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Electrical double percolation and carbon nanotubes distribution in solution processed immiscible polymer blend
Synthetic Metals 175 (2013) 75–80
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Electrical double percolation and carbon nanotubes distribution in solution processed immiscible polymer blend Mohammed H. Al-Saleh ∗ , Haya K. Al-Anid, Yazan A. Hussain Department of Chemical Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
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Article history: Received 22 February 2013 Received in revised form 7 April 2013 Accepted 7 May 2013 Available online 3 June 2013 Keywords: Carbon nanotube Polymer blend microstructure Electrical percolation Solution processing Conduction mechanism
a b s t r a c t Carbon nanotubes (CNT) were dispersed in immiscible (50/50 vol/vol) polycarbonate (PC)/polystyrene (PS) blend by solution processing. The microstructure, electrical properties and conduction mechanism of the blend were investigated as function of CNT content. Optical microscopy and transmission electron microscopy were used for microstructure analyses. At low nanofiller loadings, the PC/PS blend was found to exhibit a phase-separated morphology with a domain size in the range of tens of microns. The location of CNT in the immiscible blend was found to be function of the CNT concentration. At 0.05 wt% CNT loading, CNT was found to preferentially reside in the PS phase. However, at 5 wt%, the nanotubes were observed in both phases without preferential localization. Regardless of CNT location, good level of nanotubes dispersion was observed. The selective localization of CNT in the PS phase at low filler loading and the proper dispersion of CNT led to construction of conductive network within the polymer blend at CNT content of only 0.05 wt% (∼0.034 vol%). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Selective localization of conductive nanofiller in immiscible polymer blends is an effective method to reduce the electrical percolation threshold concentration (EPTC) of nanocomposite materials. EPTC is the critical concentration required to create conductive network within insulating materials. At the EPTC, a sudden and remarkable increase in the composite electrical conductivity occurs. Many reports have been published on the preferential localization and electrical conductivity of nanofiller/immiscible polymer mixtures following the work of Geuskens et al. in 1987 [1] and of Sumita et al. in 1991 [2]. The later work was the first in providing theoretical explanation for this phenomenon. Currently, it is widely accepted that filler location in an immiscible polymer blend is mainly controlled by the difference in thermodynamic affinity of the filler to the blend’s components. Many studies showed that the filler prefers the phase with which it has the lowest interfacial tension [3–6]. However, kinetics [3,7], polymer viscosity [8] and filler size compared to the polymer blend domains size can interfere with the thermodynamic affinity. Most of the published works about nanofillers in polymer blends utilized melt mixing for blend preparation [5,9–22]. Very limited publications are available on solution processed immiscible polymer blends. This is partly due to the need for large amounts of
∗ Corresponding author. E-mail address: [email protected] (M.H. Al-Saleh). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.05.004
solvents, long processing time, unavailability of adequate solvents for many polymers and necessity to dry the solvent to recover the nanocomposite. However, with solution processing there is a possibility of creating novel structures that are unattainable by the conventional and industrially favorable melt mixing technique. Moreover, nanocomposites prepared by solution processing have much lower percolation threshold than those prepared by melt mixing [23]. Of the few reported studies on solution processed polymer blends, Kawazoe and Ishida [24,25] demonstrated the role of size exclusion on the selective localization of nanofiller. They studied the electrical properties and structure of solution processed carbon black (CB) filled styrene butadiene rubber/acrylonitrile butadiene rubber (SBR/NBR) blend. A remarkable reduction in percolation threshold was reported due to the selective localization of CB in one the blend’s phases. The location of CB in the SBR/NBR blend was reported to change with the processing solvent because of the change in polymer chain radius of gyration. For example, in toluene solution the apparent hydrodynamic radius of NBR was 5 nm compared to 150 nm for SBR. The smaller NBR chains can be confined within the voids of CB aggregates that have an opening size of about 20–30 nm. This confinement made the NBR-rich CB particles more compatible with the NBR phase and consequently led to its migration and selective localization within the NBR phase. In chloroform, CB was observed in the SBR phase due to the same reason [24,25]. Most recently, solution processing was utilized to obtain a homogenous epoxy/PEI/CB mixture from which a unique double-percolated structure was developed [26]. After solvent removal and epoxy curing, reaction induced phase separation
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created a two-phase structure in which CB was found to selectively reside in a PEI continuous phase. As a result the electrical percolation threshold concentration was only 0.5 wt% CB [26]. In this work, carbon nanotube (CNT) filled (50/50) polycarbonate/polystyrene (PC/PS) immiscible polymer blend was prepared by solution casting. The aim of the study is to investigate the microstructure (blend morphology, CNT dispersion and CNT location) and electrical properties of the CNT/PC/PS system as function of CNT content. PS and PC were selected as a model system since chloroform is a good dispersion medium for CNT and a good solvent for both polymers. Thus, at low polymer concentration a homogeneous and single phase mixture can be formed. Several reports are available about the microstructure and properties of PC/PS blends [27–30]. However, to the best of our knowledge no published studies about the solution processed CNT/PC/PS nanocomposites exist. 2. Experimental 2.1. Materials The polycarbonate (Calibre 200-10, density 1.2 g/cm3 MFI 10 g/10 min) was supplied by Dow Chemical and the polystyrene (PS-1500, density 1.04, MFI 6 g/10 min) was supplied by Nova Chemicals. All polymers were kindly provided by the manufacturers. The nanotubes were NanocylTM NC7000 (Nanocyl S.A., Sambreville, Belgium). NC7000 are multiwall carbon nanotubes (MWCNT) produced by chemical vapor deposition with an average diameter of 9.5 nm and a length of 1.5 m. As received HPLC grade chloroform (Fischer Scientific) was used as the dispersion medium for the nanotubes and as the solvent for nanocomposite preparation. 