Tailor-Made Distribution of Nanoparticles in Blend Structure toward

Research Article www.acsami.org

Tailor-Made Distribution of Nanoparticles in Blend Structure toward Outstanding Electromagnetic Interference Shielding Sourav Biswas, Goutam Prasanna Kar, and Suryasarathi Bose* Department of Materials Engineering, Indian Institute of Science, Bangalore, India 560012 ABSTRACT: Engineering blend structure with tailor-made distribution of nanoparticles is the prime requisite to obtain materials with extraordinary properties. Herein, a unique strategy of distributing nanoparticles in different phases of a blend structure has resulted in >99% blocking of incoming electromagnetic (EM) radiation. This is accomplished by designing a ternary polymer blend structure using polycarbonate (PC), poly(vinylidene fluoride) (PVDF), and poly(methyl methacrylate) (PMMA) to simultaneously improve the structural, electrical, and electromagnetic interference shielding (EMI). The blend structure was made conducting by preferentially localizing the multi-wall nanotubes (MWNTs) in the PVDF phase. By taking advantage of “π−π stacking” MWNTs was noncovalently modified with an imidazolium based ionic liquid (IL). Interestingly, the enhanced dispersion of IL-MWNTs in PVDF improved the electrical conductivity of the blends significantly. While one key requisite to attenuate EM radiation (i.e., electrical conductivity) was achieved using MWNTs, the magnetic properties of the blend structure was tuned by introducing barium ferrite (BaFe) nanoparticles, which can interact with the incoming EM radiation. By suitably modifying the surface of BaFe nanoparticles, we can tailor their localization under the macroscopic processing condition. The precise localization of BaFe nanoparticles in the PC phase, due to nucleophilic substitution reaction, and the MWNTs in the PVDF phase not only improved the conductivity but also facilitated in absorption of the incoming microwave radiation due to synergetic effect from MWNT and BaFe. The shielding effectiveness (SE) was measured in X and Ku band, and an enhanced SE of −37 dB was noted at 18 GHz frequency. PMMA, which acted as an interfacial modifier in PC/PVDF blends further, resulting in a significant enhancement in the mechanical properties besides retaining high SE. This study opens a new avenue in designing mechanically strong microwave absorbers with a suitable combination of materials. KEYWORDS: co-continuous polymer blends, MWNT, Ionic liquids, BaFe, mechanical properties, rheology, AC electrical conductivity, EMI shielding

■

INTRODUCTION Extensive use of electronic equipment brings with it the challenge of developing proper shielding materials for their proper functioning1−3 as the emitted electromagnetic (EM) radiation from nearby devices interferes with the functioning of adjacent ones. Hence, much attention has been paid to fabricate electromagnetic interference (EMI) shielding materials.4−10 Metals are used for their high conductivity as shielding materials, but generally, metals have high density, are prone to corrosion, and are difficult to process. Further, EM radiation is not fully eliminated because EM waves are reflected from the metal surface and interfere with nearby devices, causing further interference. Electromagnetic radiation is the synchronized propagation of an electric field and a magnetic field perpendicular to each other. In this context, to circumvent such consequences, substantial efforts have been made to design lightweight EMI shielding materials where polymeric nanocomposites have already took an emergent position.11−17 The shielding of EM radiation by CPC (conducting polymer composite) is known to occur via three mechanisms: reflection, absorption, and multiple reflection. Reflection is the primary mechanism shown by conducting materials because the mobile © 2015 American Chemical Society

charge (or hole) can interact with the EM radiation. Absorption takes place when materials with electric and magnetic dipole interact with the EM radiation. However, absorption is dependent on the dimension (thickness) of the materials, and shielding effectiveness drops with decreasing thickness. Exploiting the high aspect of carbon nanotubes (CNTs), insulating polymer matrix can be made conducting and, hence, shows promising application in EMI shielding. A large number of studies have been carried out using carbon nanotube incorporated polymer matrix in this context.3,18−22 However, the intertube van der Waals force results in agglomeration, which reduces the effective aspect ratio and hence the bulk electrical conductivity. In this context, surface modification can play a key role in debundling them and enhancing the interfacial interaction between the constituents. Generally, the electrical conductivity of multi-wall nanotubes (MWNTs) mainly depends on their π-conjugation, but during covalent functionalization, this π-conjugation is greatly affected. So, the Received: September 5, 2015 Accepted: October 29, 2015 Published: October 29, 2015 25448

