Rheological and electrical properties of polypropylene composites containing functionalized multi-walled carbon nanotubes and compatibilizers
Introduction
Nanometer scale fillers have stimulated great interest on the field of composite materials which have superior mechanical and electrical properties. Carbon nanotubes have been used frequently for manufacturing of composite materials due to its excellent mechanical property and fibrous structure with high aspect ratio. The nanotube composites with extraordinary electrical properties can be used for applications in electrostatic dissipation (ESD), electromagnetic interference (EMI) shielding, radio frequency interference (RFI) shielding, and so on [1], [2], [3], [4], [5]. However, carbon nanotubes tend to form agglomerates during processing of composites because of van der Waals attraction between nanotubes. The nanotube agglomerates reduce the surface area and interrupt formation of the network structure which is essential to improve electrical and mechanical properties and to induce efficient transfer of their superior properties to the polymer matrix. Uniform dispersion of the carbon nanotubes is required to realize the potential of carbon nanotubes as reinforcing fillers [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Improved wetting and interfacial bonding between polymer matrix and carbon nanotubes are needed to produce superior carbon nanotube composites.
The synthesized carbon nanotubes usually exist as agglomerates of the size of several hundred micrometers [12]. Such entanglements make it difficult to disperse nanotubes uniformly in polymer matrix. To overcome the dispersion problem, it is necessary to tailor the chemical nature of the nanotube surface. One of the most straightforward methods for nanotube dispersion is direct mixing, but it does not always yield homogeneous distribution of nanotubes because of the lack of compatibility between MWCNTs and polymer matrix. Solution processing has been a commonly used method in fabrication of the well-dispersed carbon nanotube composites. However, it is hard to achieve homogeneous dispersion of nanotubes in polymer matrix because carbon nanotubes are insoluble and bundled.
Chemical functionalization of the MWCNT surface increases interfacial interaction between MWCNTs and the polymer matrix, enhances adhesion of the MWCNTs in various organic solvents and polymers, reduces the tendency to agglomerate, and improves dispersion [13]. The improved interactions between MWCNTs and the polymer matrix govern the load-transfer from the polymer to the nanotubes and hence increase the reinforcement efficiency. Attachment of oxygen containing functional groups (i.e., carboxyl groups, carbonyl groups, hydroxyl groups, etc.) on the surface of the MWCNTs could be achieved by applying several chemical treatments. Types of chemical functional groups that are attached to the nanotubes should depend on the nature of the bulk polymer. The presence of functional groups on the nanotube surface is convenient because a variety of chemical reactions can be induced with functional groups [14]. The chemically functionalized MWCNTs can be easily mixed with the polymer matrix. Especially, acid treatment of the nanotube is a well-known technique to remove catalytic impurities, generate functional groups on open ends or sidewalls of nanotubes, and facilitate good dispersion of MWCNTs in polymeric solutions or melts. The amine treatment brings the desired solubility to MWCNTs by using the long-chain aliphatic amine, such as octadecylamine [CH3(CH2)17NH2]. Heat treatment is usually employed for annealing or activating the MWCNT surface as well as removing amorphous carbons.
Compatibilization of polymer/MWCNT composites using a suitable coupling agent, normally a graft or block copolymer, is a well-established method to change the surface energy of the MWCNTs to improve their wetting or adhesion characteristics and to reduce their tendency to agglomerate in the polymer matrix [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Since non-polar polymer and MWCNTs are incompatible due to differences in chemical nature and polarity, it is hardly expected that uniform dispersion of the MWCNTs would be accomplished. Poor adhesion between MWCNTs and the polymer matrix restricts separation of MWCNT aggregates resulting in poor dispersion of MWCNTs and insufficient reinforcement.