2.2. Method For the effect of CNT on the electrical properties and morphology of CNT/PS/PC blends, solutions of 1.5 wt% PC/PS (50/50 vol/vol) were prepared by dissolving 0.6 g PC and 0.52 g PS in 50 mL chloroform. Different amounts of CNT (0, 0.28, 0.56, 1.12, 5.63, 11.31, 22.86, 58.95 mg) were separately dispersed in 20 mL chloroform by sonication for 10 min. The CNT-suspension was added to the polymer solution and the mixture was sonicated for 10 min. After that, the solution was cast onto a glass substrate in an oven at 100 ◦ C resulting in 40–50 m thick film. The obtained film was folded and pressed in Carver hot press (Carver Inc., Wabash, IN, USA) to produce 1.0 mm thick rectangular plates (4 cm × 2 cm). The molding was conducted at 250 ◦ C under 22 MPa pressure for 10 min. 2.3. Characterization Specimens for the optical microscopy (OM) and transmission electron microscopy (TEM) analysis were cut from the molded rectangular plates using Ultracut Reichert-Jung microtome. For the OM analysis, the sections were 1 m thick; while for the TEM study the sections were ∼70 nm thick. Differential Interference Contrast (DIC) inverted microscope (Nikon, ECLIPSE, Japan) was used to investigate the macrostructure of the blend. Zeiss EMIOCR microscope was used for TEM imaging at an acceleration voltage of 60 kV. The TEM specimens were stained with OsO4 for 18 h to enhance the contrast between the PS and PC phases. The electrical resistivity of the nanocomposites was characterized by two setups depending on the nanocomposite’s electrical resistivity. Keithley 6517B electrometer connected to Keithley 8009 test fixture setup was used to measure the electrical resistivity and investigate the current–voltage (I–V) relation for specimens with electrical resistivity higher than 106 cm. For less resistive specimens, a four-wire probe connected to Keithley 2010 digital
multimeter (DMM) was used. The I–V characteristic of the conductive specimens was characterized by connecting the outer wires of the four-wire probe to a GW Instek PPS-3635 current source and the inner wires to the Keithley 2010 DMM to measure the voltage drop. 3. Results and discussions 3.1. Morphological observations: as-cast films During solution casting, the development of the nanofiller/polymer blend microstructure with solvent evaporation depends on several factors including: the surface tension and molecular weight of the polymer pairs, nanofiller concentration, cast film thickness, and solvent removal rate [31]. Various morphologies such as cocontinuous, dispersed and even bilayer structures (complete phase separation) can be obtained by controlling the casting process variables. For applications requiring polymeric materials with low electrical percolation threshold concentration, a double percolation structure is required. In double percolation, the filler rich phase should be conductive (first percolation) and continuous (second percolation). The lowest ever percolation threshold is obtained when the blend structure is cocontinuous and the nanofiller is selectively localized at the blend interface. The cocontinuous structure can be obtained by optimizing the solvent casting conditions to kinetically trap the required microstructure or by adding a copolymer or filler to the blend/solvent mixture to kinetically or even thermodynamically stabilize the microstructure [32,33]. As an example on the second case, 2 vol% of CdSe nano-rods were required to arrest and stabilize the cocontinuous morphology of polystyrene/poly(vinyl methyl ether) blend during thermal quenching above the blend lower critical solution temperature [32]. Since the ultimate objective of this work is to produce a nanostructured polymeric material with very low percolation threshold (<0.1 vol% CNT), we believe that such low nanofiller concentration is not enough to stabilize the blend morphology. Therefore, we attempted to kinetically trap the co-continuous structure (without stabilization) by controlling the solution casting conditions. Preliminary qualitative investigations were conducted to study the effect of different solvent removal rates on the structure of ascast films of PC/PS blends filled with 0.1 wt% CNT. The removal rate was varied by changing the casting temperature from 15 ◦ C to 100 ◦ C. In general, the PS/PC blend was found to completely separate into two layers when cast at room temperature (slow solvent removal), while no phase separation was observed by the naked eye for blends cast at high temperatures (fast solvent removal). Under slow solvent removal rate, the blend had enough time to completely separate into layers. However, with fast solvent removal the polymer domains phase growth was restricted because of the rapid and significant increase in the viscosity of the polymer blend solution. Thus, in order to avoid the bi-layer structure, blends were cast at high temperature (100 ◦ C) in all subsequent experiments. 3.2. Morphological observations: after thermal annealing After casting at high temperature, the obtained films were folded and thermally annealed at 250 ◦ C as described in the experimental section. Fig. 1 shows the general macrostructure of the 50/50 PC/PS blend without CNT. It is evident that the blend exhibits a cocontinuous phase-separated morphology with a domain size in the range of tens of microns. The microstructure of each of the obtained phases was investigated by TEM as shown in Fig. 2. The PC phase was stained with OsO4 and appears as the dark phase while PS is the light phase. Again, the image clearly shows
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Fig. 1. DIC micrograph of (50/50) PC/PS blend after solvent casting and thermal annealing.
a phase-separated morphology and it is apparent that the blend has been separated into a PS rich phase encapsulating PC inclusions and a PC rich phase encapsulating PS inclusions nanometers to several microns in size. The large domain sizes of the blend (Fig. 1) and the morphology of the system (Fig. 2) confirms that the blend’s microstructure was kinetically trapped during the fast solvent removal process. PS/PC blends filled with 0.5 wt% CNT exhibited similar microstructure to that of unfilled PS/PC blend, as shown in Figs. 3 and 4a. This observation confirms that at low nanofiller concentrations the presence of the nanofiller does not affect the phase separation process.