DOI: 10.1021/acsami.5b08333 ACS Appl. Mater. Interfaces 2015, 7, 25448−25463

Research Article

ACS Applied Materials & Interfaces more efficient noncovalent functionalization preserves the structural integrity of MWNTs.23−25 It is envisaged that the conducting nanoparticles attenuate EM radiation by reflection though; multiple scattering through the network of high aspect ratio particles like MWNTs cannot be ruled out either. As we discussed earlier that absorption of EM waves is entertained only when materials with electric or magnetic dipoles interact with the EM radiation. In this context, various ferromagnetic and/or dielectric particles have been used in the polymer matrix along with the conducting particles.7,26−35However, due to agglomeration; these magnetic nanoparticles interfere with the conducting network of MWNTs, thereby reducing the overall conductivity. Blending immiscible polymers could be a superior choice to design EMI shielding materials as nanomaterials with different specific characteristic can be localized in a given phase. In addition, the effective concentration of the particles can be increased significantly by localizing them in a given phase of cocontinuous blends.36 Rohini et al.37 designed immiscible PS/ PMMA blend filled with amine, and pristine MWNTs and achieved −24 dB of SE. Pawar et al.26 designed immiscible PC/ SAN blend with MWNT grafted Fe3O4 and achieved −32.5 dB of SE in which 51% attenuation was by absorption. Kar et al.32 designed immiscible PVDF/ABS blend with IL-MWNT and BT-GO nanoparticles where the total shielding was −26 dB of which 40% attenuation was by absorption. In our recent work,11 we designed immiscible PVDF/SAN blend containing cobalt ferrite anchored onto MWNT via pyrene derivative which showed an impressive SE of −35 dB of which 70% attenuation was by absorption. One major concern using binary immiscible blends is their poor interface, which results in mechanical failure and limits their use in in structural applications. In light of the existing challenges, we employed a co-continuous PC/PVDF blends as a model system to prepare EM shielding materials using different nanoparticles and employed an interfacial modifier, PMMA in this case, to strengthen the interface. PMMA is mutually soluble in PC and PVDF and, hence, improves the interfacial adhesion between the entities.38−40 The electrical conductivity and the EMI SE was evaluated in a broad range of frequencies. The state of dispersion of the nanoparticles in the blends was assessed using electron microscopy and selective area EDS mapping. The mechanism of shielding is discussed herein.

■

Scheme 1. Schematic Representation of the Synthesis Route to Prepare Ionic Liquid and Ionic Liquid Modified MWNT

Preparation of Ionic Liquid Modified MWNT (IL-MWNT). Imidazolium based ionic liquids have very strong interaction with carbon based materials. Here MWNTs were noncovalently modified by the synthesized imidazolium based ionic liquids due to strong π−π stacking between the imidazole moieties of IL and the delocalized πcloud in MWNTs. Typically, 1:2 wt ratio of MWNTs and synthesized IL was mixed in DMF and subsequently dispersed by bath sonication for 2 h. After vigorous stirring for 10 h the resultant mixture was kept for another 10 h for aging followed by centrifugation in DMF several times to removed unattached IL. Finally, the solvent was evaporated by vacuum drying at 80 °C and IL-MWNTs was obtained (Scheme 1).32 Synthesis of BaFe-NH2 Nanoparticles. A two-step procedure (Scheme 2) is followed for synthesis of amine terminated BaFe nanoparticles. By starting with the dispersion of BaFe particles in H2O2 by bath sonication, the dispersed solution was refluxed for 4 h at 105 °C followed by vacuum drying. Later, the as prepared hydroxylated BaFe nanoparticles was refluxed again in the presence of APTS at 80 °C for 24 h. Centrifugation of the obtained mixture followed by washing with toluene (four times) to remove the excess APTS yields BaFe-NH2. Finally, the solvent was evaporated under vacuum. Synthesis of PC-BaFe Composites. Amine-terminated BaFe nanoparticles, as prepared in the previous step, can react with the ester groups of PC via nucleophilic substitution, resulting in BaFe terminated PC chain. Typically, PC was dissolved in DMF, the amine-terminated BT nanoparticles were dispersed in the solution, and the resultant solution bath was sonicated for 30 min. After 24 h of refluxing at 150 °C, the resultant reaction mixture was poured onto a Teflon sheet and vacuum-dried at 80 °C. This step was done to restrict the BaFe nanoparticles in the PC phase which otherwise might migrate to the thermodynamically favored phase, here PVDF. Blend Preparation. First, 60/40 (w/w) PC/PVDF blends were mixed at 260 °C temperature, 60 rpm rotational speed in a Haake minilab-II mini-mixer under N2 atmosphere for 20 min. The nanoparticles were added directly with the constituent polymers in the extruder and are schematically described in Scheme 3a. To restrict the nanoparticles in a given phase, we made a few compositions where we used the previously synthesized BaFe nanoparticles functionalized with PC and mixed with PVDF and MWNTs into the extruder (Scheme 3b). This will eventually lead to localization of IL-MWNTin the PVDF phase and BaFe in the PC phase. For a few compositions, 10 wt % PMMA was also added to strengthen the interface of PC/ PVDF blends. Characterizations. Tecnai G2F30 at 300 kV was used to acquire transmission electron microscopy (TEM) images on various nanoparticles. ULTRA 55 SEM with an accelerating voltage of 10 kV was used to assess the morphology of various blends. A Lakeshore Vibratory Sample Magnetometer (VSM) was used to determine the magnetic property of the samples with an applied force of −8000 to

