Analysis of MWCNT/epoxy composites at microwave frequency: reproducibility investigation
 Mauro Giorcelli^{1}Email author,
 Patrizia Savi^{2},
 Mario Miscuglio^{2},
 Muna Hajj Yahya^{2} and
 Alberto Tagliaferro^{1}
DOI: 10.1186/1556276X9168
© Giorcelli et al.; licensee Springer. 2014
Received: 4 February 2014
Accepted: 27 March 2014
Published: 5 April 2014
Abstract
A wideband microwave characterization of nanocomposites based on commercial multiwalled carbon nanotubes (MWCNTs) and epoxy resin is presented. The sample preparation method is discussed in detail. Field emission scanning electron microscopy is used for morphological sample analysis of nanocomposites and MWCNTs. The complex permittivity is measured in a wide frequency band (3 to 18 GHz) using a commercial dielectric probe (Agilent 85070D) and a network analyzer (E8361A). A statistical analysis based on oneway analysis of variance (ANOVA) technique is performed. The aim of this statistical analysis is to investigate the influence of concentration of nanoparticles inside the polymer matrix on the complex permittivity. This can be significantly different in nanocomposites even if the samples have similar electrical properties.
Keywords
Carbon nanotubes Epoxy resin Permittivity measurementsBackground
Nanocomposites (NCs) are the new frontier of materials in civil and military applications. In particular, polymer NCs are a hot spot in several research fields. As a general rule, NCs are prepared by dispersing a nanometersized filler into a polymer matrix creating a network able to improve the properties of a host polymer.
Carbon nanotubes (CNTs) and, in particular, multiwalled CNTs (MWCNTs) have been used intensively as a filler in a variety of polymers [1, 2]. Their outstanding mechanical, electrical, and thermal properties allow then to enhance the properties of the material in which they are used as a filler for matrix reinforcement [3]. Also, this increase in performance takes place even at low percentages of MWCNTs. A critical point is the MWCNT dispersion as reported by Bauhofer [4] because with an accurate dispersion, it is possible to lower the MWCNT amount required to improve host material performances. Recently, MWCNT composites have been proposed as microwave absorbers [5, 6] and for shielding applications [7–10]. For these applications, the ability to tailor the values of complex permittivity with characteristics of the matrix and MWCNT concentration is critical.
In this work, NCs based on MWCNTs and epoxy resin were prepared using an in situ polymerization process. Special care was paid to avoid any imperfection in dispersion or defects.
The complex permittivity of epoxy resin and NC with 1 and 3 wt.% MWCNTs was measured in the frequency range 3 to 18 GHz using a commercial dielectric probe (Agilent 85070D; Agilent Technologies, Sta. Clara, CA, USA) and a network analyzer (E8361A; Agilent Technologies). The sample's reproducibility was tested applying a statistical analysis based on a oneway analysis of variance (ANOVA) technique.
Methods
In the NC fabrication process, one kind of MWCNT (NTX3; Nanothinx, Rio Patras, Greece) was used as a filler at 1 and 3 wt.% concentrations. The nominal MWCNT characteristics were diameter 25 to 45 nm, length >10 μm, purity >98%. The nominal aspect ratio thus varies from 250 to 400 where an average of 325 is assumed in the following process.
Epilox, a commercial thermosetting resin produced by LeunaHarze (Leuna, Germany) was used as polymer matrix. It is a bicomponent system formed by a resin and a hardener. Resin (T1936/700) is a modified commercial matter, colorless, and lowviscosity (650 to 750 mPa s at 25°C) epoxy resin with reduced crystallization tendency with a density of 1.14 g cm^{3}. The chemical composition of Epilox resin T1936/700 is mainly bisphenol A (30 to 60 wt.%), with an addition of crystalline silica (quartz) (1 to 10 wt.%), glycidyl ether (1 to 10 wt.%), and inner fillers (10 to 60 wt.%). The hardener (H1031) is a liquid, colorless, and lowviscosity (400 to 600 mPa s at 25°C) modified cycloaliphatic polyamine epoxy adduct with a density of 1 g cm^{3}.
As the dispersion of MWCNTs inside the resin is a crucial point, it was checked using field emission scanning electron microscopy (FESEM; Zeiss Supra 40; Carl Zeiss AG, Oberkochen, Germany) by analyzing the exposed surfaces of the criofractured samples. Breaking the specimen into two pieces after flashfreezing in liquid nitrogen guaranteed that the internal structure was not affected by the fracture, avoiding internal resin elongation with subsequent MWCNT reorientation.
In our investigation, 1 and 3 wt.% correspond to 0.64 and to 1.92 vol.%, respectively. In both cases, the volume fraction was above the percolation threshold.
Further, considering timeharmonic fields, constitutive elements are a complex numbers and a complex permittivity which can be defined as $\mathit{\u03f5}$ = $\mathit{\u03f5}$  jγ/ω = $\mathit{\u03f5}$′  j$\mathit{\u03f5}$″, with γ being the conductivity and ω the angular frequency [13]. In fact, if an electromagnetic field propagates in a loss dielectric material, two kinds of electrical currents arise: displacement and conduction currents. The real part of permittivity describes the polarization effect due to the interaction with bound charges (i.e., the displacement current), and the imaginary part describes the effects due to free electron's (conduction current) increase to power loss.
The complex permittivity of pure epoxy resin and composites with 1 and 3 wt.% MWCNTs was measured in the frequency range of 3 to 18 GHz. The samples were measured using a commercial dielectric probe (Agilent 85070D) and a network analyzer (E8361A). The measurement setup is shown in Figure 1 (right panel). A standard calibration short/air/water was adopted. This type of measurements was chosen because of its widerband feasibility (200 MHz to 20 GHz) with respect to waveguide measurements or freespace measurements; moreover, the samples can be of relatively small dimensions. The drawback is that samples should have a very smooth and flat surface in order to avoid the presence of an air gap at the probe face [14, 15].
The electrical properties of the polymer were tailored by changing the concentration of MWCNTs. Four different specimens were prepared for each concentration of MWCNTs in order to give statistical significance of the permittivity results. The differences among the two concentrations of MWCNTs (1 and 3 wt.%) and pristine epoxy resin were tested through the oneway ANOVA technique. The oneway ANOVA compares the means between the groups (i.e., the different concentrations) and determines the level of significance of the null hypothesis. This method allows us to determine the impact of the nanoparticles on the electrical properties of the composites. By applying Tukey's multiple comparison tests to the data a level of confidence, p value was estimated for each compared pair (p > 0.05, p ≤ 0.01, p ≤ 0.001). The standard deviation of measurements performed on four samples is represented by error bars. The number of samples considered is representative of the statistical calculation, because the conditions of the ANOVA test (independence of the samples, normality of the data points among the population, absence of outliers in the population, and almost equality of population variances) hold. This analysis was performed with Graphpad Prism® (GraphPad Software, Inc., La Jolla, CA, USA).
Results and discussion
Multiple comparison summary  relative permittivity  real part
Tukey’s multiple comparison tests  Mean diff  95% CI of diff  Adjusted p value  Significant?  Summary 