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Fig. 3. DIC micrograph of 0.5 wt% CNT (50/50) PC/PS blend.
Fig. 3 depicts an OM image showing a cocontinuous morphology. The TEM micrograph at low magnification (Fig. 4a) shows PC inclusions dispersed in the PS rich phase. At high magnification (Fig. 4b), tens of well-dispersed CNT can be clearly seen in the PS phase (light phase) and some of them are at the interface between PS and PC. At high CNT loadings, we were not able to see the two phases by the optical microscope revealing that CNT has hindered the phase growth. Fig. 5 shows TEM micrograph of 50/50 PC/PS blend filled with 5 wt% CNT. At this loading, well-dispersed nanotubes are clearly seen in both phases without any preferential localization. It is speculated that at higher loading of CNT, the fast removal of solvent did not allow all of the nanotubes to transfer from the PC phase to the PS phase leading to kinetically trapped microstructure without selective localization of the filler. 3.3. Location of CNT in PS/PC blend
Fig. 2. TEM micrograph of 50/50 PC/PS blend stained with OsO4 . The dark phase is the PC while the light phase is the PS.
For a nanofiller/immiscible polymer blend mixture, it is widely accepted that the location of nanofiller is mainly governed by its different thermodynamic affinity for each of the blend’s components. Thermodynamically, the nanofiller reside in the phase with which it has lower interfacial tension. This concept successfully predicted the location of nanofiller in most of the immiscible blends prepared by melt processing [3]. However for polymer blends by solution processing, it is not clear yet what are the major factors that govern the nanofiller final destination. For example, the location of silver nanowires in a thin film of immiscible blend of polystyrene/poly(vinyl pyrrolidone) (PS/PVP) was found function of the silver nanowires surface chemistry [34]. Nanowires with hydrophilic surface preferred the PVP phase. However, after modifying the surface with thiols having hydrophobic tails, the nanowires preferred the PS phase [34]. On the other hand, as mentioned in the introduction section, the location of CB in a mixture of NBR/SBR/solvent was found function of the solvent type, i.e. polymer radius of gyration, and independent on the nanofiller surface chemistry and its thermodynamic affinity toward any of the blend’s components [25]. CB nanoparticles were found in the NBR-rich phase when toluene (a bad solvent for NBR) was the solvent. However, when chloroform (a bad solvent for SBR) was used as a common solvent, CB nanoparticles were observed in the SBR-rich phase [24,25]. Based on these experimental observations, it was hypothesized that a bad solvent will reduce the polymer chains radius of gyration such that it is small enough to be confined/adsorbed within the CB aggregate concaves [24,25].
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Fig. 4. TEM images of 50/50 PC/PS blend containing 0.5 wt% CNT at (a) low magnification and (b) high magnification. Scale bars are 1 m in image (a) and 100 nm in image (b).
Neither surface chemistry nor molecular conformation concepts were able to predict the uneven distribution of the CNT in the PS/PC/chloroform mixture and ultimate in a PS/PC blend after solvent removal. Both concepts predicate PC as the preferential phase for CNT in PC/PS blend. From a thermodynamic point of view, PC is the preferred phase for CNT because PC is more polar than PS. Similarly, chloroform is good solvent for PS compared to PC, thus PC are expected to have smaller radius of gyration and higher affinity toward the CNT surface. This contradicts the experimental findings that show, at low nanofiller loading, selective localization of CNT in the PS phase after solvent removal (as shown in Fig. 4b) and in the PS-rich phase in a mixture of CNT in (50/50) PS/PC/chloroform as shown in Fig. 6. Thus, further investigations are required to find
Fig. 5. TEM image of 5 wt% CNT filled 50/50 PC/PS blend.
the major factors that govern the location of nanoparticles in the immiscible polymer blend solutions. 3.4. Electrical properties and percolation The preferential localization of the filler in one of the blend phases or at the interface and the continuity of the conductive phase results in a material with low percolation threshold. Fig. 7 shows the electrical resistivity of the 50/50 PS/PC system as function of CNT content. The volume resistivity of the unfilled polymer blend is 1.4 × 1015 cm. At only 0.05 wt% CNT loading, the electrical resistivity decreased by almost 9 orders of magnitude to 3 × 106 cm indicating the formation of the first conductive network within the polymer blend. As discussed in the morphology section, for PC/PS blends with low CNT content, CNT selectively localize in the PS phase. This selective localization in addition to the good dispersion of CNT in PS is the reason for this very low electrical percolation
Fig. 6. Digital image of CNT/PC/PS/chloroform mixture. The polymer concentration in the solvent 0.14 g/g and the concentration of CNT relative to the polymer is 0.5 wt%. The upper layer is the PS-rich phase.
