Decoration of multi-walled carbon nanotubes by polymer wrapping and its application in MWCNT/polyethylene composites
© Hsiao et al.; licensee Springer. 2012
Received: 13 December 2011
Accepted: 6 May 2012
Published: 6 May 2012
We dispersed the non-covalent functionalization of multi-walled carbon nanotubes (CNTs) with a polymer dispersant and obtained a powder of polymer-wrapped CNTs. The UV–vis absorption spectrum was used to investigate the optimal weight ratio of the CNTs and polymer dispersant. The powder of polymer-wrapped CNTs had improved the drawbacks of CNTs of being lightweight and difficult to process, and it can re-disperse in a solvent. Then, we blended the polymer-wrapped CNTs and polyethylene (PE) by melt-mixing and produced a conductive masterbatch and CNT/PE composites. The polymer-wrapped CNTs showed lower surface resistivity in composites than the raw CNTs. The scanning electron microscopy images also showed that the polymer-wrapped CNTs can disperse well in composites than the raw CNTs.
Carbon nanotubes (CNTs), having excellent electrical and extraordinary mechanical properties, are suitable to serve as a conductive filler or a strengthening material in polymer composites; however, their chemical inertia and smooth surface result in their lack of solubility and poor compatibility with polymers. In addition, the CNTs have high aspect ratio and easily attract or tangle to each other due to the van der Waals force interaction. Furthermore, CNTs are inherently lightweight, occupy a lot of space, and are easily blown, thereby increasing the trouble in handling and the difficulty in processing and application.
To obtain good dispersion of CNTs in polymers, we can prepare the CNT masterbatch by mixing CNTs and carriers first, and then the masterbatch are diluted with thermoplastics to produce the CNT/polymer composites. There are three kinds of methods to prepare the CNT masterbatch. The first method is so-called in situ polymerization method ‐: the monomers and CNTs are mixed well in a solution. Then, the monomers are polymerized, and therefore, the CNTs can disperse in the polymer polymerized from the monomers. However, this method usually needs the chemical modification of CNTs for well-dispersed CNTs in the solution, and it could damage the structure of CNTs and reduce their conductivity. The second method is the solution process ‐: the CNTs and the polymer solution are mixed well, and the CNT/polymer composites are obtained by re-precipitation or removing the solvent from the mixture. This method can easily disperse the CNTs in the polymer; however, it is not suitable for mass production due to its high cost, toxicity of the solvent, and solubility limitations of the polymer in the solvent. The third method is the melt-mixing [5, 8]: the CNTs and polymer are directly mixed at high temperature (usually above the glass transition temperature of the polymer) by mechanical mixing. The melt-mixing is also an easy way to produce the CNT masterbatch, but the CNTs are difficult to disperse well in the polymer.
Non-covalent functionalization of CNTs by polymer wrapping is a feasible process to disperse CNTs in the solvent and that could not cause dramatic changes in the electronic properties of CNTs. It has been reported that a molecular structure containing functional groups can effectively absorb on the surface of CNTs and provide the dispersion of CNTs in a solution ‐. However, preparing the polymer-wrapped CNTs by in situ polymerization is not suitable to be used in CNT/polymer composites due to the residual catalysts and impurities ‐. The surface modification of CNTs with strong acids is usually used to improve the dispersion of CNTs in polar solvents (such as H2O, ethanol, and DMF), but that could also disrupt the sp2 structure and conjugation of the CNTs ‐. The CNTs functionalized with hydrophilic molecules can disperse in polar solvents, but they could not disperse well in less polar solvents (such as toluene and n-hexane) and have poor compatibility with aliphatic polymers ‐.
In this study, we reported an easy way to disperse CNTs in solvent and polymer. The surface functionalization of CNTs with a polymer dispersant, containing both aromatic group and amine group, can disperse CNTs in less polar solvents. The powder of polymer-wrapped CNTs can also re-disperse in organic solvents and be easily used in producing the CNT/polyethylene (PE) composites by melt-mixing.
Materials and instrumentations
Surface resistivity was measured on a surface resistivity meter, which was our homemade equipment and calibrated against indium tin oxide on PET as reference samples. The ultraviolet–visible (UV–vis) absorption spectra were recorded using a UV–vis spectrometer (SHIMADZU, Kyoto, Japan). The CNT/PE composites were prepared using a twin-screw extruder (MP-2015, APV, Houston, TX, USA).
All other chemicals which were not specially mentioned were commercially available and used as supplied. We used the multi-walled carbon nanotubes purchased from Nanomaterial Store (OD, 10 to 30 nm; length, 10 to 30 μm; purity, 85 %; Fremont, CA, USA). The polymer dispersant, which was synthesized form the monomers of phenyl methacrylate and glycidyl methacrylate and followed by grafting with amine, had a weight-average molecular weight of about 10,000 and an amine value (mg KOH) of 14.
Preparation of the polymer-wrapped CNTs
The polymer dispersant, CNTs, and solvent were added to a reactor, and they were mixed by ultrasonic oscillation and mechanical stirring for 1 h to form the CNT suspension. After filtering the CNT suspension and collecting the sample, the sample was baked using an oven to obtain the powder of polymer-wrapped CNTs.
Preparation of the CNT masterbatch and CNT/PE composites
The powder of polymer-wrapped CNTs and PE were blended using a kneader first, and then blended using a twin-screw extruder to form the 10 wt.% CNT masterbatch. The CNT masterbatch was further diluted with PE using the kneader and twin-screw extruder to form the CNT/PE composites with various concentrations of CNTs.
Results and discussions
Dispersion of the powder of polymer-wrapped CNTs
Decoration of CNTs by polymer wrapping is an easy way to disperse CNTs in solvent and polymer, and the powder of polymer-wrapped CNTs can re-disperse in a solvent. This technology also improves the drawbacks of CNTs of being lightweight and difficult to process. Depending on the concentration of CNTs in composites, they can meet electrical requirements such as anti-electrostatic property electrostatic discharge conductivity and electromagnetic interference The composites can be applied in an anti-electrostatic product, an electrostatic discharge product, an electromagnetic and radiation shield, and 3 C electronic equipment.
We acknowledge the financial support from the Department of Industrial Technology of the Ministry of Economic Affairs, Taiwan. AEH, on behalf of his co-authors, would like to especially thank their colleagues, Chien-Lin Huang, Jui-Chen Liu, and Ching-Yuan Hu, for their aid in the apparatus availability.
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