1 wt.% vs. 3 wt.%  2.186  2.865 to 1.507  0.0031  Yes  ** 
1 wt.% vs. Epilox  0.5255  0.09689 to 1.148  0.1233  No  ns 
3 wt.% vs. Epilox  2.712  1.870 to 3.553  0.0027  Yes  ** 
Multiple comparison summary  relative permittivity  imaginary part
Tukey’s multiple comparison tests  Mean diff  95% CI of diff  Adjusted p value  Significant?  Summary 

1 wt.% vs. 3 wt.%  0.5777  0.6655 to 0.4899  0.0002  Yes  *** 
1 wt.% vs. Epilox  0.1014  0.0446 to 0.2474  0.1265  No  ns 
3 wt.% vs. Epilox  0.6792  0.5381 to 0.8202  0.0006  Yes  *** 
Conclusions
Nanocomposites based on epoxy resin and MWCNTs in two different concentrations were made. FESEM analysis showed a discrete dispersion of MWCNTs inside material. As it was expected, the complex permittivity measured in the frequency range of 3 to 18 GHz increases with filler concentration. The statistical analysis of variance, using ANOVA technique, showed that there was no difference between pristine epoxy resin and NC with 1 wt.% of MWCNTs. The difference in permittivity, real and imaginary part, is significant only with 3 wt.% of MWCNTs. Future works will be on the application of this analysis to other types of MWCNTs in order to consolidate the present data.
Abbreviations
 CN:

Carbon nanotube
 MWCNT:

Multiwalled carbon nanotube
 NC:

Nanocomposites.
Declarations
Acknowledgements
The authors express their gratitude to Nanothinx for supplying the materials and Salvatore Guastella for FESEM analysis.
Authors’ Affiliations
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