EXPERIMENTAL SECTION

Materials. Poly(vinylidene fluoride) (KYNAR-761) (molecular weight 440 000 g mol−1) was procured from Arkema. Polycarbonate (Lexan-143R) with melt flow index of 11gm/10 min was acquired from Sabic. PMMA (Guzpol-P, 876G) with melt flow index of 6g/10 min was procured from GSFC, India. Pristine MWNTs (length 1.5 μm and diameter 9.5 nm) was procured from Nanocyl SA (Belgium). Barium ferrite (BaFe12O19) (325 mesh), 3-aminopropyltriethoxysilane (APTS),1-(3-Aminopropyl)imidazole,3-Bromopropylaminehydrobromide(98%) and H2O2(29−32 wt %) were procured from SigmaAldrich. Analytical grades of N,N-Dimethylfomamide, dry ethanol, tetrahydrofuran, chloroform, and ethyl acetate solution were acquired from commercial sources. Synthesis of Ionic Liquid. 1-(3-Aminopropyl)imidazole (10 mL) and dry ethanol (20 mL) was mixed in a two-necked round-bottom flask and stirred thoroughly under nitrogen atmosphere at room temperature for 1 h. 3-Bromopropylaminehydrobromide was subsequently added into the mixture and refluxed at 80 °C for 24 h under nitrogen atmosphere. Finally, the synthesized ionic liquid was separated, followed by precipitation with ethyl acetate solution ( Scheme 1).32 25449


Research Article

ACS Applied Materials & Interfaces Scheme 2. Synthesis of PC-BaFe Composite by Solution Blending

Scheme 3. Various Protocols Adopted during Blend Preparation

8000 Oe at room temperature. PerkinElmer GX FT-IR instrument was used to acquire FT-IR spectra in the range of 4000−400 cm−1. X-ray diffraction was recorded using a XPERT Pro from PANalytical. A Cu Kα radiation source (λ = 1.5406 Å, 40 kV and 30 mA) was used to determine the XRD profile of BaFe nanoparticles. A universal tensile testing machine (Instron) was used to determine the mechanical properties of various blends at room temperature. For tensile testing, five dumbbell-shaped compression molded samples were previously prepared in each individual blends. All the experiments were done with a constant cross head speed of 5 mm min−1. All the dumbbell-shaped specimens were kept overnight in a vacuum oven for drying at 80 °C. A discovery hybrid rheometer (DHR-3, TA Instrument) was used to determine the flow characteristics of various blends. All the experiments were done by using a 25 mm parallel plate geometry at 260 °C temperature under nitrogen atmosphere. Prior to rheological study all the specimens were kept overnight in a vacuum oven for drying at 80 °C. Alpha-N Analyzer, Novocontrol (Germany), was used to acquire AC electrical conductivity of various blends in a frequency range of 0.1 Hz to 10 MHz. The disk samples were made by compression molding at 260 °C and uniformly polished prior to measuring the AC electrical conductivity. EM shielding interference was studied by Anritsu MS4642A vector network analyzer (VNA). Damaskos MT-07 was used as toroidal sample holder and was connected with VNA for measurements. Prior to experiment, the full setup was calibrated by full SOLT (short-openload-through). Generally, 5 mm toroidal samples were prepared by compression molded at 260 °C temperature. S parameters (S11, S12, S22, and S21) were measured in X and Ku-band of frequency for all the measurements. We measured total shielding efficiency and minimum reflection loss in dB unit.

