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PE blends filled with GNP:CNT hybrid nanofiller
Synthetic Metals 217 (2016) 322–330
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Electrical, EMI shielding and tensile properties of PP/PE blends filled with GNP:CNT hybrid nanofiller Mohammed H. Al-Saleh Department of Chemical Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
A R T I C L E I N F O
Article history: Received 18 February 2016 Received in revised form 14 April 2016 Accepted 22 April 2016 Available online xxx Keywords: Polymer nanocomposite Electrical properties Mechanical properties Microstructures EMI shielding
A B S T R A C T
Polypropylene (PP)/polyethylene (PE) blends filled with 5 vol% graphene nanoplatelets: carbon nanotube (GNP:CNT) hybrid nanofiller were prepared by melt mixing. The blends’ microstructure and the influence of GNP:CNT volume ratio on the electrical, electromagnetic interference (EMI) shielding and tensile strength were investigated. The scanning electron microscopy analysis showed that the CNT and GNP are localized in the PE phase. The electrical conductivity and EMI shielding were found to increase with the increase in CNT volume fraction due to the 1D geometry of the CNT that is more effective than the 2D geometry of the GNP in building conductive networks. This finding indicates that not only the nanofiller conductivity but also the nanofiller geometry should be considered in designing hybrid nanocomposite materials. Moreover, the tensile strength was found to increase with the decrease in GNP:CNT volume ratio due to the good adhesion between the CNT particles and the PE phase compared to the almost no adhesion between the GNP particles and the PE phase. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Conductive polymer nanocomposites (CPN) have wide range of applications in the electronics and energy sectors [1]. CPNs can be used as light-weight shields to maintain the electromagnetic compatibility of electronic devices and as electrodes in batteries and fuel cells [1,2]. Nonetheless, the wide commercial use of CPN is very limited due to the relatively high cost of the high aspect ratio conductive nanofillers. Thus, CPN with high electrical conductivity should be formulated at the lowest possible nanofiller content to enhance CPN competitiveness. In order to achieve this objective many ideas have been investigated such as the double percolation of immiscible polymer blends [3–5], selective localization of nanofiller at the external surface of polymer powder [6] and using hybrid nanofiller mixture [7–11]. Herein, the focus is on investigating the concepts of double percolation and hybrid nanofiller together on the electromagnetic interference (EMI), electrical and mechanical properties of polymer polypropylene (PP)/polyethylene (PE) blends filled with graphene nanoplatelets (GNP): carbon nanotube (CNT) hybrid mixture. GNP and CNT are of the most promising carbon nanofillers due to their high electrical conductivity and aspect ratio.
E-mail addresses: [email protected], [email protected] (M.H. Al-Saleh) . http://dx.doi.org/10.1016/j.synthmet.2016.04.023 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
CPNs based on two different conductive nanofillers have been investigated by many researchers [11–18]. For example at nanofiller content of 0.5 wt%, GNP:CNT/polyetherimide (PEI) nanocomposite exhibited higher electrical conductivity than GNP/PEI and CNT/PEI nanocomposites [16]. This finding was ascribed to the creation of interconnected network, in which the CNT particles connected the GNP particles. On the other hand, for copper nanoparticles (CuNP):CNT/polypropylene (PP), no synergistic effect of using hybrid nanofiller mixture on the electrical percolation threshold was reported. However, it was observed that the affinity of CuNP towards the CNT particles facilitated the dispersion of CuNP nanoparticles. Herein, immiscible polymer blend filled with hybrid nanofiller mixture is investigated. Two carbon nanofillers, GNP and CNT, of almost similar electrical conductivity but different geometries are used. In immiscible polymer blends, nanofiller particles will selectively localize in one of the blends’ phases or at the blends’ interface leading to higher effective concentration in the nanofiller-rich phase. If the nanofiller-rich phase is continuous, the blend’s electrical percolation threshold will be lower than that of the nanofiller/single-phase composite [19–23]. There are limited number of studies about immiscible polymer blends filled with at least two different fillers [2,23–29]. For example, Besco and coworkers [29] found that for polycarbonate/acrylonitrile-butadiene-styrene filled with organically modified clay (OMC) and CNT, both nanofillers were selectively localized in the PC phase and the
M.H. Al-Saleh / Synthetic Metals 217 (2016) 322–330
addition of OMC was found to hinder the formation of CNT conductive network and consequently increased electrical percolation threshold. However, Zhang et al. [30] found that the addition of glass fiber (GF) enhances the electrical conductivity of CNT filled polyoxymethylene (POM)/maleic anhydride-grafted polyethylene (MA-PE) blend. In the CNT/POM/MA-PE mixture [30], the CNT was found to reside in the dispersed MA-PE phase. However upon the addition of 20 wt% GF, the CNT/MA-PE phase was found to coat the 3D network of GF particles leading to formation of continuous network. In this work, the influence GNP:CNT volume ratio on the microstructure, electrical, EMI shielding and mechanical properties of 50/50 and 90/10 polypropylene/polyethylene (PP/PE) blend are investigated. PP/PE blend was selected because PP, PE and their blends are of the most widely used polymeric materials due to their cost advantage and unique properties over many other materials. Since it was predicted that in a PP/PE blend the nanofillers will reside in the PE phase, the 50/50 and 90/10 PP/PE volume ratios were selected in order to investigate the properties of the blend when nanofillers/PE is a major phase (i.e. the 50/50 PP/ PE blend) and when the nanofiller/PE is a minor phase (i.e. the 90/ 10 PP/PE blend). In addition in all experiments, the nanofiller volume percent was constant at 5.0 vol%. This level of nanofiller concentration was selected in order to obtain a composite by melt mixing and with adequate level of EMI shielding [31]. 2. Experimental details 2.1. Materials and fabrications Two polymers were used in this study, namely: PP and PE. The main properties of these two polymers are listed in Table 1. The nanofillers were multi-walled CNT (NanocylTM NC7000, Nanocyl S. A., Sambreville, Belgium) and GNP (xGnP-Grade M, XG Sciences, USA). The nanotubes are 9.5 nm in diameter and 1.5 mm in length and GNP particles have a disc-shape geometry with average thickness of 7 nm and diameter of 5 mm. For the calculations of the nanofiller volume fraction, the density of CNT and GNP were set at 1.66 g/ml and 2.2 g/ml, respectively. In all experiments, the total nanofiller content was 5.0 vol%. All blends were prepared by melting mixing using a batch mixer (Plastograph EC, Brabender, Germany). Polymer pellets, prior to mixing, were dried in a vacuum oven for 16 h at 80 C. The CNT and GNP powders were also pre-dried at 130 C for 16 h. The melt mixing was conducted at 100 rpm and 180 C for 13 min. In a typical experiment, X g of polymer pellets were fed to the preheated mixer and mixed for 3.0 min. For the 50/50 (vol/vol) PP/PE blends, the amounts of PP and PE were 13.3 g and 14.1 g, respectively. While for the 90/10 (vol/vol) PP/PE blends, the PP and PE amounts were 24.6 g and 2.9 g respectively. Then, Y amount of the nanofillers were fed into the mixer. Tables 2 and 3 lists the amounts of the GNP and CNT used in preparing the 5 vol% filled 50/ 50 PP/PE and 90/10 PP/PE blends, respectively. The nanofillers were added to the mixer following three different sequences. In the first sequence, both fillers were fed at the same time. In the second sequence, GNP was first fed and after
