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Nanoscale reinforcement of polypropylene composites with carbon nanotubes and clay: dispersion state, electromagnetic and nanomechanical properties.

INTRODUCTION

The use of bifiller structures is widely discussed in the literature in the context of the nanofiller combined with macroscopic filler. As for example, multiscale reinforcements with carbon nanotubes (CNTs), for electrical and thermal properties improvement, and traditional fillers like glass or carbon fibers, for structural properties, look to be a promising solution towards the development of advanced composites [1], Recently, the individual addition of two nanofillers to be incorporated into the polymer host is studied as a possible approach for achieving the nanoscale reinforcement and multifunctionality. Few authors discussed the bifiller system based on CNTs and clay (CL) in polypropylene (PP) matrix [2, 3]. Researchers reported that the macroscopic property enhancements of the composites strongly depend on the extent to which the nanostructures are dispersed in the matrix, as well as the linkage between the nanostructures and the host matrix. Thus, the distribution/dispersion of the CNTs in conjunction with their weight percentage loading in the polymer matrix strongly influences the properties of the polymer [4-6], The clay exfoliation in the presence of CNTs is another issue that has to be discussed.

Sathyanarayana et al. [1] found that the dispersion of CNTs in thermoplastics is a simultaneous sequential process starting with: (i) the wetting of the CNT agglomerates by the polymer melt, (ii) infiltration of the polymer melt into the CNT agglomerates, (iii) disintegration of agglomerates, weakened by infiltration, into small fractals by mechanisms of rupture and/or erosion, and (iv) distribution of fractals and single nanotubes in the polymer host. This mechanism of filler dispersion is strongly influenced by the processing approach, associated process parameters and the nature of the polymer and the CNTs.

Multiwall carbon nanotubes (MWCNTs) are envisaged recently to be ideal reinforcements for polymeric matrices owing to their exceptional mechanical and physical properties. Loos et al. [7] claim that MWCNTs are the most viable strengthening option for composites based on analysis of their cost versus property relation compared to carbon fibers. MWCNTs are highly attractive filler due to their ability to transition an insulating polymer matrix to a conductive composite in addition to excellent structural properties. On the other hand, clays are more often used in enhancing thermal and barrier properties of polymers [8]. However, the presence of the two nanofillers in the polymer is not sufficient to ensure composites with excellent structural and multifunctional characteristics [2]. The extent of dispersion and distribution of nanotubes in the polymer host is considered as one of the most critical factors having an effect on the properties improvement of the polymer nanocomposite [2, 3]. Moreover, the dispersion/distribution of MWCNTs could be strongly affected by the use of clay in the bifiller system, as the presence of large scale agglomerates of any individual filler would substantially lower the efficiency of load transfer.

In the present study, the bifiller system incorporating MWCNTs and clay is investigated to obtain polypropylene composites with multifunctional properties, such as electromagnetic shielding and hardness, based on nanoscale reinforcement. In order to characterize the state of the MWCNT dispersion, as varying the nanotube content within the polypropylene matrix, with and without the presence of clay, several experimental techniques are adopted. These include transmission electron microscopy (TEM), Raman spectroscopy, and rheology. The effects of both nanofillers on the properties of nanocomposites, namely electromagnetic and nanomechanical, are elucidated in relation with the percolation concept.

EXPERIMENTAL

Materials

Nanocomposites with MWCNT in various concentrations, with and without addition of fixed amount of clay in isotactic polypropylene (Buplen[R] 6231) are studied. Commercial masterbach Plasticil (20 wt% MWCNT/PP) was diluted with polypropylene for production of PP/MWCNT composites. While the bifiller composites of polypropylene with various amounts of MWCNTs and 3 wt% clay (PP/MWCNT/3%CL) were prepared by mixing with PP of the clay masterbach (20 wt% Cloisite 30B/PP) and the Plasticil masterbatch in the presence of compatibiliser MA-g-PP (Fusabond P613). The composites were processed by melt mixing with double screw extruder at the screw speed of 45 rpm in the temperature range 180-200[degrees]C. Monofiller and bifiller compositions were prepared containing 0-5 wt% MWCNTs, with and without 3 wt% clay, respectively. The extruded mixtures were palletized and disk-shaped samples with diameter of 20 mm and thickness of 1 mm were prepared by compression molding in a press at 80[degrees]C for 6 min for further study.