Figure 1. (a) FT-IR spectra of various nanoparticles, TEM images of (b) MWNT and (c) IL-MWNT (arrow shows the attached IL on MWNT surface). (d) State of dispersion of (i) MWNT and (ii) ILMWNT in DMF. (e) XRD profile of BaFe, (inset i) TEM micrograph of BaFe, and (inset ii) lattice plane spacing of BaFe.

■

MWNTs. A prominent peak at 3494 cm−1 corresponding to the N−H stretching frequency, clearly indicate the attachment of amine in the ionic liquid. Further, peaks at 2998, 1620, 1460, and 1250 cm−1 corresponding to C−H stretching frequency of alkanes, N−H bending vibration, C−H bending vibration of alkanes, and C−N stretching frequency of aliphatic amines, respectively, clearly show the successful synthesis of IL with NH2 terminal groups. Further, the presence of IL-NH2 on the surface of MWNTs is well observed from the TEM micrograph. Figure 1b, shows that the TEM micrograph of pristine MWNTs

RESULTS AND DISCUSSION Characterization of Various Nanoparticles. Imidazolium based ionic liquids interact strongly with carbon based materials. From FT-IR and TEM images it is well evident that MWNTs were noncovalently modified by the IL due to strong π−π stacking. The synthesis of imidazolium based amine terminated ionic liquids is described in Scheme 1. FT-IR spectra (Figure 1a) support the successful synthesis of amine terminated ionic liquids and its presence on the surface of 25450


Research Article

ACS Applied Materials & Interfaces

Figure 2. Scanning electron microscope images of (a) 60/40 PC/PVDF blend (neat), (b) with 10 wt % PMMA, (c) with 2 wt % IL-MWNT, (d) with 2 wt % IL-MWNT + BaFe in PVDF, (e) with 2 wt % IL-MWNT in PVDF + BaFe in PC and (f) with 2 wt % IL-MWNT in PVDF + BaFe in PC + 10 wt % PMMA. Magnified SEM micrograph of (g) 2 wt % MWNT, and (h) 2 wt % IL-MWNT. (i) Selective dissolution test of MWNT containing sample (1) dissolved in chloroform to etch out PC phase and (2) dissolved in DMF to etch out PVDF phase.

and Figure 1c corresponds to ionic-liquid-modified MWNTs. The higher magnification TEM image shown in the inset of Figure 1c clearly supports the attachment of IL onto MWNTs.

This surface modification can improve the quality of dispersion of MWNTs substantially as see from Figure 1d. The MWNTs which was dispersed in DMF by bath sonication starts settling 25451


Research Article


Figure 3. (a) Selective dissolution of 2 wt % IL-MWNT + 5 wt % BaFe in PVDF phase (i) dissolved in chloroform to etch out PC phase and (ii) dissolved in DMF to etch out PVDF phase. (b) EDS analysis of the blend containing 2 wt % IL-MWNT + 5 wt % BaFe in PVDF phase (the samples were etched with chloroform). (c) Selective dissolution of 2 wt % IL-MWNT in PVDF + 5 wt % BaFe in PC phase (i) dissolved in chloroform to etch out PC phase, (ii) dissolved in DMF to etch out PVDF phase, and (iii) obtained chloroform solution in an external magnetic field. (d) EDS spectra of 2 wt % IL-MWNT in PVDF + BaFe in PC phase without etching any phases. (e) EDS spectra of 2 wt % IL-MWNT in PVDF + BaFe in PC after etching out PC phase by chloroform.