Manufacturer Brand name Specific Gravity Melt flow rate (g/10 min) a b
230 C and 2.16 kg load. 190 C and 5.0 kg load.
Table 2 Amounts of GNP and CNT used to formulate the 5 vol% GNP:CNT filled 50/50 PP/PE blends. GNP:CNT
GNP (g)
CNT (g)
GNP wt%
CNT wt%
GNP vol%
CNT vol%
5:0 4:1 3:2 2:3 1:4 0:5
3.420 2.710 2.020 1.340 0.660 0.000
0.000 0.515 1.030 1.550 2.065 2.580
11.1% 8.8% 6.6% 4.4% 2.2% 0.0%
0.0% 1.7% 3.4% 5.1% 6.9% 8.6%
5.0% 4.0% 3.0% 2.0% 1.0% 0.0%
0.0% 1.0% 2.0% 3.0% 4.0% 5.0%
Table 3 Amounts of GNP and CNT used to formulate the 5 vol% GNP:CNT filled 90/10 PP/PE blends. GNP:CNT
GNP (g)
CNT (g)
GNP wt%
CNT wt%
GNP vol%
CNT vol%
5:0 4:1 3:2 2:3 1:4 0:5
3.520 2.816 2.112 1.408 0.704 0.000
0.000 0.531 1.062 1.593 2.124 2.655
11.3% 9.1% 6.9% 4.6% 2.3% 0.0%
0.0% 1.7% 3.5% 5.2% 7.0% 8.8%
5.0% 4.0% 3.0% 2.0% 1.0% 0.0%
0.0% 1.0% 2.0% 3.0% 4.0% 5.0%
5.0 min the CNT was fed. In the third sequence, CNT was first introduced then after 5.0 min the GNP was fed. At the end of the mixing process, the blend was collected and sent to a compression molding machine (Carver Inc., Wabash-IN, USA) to prepare specimens for electrical, EMI shielding and mechanical properties characterization. The molding was conducted under 27.5 MPa and 200 C for 10 min. For the electrical conductivity and EMI shielding characterizations, the samples were (40 mm 20 mm 1 mm) rectangles. For tensile tests, initially (65 mm 65 mm 1 mm) plates were produced; then ASTM D628-5-IMP die was used to cut type V ASTM D638-03 specimens. 2.2. Characterization tools The microstructure of the PP/PE blends was investigated using Environmental Scanning Electron Microscope (Quanta 450 FEG, FEI). Prior to SEM analysis, samples were fractured in liquid nitrogen and sputtered with a thin layer of gold using sputtering machine (Q150R ES, Quorum Technologies, UK). The blends electrical conductivity was measured using two different setups. Conductive samples were characterized using digital multimeter (Keithley 2010 DMM, Keithley Instruments, USA) connected to a 4-wire probe test fixture, while the non-conductive samples were characterized using Keithley 6517A electrometer connected to Keithley 8009 test fixture (Keithley Instruments, USA). The reported electrical resistivity represents the average of at least six specimens. The EMI shielding effectiveness (SE) in the X-band (8.0–12.0 GHz) frequency range was conducted using E5071C ENA network analyzer connected to a WR-90 rectangular waveguide. The rectangular (2 4 cm2) specimens were inserted between the two sections of the waveguide and the S-parameters (S11, S12, S22, S21) of each sample were recorded. The total EMI SE was calculated as follows: EMISE ¼ 10log
Table 1 Information about PP and PE used in this study. PP
PE
SABIC PP 504P 0.9 3.2a
ExxonMobil Chemical HTA 001HD 0.952 0.32b
323
1 jS12 j2
¼ 10log
1 jS21 j2
ð1Þ
The tensile tests were conducted according to the ASTM standard D638-03 using WDW-20 (Jinan Testing Equipment IE Corporation, China) tensile testing machine. For each formulation, at least six specimens (Type V ASTM D638-03) were tested and the average of those was reported. The crosshead speed for all tests was 10 mm/min.
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3. Results and discussion 3.1. Microstructure SEM analysis was conducted to explore the location of the CNT and GNP within the PP/PE blend. Fig. 1 depicts SEM micrographs of
50/50 and 90/10 PP/PE blends filled with 3:2 GNP:CNT. From Fig. 1a two major observations can be drawn. The first one is the expected co-continuous structure of the 50/50 PP/PE blend, and the second observation is the selective localization of CNT and GNP in one of the blend’s phases. From this micrograph it is impossible to distinguish between the PP and PE phases since they have equal
Fig. 1. SEM micrograph of PP/PE blends filled with 5 vol% hybrid nanofiller (3:2 GNP:CNT). a) 50/50 PP/PE blend and (b) 90/10 PP/PE blend.
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g 12
4g p g p ¼ g1 þ g2 d p 1 2p d g1 þ g2 g1 þ g2 4g d1 g d2
ð2Þ
In the equation, g i is the surface tension of component i,g di is the
dispersive component of the surface tension and g pi is the polar component of the surface tension. The theoretical results listed in Table 4 shows that the GNP/PE and CNT/PE interfacial tensions are, respectively, lower than the GNP/PP and CNT/PP interfacial tensions. For example, the GNP/PE and GNP/PP interfacial tensions are 7.2 mN/m and 10.1 mN/m, respectively. The remarkable difference in interfacial tensions favors the localization of GNP and CNT in the PE phase. This theoretical predication is consistent with the experimental observation. 3.2. Electrical properties In CPN, critical amount of nanofiller, widely known as the electrical percolation threshold (EPT), is needed to create conductive network within the polymer matrix. At the EPT, the electrical conductivity of the nanocomposite increases by several orders of magnitude. The effect of using hybrid nanofiller on the EPT have been investigated by many researchers [12–14,33]. Herein the focus is on studying the effect of using hybrid nanofiller mixture at concentration above the EPT, since for the EMI shielding applications nanofiller loading above the EPT is needed to formulate CPN with adequate level of shielding (i.e. a SE 20 dB). As mentioned in the experimental section, the blends were prepared following three different mixing protocols. The results showed no influence of the order of addition on the electrical conductivity. This means that the mixing time was enough for blends to reach their steady-state microstructure. Fig. 2 shows the volume resistivity (r) of the 5 vol% GNP:CNT filled PP/PE blends as Table 4 Surface tension ðg Þ of PP, PE, CNT and GNP and interfacial tension ðg 12 Þ between the nanofiller/polymer pairs at 180 C.
g ðmN=mÞ PE [32] PP [32] CNT [41] GNP [40] CNT-PP CNT-PE GNP-PP GNP-PE a b c
26.6 20.8 23.5 45.6
g d (mN/m)a
g p (mN/m)b
26.6 20.4 23.5 41.7
0.0 0.4 0.0 3.9
Dispersive component of the surface tension. Polar component of the surface tension. Calculated based on the harmonic-mean equation.
