Toughening high performance ultrahigh molecular weight polyethylene using multiwalled carbon nanotubes
Introduction
The unusual electrical and mechanical properties of carbon nanotubes (CNTs) have motivated a flurry of interests to exploit their applications in advanced composite materials, particularly polymer composites, to improve the performance of a matrix or to achieve new properties. It has been reported that CNTs are extremely strong with the strength of tens of GPa and exceptionally stiff with Young's modulus in TPa range, yet remarkably flexible with the break strain larger than 5% [1], [2]. In addition, CNTs have high aspect ratios (>100). These properties make it interesting to use CNTs as fillers in ductile nanocomposites. The effective utilization of nanotubes in composite applications depends strongly on the ability to disperse the CNTs homogeneously throughout the matrix without destroying the integrity of the CNTs. Furthermore, a good interfacial bonding is required to achieve load transfer across the CNT–matrix interface, a necessary condition for improving the mechanical properties of polymer composites.
Some experimental studies on CNT-reinforced polymer composites have been reported [3], [4], [5]. A 20% increase of modulus in tension and 24% in compression were observed for 5 wt% multiwalled carbon nanotubes (MWCNT)/epoxy composites [3]. Using the surfactant C12EO8 as the processing aid, Gong et al. [4] obtained a 30% increase of elastic modulus with the addition of only 1 wt% CNTs into the epoxy matrix, while the addition of CNTs without the surfactant only has moderate effects on the mechanical properties. For MWCNT/PMMA composites, tensile strength increased 7–30%, toughness ∼10% and hardness ∼43% with the addition of 1–7 wt% acid-treated MWCNTs [5].
In order to optimize the performance of the CNT reinforced polymer composites, some attempts have also been made to produce alignment of CNTs in the matrix [6], [7], [8]. Drawing at temperatures above the glass transition temperatures of the polymer matrix has been shown to be effective to produce uniaxial alignment of CNTs along the draw direction [7]. A most noticeable enhancement in tensile stiffness has been reported. However, the alignment caused a significant loss of ductility. Such observations also led to speculations that the effect of CNTs on polymer composites may be similar to that of nano-carbon fiber. Hence, the resilience of CNTs does not have any effects on the composites.
In this paper, we report the effect of MWCNTs on mechanical properties of highly anisotropic ultrahigh molecular weight polyethylene (UHMWPE) films prepared by intimately mixing 1 wt% CNTs with UHMWPE followed by solution casting and hot draw. We believe this is the first report that shows the incorporation of CNTs into UHMWPE matrix enhances the ductility significantly as well as the strain energy absorption before fracture.
Section snippets
Preparation of MWCNT/UHMWPE composite
The composites used in the present study were made of UHMWPE HiFax 1900 reactor powders with molecular weight of 6×106 g mol−1 kindly supplied by Basell Ltd, USA, and MWCNTs with diameters of the order 15 nm. The as-received MWCNTs were in the form of clusters under scanning electron microscopy (SEM). All CNTs were purified by following the treatment procedures developed by Windle et al. [9] before use. The purified CNTs were dispersed in xylene by magnetic stirring for 2 h followed by a 2 h of
Morphology of MWCNTs and composite films
The as-received MWCNTs were observed in the form of clusters and had many impurities by SEM as shown in Fig. 1(a). After the acid-treatment, the content of CNTs was increased and the typical rope status of CNTs can be readily seen (Fig. 1(b)). High resolution TEM photos show that these CNTs are of diameters ∼15 nm, Fig. 1(c).
The morphology of the 1 wt% UHMWPE/MWCNT composite film is shown in Fig. 2. Several interesting observations can be made from these micrographs. First, the MWCNTs appear to
Conclusions
In summary, the incorporation of 1 wt% MWCNT reinforcement produces a remarkable increase in the tensile strength and modulus for non-drawn UHMWPE composite films of 49.7 and 38%. For the anisotropic MWCNT/UHMWPE composite films with high draw ratios, an up to 150% increase in strain energy density has been observed together with a simultaneous increase of tensile strength of ∼25% and an increase of ductility up to 140%. The tensile modulus of the composite film appears to be little effected.
Acknowledgments
This project was funded by the Research Grant Council of Hong Kong with an earmarked grant for research, grant no. HKUST 6244100P.
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