Surface modification is a good way to improve the surface activity and interfacial strength of multiwalled carbon nanotubes (MWCNTs) when used as fillers in the polymer composites. Among the reported methods for nanotube modification, mixed acid oxidation and plasma treatment is often used by introducing polar groups to the sidewall of MWCNT successfully. The purpose of this study is to evaluate the effect of different surface modification of MWCNT on the mechanical property and electrical conductivity of Fluoro-elastomer (FE)/MWCNT nanocomposites. MWCNTs were surface modified by mixed oxidation and CF4 plasma treatment and then used to reinforce the fluoro elastomer (FE, a copolymer of trifluorochloroethylene and polyvinylidene fluoride). FE/MWCNT composite films were prepared from mixture solutions of ethylacetate and butylacetate, using untreated CNTs (UCNTs), acid-modified CNTs (ACNTs), and CF4 plasma-modified CNT (FCNTs). In each case, MWCNT content was 0.01 wt%, 0.05 wt%, 0.1 wt%, and 0.2 wt% with respect to the polymer. Morphology and mechanical properties were characterized by using scanning electron microscopy (SEM), Raman spectroscopy, as well as dynamic mechanical tests. The SEM results indicated that dispersion of ACNTs and especially FCNTs in FE was better than that of UCNTs. DMA indicated mechanical properties of FCNT composites were improved over ACNT and UCNT filled FE. The resulting electrical properties of the composites ranged from dielectric behavior to bulk conductivities of 10−2 Sm−1 and were found to depend strongly on the surface modification methods of MWCNTs.
CNT/polymer nanocomposites hold the promise of delivering exceptional mechanical properties and multifunctional characteristics [
Reported methods for nanotubes modification include chemical and physical treatments [
As we all know, fluoro-elastomer is quite difficult to be reinforced due to its bad compatibility with the filler and high spatial shielding effect. The fluoro-elastomer we used in the research is a copolymer of trifluorochloroethylene and polyvinylidene fluoride with the volume ratio 1 : 1, which is one of the important materials employed in the aerospace and automotive industries. However, its application is limited by the relatively lower strength. In order to achieve optimal enhancement in the mechanical properties of FE/CNTs nanocomposites, two key issues should be considered: homogeneous dispersion of CNTs in the fluoro-elastomer and strong interfacial bonding between CNT and FE matrix [
MWCNTs prepared by chemical vapor deposition (CVD) were purchased from the Organic Chemical Limited Company, Chengdu, China. The lengths were about 50
A raw-MWCNTs sample (4.0 g) was mixed with the concentrated H2SO4 (98%) and HNO3 (65%) mixed solution (3 : 1 by volume) for 24 h at room temperature with stirring. Finally, the solution was filtered through a cellulose nitrate filter (pore size
Before being modified by CF4 plasma, the as-received MWCNT was cleaned by a classical wet method using nitric acid in order to remove the metal catalysts. The inductive coupled plasma was generated in the radio frequency plasma modification equipment (RF-600, Southwest Academy of Nuclear Physics) with a rotating barrel fixed between the two discharge electrodes. A controlled flow of CF4 gas was introduced into the chamber. The reflective frequency was 13.56 MHz. The diameter of radio-frequency plate electrode was approximately 350 mm, and the spacing of the electrode and samples was 150 mm. The CF4 plasma treatment conditions for MWCNT powder were as follows: gas flow rate of 80 sccm, operating pressure of 10 Pa, a bias of 200 V, power of 300 W, process duration of 10 minutes, and the average temperature of samples at about 100°C during the CF4 plasma treatment. Details were presented in the previous papers [
Scheme of surface modification and mixing: (a) CF4 plasma modification of MWCNT; (b) suggested hydrogen bonding of FE with FCNT.
The fabrication of FE/MWCNTs composites is based on a convenient solution process. In brief, FE was first dissolved in the mixture of ethyl acetate and butyl acetate for 2 days till the uniform solution formed. The exact amount of MWCNTs was dissolved in ethyl acetate with continuous ultrasonication for 30 minutes. Then the FE solution was added to the MWCNT solution to obtain a MWCNT-to-polymer weight ratio of 0.05–0.2 wt.% while stirring continually. The solution was then sonicated for 5 min using a high-power sonic tip (200 W) followed by a mild sonication for 2 h in a sonic bath. After careful mixing of FE solution with carbon nanotubes followed by subsequently casting and controlled solvent evaporation, free-standing FE/MWCNTs composite films were obtained by peeling off from Teflon disks. For the control sample, pure FE films were obtained under the same fabrication processing.