10.0 9.0 8.0
log[ρ(Ω·cm)]
volumes. However, from the SEM micrograph of the 90/10 PP/PE blend (Fig. 1b), it is apparent that the minor phase, which is the PE phase, is highly filled with the nanofillers, and the major PP phase is free of the nanofillers. This is a direct evidence that the CNT and GNP are selectively localized in the PE phase. Interestingly, even though the GNP particles have large dimensions, they are aligned and contained within the PE phase, revealing that the thermodynamic affinity is the controlling parameter for the nanofiller location. Similar observations were reported for poly(methylmethacrylate (PMMA)/PE) blend filled with CNT: expanded graphite mixture, where both nanofillers were found to selectively reside in the PE phase [2]. This selective localization can be attributed to the lower interfacial tension between the nanofillers (GNP and CNT) and the PE phase compared to that with the PP phase as listed in Table 4. The table lists the surface tension of nanofillers and polymers and the interfacial tension ðg 12 Þ between the nanofiller/polymer pairs. The interfacial tension was calculated using the harmonic-mean equation [32]:
325
7.0
50/50 PP/PE
6.0
90/10 PP/PE
5.0 4.0 3.0 2.0 1.0 0.0 -1.0 5:0
4:1
3:2
2:3
1:4
0:5
GNP:CNT Fig. 2. Electrical resistivity of 5 vol% GNP:CNT filled PP/PE blends as function of GNP:CNT and PP/PE volume ratios.
function of PP/PE and GNP:CNT volume ratios. It is apparent that the electrical resistivity of the 90/10 PP/PE blends is lower than that of the 50/50 PP/PE blends. This finding is expected because of the higher effective concentration of the conductive nanofillers in the PE phase with the decrease of PE volume fraction in the cocontinuous PP/PE blend. For the effect of GNP:CNT volume ratio, it is evident that the electrical resistivity decreases with the decrease in GNP:CNT volume ratio. This is mainly due to the 1-D geometry of the CNT that is more effective than the 2-D geometry of the GNP in creating conductive networks. Based on the dimensions of the GNP and CNT used in this study, the electrical percolation threshold for nanocomposites filled with rod-like nanofiller (Eq. (1)) and disklike nanofiller (Eq. (2)) can be estimated as follows [34,35]:
Fc ¼ 0:5=AR
Fc ¼
27pD2 t 4ðD þ IPDÞ3
ð3Þ
ð4Þ
In the equations (3) and (4), Fc is the nanofiller volume fraction at the electrical percolation threshold, AR is the nanofiller aspect ratio, t is the nanofiller thickness, D is the nanofiller diameter, and IPD is the inter-particle distance. For the GNP used in this study, t is 7 nm and the equivalent D is 5 mm. For the IPD it can be set at 10 nm, since for composites an IPD less than or equal to 10 nm is required to permit electron conduction by tunneling [34]. While for the CNT used in the study, the AR after the melt mixing can be assumed 80 [36–38]. Thus, Fc for the PE phase is expected to be 3 vol% GNP or 0.63 vol% CNT. This remarkable difference indicates that nanofillers of rod-like structures are more effective than nanofiller of disk-like structures in creating conductive networks.
g 12 (mN/m)c
3.3. EMI shielding properties
0.6 0.2 10.1 7.2
Nowadays, there is a remarkable demand for EMI shielding materials in order to maintain the electromagnetic compatibility of the rapidly growing number of portable electronic devices. Due to their light-weight and design-flexibility, CPNs are of the most promising candidates to attenuate EMI. The EMI SE of a CPN depends on many factors of which the nanocomposites’ electrical conductivity and shielding plate thickness are of the most significance. Fig. 3 depicts the effect of GNP:CNT volume ratio and EMI frequency on the total SE and Fig. 4 depicts the average
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30
a) 50/50 PP/PE
25
GNP-CNT 0-5
20
EMI SE (dB)
1-4 2-3
15
EMISE ¼ SER þ SEA
ð5Þ
3-2
10
4-1
5
5-0
0 8
9
10
11
35
s
ð6Þ
2pf m
ð7Þ
3.4. Tensile properties
30
b) 90/10 PP/PE
GNP-CNT 0-5
25
1-4
20
2-3
15
3-2
10
4-1
5
5-0
0
SER ¼ 39:5 þ 10log
pffiffiffiffiffiffiffiffiffiffiffiffiffi SEA ¼ 8:7d pf ms
12
Frequency (GHz)
EMI SE (dB)
shielding by reflection ðSER Þ and shielding by absorption ðSEA Þ increases with the increase in the electrical conductivity ðs Þ. In the equations, m is the magnetic permeability, f is the frequency and d is the thickness of the shielding plate.
8
9
10
11
12
Frequency (GHz) Fig. 3. EMI SE of 5 vol% GNP:CNT filled PP/PE blends as function of GNP:CNT volume ratio and frequency a) 50/50 PP/PE and b) 90/10 PP/PE.
Before discussing the properties of the filled PP/PE blends it is worth mentioning that the tensile strength of the unfilled 50/ 50 and 90/10 PP/PE are 25 0.4 MPa and 27.7 0.9 MPa, respectively. Fig. 5 depicts the effect of GNP:CNT volume ratio on the tensile strength of the PP/PE blends as function of PP/PE and GNP: CNT volume ratios. It is evident that the tensile strength was increased with the decrease in GNP:CNT volume ratio. For example, for the 50/50 PP/PE blends, the tensile strength increased from 24.6 MPa for the 5:0 GNP:CNT mixture (which is similar to that of the unfilled 50/50 PP/PE blend) to 29.4 MPa for the 0:5 GNP: CNT mixture, corresponding to 20% enhancement in the tensile strength. In a previous work using similar CNT and PE [39], it was found that the tensile strength of 5 vol% CNT/PE is 26% higher than that of unfilled PE. Thus the tensile strength of GNP:CNT/PP/PE blends are consistent with the properties of CNT/PE composite and with the microstructure analysis that shows good adhesion between the CNT and PE, as shown in Fig. 6a, and no adhesion between the nanofiller and the PE phase, as shown in Fig. 6b. In the later micrograph, the surface of GNP particles is clearly seen free of polymer chain. Moreover, the microstructure analysis showed clear voids between the GNP particles (Fig. 7a) and between the particles GNP and polymer matrix (Fig. 7b). Voids are expected to have detrimental effect on the tensile properties since they can act as stress concentrators. These SEM observations are consistent with the theoretical predications base on the interfacial tension of
40 50/50 PP/PE 90/10 PP/PE
35
EMI SE in the X-band range of the 50/50 and 90/10 PP/PE blends as function of GNP:CNT volume ratio. It is apparent that within the Xband frequency range, the EMI SE is marginally affected by the frequency. However, at any given frequency it is evident that the EMI SE increases with the decrease of GNP:CNT volume ratio (i.e. increase in CNT fraction) indicating no synergistic effect of using hybrid mixture on the EMI SE. From Fig. 4 it is apparent that the 0:5 GNP:CNT blends have EMI SE that is 5–6 times that of the 5:0 GNP:CNT blends. This clearly indicates that CNT has superior properties compared to GNP. The higher EMI SE for the CNT dominated blends is related to the higher electrical conductivity of these blends. Theoretically, EMI SE increases with the increase in electrical conductivity, as shown in Eqs. (5)–(7) [31]. In these equations, it is apparent that both the
Tensile Strenght (MPa)
30 Fig. 4. Average EMI SE of 5 vol% GNP:CNT filled PP/PE blends as function of GNP: CNT volume ratio and PP/PE volume ratio.
