Physical properties of novel free-standing polymer–nanotube thin films
Introduction
Due to their highly desirable physical and chemical properties, carbon nanotubes have been extensively investigated since their discovery in 1991 [1]. Probably the most readily attainable application for these materials is as a filler in polymer or epoxy-based composite systems. Polymer–CNT composites can be used for a large variety of applications including solar cells [2], flat panel displays [3], [4], electromechanical actuators [5], electrostatic dissipation [6], electromagnetic interface shielding [6], flexible electronics [7], and nanosensors within a polymer to monitor deformations [8] and polymer transitions [9]. Many of the advantages of CNTs over metal fillers are associated with their greater flexibility, lower density, better durability and of course their one-dimensional nature. However, problems connected with the commercialisation of CNTs include difficulties scaling up the production process, and the challenges of handling and selectively positioning such a low density, nano-sized material. One of the most likely methods for commercial growth is chemical vapour deposition (CVD) [10], [11], [12]. CVD allows for the in situ growth of CNTs onto a range of substrates. This technique is also scalable and can be used to produce a large number of CNT arrays at once.
The incorporation of CNTs into a polymer matrix allows us to combine the desirable properties of the nanotubes with the processing advantages of the polymer. Many composite production techniques have been demonstrated including extrusion [13], intercalation into Buckypaper [14], electrospinning [15] and solution processing [16]. However, in all cases achieving controlled dispersion of the nanotubes in the matrix is a non-trivial problem. We address this issue by introducing a novel composite formation method whereby the polymer matrix is incorporated around a pre-formed nanotube network. Using this technique we can selectively position the CNTs within the composite, as previously demonstrated by the authors [17]. This technique results in a uniform nanotube distribution as controlled by the pre-deposited catalyst and growth process. In this letter we report initial studies on the properties of a CNT–polymer composite system produced by this technique.
Section snippets
Experimental
Nanotubes to be used in the composite were grown by chemical vapour deposition. This process involves the thermal decomposition of a carbon source which can then restructure and grow as CNTs in the presence of a catalyst material. We focus on acetylene as the carbon source. The catalyst we use is poly(styrene–vinylferrocene). The vinylferrocene part of this copolymer contains an iron core which is active as the catalyst for CNT growth. The copolymer was dissolved in toluene to form a solution
Results and discussion
Fig. 1 shows a scanning electron microscopy (SEM) image of as-grown CNTs on a SiO2 substrate. Nanotubes cover the surface of the SiO2. The average tube diameter in the SEM below is in the region of 90 nm, which includes a 2 × 15 nm gold layer deposited for imaging purposes. (This gold layer is absent from CNT arrays incorporated into composites.) In this work we choose to grow nanotubes as an entangled mat rather than as aligned arrays. This will result in a composite film with an isotropic
Conclusion
In summary we have demonstrated a novel technique for polymer–nanotube composite production which results in both mechanical and electrical enhancement of the polymer thin film. Further studies are under way to determine the effects of CNT volume fraction on the properties of the composite. For a loading of 0.4 wt%, we observed increases in surface and bulk conductivities by factors of 107 and 108, respectively. The Young’s Modulus increased by a factor of 2.1 ± 0.9. These values indicate that we
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