Hierarchical Structures Composed of Confined Carbon Nanotubes in

Article pubs.acs.org/Macromolecules

Hierarchical Structures Composed of Confined Carbon Nanotubes in Cocontinuous Ternary Polymer Blends Eyal Cohen,*,†,§ Lior Zonder,†,§ Amos Ophir,‡ Samuel Kenig,‡ Stephen McCarthy,† Carol Barry,† and Joey Mead† †

Department of Plastics Engineering, University of Massachusetts Lowell, 1 University Ave., Lowell, Massachusetts 01854, United States ‡ Department of Plastics Engineering, Shenkar College of Engineering and Design, 12 Anna Frank St., Ramat Gan 52526, Israel S Supporting Information *

ABSTRACT: An hierarchical structure, composed of a ternary cocontinuous polymer blend, where carbon nanotubes are mostly localized in one of the phases through π−π interactions, is fabricated by direct melt mixing of polyamide 12 and polypropylene, as the two major components of the ternary blend, together with pyridine-modified poly(ethyleneco-methacrylic acid) as the minor component that can form strong interactions with the CNTs via π−π interactions and confined the percolated network at the polyamide/polypropylene interface. The hierarchical structure was designed by means of surface energies, and the obtained morphology was verified using electron microscopy. This ternary structure has lower electrical resistivity as compared to cocontinuous binary composites. Different polymer viscosities were used in this study in order to emphasize the importance of kinetics during cocontinuous morphology formation.

1. INTRODUCTION Melt mixing is currently the most convenient and costeffective method for incorporating conductive fillers, such as carbon nanotubes (CNTs), into a polymer matrix in order to fabricate conductive polymer composites.1 Conductivity is achieved through the formation of a three-dimensional network of conductive particles which enables electron transport across the polymer matrix.2 A promising method to reduce the percolation threshold is by melt mixing an immiscible polymer blend together with a conductive particle and generating a cocontinuous morphology where the conductive particles are preferentially localized inside one of the phases or at the interface (this is known as double percolation).3−11 Advanced structures are based on the localization of the conductive particle at the interface of a binary polymer blend. Several studies12,13 have shown that carbon black particles can be located at the interface of a binary blend during melt mixing by tuning either the thermodynamic conditions or the kinetics of the mixing process. Interface localization of CNTs in polymer blends was addressed by some research groups. Pötschke and co-workers14,15 used reactive modifiers in different polymer blends to control the CNT localization and change the blend kinetics toward interfacial localization. Yet, confining the CNTs at the interface itself remained difficult to achieve and the CNTs tended to migrate to one of the phases eventually. From a thermodynamic point of view it is unlikely for high aspect ratio particles, such as CNTs, to be located at the interface, as was shown by Göldel et al.16−18 and by Krasovitski and Marmur.19 For example, in © 2013 American Chemical Society

polycarbonate/poly(styrene−acrylonitrile) blends, where the CNTs were premixed in the polystyrene, the CNTs were located in the polycarbonate phase even though only minor differences in surface energies favor the polycarbonate phase. It was claimed that high aspect ratio particles form an unstable curvature at the polymers/particle intersection, which acts to increase wetting angle of the preferred phase. In order for equilibrium to be restored, molecules of the preferred phase advance along the surface and cover the particle. An exception is the work by Baudouin et al.20 which showed an interfacial localization of CNTs in polyamide/poly(ethylene−methyl acrylate) melt blends under certain mixing conditions. A possible route to fabricate a stable microstructure where CNTs are localized at the interface of an immiscible blend is by the utilization of a ternary blend. In this ternary blend, the two major phases are cocontinuous; the third minor component selectively contains the CNTs and is localized at the interface in a continuous manner. Morphologies, thermodynamic characteristics, and kinetics aspects of ternary polymer blends composed of two major phases and one minor phase are well demonstrated in the literature.21−24 The suggested hierarchical structure is similar to the cellular structure suggested by Winey et al.,25,26 who prepared a percolated CNT framework either by pressing coated polymer pellets or by infiltration of polymer resin onto a CNTs framework. The ternary blend strategy Received: September 11, 2012 Revised: January 24, 2013 Published: February 18, 2013 1851