25 20 15 10 5 0 5:0
4:1
3:2
2:3
1:4
0:5
GNP:CNT Fig. 5. Tensile strength of 5 vol% filled PP/PE blends as function of GNP:CNT volume ratio.
M.H. Al-Saleh / Synthetic Metals 217 (2016) 322–330
Fig. 6. SEM micrographs of 50/50 PP/PE blends filled with 5 vol% hybrid nanofiller (3:2 GNP:CNT) showing a) CNT-rich zone and b) GNP-rich zone.
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Fig. 7. SEM micrographs of 50/50 PP/PE blends filled with 5 vol% hybrid nanofiller (3:2 GNP:CNT) showing the a) the voids between the GNP and polymer matrix (b) voids between GNP particles.
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CNT/PE and GNP/PE pairs, as listed in Table 4. The GNP/PE interfacial tension is 7.2 mN/m compared to only 0.2 mN/m for the CNT/PE system. This remarkable difference reveals better adhesion at the CNT/PE interface compared to the GNP/PE interface. 4. Conclusions The effect of GNP:CNT hybrid nanofiller on the microstructure and properties of 50/50 and 90/10 PP/PE blends were investigated. All blends were prepared by melt mixing. The CNT and GNP were found to selectively localize within the PE phase. At 5 vol% GNP: CNT loading, the electrical conductivity and EMI SE were increased with the increase in the fraction of CNT because the 1-D geometry of the CNT is more effective than the 2-D geometry of the GNP in building three dimensional networks. In addition an enhancement in tensile strength was found with the increase in CNT content. This enhancement is ascribed to the good adhesion at the CNT/PE interfaces compared to bad adhesion at the GNP/PE interface. Acknowledgment This work was financially supported by the Scientific Research Support Fund, Ministry of Higher Education – Amman – Jordan (Grant Number Bas/2/05/2010). References [1] M.H. Al-Saleh, U. Sundararaj, A review of vapor grown carbon nanofiber/ polymer conductive composites, Carbon 47 (2009) 2–22, doi:http://dx.doi.org/ 10.1016/j.carbon.2008.09.039. [2] A. Raulo, S. Suin, S. Paria, Expanded graphite (EG) as a potential filler in the reduction of percolation threshold of multiwall carbon nanotubes (MWCNT) in the PMMA/HDPE/EG/MWCNT nanocomposites, Polym Compos. (2015) n/a–n/ a, doi:http://dx.doi.org/10.1002/pc.23385. [3] M. Sumita, K. Sakata, S. Asai, K. Miyasaka, H. Nakagawa, Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black, Polym. Bull. 25 (1991) 265–271, doi:http://dx.doi.org/10.1007/BF00310802. [4] F. Gubbels, R. Jerome, P. Teyssie, E. Vanlathem, R. Deltour, A. Calderone, et al., Selective localization of carbon black in immiscible polymer blends: a useful tool to design electrical conductive composites, Macromolecules 27 (1994) 1972–1974, doi:http://dx.doi.org/10.1021/ma00085a049. [5] M.H. Al-Saleh, U. Sundararaj, An innovative method to reduce percolation threshold of carbon black filled immiscible polymer blends, Compo. Part A: Appl. Sci. Manufact. 39 (2008) 284–293, doi:http://dx.doi.org/10.1016/j. compositesa.2007.10.010. [6] M.H. Al-Saleh, Influence of conductive network structure on the EMI shielding and electrical percolation of carbon nanotube/polymer nanocomposites, Synth. Met. 205 (2015) 78–84, doi:http://dx.doi.org/10.1016/j. synthmet.2015.03.032. [7] L. Liu, J.C. Grunlan, Clay assisted dispersion of carbon nanotubes in conductive epoxy nanocomposites, Adv. Funct. Mater. 17 (2007) 2343–2348, doi:http://dx. doi.org/10.1002/adfm.200600785. [8] H.-D. Bao, Z.-X. Guo, J. Yu, Effect of electrically inert particulate filler on electrical resistivity of polymer/multi-walled carbon nanotube composites, Polymer 49 (2008) 3826–3831, doi:http://dx.doi.org/10.1016/j. polymer.2008.06.024. [9] M. Kotaki, K. Wang, M.L. Toh, L. Chen, S.Y. Wong, C. He, Electrically conductive Epoxy/Clay/Vapor grown carbon fiber hybrids, Macromolecules 39 (2006) 908–911, doi:http://dx.doi.org/10.1021/ma0522561. [10] J.F. Feller, S. Bruzaud, Y. Grohens, Influence of clay nanofiller on electrical and rheological properties of conductive polymer composite, Mater. Lett. 58 (2004) 739–745, doi:http://dx.doi.org/10.1016/j.matlet.2003.07.010. [11] J. Sumfleth, X. Adroher, K. Schulte, Synergistic effects in network formation and electrical properties of hybrid epoxy nanocomposites containing multiwall carbon nanotubes and carbon black, J. Mater. Sci. 44 (2009) 3241–3247, doi:http://dx.doi.org/10.1007/s10853-009-3434-7. [12] R. Socher, B. Krause, S. Hermasch, R. Wursche, P. Pötschke, Electrical and thermal properties of polyamide 12 composites with hybrid fillers systems of multiwalled carbon nanotubes and carbon black, Compos. Sci. Technol. 71 (2011) 1053–1059, doi:http://dx.doi.org/10.1016/j.compscitech.2011.03.004. [13] P.-G. Ren, Y.-Y. Di, Q. Zhang, L. Li, H. Pang, Z.-M. Li, Composites of ultrahighMolecular-Weight polyethylene with graphene sheets and/or MWCNTs with segregated network structure: preparation and properties, Macromol. Mater. Eng. 297 (2012) 437–443, doi:http://dx.doi.org/10.1002/mame.201100229. [14] H. Palza, Modifying the electrical behaviour of polypropylene/carbon nanotube composites by adding a second nanoparticle and by annealing processes, Express Polym. Lett. 6 (2012) 639–646, doi:http://dx.doi.org/ 10.3144/expresspolymlett.2012.68.
