Three-dimensional heterostructure of metallic nanoparticles and carbon nanotubes as potential nanofiller
© Kim et al; licensee Springer. 2012
Received: 26 January 2012
Accepted: 29 March 2012
Published: 29 March 2012
The effect of the dimensionality of metallic nanoparticle-and carbon nanotube-based fillers on the mechanical properties of an acrylonitrile butadiene styrene (ABS) polymer matrix was examined. ABS composite films, reinforced with low dimensional metallic nanoparticles (MNPs, 0-D) and carbon nanotubes (CNTs, 1-D) as nanofillers, were fabricated by a combination of wet phase inversion and hot pressing. The tensile strength and elongation of the ABS composite were increased by 39% and 6%, respectively, by adding a mixture of MNPs and CNTs with a total concentration of 2 wt%. However, the tensile strength and elongation of the ABS composite were found to be significantly increased by 62% and 55%, respectively, upon addition of 3-D heterostructures with a total concentration of 2 wt%. The 3-D heterostructures were composed of multiple CNTs grown radially on the surface of MNP cores, resembling a sea urchin. The mechanical properties of the ABS/3-D heterostructured nanofiller composite films were much improved compared to those of an ABS/mixture of 0-D and 1-D nanofillers composite films at various filler concentrations. This suggests that the 3-D heterostructure of the MNPs and CNTs plays a key role as a strong reinforcing agent in supporting the polymer matrix and simultaneously serves as a discrete force-transfer medium to transfer the loaded tension throughout the polymer matrix.
Nanostructured materials offer the advantages of a large specific surface area and strong mechanical properties. Thus, they have been used as fillers to reinforce polymer matrices. The interactions that occur at the molecular level owing to the large interfacial contact area between a polymer and nanofiller play a major role in dramatically enhancing the mechanical properties of hybrid polymer-nanofiller composites. Among the various characteristics of nanofillers, the dimensionality of nanofillers dispersed in polymer matrices has been of particular interest in recent years. Numerous research groups have examined the effect of nanofillers with dimensionalities of 0 or 1 on the mechanical properties of polymer composites. For example, the addition of small amounts of metallic nanoparticles (MNPs, 0-D) and carbon nanotubes (CNTs, 1-D) was found to significantly enhance the mechanical properties of polymer composites by as much as approximately 3% and approximately 100%, respectively [1–9]. We expect that MNPs added to a polymer matrix can only interact at several points along a particular polymer chain, whereas CNTs added to a polymer matrix can interact over the length of the polymer chain. Here, we posed the question of whether the combination of the 3-D heterostructures of both MNP-and CNT-based nanofillers in a polymer matrix can synergistically affect the mechanical properties of polymer-nanofiller composites. In order to answer this question, we systematically investigated the effect of nanofillers with the 3-D heterostructure of MNPs and CNTs on the mechanical properties of polymer composites. The heterostructure of MNPs and CNTs is composed of multiple CNTs grown radially on the entire surface of an MNP. These CNTs resemble a sea urchin and are hereafter referred to as sea urchin-like CNTs, or SU-CNTs [10, 11]. In this study, an acrylonitrile butadiene styrene (ABS) was chosen as a specific example of polymer matrix because it is considered superior for its hardness, gloss, and toughness. We would intuitively expect that the SU-CNTs could have a novel enhancement effect on the mechanical properties of ABS polymer matrices due to their unique 3-D carbon nanostructure.
The SU-CNTs were then dispersed in the ABS solution at various concentrations, i.e., 0.25, 0.5, 1, and 2 wt%, by sonication (170 W, 40 kHz) for 3 h. The ABS/SU-CNT solution was mixed with filtered water to initiate the wet phase inversion process, in which a homogeneous mixture of ABS/SU-CNT was rapidly precipitated because of the solubility difference . After the precipitated ABS/SU-CNT composites were dried at 80°C for 5 h, they were transformed into a thin film (diameter 58 mm; thickness 200 μm) using a hot press system (Model No. Mounting Press I, SSAUL BESTECH, Inc., Seoul, South Korea) at 180°C and 150 bar for 8 min. The ABS/SU-CNT composite thin film was then cut into specimens (40 mm × 6 mm × 200 μm) for tensile strength measurements (Model No. LRX Plus, Lloyd Inst. Ltd., Fareham, Hampshire, UK) following the standard tensile strength testing guidelines, ASTM D882 . Seven tensile strength measurements were made for each sample, and the values were averaged to obtain the tensile strength of the fabricated ABS/SU-CNT composite. Here, the fabrication of ABS/MNP/CNT composite thin films and their tensile strength measurements were also made for comparison purpose. The morphologies of the SU-CNTs were characterized using a scanning electron microscope (SEM, S-4200, Hitachi, Chiyoda, Japan) operated at 15-20 kV and a transmission electron microscope (TEM, JEM-2100 F, Jeol Ltd., Tokyo, Japan) operated at 200 kV. The relative fraction of MNPs and CNTs in the prepared SU-CNTs was determined using thermogravimetric analysis (TGA) (TGA Q50, TA Inst., New Castle, DE, USA) with a heating rate of 10°C/min.
Results and discussion
We first removed the MNP cores from the SU-CNTs using nitric acid treatment so that pure CNTs remained as shown in Figure 2c. We also separately prepared MNP cores using a spray pyrolysis process as shown in Figure 2d. We then added a mixture of MNPs and CNTs (MNPs/CNTs) or SU-CNTs to the polymer matrices as reinforcing agents to examine the effect of nanofiller dimensionality on the mechanical properties of polymer composites. Figure 2e shows an image of the ABS/SU-CNT composite thin film prepared by hot pressing, and Figure 2f shows an SEM image of the SU-CNTs dispersed in the ABS matrix.
We have examined the effect of nanofiller dimensionality on the mechanical properties of a polymer composite. We found that 3-D heterostructures of MNPs and CNTs (i.e., SU-CNTs) were much better reinforcing agents than the mixture of 0-D MNPs and 1-D CNTs and that they significantly improved the mechanical properties of the polymer composites. The unique structure of 3-D SU-CNTs prepared by this approach using a combination of spray pyrolysis and thermal CVD processes accounted for the enhancement of the mechanical properties of the polymer composites. The strong interaction of multiple CNTs with the large surface of the MNP core in the 3-D SU-CNT nanofillers facilitated strong coupling forces between the CNTs and polymer molecules. Simultaneously, the discrete CNTs grown radially on the entire surface of the MNP core allowed homogeneous transfer of the loaded tension throughout the polymer composite matrix.
This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0013114). This work was also partially supported by the Converging Research Center Program through the NRF, funded by the Ministry of Education, Science, and Technology (2009-0081928).
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