down after 30 min whereas, IL modified MWNTs remain quite well suspended even after 30 days after the initial bath

sonication (Figure 1d, vial ii). The synthesis of amine terminated BaFe nanoparticles, as described in Scheme 2, was 25452


Research Article

ACS Applied Materials & Interfaces also confirmed by FT-IR and TEM. A broad peak at 3400 cm−1 corresponding to N−H stretching is clearly evident in BaFeNH2 and which is evidently absent in BaFe nanoparticles. The peak at 3410 cm−1 corresponds to the symmetric and asymmetric stretching vibration of weakly bound water molecules. Peaks at 1570, 1460, and 1120 cm−1 appear only in BaFe-NH2 corresponding to N−H bending vibration, C−H bending vibration of alkanes, and C−N stretching frequency of aliphatic amines, respectively. This clearly supports the successful synthesis of amine-terminated BaFe nanoparticles. Figure 1e shows the XRD profile of BaFe particles. The main diffraction peaks appears at 2θ value of 31.36, 33.22, 35.16, 38.12, 41.32, 43.46, 56.08, and 64.12°, which correspond to the (110), (107), (114), (203), (205), (206), (217), and (220) reflections. The average diameter of BaFe nanoparticles are about 100 nm, as shown inset i of Figure 1e. The XRD result is also supported by the lattice plane spacing of 0.26 nm, as shown in inset ii of Figure 1e, corresponding to the (114) plane of BaFe.41 Selective Localization of Nanoparticle and Refinement in Morphology. Blending of polymers is an efficient way not only for developing materials with precise control in properties but also for recycling. Immiscibility, either chemical incompatibility or lack of specific interaction usually leads to heterogeneous morphologies. The final properties of the blends are contingent upon specific interaction between constituents and the obtained morphology. So, monitoring the phase morphology during blending is the deciding factor of the final properties of the blend. Co-continuous morphology is observed here in case of control 60/40 (w/w) PC/PVDF blends (Figure 2a). It should be noted that in all the cases, SEM morphology was obtained after selectively etching the PC phase by chloroform (for 72 h) to improve the contrast between the phases. A refinement in co-continuous morphology was observed after incorporation of 10 wt % PMMA, asserting the key role of PMMA as an interfacial modifier in the blends (Figure 2b). It is important to note that after addition of different nanoparticles, the co-continuous morphology was well retained (Figure 2c−f). The preferential localization of nanoparticles in a given phase of immiscible polymer blend is generally governed by the various parameters such as specific interaction, flow properties, and thermodynamics. Sumita et al.36 proposed that the localization of nanoparticles is greatly controlled by the surface free energies associated with the blend components and the fraction can be estimated by the wetting coefficient. From our previous study, we observed that, though PC is the thermodynamically favored phase, nanoparticles are localized at PVDF phase due to its high polarity.22 The magnified SEM micrographs reveal that MWNTs and ILMWNTs are preferentially dispersed in the PVDF phase of the blend (Figure 2g,h). The selective dissolution experiments also support this observation (Figure 2i). Here, MWNTs containing blend sample was dissolved in chloroform (to remove the PC phase) and sonicated for 10 min to examine the change in color. The obtained solution remains colorless; however, the solution turned black immediately after dissolving in DMF, revealing the fact that MWNTs are selectively localized in the PVDF phase. The higher magnification SEM micrographs also revealed better dispersion of IL-MWNTs as compared to MWNTs in the PVDF phase. Figure 2g clearly shows the agglomeration of MWNTs in the PVDF phase whereas, ILMWNT are nicely distributed (Figure 2h, arrows indicates the location of IL-MWNTs). So, it is clear that strong π−π stacking