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Electrical, EMI shielding and tensile properties of PP/PE blends filled with GNP:CNT hybrid nanofiller Mohammed H. Al-Saleh Department of Chemical Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
A R T I C L E I N F O
Article history: Received 18 February 2016 Received in revised form 14 April 2016 Accepted 22 April 2016 Available online xxx Keywords: Polymer nanocomposite Electrical properties Mechanical properties Microstructures EMI shielding
A B S T R A C T
Polypropylene (PP)/polyethylene (PE) blends filled with 5 vol% graphene nanoplatelets: carbon nanotube (GNP:CNT) hybrid nanofiller were prepared by melt mixing. The blends’ microstructure and the influence of GNP:CNT volume ratio on the electrical, electromagnetic interference (EMI) shielding and tensile strength were investigated. The scanning electron microscopy analysis showed that the CNT and GNP are localized in the PE phase. The electrical conductivity and EMI shielding were found to increase with the increase in CNT volume fraction due to the 1D geometry of the CNT that is more effective than the 2D geometry of the GNP in building conductive networks. This finding indicates that not only the nanofiller conductivity but also the nanofiller geometry should be considered in designing hybrid nanocomposite materials. Moreover, the tensile strength was found to increase with the decrease in GNP:CNT volume ratio due to the good adhesion between the CNT particles and the PE phase compared to the almost no adhesion between the GNP particles and the PE phase. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Conductive polymer nanocomposites (CPN) have wide range of applications in the electronics and energy sectors [1]. CPNs can be used as light-weight shields to maintain the electromagnetic compatibility of electronic devices and as electrodes in batteries and fuel cells [1,2]. Nonetheless, the wide commercial use of CPN is very limited due to the relatively high cost of the high aspect ratio conductive nanofillers. Thus, CPN with high electrical conductivity should be formulated at the lowest possible nanofiller content to enhance CPN competitiveness. In order to achieve this objective many ideas have been investigated such as the double percolation of immiscible polymer blends [3–5], selective localization of nanofiller at the external surface of polymer powder [6] and using hybrid nanofiller mixture [7–11]. Herein, the focus is on investigating the concepts of double percolation and hybrid nanofiller together on the electromagnetic interference (EMI), electrical and mechanical properties of polymer polypropylene (PP)/polyethylene (PE) blends filled with graphene nanoplatelets (GNP): carbon nanotube (CNT) hybrid mixture. GNP and CNT are of the most promising carbon nanofillers due to their high electrical conductivity and aspect ratio.
E-mail addresses: [email protected], [email protected] (M.H. Al-Saleh) . http://dx.doi.org/10.1016/j.synthmet.2016.04.023 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
CPNs based on two different conductive nanofillers have been investigated by many researchers [11–18]. For example at nanofiller content of 0.5 wt%, GNP:CNT/polyetherimide (PEI) nanocomposite exhibited higher electrical conductivity than GNP/PEI and CNT/PEI nanocomposites [16]. This finding was ascribed to the creation of interconnected network, in which the CNT particles connected the GNP particles. On the other hand, for copper nanoparticles (CuNP):CNT/polypropylene (PP), no synergistic effect of using hybrid nanofiller mixture on the electrical percolation threshold was reported. However, it was observed that the affinity of CuNP towards the CNT particles facilitated the dispersion of CuNP nanoparticles. Herein, immiscible polymer blend filled with hybrid nanofiller mixture is investigated. Two carbon nanofillers, GNP and CNT, of almost similar electrical conductivity but different geometries are used. In immiscible polymer blends, nanofiller particles will selectively localize in one of the blends’ phases or at the blends’ interface leading to higher effective concentration in the nanofiller-rich phase. If the nanofiller-rich phase is continuous, the blend’s electrical percolation threshold will be lower than that of the nanofiller/single-phase composite [19–23]. There are limited number of studies about immiscible polymer blends filled with at least two different fillers [2,23–29]. For example, Besco and coworkers [29] found that for polycarbonate/acrylonitrile-butadiene-styrene filled with organically modified clay (OMC) and CNT, both nanofillers were selectively localized in the PC phase and the
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addition of OMC was found to hinder the formation of CNT conductive network and consequently increased electrical percolation threshold. However, Zhang et al. [30] found that the addition of glass fiber (GF) enhances the electrical conductivity of CNT filled polyoxymethylene (POM)/maleic anhydride-grafted polyethylene (MA-PE) blend. In the CNT/POM/MA-PE mixture [30], the CNT was found to reside in the dispersed MA-PE phase. However upon the addition of 20 wt% GF, the CNT/MA-PE phase was found to coat the 3D network of GF particles leading to formation of continuous network. In this work, the influence GNP:CNT volume ratio on the microstructure, electrical, EMI shielding and mechanical properties of 50/50 and 90/10 polypropylene/polyethylene (PP/PE) blend are investigated. PP/PE blend was selected because PP, PE and their blends are of the most widely used polymeric materials due to their cost advantage and unique properties over many other materials. Since it was predicted that in a PP/PE blend the nanofillers will reside in the PE phase, the 50/50 and 90/10 PP/PE volume ratios were selected in order to investigate the properties of the blend when nanofillers/PE is a major phase (i.e. the 50/50 PP/ PE blend) and when the nanofiller/PE is a minor phase (i.e. the 90/ 10 PP/PE blend). In addition in all experiments, the nanofiller volume percent was constant at 5.0 vol%. This level of nanofiller concentration was selected in order to obtain a composite by melt mixing and with adequate level of EMI shielding [31]. 2. Experimental details 2.1. Materials and fabrications Two polymers were used in this study, namely: PP and PE. The main properties of these two polymers are listed in Table 1. The nanofillers were multi-walled CNT (NanocylTM NC7000, Nanocyl S. A., Sambreville, Belgium) and GNP (xGnP-Grade M, XG Sciences, USA). The nanotubes are 9.5 nm in diameter and 1.5 mm in length and GNP particles have a disc-shape geometry with average thickness of 7 nm and diameter of 5 mm. For the calculations of the nanofiller volume fraction, the density of CNT and GNP were set at 1.66 g/ml and 2.2 g/ml, respectively. In all experiments, the total nanofiller content was 5.0 vol%. All blends were prepared by melting mixing using a batch mixer (Plastograph EC, Brabender, Germany). Polymer pellets, prior to mixing, were dried in a vacuum oven for 16 h at 80 C. The CNT and GNP powders were also pre-dried at 130 C for 16 h. The melt mixing was conducted at 100 rpm and 180 C for 13 min. In a typical experiment, X g of polymer pellets were fed to the preheated mixer and mixed for 3.0 min. For the 50/50 (vol/vol) PP/PE blends, the amounts of PP and PE were 13.3 g and 14.1 g, respectively. While for the 90/10 (vol/vol) PP/PE blends, the PP and PE amounts were 24.6 g and 2.9 g respectively. Then, Y amount of the nanofillers were fed into the mixer. Tables 2 and 3 lists the amounts of the GNP and CNT used in preparing the 5 vol% filled 50/ 50 PP/PE and 90/10 PP/PE blends, respectively. The nanofillers were added to the mixer following three different sequences. In the first sequence, both fillers were fed at the same time. In the second sequence, GNP was first fed and after