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Fig. 7. Electrical Resistivity of 50/50 PC/PS blend as function of CNT content.
threshold concentration. At 0.05 wt% CNT, the CNT/PC/PS blend is suitable for applications in the field of electrostatic charge dissipation. Increasing CNT loading from 0.05 wt% to 0.1 wt% did not have significant effect on the electrical resistivity. However, further addition of CNT (beyond the 0.1 wt%) created more conductive networks and enhanced the system electrical conductivity. At 5 wt% CNT, the system electrical resistivity was 1.3 cm. This level of electrical resistivity makes this composite suitable for applications in the electromagnetic interference shielding field. Composite materials conduct electric current by direct contact between filler particles and/or by tunneling. Tunneling is assumed to occur when the filler particles are separated by less than 10 nm [35]. The conduction mechanisms can be identified by investigating the composite current–voltage (I–V) characteristic. A linear I–V relation means that direct contact is the dominant mechanism; while a nonlinear I–V relation means that tunneling is taking place [36]. Fig. 8 shows a power law relation for a sample containing 0.05 wt% CNT. The nonlinear I–V behavior indicates that at such low concentrations the electrons move by tunneling in addition to direct contact. Fitting the I–V data of the 0.05 wt% CNT nanocomposite, as shown in Fig. 8, proved that there is a power law
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Fig. 8. Current–voltage relationship of 0.05 wt% CNT filled 50/50 PC/PS blend.
relation (I ∝ Vn ) between current and voltage with exponent value (n) equal to 2.6. The reported values of n for graphitized carbon black/rubber composites below the electrical percolation threshold range between 2 and 3 [37]. At higher CNTs loading, e.g., 5 wt%, TEM observations (Fig. 5) clearly showed that there is contact between the filler particles. The I–V characteristic showed a linear I–V behavior (Fig. 9) indicating that conduction at this concentration is indeed by direct contact between the CNT particles. Several reports have been published concerning the electrical percolation threshold and the microstructure of CNT filled immiscible polymer blends. Table 1 summarizes most of the published works in this area. The table lists the system, the location of nanotubes and the reported percolation threshold. It is clear that for most systems, CNT was found to selectively localize in one of the blend phases. Only one study reported selective localization of CNT at the interface [38]. It is worth mentioning that all of the reported blends in Table 1 were prepared by melt mixing. Comparing the percolation threshold obtained in this study with the reported values, it is evident that the percolation threshold of the solution processed CNT/PS/PC system is at least 5 times lower than the lowest reported percolation threshold for a blend by melt mixing.
Table 1 Summary literature reported results for some polymer blends and the location of CNT. Polymer system
Location of CNT
Percolation threshold
Reference
PC/PVDFa PA6/ABS PC/SAN PC/ABS1 b PC/ABS2 c PC/ABS3 d PC/PE PET/PVDF PET/PP PVDF/PA6 HDPE/PA P␣MSAN/PMMA PS/SEBS-MA PA12/HDPE PA12/EA PA12/EA e PA6/ABS; PA6/PP/ABS; PA6/PP/ABS/HDPE
PC PA6 PC PC Mainly in ABS and some in PC ABS PC PET PET PA6 PA Mainly in P␣MSAN, some at the interface SEBS-MA PA PA Interface PA6
0.95 vol% (50/50 blend)0.8 vol% (30/70 blend) 0.25 wt% (50/50 blend) <1 wt% (60/40 blend) – – – 1–1.5 wt% – – – – 2 wt% – 0.75 wt% – – –
[18] [39] [40] [41]
a b c d e
PVDF: poly vinylidene fluoride. ABS1 : 5% rubber content. ABS2 : 20% rubber content. ABS1 : 60% rubber content. Polymer grafted MWNTs were used.
[17] [20] [42] [43] [44] [45] [46] [47] [38] [48]
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References
Fig. 9. I–V characteristic of 5 wt% CNT filled (50/50) PC/PS blend.
4. Conclusions The microstructure and electrical percolation behavior of solution processed (50/50) PC/PS polymer blend filled with CNT were investigated. The nanocomposites were prepared by solution casting at 100 ◦ C followed by compression molding at high temperature and pressure. The unfilled PS/PC blend exhibited a cocontinuous phase-separated morphology into PS-rich phase and a PC-rich phase. At low CNT loading, CNT was found to selectively localize in the PS phase. However, at higher CNT loadings, CNT was observed in the PC and PS phase without any preferential localization. The localizing of CNT in the PS phase, at low nanofiller loading, contradicts with the chemical affinity that predicts higher affinity of CNT toward the PC phase. Regardless of the CNT concentration, good level of nanotubes dispersion was observed. As a result of the good dispersion and selective localization of CNT in the PS phase at low CNT content, a low electrical percolation threshold of 0.05 wt% was obtained. At the percolation concentration, The I–V characteristic showed a nonlinear I–V behavior indicating that conduction at the percolation point is by tunneling in addition to direct contact between the CNT particles. However, a linear I–V characteristic was obtained at high nanofiller loading revealing that conduction at high concentration is due direct contact between nanoparticles. Acknowledgement The authors would like to thank the staff of the Nanotechnology Research Centre at Jordan University of Science and Technology for helping with the optical microscopy characterization. The financial support from the Deanship of Scientific Research at Jordan University of Science and Technology (grant number 24/2012) is acknowledged.