Characterization Methods

A Tecnai-TF30 microscope with S-Twin lens and a FEG (Field Emission Gun) source by FEI was used for TEM analyses. TEM samples were prepared by focused ion beam (FIB) technique, instead by traditional ultramicrotome sectoring at low temperature. The FIB allows obtaining fine lamina of about 300 nm thickness, by Ga ions in a low current regime so that the sample is able to sustain the reached temperature without damage, fact proven also by the obtained image sharpness quality.

TEM images are electron-density distributions produced when a thin specimen scatters electrons. We detect and display such distributions in different ways depending on whether we are using a TEM or a scanning transmission electron microscopy (STEM). In STEM image the high angle annular dark field (HAADF) detector has the advantage to create a Z-contrast image. In a Z-contrast image higher atomic number elements have a bright contrast with respect to low atomic number elements. The dark field (DF) image contains specific orientation information, not just general scattering information, as is the case for a Z-contrast image.

Raman spectroscopy technique was applied for the study of the molecular structure of samples. In the related experiments the UV-Vis Labram HR-800 spectrograph was used for the analysis of the scattered Raman radiation excited with the 441.6 nm line of an air-cooled HeCd laser of Kimmon Electric (Dual, 325/442 nm, UV/blue, 20/80 mW, IK5651R-G model laser). A narrow-bandpass interference filter is used for the elimination of the laser plasma lines. A microscope was used for the delivery of the excitation laser beam on the sample and the collection of the backscattered light with a focusing objective 50X (spatial resolution <2 [micro]m). The spectra were obtained using 0.4 mW laser power on the specimen for a total integration time of 400 s. An 1800-grooves/mm grating and a 2D CCD detector (operating at 140 K) did the dispersion and the detection of the Raman photons, respectively (spectral resolution ~6 [cm.sup.-1]).

The rheological characteristics were tested using AR-G2 Rheometer (TA Instruments) with electrically heated plate geometry 25 mm. Dynamic viscosity was measured in the angular frequency range of 0.03-100 rad/s at low strain amplitude of 0.01 (linear viscoelastic range) and a gap size of 500 [micro]m between plates at temperatures of 175, 180, 185, and 190[degrees]C. To perform the measurements, disk-shaped samples with diameter of 20 mm and thickness of 1 mm was used. The TA Advantage Software was used for data analysis and calculations.

The microwave measurements were carried out with a scalar network analyzer R2-408R (ELM1KA, Vilnius, Lithuania). The IEC 62431:2008(E) standard specifying the measurement method for the reflectivity of electromagnetic (EM) materials for normal incidence was used. The EM response of samples as ratios of transmitted/input ([S.sub.21]) and reflected/input ([S.sub.11]) signals was measured within 26-37 GHz frequency range (Ka-band). The frequency stability of the oscillator was controlled by frequency meter and it was as high as [10.sup.-6.] The power stabilization was provided on the level of 7.0 mW [+ or -] 10 [micro]W. Measurement range of EM attenuation was from 0-40 dB with a basic measurement error of 7% over the range 0-25 dB. The accuracy was controlled by repetitive measurements for different orientations of the sample in the waveguide cross-section. The samples were cut precisely to fit the waveguide of cross-section 7.2-34 mm. The measurements were performed for free standing 1 mm thick bulk samples.

Nanoindentations were carried out using Berkovich Diamond Nanoindenter (Bruker) with a Tip Radius 70 nm. The nanohead perform indentation tests, where the applied load and displacement are continuously monitored, and generating load versus displacement data for a test specimen. Prior to indentation, the sample surface was polished using Leica RM2245 microtome with a diamond knife. The test consists of total 48 indentations and spacing of 80 pm. A typical indentation experiment consisted of subsequent steps: (i) approaching the surface; (ii) loading to the maximum force (100 mN) for 15 s; (iii) holding the indenter at peak load for 10 s; (iv) unloading from maximum force (100 mN) to 10% for 15 s; (v) holding at 10% of maximum force for 15 s; (vi) final complete unloading for 1 s. The hold step was included to avoid the influence of creep on the unloading characteristics since the unloading curve was used to obtain the elastic modulus of material.