dx.doi.org/10.1021/ma301903n | Macromolecules 2013, 46, 1851−1859

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chosen because (1) we found, in a separated experiment, that the percolation threshold of the PA is about 2 wt % and (2) the PEAA matrix could not be effectively premixed with more than 14 wt % CNT due to limitations with our equipment. The nanocomposite pellets were compression molded into 2 mm thick rectangular plates at 215 °C for 7 min. In addition to the CNT composites, neat ternary blends prepared without any CNTs were used as control samples. Characterization. FTIR spectra of EMAA and mEMAA were recorded in order to verify the chemical modification. The analysis (Bruker Alpha) was performed between 375 and 2000 cm−1, the resolution was 2 cm−1, and the results were based on an average of 22 scans. FTIR samples were prepared by casting 1% w/v polymer/THF solutions on KBr windows. The polar and disperse components of the surface tension for all blend constituents were obtained by measuring the contact angle between a solid surface sample of the polymers and three testing liquids: deionized, ultrafiltered water (0.2 μm filter), diiodomethane (DIM), and ethylene glycol (EG). Measurements were performed according to the sessile drop method using a video-based, softwarecontrolled, CA analyzer (OCA 20, Dataphysics Instruments, Germany). 5 μL drops from the three liquids were used to calculate the surface tensions according to the Owens−Wendt geometric mean equation. Standard surface tensions (mN/m) for these liquids were obtained from the OCA 20 database. For water, γ = 72.1, γd = 19.9, and γp = 52.2; for DIM γ = 50.8, γd = 49.5, and γp = 1.3; and for EG γ = 48, γd = 29, and γp = 19 (where γ is the total surface energy and γd and γp are the disperse and polar components, respectively). The measured surface energies, along with their dispersive and polar components for the polymers used in this study, are presented as Supporting Information. Since the temperature coefficients for CNTs and for some of the polymers are not available in the literature, the data presented are for ambient temperature, and no temperature corrections were performed. Interfacial tensions between the different components were calculated according to the harmonic mean equation. A solvent extraction method was used to characterize the existence of a cocontinuous morphology of the different composites. Three 100 mg specimens from each nanocomposite were dissolved in m-cresol for 48 h, then washed in formic acid and in ethanol, and finally vacuum-dried in order to selectively dissolve the PA phase. Weight loss after extraction was used to determine the degree of continuity of the PA within the composites. The EMAA proportion and continuity along the PA/PP interphase were obtained using the same selective extraction technique. The same 100 mg samples were used after the extraction of the PA phase. The samples were immersed in THF for 24 h, which dissolved the EMAA phase without dissolving the PP phase. The proportion of the EMAA along the PA/PP interface was calculated by comparing the lost weight of the EMAA to the weight of the EMAA according to the composite composition. It was presumed that not all of the EMAA will be located at the PA/PP interface due to thermodynamic and kinetic of the blend formation. Morphology analysis was performed using high-resolution SEM (Zeiss Gemini Ultra-55). SEM image samples were cryo-fractured in liquid nitrogen, chrome sputtered, and mounted on SEM sample holders. SEM images were scanned at 5 kV. TEM (FEI, Tecnai G2 12 Twin) images were performed on CNT composites at an accelerating voltage of 150 kV on 110 nm thick samples. The contrast between the different phases was obtained by exploiting the different tendencies of CNTs to be localized at the different phases and therefore “stained” the phases differently. Electrical volume resistivity was recorded using Keithley electrometer (model 6517B) equipped with 8009 high-resistance test apparatus on 2 mm thick samples. The voltage was set to 100 V for all samples. At least three specimens were measured from each compound. Viscoelastic behavior of the different composites was characterized in the melt state by a dynamic oscillatory shear rheometer (TA Instruments’ ARES ex2000) in parallel plate geometry with a diameter of 25 mm and plate gap of 1.9−2.0 mm. The measurements were carried out at 230 °C, using a constant strain of 1% and with angular