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Electrical double percolation and carbon nanotubes distribution in solution processed immiscible polymer blend Mohammed H. Al-Saleh ∗ , Haya K. Al-Anid, Yazan A. Hussain Department of Chemical Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan
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Article history: Received 22 February 2013 Received in revised form 7 April 2013 Accepted 7 May 2013 Available online 3 June 2013 Keywords: Carbon nanotube Polymer blend microstructure Electrical percolation Solution processing Conduction mechanism
a b s t r a c t Carbon nanotubes (CNT) were dispersed in immiscible (50/50 vol/vol) polycarbonate (PC)/polystyrene (PS) blend by solution processing. The microstructure, electrical properties and conduction mechanism of the blend were investigated as function of CNT content. Optical microscopy and transmission electron microscopy were used for microstructure analyses. At low nanofiller loadings, the PC/PS blend was found to exhibit a phase-separated morphology with a domain size in the range of tens of microns. The location of CNT in the immiscible blend was found to be function of the CNT concentration. At 0.05 wt% CNT loading, CNT was found to preferentially reside in the PS phase. However, at 5 wt%, the nanotubes were observed in both phases without preferential localization. Regardless of CNT location, good level of nanotubes dispersion was observed. The selective localization of CNT in the PS phase at low filler loading and the proper dispersion of CNT led to construction of conductive network within the polymer blend at CNT content of only 0.05 wt% (∼0.034 vol%). © 2013 Elsevier B.V. All rights reserved.
1. Introduction Selective localization of conductive nanofiller in immiscible polymer blends is an effective method to reduce the electrical percolation threshold concentration (EPTC) of nanocomposite materials. EPTC is the critical concentration required to create conductive network within insulating materials. At the EPTC, a sudden and remarkable increase in the composite electrical conductivity occurs. Many reports have been published on the preferential localization and electrical conductivity of nanofiller/immiscible polymer mixtures following the work of Geuskens et al. in 1987 [1] and of Sumita et al. in 1991 [2]. The later work was the first in providing theoretical explanation for this phenomenon. Currently, it is widely accepted that filler location in an immiscible polymer blend is mainly controlled by the difference in thermodynamic affinity of the filler to the blend’s components. Many studies showed that the filler prefers the phase with which it has the lowest interfacial tension [3–6]. However, kinetics [3,7], polymer viscosity [8] and filler size compared to the polymer blend domains size can interfere with the thermodynamic affinity. Most of the published works about nanofillers in polymer blends utilized melt mixing for blend preparation [5,9–22]. Very limited publications are available on solution processed immiscible polymer blends. This is partly due to the need for large amounts of
∗ Corresponding author. E-mail address: [email protected] (M.H. Al-Saleh). 0379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2013.05.004
solvents, long processing time, unavailability of adequate solvents for many polymers and necessity to dry the solvent to recover the nanocomposite. However, with solution processing there is a possibility of creating novel structures that are unattainable by the conventional and industrially favorable melt mixing technique. Moreover, nanocomposites prepared by solution processing have much lower percolation threshold than those prepared by melt mixing [23]. Of the few reported studies on solution processed polymer blends, Kawazoe and Ishida [24,25] demonstrated the role of size exclusion on the selective localization of nanofiller. They studied the electrical properties and structure of solution processed carbon black (CB) filled styrene butadiene rubber/acrylonitrile butadiene rubber (SBR/NBR) blend. A remarkable reduction in percolation threshold was reported due to the selective localization of CB in one the blend’s phases. The location of CB in the SBR/NBR blend was reported to change with the processing solvent because of the change in polymer chain radius of gyration. For example, in toluene solution the apparent hydrodynamic radius of NBR was 5 nm compared to 150 nm for SBR. The smaller NBR chains can be confined within the voids of CB aggregates that have an opening size of about 20–30 nm. This confinement made the NBR-rich CB particles more compatible with the NBR phase and consequently led to its migration and selective localization within the NBR phase. In chloroform, CB was observed in the SBR phase due to the same reason [24,25]. Most recently, solution processing was utilized to obtain a homogenous epoxy/PEI/CB mixture from which a unique double-percolated structure was developed [26]. After solvent removal and epoxy curing, reaction induced phase separation
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created a two-phase structure in which CB was found to selectively reside in a PEI continuous phase. As a result the electrical percolation threshold concentration was only 0.5 wt% CB [26]. In this work, carbon nanotube (CNT) filled (50/50) polycarbonate/polystyrene (PC/PS) immiscible polymer blend was prepared by solution casting. The aim of the study is to investigate the microstructure (blend morphology, CNT dispersion and CNT location) and electrical properties of the CNT/PC/PS system as function of CNT content. PS and PC were selected as a model system since chloroform is a good dispersion medium for CNT and a good solvent for both polymers. Thus, at low polymer concentration a homogeneous and single phase mixture can be formed. Several reports are available about the microstructure and properties of PC/PS blends [27–30]. However, to the best of our knowledge no published studies about the solution processed CNT/PC/PS nanocomposites exist. 2. Experimental 2.1. Materials The polycarbonate (Calibre 200-10, density 1.2 g/cm3 MFI 10 g/10 min) was supplied by Dow Chemical and the polystyrene (PS-1500, density 1.04, MFI 6 g/10 min) was supplied by Nova Chemicals. All polymers were kindly provided by the manufacturers. The nanotubes were NanocylTM NC7000 (Nanocyl S.A., Sambreville, Belgium). NC7000 are multiwall carbon nanotubes (MWCNT) produced by chemical vapor deposition with an average diameter of 9.5 nm and a length of 1.5 m. As received HPLC grade chloroform (Fischer Scientific) was used as the dispersion medium for the nanotubes and as the solvent for nanocomposite preparation. 