Manufacturer Brand name Specific Gravity Melt flow rate (g/10 min) a b
230 C and 2.16 kg load. 190 C and 5.0 kg load.
Table 2 Amounts of GNP and CNT used to formulate the 5 vol% GNP:CNT filled 50/50 PP/PE blends. GNP:CNT
GNP (g)
CNT (g)
GNP wt%
CNT wt%
GNP vol%
CNT vol%
5:0 4:1 3:2 2:3 1:4 0:5
3.420 2.710 2.020 1.340 0.660 0.000
0.000 0.515 1.030 1.550 2.065 2.580
11.1% 8.8% 6.6% 4.4% 2.2% 0.0%
0.0% 1.7% 3.4% 5.1% 6.9% 8.6%
5.0% 4.0% 3.0% 2.0% 1.0% 0.0%
0.0% 1.0% 2.0% 3.0% 4.0% 5.0%
Table 3 Amounts of GNP and CNT used to formulate the 5 vol% GNP:CNT filled 90/10 PP/PE blends. GNP:CNT
GNP (g)
CNT (g)
GNP wt%
CNT wt%
GNP vol%
CNT vol%
5:0 4:1 3:2 2:3 1:4 0:5
3.520 2.816 2.112 1.408 0.704 0.000
0.000 0.531 1.062 1.593 2.124 2.655
11.3% 9.1% 6.9% 4.6% 2.3% 0.0%
0.0% 1.7% 3.5% 5.2% 7.0% 8.8%
5.0% 4.0% 3.0% 2.0% 1.0% 0.0%
0.0% 1.0% 2.0% 3.0% 4.0% 5.0%
5.0 min the CNT was fed. In the third sequence, CNT was first introduced then after 5.0 min the GNP was fed. At the end of the mixing process, the blend was collected and sent to a compression molding machine (Carver Inc., Wabash-IN, USA) to prepare specimens for electrical, EMI shielding and mechanical properties characterization. The molding was conducted under 27.5 MPa and 200 C for 10 min. For the electrical conductivity and EMI shielding characterizations, the samples were (40 mm 20 mm 1 mm) rectangles. For tensile tests, initially (65 mm 65 mm 1 mm) plates were produced; then ASTM D628-5-IMP die was used to cut type V ASTM D638-03 specimens. 2.2. Characterization tools The microstructure of the PP/PE blends was investigated using Environmental Scanning Electron Microscope (Quanta 450 FEG, FEI). Prior to SEM analysis, samples were fractured in liquid nitrogen and sputtered with a thin layer of gold using sputtering machine (Q150R ES, Quorum Technologies, UK). The blends electrical conductivity was measured using two different setups. Conductive samples were characterized using digital multimeter (Keithley 2010 DMM, Keithley Instruments, USA) connected to a 4-wire probe test fixture, while the non-conductive samples were characterized using Keithley 6517A electrometer connected to Keithley 8009 test fixture (Keithley Instruments, USA). The reported electrical resistivity represents the average of at least six specimens. The EMI shielding effectiveness (SE) in the X-band (8.0–12.0 GHz) frequency range was conducted using E5071C ENA network analyzer connected to a WR-90 rectangular waveguide. The rectangular (2 4 cm2) specimens were inserted between the two sections of the waveguide and the S-parameters (S11, S12, S22, S21) of each sample were recorded. The total EMI SE was calculated as follows: EMISE ¼ 10log
Table 1 Information about PP and PE used in this study. PP
PE
SABIC PP 504P 0.9 3.2a
ExxonMobil Chemical HTA 001HD 0.952 0.32b
323
1 jS12 j2
¼ 10log
1 jS21 j2
ð1Þ
The tensile tests were conducted according to the ASTM standard D638-03 using WDW-20 (Jinan Testing Equipment IE Corporation, China) tensile testing machine. For each formulation, at least six specimens (Type V ASTM D638-03) were tested and the average of those was reported. The crosshead speed for all tests was 10 mm/min.
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3. Results and discussion 3.1. Microstructure SEM analysis was conducted to explore the location of the CNT and GNP within the PP/PE blend. Fig. 1 depicts SEM micrographs of
50/50 and 90/10 PP/PE blends filled with 3:2 GNP:CNT. From Fig. 1a two major observations can be drawn. The first one is the expected co-continuous structure of the 50/50 PP/PE blend, and the second observation is the selective localization of CNT and GNP in one of the blend’s phases. From this micrograph it is impossible to distinguish between the PP and PE phases since they have equal
Fig. 1. SEM micrograph of PP/PE blends filled with 5 vol% hybrid nanofiller (3:2 GNP:CNT). a) 50/50 PP/PE blend and (b) 90/10 PP/PE blend.
M.H. Al-Saleh / Synthetic Metals 217 (2016) 322–330
g 12
4g p g p ¼ g1 þ g2 d p 1 2p d g1 þ g2 g1 þ g2 4g d1 g d2
ð2Þ
In the equation, g i is the surface tension of component i,g di is the
dispersive component of the surface tension and g pi is the polar component of the surface tension. The theoretical results listed in Table 4 shows that the GNP/PE and CNT/PE interfacial tensions are, respectively, lower than the GNP/PP and CNT/PP interfacial tensions. For example, the GNP/PE and GNP/PP interfacial tensions are 7.2 mN/m and 10.1 mN/m, respectively. The remarkable difference in interfacial tensions favors the localization of GNP and CNT in the PE phase. This theoretical predication is consistent with the experimental observation. 3.2. Electrical properties In CPN, critical amount of nanofiller, widely known as the electrical percolation threshold (EPT), is needed to create conductive network within the polymer matrix. At the EPT, the electrical conductivity of the nanocomposite increases by several orders of magnitude. The effect of using hybrid nanofiller on the EPT have been investigated by many researchers [12–14,33]. Herein the focus is on studying the effect of using hybrid nanofiller mixture at concentration above the EPT, since for the EMI shielding applications nanofiller loading above the EPT is needed to formulate CPN with adequate level of shielding (i.e. a SE 20 dB). As mentioned in the experimental section, the blends were prepared following three different mixing protocols. The results showed no influence of the order of addition on the electrical conductivity. This means that the mixing time was enough for blends to reach their steady-state microstructure. Fig. 2 shows the volume resistivity (r) of the 5 vol% GNP:CNT filled PP/PE blends as Table 4 Surface tension ðg Þ of PP, PE, CNT and GNP and interfacial tension ðg 12 Þ between the nanofiller/polymer pairs at 180 C.
g ðmN=mÞ PE [32] PP [32] CNT [41] GNP [40] CNT-PP CNT-PE GNP-PP GNP-PE a b c
26.6 20.8 23.5 45.6
g d (mN/m)a
g p (mN/m)b
26.6 20.4 23.5 41.7
0.0 0.4 0.0 3.9
Dispersive component of the surface tension. Polar component of the surface tension. Calculated based on the harmonic-mean equation.