could lead to the same CNT structures, by applying conventional melt mixing techniques, and without using prior steps to produce the CNT framework. In this work we demonstrate that hierarchical structures composed of a cocontinuous ternary polymer blend, where CNTs are mostly confined in the continuous minor phase, are feasible and can overcome the low tendency of high aspect ratio particles to be located at the interface of two polymers. These hierarchical structures can be exploited as a route to lowered CNT percolation thresholds. Instead of trying to locate the CNTs at the interface, the CNTs were simply confined into an interphase of a minor third polymeric component, which was continuous throughout the blend. To meet this goal, we designed and fabricated a cocontinuous ternary polymer blend composed of polyamide 12 (PA) and polypropylene (PP) as the major phases while a poly(ethylene-co-methacrylic acid) copolymer (EMAA) with two different viscosities was used as the minor phase that has the potential to be located at the PA/ PP interface. In order to confine the CNTs in the minor phase, not only were the CNTs premixed with the minor phase but also the EMAA minor phase was chemically modified with 4aminomethylpyridine (AMP) in such a way that π−π interactions could be formed between the modified EMAA and the CNTs, as we had already shown in our previous study.27

2. EXPERIMENTAL SECTION Materials. Two polymers were used as the major components in the ternary blends. Homopolymer grade PP was purchased from Carmel Olefins (Capilene E50E) and PA from EMS-Grivory (Grilamid L25); the PA was vacuum-dried overnight before use. For the minor phase two commercial grades of EMAA with an 11.5 wt % concentration of methacrylic acid comonomer were used. The first was a high-viscosity EMAA (DuPont Nucrel1202HC, melt flow index of 1.5 g/10 min), and the second was a low-viscosity EMAA (DuPont Nucrel699, melt flow index of 95 g/10 min). The high- and lowviscosity minor phase polymers were designated respectively as EMAA1 and EMAA95. The CNTs used in this study were multiwalled carbon nanotubes purchased from Nanocyl (NC7000). The CNTs were used as received without any purification but were reported to have 90% purity and an average surface area of 250−300 m2/g. An XPS survey indicated that the CNTs were composed of 98.86% carbons atoms (63.4% sp2; 14.8% sp3) and 1.14% oxygen atoms. The 4-aminomethylpyridine (AMP), tetrahydrofuran (THF), m-cresol, formic acid (80%), and methanol were obtained from Sigma-Aldrich Israel and were used as received. Modification of Poly(ethylene-co-methacrylic acid) with Aminopyridine. Both the EMAA1 and EMAA95 were modified with AMP according to the procedure described elsewhere.27 EMAAs (50 g) were dissolved in THF (850 mL) at 67 °C for 24 h in a 1 L reaction flask. A stoichiometric amount of AMP (4:1 molar ratio with respect to the methacrylic acid content) was added, and the reaction continued for 24 h. The modified polymers (mEMAA1 and mEMAA95) were precipitated with cold methanol, filtered, washed extensively with methanol again, and vacuum-dried at 60 °C for 24 h. The modified polymers were then pressed at 180 °C and cut into pellets. Blend Preparation. The four different minor components (EMAA1, mEMAA1, EMAA95, and mEMAA95) were melt blended with 14.3 wt % CNTs in a Brabender mixer at 80 rpm and 200 °C for 8 min to form four different master batches. Ternary polymer blends were prepared using a 16 mm corotating twin-screw extruder (Prism, Eurolab) operating at 250 rpm with a barrel temperature of 230 °C. The PA and PP were dry-mixed with the different EMAA master batches in order to produce 47/47/6 v/v/v ternary blends. Each dry blend was fed directly into the feed throat of the twin-screw extruder. The CNT loading was kept constant at 1 wt %. This CNT loading was 1852


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Scheme 1. Reaction Scheme for the EMAA Modification with Aminopyridinea

a

At low temperature a salt is formed. As the salt complex is exposed to high temperature by hot press the carboxylate condensed into the amide bond.

Figure 1. FTIR spectra for (a) EMAA1 modification and (b) EMAA95 modification. After the reaction the aminopyridine forms a carboxylate complex. As the salt is heated (hot press) the carboxylate condensed into an amide bond. frequency range of 0.01−100 Hz; these conditions were confirmed to be within the linear viscoelastic region of the materials.

similar FTIR spectra after modification which indicates that the same degree of modification was achieved regardless of the EMAA viscosity. 3.2. Morphology of Ternary Blends. Four morphologies are available for a ternary polymer blend composed of two major phases and one minor phase. In addition to morphologies where the minor phase is dispersed as droplets inside one of the two major phases, two other unique morphologies are also possible. The first is a state where the minor phase is spread at the interface of the two major phases, whereas in the second state, none of the phases is completely spread on the interface of the other two; the second state is also known as partial wetting where droplets of the minor phase are localized at the interface. The spread state and the partial wetting state are well characterized in the literature.21−24 The morphology of a ternary blend system can be predicted by a set of three values which consider the tendency of each component to spread at the interface of the two other components. Three spreading coefficients (λikj), which indicate the tendency of component k to spread at the interface ij, can be predicted from the measured interfacial tensions (γ), following the procedure described in ref 24, according to eq 1:

3. RESULTS AND DISCUSSION 3.1. EMAA Modification. The chemical reaction, in which the amine end of the AMP reacts with the carboxylic moieties of the EMAA, is shown in Scheme 1. The reaction includes both the formation of the salt complex at low temperature and the condensation of the salt complex into an amide bond at elevated processing temperatures. The FTIR spectra for the modified EMAA1 and EMAA95 are presented in Figures 1a and 1b, respectively. Both EMAA1 and EMAA95 undergo an acid−base reaction where the aminopyridine reacts with the carboxylic acid to form an ammonium salt as evident by the disappearance of the carboxylic acid peak (1700 cm−1) and by the formation of the carboxylate peak (1540 cm−1). As the pyridine-modified EMAAs were exposed to elevated temperatures, the carboxylate peak disappeared and a typical amide bond peak was formed at 1697 cm−1. The characteristic aminopyridine peak is noticeable at 1590 cm−1 for both the salt complex and the amide bond modification. Both the EMAA1 and the EMAA95 exhibited 1853


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Table 1. Spreading Coefficients Values and Selective Extraction Results composite components PP/PA PP/PA/EMAA1

PP/PA/mEMAA1

PP/PA/EMAA95

PP/PA/mEMAA95

spreading coeff [mN/m]a λPP/EMAA1/PA = 1.9 ± 2 λEMAA1/PP/PA = −5.4 ± 2 λEMAA1/PA/PP = −5.5 ± 2 λPP/mEMAA1/PA = 1.4 ± 2 λmEMAA1/PP/PA = −8.5 ± 2 λmEMAA1/PA/PP = −2.5 ± 2 λPP/EMAA95/PA = 1.4 ± 2.2 λEMAA95/PP/PA = −4.0 ± 2.2 λEMA95/PA/PP = −7.0 ± 2.2 λPP/mEMAA95/PA = 0.50 ± 2.1 λmEMAA95/PP/PA = −9.4 ± 2.1 λmEMAA95/PA/PP = −1.6 ± 2.1

continuity of PA [%]b

proportion of EMAA at the interface [%]b

99.2 ± 1.2 94.7 ± 1.4

39.5 ± 2.3

98.9 ± 1.5

78.9 ± 2.1

98.7 ± 1.3

72.5 ± 2.7

99.2 ± 1.8

74.6 ± 1.8

Errors were calculated based on a set of five drops from each liquid and includes error propagation throughout the surface energies, surface tensions and spreading coefficients. bErrors are based on three samples measurements. a

Figure 2. SEM images for the different CNT ternary blend nanocomposites, taken at low magnification in order to characterize the blend morphology: (a) PA/PP blend; (b) PA/PP/EMAA1; (c) PA/PP/mEMAA1; (d) PA/PP/EMAA95; (e) PA/PP/mEMAA95.

λikj = γuj − (γik + γjk)

major phases. Other combinations of the three calculated coefficients would give rise to other undesirable morphologies, such as droplets of the minor phase in one of the two major phases and a partial wetting morphology.24 Table 1 lists the spreading coefficients, continuity values, and proportion of EMAA at the interface in the ternary blends. The spreading coefficients for the different polymer compositions indicated that, for the four ternary blends studied in this research, the desired morphologywhere the two major

(1)

In order for k to spontaneously spread on the ij interface, the spreading coefficient must be positive. For a ternary blend to take the desired morphology, where the minor phase is spread between the two major phases, the spreading coefficient for the EMAA need to have a positive value (spread), while the coefficients of the two other component needs to have negative values, in order not to form a spread interface between the two 1854