2.2. Method For the effect of CNT on the electrical properties and morphology of CNT/PS/PC blends, solutions of 1.5 wt% PC/PS (50/50 vol/vol) were prepared by dissolving 0.6 g PC and 0.52 g PS in 50 mL chloroform. Different amounts of CNT (0, 0.28, 0.56, 1.12, 5.63, 11.31, 22.86, 58.95 mg) were separately dispersed in 20 mL chloroform by sonication for 10 min. The CNT-suspension was added to the polymer solution and the mixture was sonicated for 10 min. After that, the solution was cast onto a glass substrate in an oven at 100 ◦ C resulting in 40–50 m thick film. The obtained film was folded and pressed in Carver hot press (Carver Inc., Wabash, IN, USA) to produce 1.0 mm thick rectangular plates (4 cm × 2 cm). The molding was conducted at 250 ◦ C under 22 MPa pressure for 10 min. 2.3. Characterization Specimens for the optical microscopy (OM) and transmission electron microscopy (TEM) analysis were cut from the molded rectangular plates using Ultracut Reichert-Jung microtome. For the OM analysis, the sections were 1 m thick; while for the TEM study the sections were ∼70 nm thick. Differential Interference Contrast (DIC) inverted microscope (Nikon, ECLIPSE, Japan) was used to investigate the macrostructure of the blend. Zeiss EMIOCR microscope was used for TEM imaging at an acceleration voltage of 60 kV. The TEM specimens were stained with OsO4 for 18 h to enhance the contrast between the PS and PC phases. The electrical resistivity of the nanocomposites was characterized by two setups depending on the nanocomposite’s electrical resistivity. Keithley 6517B electrometer connected to Keithley 8009 test fixture setup was used to measure the electrical resistivity and investigate the current–voltage (I–V) relation for specimens with electrical resistivity higher than 106 cm. For less resistive specimens, a four-wire probe connected to Keithley 2010 digital
multimeter (DMM) was used. The I–V characteristic of the conductive specimens was characterized by connecting the outer wires of the four-wire probe to a GW Instek PPS-3635 current source and the inner wires to the Keithley 2010 DMM to measure the voltage drop. 3. Results and discussions 3.1. Morphological observations: as-cast films During solution casting, the development of the nanofiller/polymer blend microstructure with solvent evaporation depends on several factors including: the surface tension and molecular weight of the polymer pairs, nanofiller concentration, cast film thickness, and solvent removal rate [31]. Various morphologies such as cocontinuous, dispersed and even bilayer structures (complete phase separation) can be obtained by controlling the casting process variables. For applications requiring polymeric materials with low electrical percolation threshold concentration, a double percolation structure is required. In double percolation, the filler rich phase should be conductive (first percolation) and continuous (second percolation). The lowest ever percolation threshold is obtained when the blend structure is cocontinuous and the nanofiller is selectively localized at the blend interface. The cocontinuous structure can be obtained by optimizing the solvent casting conditions to kinetically trap the required microstructure or by adding a copolymer or filler to the blend/solvent mixture to kinetically or even thermodynamically stabilize the microstructure [32,33]. As an example on the second case, 2 vol% of CdSe nano-rods were required to arrest and stabilize the cocontinuous morphology of polystyrene/poly(vinyl methyl ether) blend during thermal quenching above the blend lower critical solution temperature [32]. Since the ultimate objective of this work is to produce a nanostructured polymeric material with very low percolation threshold (<0.1 vol% CNT), we believe that such low nanofiller concentration is not enough to stabilize the blend morphology. Therefore, we attempted to kinetically trap the co-continuous structure (without stabilization) by controlling the solution casting conditions. Preliminary qualitative investigations were conducted to study the effect of different solvent removal rates on the structure of ascast films of PC/PS blends filled with 0.1 wt% CNT. The removal rate was varied by changing the casting temperature from 15 ◦ C to 100 ◦ C. In general, the PS/PC blend was found to completely separate into two layers when cast at room temperature (slow solvent removal), while no phase separation was observed by the naked eye for blends cast at high temperatures (fast solvent removal). Under slow solvent removal rate, the blend had enough time to completely separate into layers. However, with fast solvent removal the polymer domains phase growth was restricted because of the rapid and significant increase in the viscosity of the polymer blend solution. Thus, in order to avoid the bi-layer structure, blends were cast at high temperature (100 ◦ C) in all subsequent experiments. 3.2. Morphological observations: after thermal annealing After casting at high temperature, the obtained films were folded and thermally annealed at 250 ◦ C as described in the experimental section. Fig. 1 shows the general macrostructure of the 50/50 PC/PS blend without CNT. It is evident that the blend exhibits a cocontinuous phase-separated morphology with a domain size in the range of tens of microns. The microstructure of each of the obtained phases was investigated by TEM as shown in Fig. 2. The PC phase was stained with OsO4 and appears as the dark phase while PS is the light phase. Again, the image clearly shows
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Fig. 1. DIC micrograph of (50/50) PC/PS blend after solvent casting and thermal annealing.
a phase-separated morphology and it is apparent that the blend has been separated into a PS rich phase encapsulating PC inclusions and a PC rich phase encapsulating PS inclusions nanometers to several microns in size. The large domain sizes of the blend (Fig. 1) and the morphology of the system (Fig. 2) confirms that the blend’s microstructure was kinetically trapped during the fast solvent removal process. PS/PC blends filled with 0.5 wt% CNT exhibited similar microstructure to that of unfilled PS/PC blend, as shown in Figs. 3 and 4a. This observation confirms that at low nanofiller concentrations the presence of the nanofiller does not affect the phase separation process.