10.0 9.0 8.0
log[ρ(Ω·cm)]
volumes. However, from the SEM micrograph of the 90/10 PP/PE blend (Fig. 1b), it is apparent that the minor phase, which is the PE phase, is highly filled with the nanofillers, and the major PP phase is free of the nanofillers. This is a direct evidence that the CNT and GNP are selectively localized in the PE phase. Interestingly, even though the GNP particles have large dimensions, they are aligned and contained within the PE phase, revealing that the thermodynamic affinity is the controlling parameter for the nanofiller location. Similar observations were reported for poly(methylmethacrylate (PMMA)/PE) blend filled with CNT: expanded graphite mixture, where both nanofillers were found to selectively reside in the PE phase [2]. This selective localization can be attributed to the lower interfacial tension between the nanofillers (GNP and CNT) and the PE phase compared to that with the PP phase as listed in Table 4. The table lists the surface tension of nanofillers and polymers and the interfacial tension ðg 12 Þ between the nanofiller/polymer pairs. The interfacial tension was calculated using the harmonic-mean equation [32]:
325
7.0
50/50 PP/PE
6.0
90/10 PP/PE
5.0 4.0 3.0 2.0 1.0 0.0 -1.0 5:0
4:1
3:2
2:3
1:4
0:5
GNP:CNT Fig. 2. Electrical resistivity of 5 vol% GNP:CNT filled PP/PE blends as function of GNP:CNT and PP/PE volume ratios.
function of PP/PE and GNP:CNT volume ratios. It is apparent that the electrical resistivity of the 90/10 PP/PE blends is lower than that of the 50/50 PP/PE blends. This finding is expected because of the higher effective concentration of the conductive nanofillers in the PE phase with the decrease of PE volume fraction in the cocontinuous PP/PE blend. For the effect of GNP:CNT volume ratio, it is evident that the electrical resistivity decreases with the decrease in GNP:CNT volume ratio. This is mainly due to the 1-D geometry of the CNT that is more effective than the 2-D geometry of the GNP in creating conductive networks. Based on the dimensions of the GNP and CNT used in this study, the electrical percolation threshold for nanocomposites filled with rod-like nanofiller (Eq. (1)) and disklike nanofiller (Eq. (2)) can be estimated as follows [34,35]:
Fc ¼ 0:5=AR
Fc ¼
27pD2 t 4ðD þ IPDÞ3
ð3Þ
ð4Þ
In the equations (3) and (4), Fc is the nanofiller volume fraction at the electrical percolation threshold, AR is the nanofiller aspect ratio, t is the nanofiller thickness, D is the nanofiller diameter, and IPD is the inter-particle distance. For the GNP used in this study, t is 7 nm and the equivalent D is 5 mm. For the IPD it can be set at 10 nm, since for composites an IPD less than or equal to 10 nm is required to permit electron conduction by tunneling [34]. While for the CNT used in the study, the AR after the melt mixing can be assumed 80 [36–38]. Thus, Fc for the PE phase is expected to be 3 vol% GNP or 0.63 vol% CNT. This remarkable difference indicates that nanofillers of rod-like structures are more effective than nanofiller of disk-like structures in creating conductive networks.
g 12 (mN/m)c
3.3. EMI shielding properties
0.6 0.2 10.1 7.2
Nowadays, there is a remarkable demand for EMI shielding materials in order to maintain the electromagnetic compatibility of the rapidly growing number of portable electronic devices. Due to their light-weight and design-flexibility, CPNs are of the most promising candidates to attenuate EMI. The EMI SE of a CPN depends on many factors of which the nanocomposites’ electrical conductivity and shielding plate thickness are of the most significance. Fig. 3 depicts the effect of GNP:CNT volume ratio and EMI frequency on the total SE and Fig. 4 depicts the average
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30
a) 50/50 PP/PE
25
GNP-CNT 0-5
20
EMI SE (dB)
1-4 2-3
15
EMISE ¼ SER þ SEA
ð5Þ
3-2
10
4-1
5
5-0
0 8
9
10
11
35
s
ð6Þ
2pf m
ð7Þ
3.4. Tensile properties
30
b) 90/10 PP/PE
GNP-CNT 0-5
25
1-4
20
2-3
15
3-2
10
4-1
5
5-0
0
SER ¼ 39:5 þ 10log
pffiffiffiffiffiffiffiffiffiffiffiffiffi SEA ¼ 8:7d pf ms
12
Frequency (GHz)
EMI SE (dB)
shielding by reflection ðSER Þ and shielding by absorption ðSEA Þ increases with the increase in the electrical conductivity ðs Þ. In the equations, m is the magnetic permeability, f is the frequency and d is the thickness of the shielding plate.
8
9
10
11
12
Frequency (GHz) Fig. 3. EMI SE of 5 vol% GNP:CNT filled PP/PE blends as function of GNP:CNT volume ratio and frequency a) 50/50 PP/PE and b) 90/10 PP/PE.
Before discussing the properties of the filled PP/PE blends it is worth mentioning that the tensile strength of the unfilled 50/ 50 and 90/10 PP/PE are 25 0.4 MPa and 27.7 0.9 MPa, respectively. Fig. 5 depicts the effect of GNP:CNT volume ratio on the tensile strength of the PP/PE blends as function of PP/PE and GNP: CNT volume ratios. It is evident that the tensile strength was increased with the decrease in GNP:CNT volume ratio. For example, for the 50/50 PP/PE blends, the tensile strength increased from 24.6 MPa for the 5:0 GNP:CNT mixture (which is similar to that of the unfilled 50/50 PP/PE blend) to 29.4 MPa for the 0:5 GNP: CNT mixture, corresponding to 20% enhancement in the tensile strength. In a previous work using similar CNT and PE [39], it was found that the tensile strength of 5 vol% CNT/PE is 26% higher than that of unfilled PE. Thus the tensile strength of GNP:CNT/PP/PE blends are consistent with the properties of CNT/PE composite and with the microstructure analysis that shows good adhesion between the CNT and PE, as shown in Fig. 6a, and no adhesion between the nanofiller and the PE phase, as shown in Fig. 6b. In the later micrograph, the surface of GNP particles is clearly seen free of polymer chain. Moreover, the microstructure analysis showed clear voids between the GNP particles (Fig. 7a) and between the particles GNP and polymer matrix (Fig. 7b). Voids are expected to have detrimental effect on the tensile properties since they can act as stress concentrators. These SEM observations are consistent with the theoretical predications base on the interfacial tension of
40 50/50 PP/PE 90/10 PP/PE
35
EMI SE in the X-band range of the 50/50 and 90/10 PP/PE blends as function of GNP:CNT volume ratio. It is apparent that within the Xband frequency range, the EMI SE is marginally affected by the frequency. However, at any given frequency it is evident that the EMI SE increases with the decrease of GNP:CNT volume ratio (i.e. increase in CNT fraction) indicating no synergistic effect of using hybrid mixture on the EMI SE. From Fig. 4 it is apparent that the 0:5 GNP:CNT blends have EMI SE that is 5–6 times that of the 5:0 GNP:CNT blends. This clearly indicates that CNT has superior properties compared to GNP. The higher EMI SE for the CNT dominated blends is related to the higher electrical conductivity of these blends. Theoretically, EMI SE increases with the increase in electrical conductivity, as shown in Eqs. (5)–(7) [31]. In these equations, it is apparent that both the
Tensile Strenght (MPa)
30 Fig. 4. Average EMI SE of 5 vol% GNP:CNT filled PP/PE blends as function of GNP: CNT volume ratio and PP/PE volume ratio.
