledgments
This work was financially supported by Australia Research Council (ARC). The authors would like to thank Dr. S.C. Dai for assistance in the operation of rheometer.
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