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Fig. 3. DIC micrograph of 0.5 wt% CNT (50/50) PC/PS blend.
Fig. 3 depicts an OM image showing a cocontinuous morphology. The TEM micrograph at low magnification (Fig. 4a) shows PC inclusions dispersed in the PS rich phase. At high magnification (Fig. 4b), tens of well-dispersed CNT can be clearly seen in the PS phase (light phase) and some of them are at the interface between PS and PC. At high CNT loadings, we were not able to see the two phases by the optical microscope revealing that CNT has hindered the phase growth. Fig. 5 shows TEM micrograph of 50/50 PC/PS blend filled with 5 wt% CNT. At this loading, well-dispersed nanotubes are clearly seen in both phases without any preferential localization. It is speculated that at higher loading of CNT, the fast removal of solvent did not allow all of the nanotubes to transfer from the PC phase to the PS phase leading to kinetically trapped microstructure without selective localization of the filler. 3.3. Location of CNT in PS/PC blend
Fig. 2. TEM micrograph of 50/50 PC/PS blend stained with OsO4 . The dark phase is the PC while the light phase is the PS.
For a nanofiller/immiscible polymer blend mixture, it is widely accepted that the location of nanofiller is mainly governed by its different thermodynamic affinity for each of the blend’s components. Thermodynamically, the nanofiller reside in the phase with which it has lower interfacial tension. This concept successfully predicted the location of nanofiller in most of the immiscible blends prepared by melt processing [3]. However for polymer blends by solution processing, it is not clear yet what are the major factors that govern the nanofiller final destination. For example, the location of silver nanowires in a thin film of immiscible blend of polystyrene/poly(vinyl pyrrolidone) (PS/PVP) was found function of the silver nanowires surface chemistry [34]. Nanowires with hydrophilic surface preferred the PVP phase. However, after modifying the surface with thiols having hydrophobic tails, the nanowires preferred the PS phase [34]. On the other hand, as mentioned in the introduction section, the location of CB in a mixture of NBR/SBR/solvent was found function of the solvent type, i.e. polymer radius of gyration, and independent on the nanofiller surface chemistry and its thermodynamic affinity toward any of the blend’s components [25]. CB nanoparticles were found in the NBR-rich phase when toluene (a bad solvent for NBR) was the solvent. However, when chloroform (a bad solvent for SBR) was used as a common solvent, CB nanoparticles were observed in the SBR-rich phase [24,25]. Based on these experimental observations, it was hypothesized that a bad solvent will reduce the polymer chains radius of gyration such that it is small enough to be confined/adsorbed within the CB aggregate concaves [24,25].
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Fig. 4. TEM images of 50/50 PC/PS blend containing 0.5 wt% CNT at (a) low magnification and (b) high magnification. Scale bars are 1 m in image (a) and 100 nm in image (b).
Neither surface chemistry nor molecular conformation concepts were able to predict the uneven distribution of the CNT in the PS/PC/chloroform mixture and ultimate in a PS/PC blend after solvent removal. Both concepts predicate PC as the preferential phase for CNT in PC/PS blend. From a thermodynamic point of view, PC is the preferred phase for CNT because PC is more polar than PS. Similarly, chloroform is good solvent for PS compared to PC, thus PC are expected to have smaller radius of gyration and higher affinity toward the CNT surface. This contradicts the experimental findings that show, at low nanofiller loading, selective localization of CNT in the PS phase after solvent removal (as shown in Fig. 4b) and in the PS-rich phase in a mixture of CNT in (50/50) PS/PC/chloroform as shown in Fig. 6. Thus, further investigations are required to find
Fig. 5. TEM image of 5 wt% CNT filled 50/50 PC/PS blend.
the major factors that govern the location of nanoparticles in the immiscible polymer blend solutions. 3.4. Electrical properties and percolation The preferential localization of the filler in one of the blend phases or at the interface and the continuity of the conductive phase results in a material with low percolation threshold. Fig. 7 shows the electrical resistivity of the 50/50 PS/PC system as function of CNT content. The volume resistivity of the unfilled polymer blend is 1.4 × 1015 cm. At only 0.05 wt% CNT loading, the electrical resistivity decreased by almost 9 orders of magnitude to 3 × 106 cm indicating the formation of the first conductive network within the polymer blend. As discussed in the morphology section, for PC/PS blends with low CNT content, CNT selectively localize in the PS phase. This selective localization in addition to the good dispersion of CNT in PS is the reason for this very low electrical percolation
Fig. 6. Digital image of CNT/PC/PS/chloroform mixture. The polymer concentration in the solvent 0.14 g/g and the concentration of CNT relative to the polymer is 0.5 wt%. The upper layer is the PS-rich phase.