RESULTS

Structural and Chemical Characterization at Nanoscale

Characterization of the dispersion state of the bifiller system of MWCNTs and clay within the polypropylene matrix, the homogeneity of the dispersion and agglomeration are studied by TEM. Sample incorporating 1 wt% MWCNTs and 3 wt% clay is tested. After lamina preparation by FIB, a TEM analyses was performed. Figure 1 shows the TEM micrographs of worm-hole shaped MWCNTs in PP matrix at magnification scale of 100 nm with two types of imaging: HAADF detector and dark field. As seen from the TEM pictures, the MWCNTs are relatively well dispersed in single nanotubes and small floccules in the polypropylene matrix. The nanotubes look well wetted by the matrix polymer. Clay layers are not clearly seen in the micrographs in Figure 1 at the low magnifications scale of 100 nm.

TEM pictures in Figure 2 at higher magnification of 20 nm scale demonstrate that the MWCNTs are well dispersed into small fractals weakened by infiltration of polypropylene matrix (Fig. 2a). The distribution of single nanotubes and small clay stacks consisting of few silicate layers in the polymer host is also optional (Fig. 2b). The TEM micrographs at high magnification (Fig. 2a) provide information about the presence of impurities embedded in carbon nanotubes, visible as black spots inside the nanotubes. Those are Mg and Fe found as traces by EDX analysis (not shown here), which are indeed used to grow MWCNTs by CVD method.

Based on TEM analysis, we may conclude about the mechanism of dispersion of the chosen bifiller system in the polypropylene. During the mixing, the MWCNT agglomerates are obviously well wetted by the polymer melt. As a result of the extrusion processing, the agglomerates are weakened by the polymer infiltration and they are disintegrated into small fractals. As seen from the micrographs, the next step of mixing leads to dispersion of MWCNTs in small fractals and single nanotubes in polypropylene in the final composites. The second nanofiller, (the clay), is well dispersed in few layers stacks with the absence of large agglomerates. Fine stacks of clay are not infiltrated within the nanotube fractals, but they are homogeneously distributed in the polypropylene matrix around nanotube fractals. Clay exfoliation or intercalation is not visible in the micrographs at high magnification, but the separation of clay stacks in small (fine) pieces appears during the melt mixing of the bifiller system in the polypropylene. Figure 3 presents schematically the dispersion state of both monofiller and bifiller composites, being resulted from the proposed mechanism of dispersion.

Raman Spectra

Controlling or characterizing distribution of nanofillers in the matrix has so far been almost impossible, but is very critical to tune composite functionality. Raman imaging offers a unique tool for direct characterization of the CNTs dispersion and loading in the polymer matrix by observation of the distribution of the intensity of CNTs bands on the composite's surface [4, 9], Therefore, we apply Raman spectroscopy in order to study the structure of PP/MWCNT and PP/MWCNT/3%CL samples at molecular scale. Figure 4 compares the spectra of PP and composites incorporating 1 and 3 wt% MWCNTs, without and with 3 wt% clay, as processed in a wide spectral range (650-3100 [cm.sup.-1]). The relative intensity of Raman peaks of MWCNTs and PP is generally proportional to the rate of the percentage of MWCNTs in samples. The bands at 1372 and 1585 [cm.sup.-1] indicated with arrows, are attributed to the D and G band in MWCNTs. The remaining peaks are attributed to polypropylene, while the contribution of clay in these spectra is negligible. Also the G' (=2D) band is identified in the spectra of samples containing MWCNTs, this band is the overtone of the D band and appears at ~2740 [cm.sup.-1] (Fig. 5b).

Figure 5a gives the processed spectra in a weak spectral range of 1300-1650 [cm.sup.-1], which distinguished MWCNTs peaks (D and G bands) for the same composites, as in Figure 4. The Raman spectra of the composites are similar in shape and they exhibit two strong bands, typical for the MWCNTs, D mode and the tangential stretching G mode. In our spectra, D and G bands are broad with a similar intensity proving a lattice of the CNTs with many defects. Both G and D bands intensities are increased simultaneously with increasing nanotube contents. In general, the less disordered the graphite-based systems are, the weaker the intensity of the D band (relative to the intensity of the G band) is expected to be. The D/G-band intensity ratio ([I.sub.D]/[I.sub.G]) has values close to 1, which provides a sensitive metric for the degree of disorder in sp2 carbon materials. The D-band which appears at the frequency of ~1373 [cm.sup.-1] is a double-resonance Raman mode affected by defects in the graphene structure. The G-band appears at ~1582 [cm.sup.-1], while the shoulder appearing at ~1620 [cm.sup.-1] is attributed to the D' band (defect-induced Raman features) [4, 10],