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phase A, and when ω < −1, the CNTs are located in phase B. With −1 < ω < 1, the particle should be located at the AB interface, but as was mentioned earlier, high aspect ratio particles such as CNTs are unlikely to remain at the interface. In such cases, the CNT localization was determined according to the CNT−polymer interfacial tension (i.e., whether ω is positive or negative). The wetting coefficients presented in Table 2 indicated that CNTs have a tendency to be located in the PA phase rather

phases are continuous and the minor phase spreads at the interface to form a continuous pathis possible from a thermodynamic point of view. By considering the error propagation thorough out the calculations, it was predicted that some of the ternary blends (such as the mEMAA95-based blend) could exhibit a partial wetting morphology instead of the desired interfacial spreading of the minor component. The morphology of the blends was verified by the selective extraction technique. By measuring the weight difference before and after the exposure of the different samples to mcresol, the degree of continuity of the PA was calculated. As shown in Table 1, the PA phase is 94.7−99.2% continuous in all of the nanocomposites. The solvent extraction technique was also used to calculate the proportion of the EMAA phase between the PA/PP phases (Table 1). For four of the five CNT composites, EMAA was mostly (about 80%) located at the PA/ PP interface. The only exception was for the EMAA1-based composite where the high-viscosity EMAA1 existed in a smaller fraction (40%) at the interface. This difference was probably due to kinetic factors which changed the blend morphology; the higher viscosity EMAA1 failed to fully spread on the PA/PP interface. It has been already shown that kinetics effects, mostly related to viscosity and elasticity of the materials, can sometimes lead to discrepancies from the spreading coefficients prediction.28 Further evidence for the ternary blend morphology was demonstrated in the SEM images for the different blends. As shown in Figure 2a, the binary blend exhibited a cocontinuous morphology. For the blend based on EMAA1 (Figure 2b), the cocontinuous morphology was disrupted and turned into droplets. In contrast, the use of pyridine-modified mEMAA1 (Figure 2c) not only preserved the cocontinuous morphology but also changed the morphology from a coarse cocontinuous structure into much finer cocontinuous structure. The change in the PA/PP structure in the presence of the mEMAA1 could only be attributed to the localization of the EMAA between the PA and the PP phases. As the EMAA is spread at the PA/PP interface the coalescence and coarsening behaviors are reduced. Those SEM images (Figures 2b and 2c) were consistent with the selective extraction analysis. The changes in the morphology between EMAA1 and mEMAA1 blends was related to the pyridine modification of the EMAA which results in a viscosity reduction (which will be discussed later) due to the replacement of the hydrogen bonds with pyridine bulky group.27 Interfacial localization and spreading, indicated by finer cocontinuous morphology, also was observed with EMAA95 and pyridine-modified mEMAA95 (Figures 2d and 2e). Thus, low-viscosity minor component (i.e., EMAA95 compared to EMAA1) favors the possibility of the EMAA forming a continuous interphase at the PA/PP interface during melt mixing, even though both EMAA grades had the same thermodynamic tendencies to do so. 3.3. CNT Selective Localization. The specific localization of particle in a polymer blend can be predicted from the wetting coefficient which describes the tendency of a particle toward a specific polymer phase, as described elsewhere,16 according to eq 2: ωAB−CNT =

Table 2. Wetting Coefficients for the CNTs with the Different Polymers interpretation

PP/PA EMAA1/PP

−1.0 3.2

CNTs located in the PA phase CNTs located in the PA phase or EMAA1 phase or at the PA/EMAA1 interface

EMAA1/PA mEMAA1/PP mEMAA1/PA EMAA95/PP

−0.1 1.9 1.7 4.0

EMAA95/PA mEMAA95/ PP mEMAA95/ PA

−0.2 1.7

a

CNTs located in the mEMAA1 phase CNTs located in the PA phase or EMAA95 phase or at the PA/EMAA95 interface CNTs located in mEMAA95 phase

3.6

CNT surface tension data taken from ref 29.