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Fig. 7. Electrical Resistivity of 50/50 PC/PS blend as function of CNT content.
threshold concentration. At 0.05 wt% CNT, the CNT/PC/PS blend is suitable for applications in the field of electrostatic charge dissipation. Increasing CNT loading from 0.05 wt% to 0.1 wt% did not have significant effect on the electrical resistivity. However, further addition of CNT (beyond the 0.1 wt%) created more conductive networks and enhanced the system electrical conductivity. At 5 wt% CNT, the system electrical resistivity was 1.3 cm. This level of electrical resistivity makes this composite suitable for applications in the electromagnetic interference shielding field. Composite materials conduct electric current by direct contact between filler particles and/or by tunneling. Tunneling is assumed to occur when the filler particles are separated by less than 10 nm [35]. The conduction mechanisms can be identified by investigating the composite current–voltage (I–V) characteristic. A linear I–V relation means that direct contact is the dominant mechanism; while a nonlinear I–V relation means that tunneling is taking place [36]. Fig. 8 shows a power law relation for a sample containing 0.05 wt% CNT. The nonlinear I–V behavior indicates that at such low concentrations the electrons move by tunneling in addition to direct contact. Fitting the I–V data of the 0.05 wt% CNT nanocomposite, as shown in Fig. 8, proved that there is a power law
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Fig. 8. Current–voltage relationship of 0.05 wt% CNT filled 50/50 PC/PS blend.
relation (I ∝ Vn ) between current and voltage with exponent value (n) equal to 2.6. The reported values of n for graphitized carbon black/rubber composites below the electrical percolation threshold range between 2 and 3 [37]. At higher CNTs loading, e.g., 5 wt%, TEM observations (Fig. 5) clearly showed that there is contact between the filler particles. The I–V characteristic showed a linear I–V behavior (Fig. 9) indicating that conduction at this concentration is indeed by direct contact between the CNT particles. Several reports have been published concerning the electrical percolation threshold and the microstructure of CNT filled immiscible polymer blends. Table 1 summarizes most of the published works in this area. The table lists the system, the location of nanotubes and the reported percolation threshold. It is clear that for most systems, CNT was found to selectively localize in one of the blend phases. Only one study reported selective localization of CNT at the interface [38]. It is worth mentioning that all of the reported blends in Table 1 were prepared by melt mixing. Comparing the percolation threshold obtained in this study with the reported values, it is evident that the percolation threshold of the solution processed CNT/PS/PC system is at least 5 times lower than the lowest reported percolation threshold for a blend by melt mixing.
Table 1 Summary literature reported results for some polymer blends and the location of CNT. Polymer system
Location of CNT
Percolation threshold
Reference
PC/PVDFa PA6/ABS PC/SAN PC/ABS1 b PC/ABS2 c PC/ABS3 d PC/PE PET/PVDF PET/PP PVDF/PA6 HDPE/PA P␣MSAN/PMMA PS/SEBS-MA PA12/HDPE PA12/EA PA12/EA e PA6/ABS; PA6/PP/ABS; PA6/PP/ABS/HDPE
PC PA6 PC PC Mainly in ABS and some in PC ABS PC PET PET PA6 PA Mainly in P␣MSAN, some at the interface SEBS-MA PA PA Interface PA6
0.95 vol% (50/50 blend)0.8 vol% (30/70 blend) 0.25 wt% (50/50 blend) <1 wt% (60/40 blend) – – – 1–1.5 wt% – – – – 2 wt% – 0.75 wt% – – –
[18] [39] [40] [41]
a b c d e
PVDF: poly vinylidene fluoride. ABS1 : 5% rubber content. ABS2 : 20% rubber content. ABS1 : 60% rubber content. Polymer grafted MWNTs were used.
[17] [20] [42] [43] [44] [45] [46] [47] [38] [48]
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References
Fig. 9. I–V characteristic of 5 wt% CNT filled (50/50) PC/PS blend.
4. Conclusions The microstructure and electrical percolation behavior of solution processed (50/50) PC/PS polymer blend filled with CNT were investigated. The nanocomposites were prepared by solution casting at 100 ◦ C followed by compression molding at high temperature and pressure. The unfilled PS/PC blend exhibited a cocontinuous phase-separated morphology into PS-rich phase and a PC-rich phase. At low CNT loading, CNT was found to selectively localize in the PS phase. However, at higher CNT loadings, CNT was observed in the PC and PS phase without any preferential localization. The localizing of CNT in the PS phase, at low nanofiller loading, contradicts with the chemical affinity that predicts higher affinity of CNT toward the PC phase. Regardless of the CNT concentration, good level of nanotubes dispersion was observed. As a result of the good dispersion and selective localization of CNT in the PS phase at low CNT content, a low electrical percolation threshold of 0.05 wt% was obtained. At the percolation concentration, The I–V characteristic showed a nonlinear I–V behavior indicating that conduction at the percolation point is by tunneling in addition to direct contact between the CNT particles. However, a linear I–V characteristic was obtained at high nanofiller loading revealing that conduction at high concentration is due direct contact between nanoparticles. Acknowledgement The authors would like to thank the staff of the Nanotechnology Research Centre at Jordan University of Science and Technology for helping with the optical microscopy characterization. The financial support from the Deanship of Scientific Research at Jordan University of Science and Technology (grant number 24/2012) is acknowledged.
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