Table 1 gives the designations of the parameters of the processed spectra, the position and the intensity of the spectra, as well as determining the intensity ratio [I.sub.D]/[I.sub.G]. The effect of concentration of MWCNTs is expressed in an upward shift of the spectra and hence growth of peak area. The intensities of both D and G peaks increase simultaneously by increasing the nanotube contents (1-3 wt%); moreover, the effect is higher by addition of second filler 3 wt% clay, which increases of the total nanofiller contents. A fitting with two Lorentzian line profiles was performed for the extraction of the Raman shifts of the G band. Small changes of the Raman shift of D and G bands after the addition of the clay is observed, which is not safe to attribute to a better dispersion of MWCNTs in the polymer matrix, due to the presence of clay, since the spectral resolution is ~7 cnT1 [11].

Rheological Properties

Rheology can provide information for both the degree of nanofiller dispersion and the percolated network structure of interacted filler particles [12-15]. Figure 6a and 6b show the dynamic viscosity vs. angular frequency at 190[degrees]C, of: (a) monofiller composites incorporating MWCNTs, and (b) bifiller composites containing 3 wt% clay as a second filler, as varying the MWCNT contents. The rheological behavior of the bifiller PP/ MWCNT/3%CL composites (Fig. 6b) is similar to that of the monofiller PP/MWCNT composites (Fig. 6a), but demonstrating slightly higher viscosity values within the studied frequency range, this attributed to higher total amount of filler. The low shear viscosity ([omega] < 1 [s.sup.-l]) Newtonian plateau, typical for the neat PP, appears only for the PP/3wt% clay and the composites with very low contents of MWCNTs below 0.5 wt%. However, at nanotube contents above 0.5 wt% MWCNTs and particularly above 1.5 wt% MWCNTs the composites demonstrate pseudoplastic flow behavior with a yield stress. This could be related with the formation of a network structure of MWCNTs, associated with percolation [12-16].

In order to determine the rheological flocculation and percolation thresholds [12, 16], Figure 7 presents the dynamic viscosity at a high shear ([omega] = 100 [s.sup.-1]) vs. nanotube contents of both monofiller (PP/MWCNTs) and bifiller (PP/MWCNTs/3%CL) composites. The viscosity function linearly increases at very low nanotube contents, but loses the linearity above the rheological flocculation threshold ([[phi].sub.c] = 0.5 wt%). Further increase of the nanofiller contents result in a sharp increase of the viscosity function, related to rheological percolation. The percolation threshold was determined of [[phi].sub.p1] = 1.5 wt% (for the monofiller systems) and [[phi].sub.p2] = 2 wt% MWCNTs (for the bifiller systems), this associated with the network formation of interconnected carbon nanotubes.

The percolation structure of conductive and high strength carbon nanotubes formed within the nonconductive viscoelastic polypropylene matrix is expected to dominate the physical and mechanical properties of nanocomposites, producing multifunctionality. In order to prove this assumption, we investigate the electromagnetic and nanomechanical properties of monofiller and bifiller composites in relation with the percolation concept.

Microwave Probing

In spite of a long history of electromagnetic (EM) shielding materials, searching of new EM coatings effective in GHz frequency range is still a challenging task due to exciting progress in microwave technique and instrumentation, what continuously generates advanced requirements for shielding materials. Furthermore, the problem of electromagnetic compatibility, i.e., parasitic interference of EM waves, has to be solved without degradation of other customer properties of coating material, like processability, moldability, high resistivity to environmental erosion, high mechanical strength, small thickness, etc. That is why it is extremely important to couple the potential of well-known materials, i.e. polypropylene, with the new properties originating from the fillers and providing the multifunctionality of the coating. As compared to conventional metal-based EM shielding materials, electrically conducting polymer composites have gained popularity recently because of light weight, low-cost, resistance to corrosion, flexibility, and processing advantage.