than the PP phase. The differences in interfacial tension between the CNT−EMAA and the CNT−PA were too small to predict in which phase the CNTs would be confined. On the other hand, when the copolymer was modified with 4aminopyridine (mEMAA), the CNTs demonstrated a strong thermodynamic preference of toward the mEMAA phase, which was the purpose of introducing π-electron moieties onto the EMAA chain. Based on the spreading and wetting coefficients, a blend composed of PA/PP and pyridine-modified mEMAA as the minor phase has the potential (neglecting kinetic considerations, as in the case of the high viscosity EMAA1) to form an hierarchical structure, where the mEMAA is spread at the PA/ PP interface, due to spreading coefficients, and the CNTs are confined in that mEMAA phase due to wetting coefficients. The specific localization of the CNTs within the different blends is shown by the SEM images in Figure 3. On the basis of our experience, we could differ between the PA phase, which is the ductile fracture phase, and the PP phase. The CNTs in the binary PA/PP blend were exclusively localized in the PA phase (Figure 3a), as was predicted form the wetting coefficients analysis. The addition of unmodified high-viscosity EMAA1 caused droplet morphology (Figure 3b); the causes of this morphology were discussed earlier. A high concentration of CNTs was confined at the interphase when pyridine-modified mEMAA1 was used (Figure 3c). SEM images of the lowviscosity EMAA95 and modified mEMAA95 (Figures 3d and 3e, respectively) showed a concentration of CNTs in a thin layer located between the PA and the PP main phases. This thin layer was the methacrylic acid copolymer. The impressive confirmation of the claimed hierarchical structure is illustrated in a series of TEM images. Figure 4a shows that CNTs are localized in the PA phase in a binary PA/ PP blend. When a pyridine-modified mEMAA1 was introduced to the blend, most of the CNTs were confined within the

γCNT−B − γCNT−A γAB

ωAB−CNTa

polymers

(2)

where γCNT‑i is the interfacial tension between the CNTs and the different polymers. When ω > 1, the CNTs are located in 1855


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Figure 3. SEM images for the different CNTs ternary blends nanocomposites, taken at high magnification for the characterization of the CNT localization: (a) PA/PP blends, (b) PA/PP/EMAA1, (c) PA/PP/mEMAA1, (d) PA/PP/EMAA95, and (e) PA/PP/mEMAA95.

the pyridine pendent group, improved the percolation network structure and enabled lower volume resistivity. The storage modulus of the molten polymer, in the lowfrequency region, is very sensitive to changes in the microstructure of the material, and as such, may serve as an indication for changes in blend morphology as well as the state of dispersion of the CNTs. Figures 6a and 6c present the storage modulus and complex viscosity of different blend compositions without CNTs and with 1 wt % CNT loadings. First, changes in the behavior of the G′ and |η*| curves in the low frequency range were observed for the EMAA1-based composite and, to a greater extent, for the mEMAA1-based composite. The increase in the storage modulus for cocontinuous blends is usually attributed to the refinement of the cocontinuous structure induced by the third component.30 The EMAA1, however, seemed to bring about a morphological transformation from fully cocontinuous to partially dispersed domains (as seen in the SEM images and selective extraction results); this transformation also may contribute to the increased elasticity.31 In contrast, the mEMAA1 polymer kept the cocontinuous structure intact and further refined it by spreading at the interface. This change was expressed by the appearance of a plateau in the storage modulus curve. The same trend was observed for the 1 wt % CNT-filled ternary blends, although the elasticity plateau produced by the mEMAA1 composite was significantly higher compared to the EMAA1 composite and the binary PA/PP composite. The viscoelastic behavior of the mEMAA1 composite correlated well with its

mEMAA1 phase (Figures 4b, 4c, and 4d). The mEMAA1 phase by itself was cocontinuous and spread along the PA/PP interface. Because of density differences between the mEMAA1, PA, and PP phases, a high-magnification image (Figure 4c) clearly showed a clear boundary, confirming that the mEMAA1 was spread at the PA/PP interface and that the CNTs preferred to be localized in the mEMAA1 phase or close to it. These results indicate that such hierarchical structures are, in fact, feasible using conventional melt mixing techniques. 3.4. Electrical and Rheological Properties. The existence of an effective CNT pathway along the interface was reflected in the electrical resistivity of the different composites (Figure 5). Three important results should be noticed. First, the blend composed of the high-viscosity EMAA (EMAA1) exhibited very high resistivity compared to the binary blend. This higher resistivity was attributed to poorer percolation network formation with the EMAA1-based blend’s droplet morphology. Second, confining the CNTs into a specific thin layer of EMAA caused a significant reduction in the volume resistivity of the composite from 1.6 × 1011 to 3.5 × 109 Ω·cm. This behavior could be attributed to the formation of a confined and efficient CNT percolation network along the EMAA pathway, as compared to a percolation network through the volume of the PA phase. Third, the modified EMAA (such as mEMAA95)-based composites exhibited lower volume resistivity than composites based on unmodified EMAA (4 × 106 compared to 3.5 × 109 Ω·cm). Thus, the efficient confinement of the CNTs, induced by π−π interactions with 1856