In order to determine the surface chemical changes during the treatments, XPS measurements were used with a hemispherical electron energy analyzer (ESCALAB250, England). A mono-chromatized Al K
Zeta-potential of different kinds of CNTs was measured by Malvem nanoparticles analyzer. CNTs were dispersed in distilled water at the same percent (0.1%) and vibrated for 10 minutes.
Samples were fractured in liquid nitrogen, and the fracture surface was observed under an acceleration voltage of 20 kV with a JEOL JSM-5900LV for SEM experiment.
Micro-Raman spectra were recorded on a Renishaw system 2000 micro-Raman spectrometer with Ar (514 nm wavelength) as excitation source. The incident light was introduced to the sample through a 50X objective as a spot less than 2
The investigation of the thermo-mechanical behavior was performed by dynamic-mechanical thermal analysis, DMA, using TA RSA3 8500-0001 system. For the measurements, rectangular specimens of 50 mm length, 5 mm width, and 2 mm thickness were prepared. The tests were performed in tensile mode at a frequency of 10 Hz with a static strain of 0.6% and a dynamic strain of 0.1%, in a temperature range between −20°C and 70°C with a heating rate of 2°C/min.
DC conductivity was measured with a Keithley 6514 Digital Electrostatic Charge Meter in a four-probe setup at room temperature and reported as an average of three readings (see Table
Experimental values of the conductivity for FE/CNT nanocomposites.
Sample | Film thickness, | Resistivity, | Conductivity, |
---|---|---|---|
FE/CNT0.01 | 102.0 | — | — |
FE/CNT0.05 | 13.1 | ||
FE/CNT0.1 | 73.0 | ||
FE/CNT0.2 | 69.6 | ||
FE/ACNT0.01 | 82.7 | ||
FE/ACNT0.05 | 44.0 | ||
FE/ACNT0.1 | 30.0 | ||
FE/ACNT0.2 | 56.3 | 26.9 | |
FE/FCNT0.01 | 54.0 | ||
FE/FCNT0.05 | 100.0 | ||
FE/FCNT0.1 | 210.0 | 33.8 | |
FE/FCNT0.2 | 180.0 | 28.3 |
First, the effects of surface modification on the chemical states and morphology of the MWCNTs were estimated. XPS C1s spectrum of untreated MWCNTs has three chemical states, as shown in Figure
XPS C1s spectra: (a) pristine; (b) acid-oxidation-treated MWCNTs; (c) CF4 plasma-treated MWCNTs. Also shown are the corresponding SEM micrographs.
In general, the stability of colloidal particles in solution is important for casting process and is greatly affected by their surface charge density. The electrostatic potential of charged particles dispersed in a liquid medium is governed mainly by surface functionality, especially by its ionization ability to produce a charged surface, and the preferential absorption of ions of one charge sign from the solution [
Direct observations of the improved dispersion after adding FCNT are shown in Figure
SEM photographs of nanocomposite fracture surfaces showing the dispersion state of MWCNT. (a) 0.2 wt% UCNT. (b) 0.2 wt% ACNT. (c) 0.2 wt% FCNT.
For a further study on the interaction between CNTs and polymer matrix, Raman was used to detect the interaction between CNTs and the FE matrix. From Figure
Raman spectra for MWCNT under different treatment.
Micro-Raman spectra of FE and FE/MWCNT nanocomposites: (a) FE/FCNT, (b) FE/ACNT, (c) FE/MWCNT, and (d) FE.
The DMA properties were measured for neat FE and nanocomposite films, as shown in Figures
The storage modulus as a function of temperature for the FE/MWCNT with different MWCNT loadings.
The storage modulus as a function of temperature for the FE/ACNT with different ACNT loadings.
The storage modulus as a function of temperature for the FE/FCNT with different FCNT loadings.
Figure
The conductivity as a function of CNT loading for CNT/FE composites.
Carbon nanotubes modified by CF4 plasma can reinforce the fluoro elastomer matrix to yield the highest increase in modulus due to better dispersion and enhanced chemical compatibility by introducing electron-rich fluorine atoms. However, for electrical properties, carbon nanotubes modified by mixed acid are predominant in the polymer matrix by removing the amorphous carbon effectively, which results in lower percolation threshold and higher conductivity. This difference in the mechanical and electrical properties would be understood in selecting suitable surface modification.
This work was financially supported by the Special Foundation of CAEP (no. 2008B0302029).