The extremely attractive electrical characteristics of carbon nanotubes have lived up to their potential as excellent conductive fillers for the insulating polymer matrices. Substantially high electrical properties have been widely reported on polymer-CNT composites [17]. The analyses of electromagnetic interference shielding efficiency of those composites [17, 18] show that the well-dispersed CNTs formed electrically conductive network in polymer. The energy of electromagnetic waves was attenuated in network resistors, which is similar to the resistive type of wave absorption materials [19, 20].

The dispersion/agglomeration state of carbon nanotubes in the polypropylene matrix and the structural disorder of MWCNTs would be determinant for the physical properties improvement of the nanocomposites. Therefore, we have studied the potential applications of polypropylene composites with carbon nanotubes as novel electromagnetic materials.

The permittivity of the investigated composites was reconstructed as presented in Figure 8a and 8b from transmittance (T = [S.sup.2.sub.21]) {Eq. 1) and reflectance (R = [S.sup.2.sub.11]) (Eq. 2) measured in a rectangular waveguide cross-section a x b (in x and y directions, correspondingly) using the following procedure developed in Refs. [21, 22]:

[T.sup.1/2] = 2([k.sub.z2]/[k.sub.z])/-2([k.sub.z2]/[k.sub.z]) cos ([k.sub.z2[tau]]) + i[[([k.sub.z2]/[k.sub.z]).sup.2]] + 1] sin ([k.sub.z2[tau]), (1)

[R.sup.1/2] = -i[[([k.sub.z2]/[k.sub.z]).sup.2] - 1] sin ([k.sub.z2[tau]])/2i([k.sub.z2]/[k.sub.z]) cos ([k.sub.z2[tau]]) + [[([k.sub.z2]/[k.sub.z]).sup.2] + 1] sin ([k.sub.z2[tau]]), (2)

where [tau] is the thickness of the polymer/CNT layer, [k.sub.z] is the longitudinal wave number, z is the direction along the waveguide, [[k.sub.z].sub.(1,3)] = [pi]/[lambda]a [square root of (4[a.sup.2] - [[lambda].sup.2])], [lambda] is the free space wavelength, [k.sub.z2] = [pi]/[lambda]a [square root of (4[epsilon][a.sup.2] - [[lambda].sup.2])], [epsilon] is the complex permittivity of the investigated sample [epsilon] = [epsilon]' + i[epsilon]". Experimental data for measured reflectance R, transmittance T, and absorbance A = 1 - R - T in [K.sub.a]-band are presented in Figure 8c.

In both cases (1 and 3 wt% of MWCNT) in PP host some, not determinative, impact of addition of clay into the EM properties has been experimentally founded at low frequencies of [K.sub.a]-band (26-30 GHz). In particular, polymers filled with 3 wt% of carbon nanotubes and 3 wt% of clay show dielectric constant equal to 6-28 GHz, and more than seven for samples produced without clay. Nevertheless, these difference is very light, and became invisible if we switch to "optical parameters", i.e., A, T, R, for composites being 1-mm thick (see Fig. 8c).

As summarized in Figure 8c, the PP/MWCNT composites show considerable electromagnetic shielding efficiency at small layer thickness of 1.0 mm, by increasing the contents of the MWCNTs. The rise of MWCNTs content around and above the percolation threshold, i.e., above 1 wt%, leads to considerable monotonically increase of absorption ability of PP composites to 30%, and at the same time to sufficient improvement of reflectance ability, from 20 to 52% for 1.0-mm-thick PP/MWCNT. As a result, 3 wt% MWCNT/PP allows less than 20% microwave signal pass through the sample.

Nanoindentation

The dispersion/agglomeration state of carbon nanotubes and clay in the polypropylene matrix, as well as the rheological flocculation and percolation thresholds, characterized above (Fig. 7) are determinant for the mechanical properties improvement of the nanocomposites [13-15, 23], In order to demonstrate the role of the dispersion state and the percolation structure on the nanoscale reinforcement of polypropylene, the nanomechanical properties are investigated by instrumented nanoindentation. The nanoindentation was chosen instead of microindentation in order to better identify the effect of nanofiller dispersion on hardness improvements by determining the surface mechanical properties at the nanoscale, as validated from 48 indentations at different spots of the sample.