between the imidazolium based ionic liquids and the delocalized π-cloud in MWNTs decreases the intertube van der Walls force of attraction between the MWNTs. From the above discussion, it is confirmed that IL-MWNTs are preferentially localized in the PVDF phase and the addition of BaFe nanoparticles does not alter the obtained cocontinuous morphology in the blends. Therefore, in this section we focus our discussion on the localization of BaFe nanoparticles using EDS elemental analysis and selective dissolution tests. It is envisaged that if all the components are mixed together in the melt-mixer (as described in Scheme 3a), BaFe and MWNTs will localized in the PVDF phase due to its higher polarity as compared to PC. Hence, to confirm this hypothesis we have prepared a composition where both MWNTs and BaFe has been mixed along with PC and PVDF in a one-step mixing protocol. The selective dissolution test performed on this particular blend clearly indicated that BaFe nanoparticles are preferentially localized in the PVDF phase (Figure 3a). The presence of BaFe in PVDF is further confirmed by EDS analysis as well (Figure 3b). So, to localize the BaFe nanoparticles in the PC phase, we first synthesized PC nanocomposites, taking the advantage of nucleophilic substitution reaction between BaFe-NH2 and the ester groups in PC. This PC/BaFe composite was then mixed with PVDF during blending in the next step (as described in Scheme 3b). It is worth noting that this two-step protocol did not affect the cocontinuous morphology, as discussed earlier (Figure 2d). The selective dissolution of the PC phase resulted in a gray solution, which clearly indicated the presence of BaFe nanoparticles in the PC phase of the blend. Further support comes from the magnet tests. We extracted the gray solution after dissolving the PC phase and exposed the vial to a magnet, and the BaFe nanoparticles, which were not seen earlier, can be clearly seen to be attracted to the magnet. This test clearly asserts the preferential localization of BaFe nanoparticles in the PC phase (Figure 3c). More interestingly, when the PVDF phase was selectively dissolved, the solution turned black, indicating the presence of MWNTs in the PVDF phase. The EDS elemental analysis further supported the selective localization of the nanoparticles. Figure 3d depicted the EDS mapping obtained from the blend without etching any of the phases and the corresponding SEM micrograph is shown as an inset. The EDS mapping clearly indicates the presence of BaFe nanoparticles whereas, Figure 3e confirms the absence of any BaFe nanoparticles in the remaining PVDF phase. In the corresponding SEM micrograph, shown as an inset in Figure 3e, the PC phase was etched out. Taken together, it is evident that if all the entities are mixed together in a one-step mixing protocol, both the nanoparticles will localize in the PVDF phase. This will eventually reduce the bulk electrical conductivity of the blends as the inclusion of BaFe interferes with the network formation of MWNTs in any given phase. However, by compartmentalizing the nanoparticles in different phases, the bulk electrical conductivity can be retained and will be discussed later. Effect of Interfacial Modifier and Nanoparticles on the Viscoelastic Behavior. Melt rheology is a useful tool to understand the flow characteristics which is important from the processability of the composites.42−47 But in immiscible polymer blends, the characteristics flow behavior is complex due to the interdependence of viscoelastic properties and obtained micro structures during blending. The state of dispersion of the nanoparticles can also be comprehended 25453


Research Article


Figure 4. (a) Complex viscosity of neat PVDF, neat PC, neat 60/40 PC/PVDF and 60/40 PC/PVDF blends with 10 wt % PMMA. (b) Complex viscosity of various 60/40 PC/PVDF blends with or without fillers. (c) Cartoon illustrating the effect of PMMA on the refinement of co-continuous morphology.

from the viscoelastic properties of the blend. Figure 4a,b shows the complex viscosity of the constituents and various PC/ PVDF blends at the processing temperature. It is observed that at 260 °C, all the investigated components exhibit shear thinning behavior at lower frequency except PC, which showed almost Newtonian behavior in the measured frequency. It is also well observed that the complex viscosity of PVDF is much higher than PC however, the particles prefer to localize in the PVDF phase due to its higher polarity. Now, interestingly, after the addition of 10 wt % PMMA, the complex viscosity marginally decreases with respect to the neat blends. Generally, the less viscous component tends to encapsulate the higher one48 and it is envisaged that here PMMA encapsulates the PVDF phase (as ηPVDF > ηPMMA, from Figure 4a) and prevents coarsening during melt blending (as shown in Figure 4c). Furthermore, by using Harkins spreading theory, it can be calculated whether the phases are dispersing individually or complete engulfing using the following equation: λPMMA/PVDF = γPC/PVDF − γPMMA/PC − γPMMA/PVDF

Table 1. Surface Free Energies of Various Components at 260 °C

(1)

(2)

where, γ is the dispersing component and γ is the polar component of the total interfacial tension. According to the surface free energies listed in Table 1, λPMMA/PVDF is 1.308 which clearly indicates the presence of PMMA at the blend d