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Figure 4. TEM images of (a) PA/PP composite where CNTs are located only in the PA phase with no preference toward the interface and (b−d) PA/PP/mEMMA1 composite where CNTs are located in the PA phase, with a high concentration in the mEMAA1 interphase due to the spreading of the pyridine-modified mEMAA1 and its high attraction toward CNTs.

modulus in the low-frequency region was magnified and less frequency dependent for all the blends. The low-frequency G′ and complex viscosity curves of the mEMAA95 third component in the case of the composite ternary blends are distinct in comparison to the CNT-filled EMAA95 and PA/PP blends; this blend exhibits higher viscosity, higher elasticity, and more of a solidlike behavior. As discussed earlier, the AMP modification decreases the total viscosity of the blend; therefore, the behavior observed for the mEMAA95 must be related to the state of dispersion and localization of the CNT at the blends interface. The effect of the pyridine modification on the rheological response is summarized in Figure 7. The addition of EMAA only slightly increased the storage modulus of the composite compared to a simple binary PA/PP CNT composite. In contrast, when a pyridine-modified EMAA minor phase was introduced, there was a dramatic increase in the storage modulus values as well as a flatting of the curve at the terminal region (low-frequency region).

Figure 5. Electrical volume resistivity for the different composites studied in this work. All samples were loaded with 1 wt % CNTs, and differ by the polymer composition (either 50/50 v/v PA/PP or 47/47/ 6 v/v/v PA/PP/EMAA).

low electrical resistivity and serves to support our hypothesis that, in this case, well-dispersed CNTs are localized in the mEMAA1 interphase, which is spread at the interface between the PA and PP major phases. The addition of EMAA95 (Figure 6b) resulted in minor changes to the G′ and |η*| curves. Conversely, the addition of mEMAA95 (Figure 6d) yielded a general reduction in the elasticity and viscosity of the melt. It also produced a slight flattening of the storage modulus curve in the terminal frequency range. When 1 wt % CNTs were added, the storage

4. CONCLUSIONS An hierarchical structure based on a ternary polymer blend composed of PA and PP as the two major continuous phases and EMMA as a third minor component which was also continuous along the PA/PP interface was fabricated by direct melt mixing. In order to confine CNTs in the EMAA phase and form a low percolation threshold network along the blend 1857


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Figure 6. Storage modulus and complex viscosity for the different ternary blends, without CNTs (open symbols) and with 1 wt % CNTs (filled symbols).

namic theories such as the spreading coefficient. Pyridine modification of the higher viscosity EMAA enabled spreading of EMAA at the interface because the modification reduced the viscosity of the copolymer. This work set the concept and guidelines toward the fabrication of other hierarchical structures with unique physical properties, composed of ternary polymer blend and nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

Figure 7. Effect of the third polymeric component on the storage modulus of the different ternary blend composites containing 1 wt % CNTs.

*E-mail: [email protected]. Author Contributions §

These authors contributed equally.

Notes

interfaces, the CNTs were premixed with the EMAA and the EMAA was modified with amino(methylpyridine), which forms noncovalent π−π interactions with the CNTs. The hierarchical ternary polymer blend morphology, where EMAA is spread on the PA/PP phase, was predicted using spreading coefficient theory, while wetting theory predicted 2.6 mN/m lower interfacial tensions between CNTs and pyridine-modified EMAA compared with pristine EMAA. It was found that viscosity variations affected the resultant morphology. Higher viscosity of the minor EMAA component depressed spreading at the PA/PP interface, and low-viscosity EMAA encouraged interfacial spreading; thus, kinetics considerations affect the theoretically obtained morphology as predicted by thermody-

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Mark Schnider and Mr. Guy Goldberg from the Weizmann Institute for their help with SEM imaging. The authors thank Yona Lichtenfeld and Dr. Einat Nativ-Roth from the Ilse Katz Institute (IKI - Ben Gurion University) for the TEM imaging.

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REFERENCES

(1) Kim, J. Y.; Park, H. S.; Kim, S. H. J. Appl. Polym. Sci. 2007, 103, 1450−1457. 1858


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Hierarchical Structures Composed of Confined Carbon Nanotubes in

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