Young's modulus and hardness are derived from the unload data segments through in situ monitoring of the force vs. displacement plot and automatic calculations by utilizing the Oliver-Pharr method (Eqs. 3 and 4) [24], Hardness, H, is defined as the mean contact pressure, calculated by dividing the indenter load, P, by the projected contact area, A, at that load:

H = P/A (3)

The elastic modulus E is calculated using the Poisson's ratio of the test material, v, the modulus of the indenter, [E.sub.i], the Poisson's ratio of the indenter, [v.sub.i] and the reduced modulus [E.sub.r]:

1/[E.sub.r] = [1 - [v.sup.2]]/E + [1 - [v.sup.2.sub.i]]/[E.sub.i] (4)

For a diamond-tipped indenter, [E.sub.i] = 1141 GPa and [v.sub.i] = 0.07 GPa. The aforementioned procedure measures hardness and modulus at the maximum penetration depth of a single load-unload indent cycle.

Monofiller and bifiller composites containing 0-5 wt% MWCNTs, without and with 3 wt% clay are studied. Representative load-displacement curves for the example PP/1wt% MWCNT composites, obtained from the nanoindentation tests at loading of 100 mN, are shown in Figure 9a. The calculated hardness and Young's modulus by Oliver-Farr model are presented in Figure 9b. The resistance to penetration during indentation was compared for all samples studied and the effect of filler type and filler content was discussed. By increasing the nanotube contents, the resistance to penetration increases for both the monofiller and the bifiller composites, this associated with a hardening effect of the nanofillers.

The apparent elastic modulus and hardness measured for different MWCNT contents in both monofiller (PP/MWCNTs) and bifiller (PP/MWCNTs/3%CL) composites are presented in Figure 10a and 10b. Generally, the hardness and Young's modulus grow with increasing the content of MWCNTs in both types of composites, without and with clay. For the monofiller composites (PP/MWCNTs), a small addition of MWCNTs around the rheological flocculation threshold (([[phi].sub.c] = 0.5 wt%) results in a significant improvement of the hardness (~14%), with a gradually increase of the Young's modulus, this associated with both the nucleation effect of MWCNTs on crystallization behavior of polypropylene [13, 14] and the formation of a flocculation structure of MWCNTs. Above the percolation threshold ([[phi].sub.p1] = 1.5 wt%), both hardness and Young's modulus started to increase gradually, thus at maximum nanotube loading of 5 wt% MWCNTs the reinforcement in the monofiller composites is of 23% (for hardness) and 16% (for elastic modulus).

The presence of clay in the bifiller systems (PP/MWCNTs/ 3%CL) slightly suppresses the effect of MWCNTs around the flocculation threshold ([[phi].sub.c] = 0.5 wt%), but strongly increases the nanofiller reinforcement effects above the flocculation and around the percolation threshold ([[phi].sub.p2] = 2 wt%). Thus, the bifiller systems become stronger and harder than the monofiller systems when the percolation structure is formed in the nanocomposites by the MWCNTs and the 3% clay platelets. Thus, at 5 wt% MWCNTs the reinforcement effect for the bifiller composites is higher than for the monofiller one, reaching values of 25% (for hardness) and 21% (for elastic modulus). The improvement of mechanical properties was validated by the small standard deviation in the concentration range studied (0-5% MWCNTs), which is about [+ or -] 0.0046 for hardness and [+ or -] 0.0485 for Young's modulus for the monofiller systems, and, respectively, [+ or -] 0.0047 and [+ or -] 0.0544 for the bifiller composites.

The synergic effect of the bifiller system (MWCNT and clay) on the nanoscale reinforcement observed above the percolation threshold, in comparison with the monofiller MWCNTs, is probably due to the continuous network structure formed by the interacted fractals of MWCNTs and homogeneously dispersed fine clay stacks (as shown in Fig. 3).

CONCLUSIONS

Bifiller (MWCNTs and 3 wt% clay) and monofiller (MWCNTs) polypropylene composites were compared as varying the nanotube contents within the range 0-5 wt% MWCNTs. The dispersion structure of bifiller systems, characterized by TEM and Raman spectroscopy, shows well dispersed MWCNTs in small fractals and single nanotubes within the polypropylene host, as well as fine clay stacks homogeneously dispersed in between. Clay exfoliation in single platelets is not visible from the TEM micrographs at high magnifications.