25 20 15 10 5 0 5:0
4:1
3:2
2:3
1:4
0:5
GNP:CNT Fig. 5. Tensile strength of 5 vol% filled PP/PE blends as function of GNP:CNT volume ratio.
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Fig. 6. SEM micrographs of 50/50 PP/PE blends filled with 5 vol% hybrid nanofiller (3:2 GNP:CNT) showing a) CNT-rich zone and b) GNP-rich zone.
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Fig. 7. SEM micrographs of 50/50 PP/PE blends filled with 5 vol% hybrid nanofiller (3:2 GNP:CNT) showing the a) the voids between the GNP and polymer matrix (b) voids between GNP particles.
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CNT/PE and GNP/PE pairs, as listed in Table 4. The GNP/PE interfacial tension is 7.2 mN/m compared to only 0.2 mN/m for the CNT/PE system. This remarkable difference reveals better adhesion at the CNT/PE interface compared to the GNP/PE interface. 4. Conclusions The effect of GNP:CNT hybrid nanofiller on the microstructure and properties of 50/50 and 90/10 PP/PE blends were investigated. All blends were prepared by melt mixing. The CNT and GNP were found to selectively localize within the PE phase. At 5 vol% GNP: CNT loading, the electrical conductivity and EMI SE were increased with the increase in the fraction of CNT because the 1-D geometry of the CNT is more effective than the 2-D geometry of the GNP in building three dimensional networks. In addition an enhancement in tensile strength was found with the increase in CNT content. This enhancement is ascribed to the good adhesion at the CNT/PE interfaces compared to bad adhesion at the GNP/PE interface. Acknowledgment This work was financially supported by the Scientific Research Support Fund, Ministry of Higher Education – Amman – Jordan (Grant Number Bas/2/05/2010). References [1] M.H. Al-Saleh, U. Sundararaj, A review of vapor grown carbon nanofiber/ polymer conductive composites, Carbon 47 (2009) 2–22, doi:http://dx.doi.org/ 10.1016/j.carbon.2008.09.039. [2] A. Raulo, S. Suin, S. Paria, Expanded graphite (EG) as a potential filler in the reduction of percolation threshold of multiwall carbon nanotubes (MWCNT) in the PMMA/HDPE/EG/MWCNT nanocomposites, Polym Compos. (2015) n/a–n/ a, doi:http://dx.doi.org/10.1002/pc.23385. [3] M. Sumita, K. Sakata, S. Asai, K. Miyasaka, H. Nakagawa, Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black, Polym. Bull. 25 (1991) 265–271, doi:http://dx.doi.org/10.1007/BF00310802. [4] F. Gubbels, R. Jerome, P. Teyssie, E. Vanlathem, R. Deltour, A. Calderone, et al., Selective localization of carbon black in immiscible polymer blends: a useful tool to design electrical conductive composites, Macromolecules 27 (1994) 1972–1974, doi:http://dx.doi.org/10.1021/ma00085a049. [5] M.H. Al-Saleh, U. Sundararaj, An innovative method to reduce percolation threshold of carbon black filled immiscible polymer blends, Compo. Part A: Appl. Sci. Manufact. 39 (2008) 284–293, doi:http://dx.doi.org/10.1016/j. compositesa.2007.10.010. [6] M.H. Al-Saleh, Influence of conductive network structure on the EMI shielding and electrical percolation of carbon nanotube/polymer nanocomposites, Synth. Met. 205 (2015) 78–84, doi:http://dx.doi.org/10.1016/j. synthmet.2015.03.032. [7] L. Liu, J.C. Grunlan, Clay assisted dispersion of carbon nanotubes in conductive epoxy nanocomposites, Adv. Funct. Mater. 17 (2007) 2343–2348, doi:http://dx. doi.org/10.1002/adfm.200600785. [8] H.-D. Bao, Z.-X. Guo, J. Yu, Effect of electrically inert particulate filler on electrical resistivity of polymer/multi-walled carbon nanotube composites, Polymer 49 (2008) 3826–3831, doi:http://dx.doi.org/10.1016/j. polymer.2008.06.024. [9] M. Kotaki, K. Wang, M.L. Toh, L. Chen, S.Y. Wong, C. He, Electrically conductive Epoxy/Clay/Vapor grown carbon fiber hybrids, Macromolecules 39 (2006) 908–911, doi:http://dx.doi.org/10.1021/ma0522561. [10] J.F. Feller, S. Bruzaud, Y. Grohens, Influence of clay nanofiller on electrical and rheological properties of conductive polymer composite, Mater. Lett. 58 (2004) 739–745, doi:http://dx.doi.org/10.1016/j.matlet.2003.07.010. [11] J. Sumfleth, X. Adroher, K. Schulte, Synergistic effects in network formation and electrical properties of hybrid epoxy nanocomposites containing multiwall carbon nanotubes and carbon black, J. Mater. Sci. 44 (2009) 3241–3247, doi:http://dx.doi.org/10.1007/s10853-009-3434-7. [12] R. Socher, B. Krause, S. Hermasch, R. Wursche, P. Pötschke, Electrical and thermal properties of polyamide 12 composites with hybrid fillers systems of multiwalled carbon nanotubes and carbon black, Compos. Sci. Technol. 71 (2011) 1053–1059, doi:http://dx.doi.org/10.1016/j.compscitech.2011.03.004. [13] P.-G. Ren, Y.-Y. Di, Q. Zhang, L. Li, H. Pang, Z.-M. Li, Composites of ultrahighMolecular-Weight polyethylene with graphene sheets and/or MWCNTs with segregated network structure: preparation and properties, Macromol. Mater. Eng. 297 (2012) 437–443, doi:http://dx.doi.org/10.1002/mame.201100229. [14] H. Palza, Modifying the electrical behaviour of polypropylene/carbon nanotube composites by adding a second nanoparticle and by annealing processes, Express Polym. Lett. 6 (2012) 639–646, doi:http://dx.doi.org/ 10.3144/expresspolymlett.2012.68.
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