Compatibilizers containing maleic anhydride functionalities are effective in improving physical properties of composites. The interaction between functional groups of the compatibilizer and carboxyl or amine groups of MWCNTs has stabilized the morphology and improved the interfacial interaction between MWCNTs and the polymer matrix [20]. Especially maleic anhydride grafted polypropylene (MA-g-PP) is one of the most promising candidates that can improve the PP/MWCNT composite properties. In general, the nanoscale dispersion of MWCNTs in polypropylene matrix is achieved by strong hydrogen bonding between hydroxyl groups of the MWCNTs and maleic anhydride groups of MA-g-PP, depending upon the chemical similarity of polypropylene matrix and the grafted polypropylene [12]. MA-g-SEBS is a triblock copolymer with styrene blocks at both ends and an ethylene/butylene block in the middle. It would be expected that the two ends containing aromatic rings would be compatible with the MWCNTs and the rubbery EB block would be compatible with the polypropylene matrix. Accordingly, the presence of MA-g-SEBS is helpful to promote interfacial adhesion and dispersion of the MWCNTs [16].
PP/MWCNT composites have been prepared and investigated for various applications. However, how to improve MWCNT dispersion, to enhance the adhesion between MWCNTs and PP matrix, and to characterize the MWCNT dispersion and interaction exactly are still unclear and remain as a technical challenge. In addition, there is only limited rheological data in the literature about the effect of compatibilizer on the rheological behavior of PP/MWCNT composite melt. Moreover, few publications existed on the extensional flow behavior of polymer/MWCNT composites in contrast to those on the shear flow.
Therefore, the majority of this study is concerned about effects of the compatibilizer and chemical functionalization of MWCNTs on rheological and electrical properties of the PP/MWCNT composites which are prepared by melt compounding. In this study, chemical functionalization is performed through acid, amine, and heat treatments. In addition, two different compatibilizers, MA-g-PP and MA-g-SEBS, are also used. Fuctionalization of MWCNTs and morphology of the PP/MWCNT composites were characterized by using FT-IR and FE-SEM. The rheological properties of PP/MWCNT composites were measured at shear and uniaxial elongational flows by using AR2000 and RME, respectively, and electrical conductivity was measured by the four-probe method. It is understood that the electrical and rheological percolation thresholds are correlated to the network structure of PP/MWCNT composites.
Section snippets
Materials and chemical functionalization of MWCNTs
Polypropylene (HP450J, MFI 3.25 g/10 min, supplied by Polymirae Co., KOREA) was used as the polymer matrix and multi-walled carbon nanotubes (supplied by Iljin Nanotech, KOREA) were used as reinforcing fillers. The MWCNTs were manufactured by chemical vapor deposition (CVD) process with the average diameter of 13 nm, the length of 10 μm, and the aspect ratio (L/D) of about 800. MA-g-PP (POLYBOND-3150, supplied by Crompton Co., USA) and MA-g-SEBS (KRATON FG 1901X, supplied by Shell Chemical Co.,
FT-IR spectra
The FT-IR spectra of the surface treated MWCNTs are shown in Fig. 1. The characteristic bands due to generated functional groups are observed in the spectrum of each chemically treated MWCNTs. As shown in Fig. 1a, the acid treated MWCNTs show new peaks in comparison with the FT-IR spectrum of the untreated MWCNTs which lack hydroxyl and carbonyl groups. The peaks around 3421 cm−1 and 1453 cm−1 are assigned to the O–H band, and the peaks at 1654 cm−1 and 1710 cm−1 are assigned to the CO band for
Conclusion
Polypropylene composites filled with chemically functionalized MWCNTs and two types of compatibilizers were prepared by melt compounding using a twin-screw extruder and effects of dispersion and interfacial interaction on morphological, rheological, and electrical properties of the composites were investigated. FT-IR spectra showed that various functional groups were formed on MWCNTs through chemical functionalization such as acid, amine, and heat treatments. It was observed by an FE-SEM that
Acknowledgements
This study was supported by the Korea Science and Engineering Foundation through the Applied Rheology Center (ARC). The authors are grateful for the support.
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