The state of dispersion was evaluated by rheological methods, as well as the rheological flocculation and percolation thresholds were determined at [[phi].sub.c] = 0.5 wt%, [[phi].sub.p1] = 1.5 wt% (for the monofiller systems) and [[phi].sub.p2] = 2 wt% (for the bifiller systems). The rheological thresholds are related with the formation of a dispersion structure of fractals (at [[phi].sub.c]) and a continuous network (at [[phi].sub.p]), by the nanofillers in the polymer host. Both structural types formed in the dispersions around the rheological thresholds are expected to dominate the physical and mechanical properties of the bifiller and monofiller composites, respectively.

Thus, considerable electromagnetic shielding efficiency at small sample thickness of 1 mm was observed for both composites, in favor of monofiller system, at MWCNT contents around and above the rheological percolation. At 3 wt% MWCNTs, the EM attenuation of level ~ 80% is obtained due to 30% absorption and 50% reflection of microwave power, with only 20% transmission of microwave radiation.

The hardness and Young's modulus grow significantly with increasing the content of MWCNTs in both types of composites, with and without clay. Around the flocculation thresholds the monofiller composites show a significant improvement of the hardness and a gradually increase of the Young's modulus. While bifiller composites demonstrate strong enhancement of both characteristics at nanofiller contents above the flocculation and around the percolation threshold. The bifiller systems become stronger and harder than the monofiller systems above the percolation threshold due to the continuous network structure formed by the interacted fractals of MWCNTs and homogeneously dispersed fine clay stacks, which produces a synergic effect on the nanoscale reinforcement.

ACKNOWLEDGMENTS

This study is inspirited and supported by the FP7-NMP-2011-SME-280987 NanoXCT "Compact X-ray computed tomography system for nondestructive characterization of nano materials". The samples are prepared and characterized for use as reference materials for verification of the NanoXCT device and methods.

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I. Petrova, (1) R. Kotsilkova, (1) E. Ivanov, (1) P. Kuzhir, (2) D. Bychanok, (2,3) K. Kouravelou, (4) Th. Karachalios, (4) A. Soto Beobide, (5) G. Voyiatzis, (5) D. Codegoni, (6) F. Somaini, (6) L. Zanotti (6)

(1) OLEM, Institute of Mechanics, Sofia, Bulgaria, Bulgarian Academy of Sciences, Sofia, Bulgaria

(2) Research Institute for Nuclear Problems, Belarusian State University, Minsk, Belarus

(3) Ryazan State Radio Engineering University, Gagarina St. 59/1, Ryazan 390005, Russia

(4) Nanothinx S.A, Stadiou St, Platani, Rio Patras, 26504, Greece

(5) Foundation for Research and Technology-Hellas (FORTH)/Institute of Chemical Engineering Sciences (ICE-HT), P.O. Box 1414, GR-265 04, Rio Patras, Greece

(6) STMicroelectronics, via C.Olivetti 2, 2064 Agrate Brianza (MB), Italy

Correspondence to: I. Petrova; e-mail: ivanka.petrova01@gmail.com

Contract grant sponsor: Ministry of Education and Science of Russian Federation; contract grant number: 14.577.21.0006; contract grant sponsor: The EU FET Flagship Project; contract grant number: FP7-604391-GRAPHENE.

DOI 10.1002/pen.24247

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. D and G band parameters for the polypropylene
composites as varying the nanofiller contents, MWCNTs
and clay.

                   CNTs    Clay    Filler content
Composition        (wt%)   (wt%)       (wt%)

PP/1%MWCNT           1      --           1
PP/1%MWCNT/3%CL      1       3           4
PP/3%MWCNT           3      --           3
PP/3%MWCNT/3%CL      3       3           6

                      D band          G band      R = [I.sub.D/
Composition        ([cm.sup.-1])   ([cm.sup-1])     [I.sub.G]

PP/1%MWCNT             1375            1584           1.007
PP/I%MWCNT/3%CL        1371            1584           0,987
PP/3%MWCNT             1371            1586           0,989
PP/3%MWCNT/3%CL        1371            1586           0.981
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Author:Petrova, I.; Kotsilkova, R.; Ivanov, E.; Kuzhir, P.; Bychanok, D.; Kouravelou, K.; Karachalios, Th.;
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2016
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