γ (mJ m−2)

γd (mJ m−2)

γp (mJ m−2)

PC PVDF PMMA

26 19.52 22.86

20.06 14.12 17.86

5.94 5.4 5

interface by encapsulating the PVDF phase. This explains the refinement of co-continuous morphology as observed from the SEM micrographs. Interestingly, addition of 1 wt % MWNT enhances the complex viscosity significantly. It is also well observed that the enhancement is more pronounced at lower frequency region where sufficient time is offered for chain relaxation. This observation clearly indicates that network of MWNTs can constrain the macromolecular motion in a given flow field. It is also observed that complex viscosity of the blends scales with MWNT concentration, and interestingly, the addition of ILmodified MWNTs further enhances the complex viscosity. The three-dimensional network structures of MWNTs facilitated by IL impede the motion of macromolecules toa greater extent. Introducing BaFe nanoparticles further enhanced the complex viscosity; however, with the addition of PMMA the viscosity decreased marginally as described earlier. Synergistic Improvement in Structural Properties: Effect of Nanoparticles and PMMA. Mechanical properties of multi component system are strongly contingent on the phase morphology and interfacial adhesion between the

where γ corresponds to the interfacial tension between the pair of polymers and diffusion coefficient is λ. The interfacial tension can be evaluated using the following equation: γij = γi + γj − 4[(γi dγjd /γi d + γjd) + (γi pγj p/γi p + γj p)]

components

p

25454


Research Article


Figure 5. (a) Stress−strain behavior, (b) Young’s modulus, (c) tensile strength, and (d) elongation at break of various blends. (e) Cartoon illustrating the effect of PMMA in strengthening the interface.

components.49−52 Elongation at break is a measure of interfacial adhesion, which decides the fracture toughness. Chemical and thermodynamic incompatibility in uncompatibilized blend results in poor interfacial adhesion between the constituents. So, the inefficient stress transfer at the interface yield in poor mechanical properties. In this context, an interfacial modifier (compatibilizer) can reduce the interfacial tension and render finer structures.44,53−55 The 60/40 PC/ PVDF blend displayed very poor mechanical properties due to its chemical incompatibility and poor interfacial adhesion (Figure 5a−d). However, incorporation of 10 wt % PMMA significantly improved the mechanical properties of the blend. For instance, the tensile strength improved by ca. 14%, the Young’s modulus increased by ca. 6%, and impressively, the elongation at break increased by ca. 33%, as compared to neat blend. This clearly indicates the role of PMMA as a compatibilizer in immiscible PC/PVDF blend, as described schematically in Figure 5e. Further, it is well-known that mechanical properties in polymer nanocomposites is directly related to the hierarchical microstructure, which is controlled by filler properties.56,57 The reinforcing effect of MWNTs in PC/PVDF blend can be well appreciated from the tensile test. A significant improvement in tensile strength (ca. 10%) and

Young’s modulus (ca. 39%) was observed with addition of MWNTs whereas, a lack of interfacial adhesion between the constituents led to premature failure. The results are even more pronounced with the addition of IL-modified MWNT. In this case, the tensile strength enhanced by ca. 41% and Young’s modulus increased by ca. 15%. At higher concentration of MWNTs the Young’s modulus increases significantly. However, the addition of BaFe nanoparticles slightly decreases the mechanical properties due to chain scission of PC; though by addition of PMMA this effect can be minimized. Taken together, better interaction and efficient stress transfer at the interface region due to the addition of PMMA resulted in improved mechanical properties. Precise Location of Nanoparticles in the Blend: Effect on Bulk Electrical Conductivity. An interconnected network of nanoparticles is a primary requisite in enhancing the charge transport in insulating polymer blend.58−60 Selective localization of conducting nanoparticles in a given phase of an immiscible polymer blend enhances the effective concentration of the particles, which, in turn, results in improved conductivity.18,26,61From SEM micrograph and selective dissolution tests, it is well observed that MWNTs are selectively localized in the PVDF phase. From Figure 6a it is observed that 25455


Research Article


Figure 6. (a) AC electrical conductivity of various composites (conductivity of neat 60/40 PC/PVDF blends and a cartoon illustrating the percolating network is shown as inset). (b) AC electrical conductivity of various 60/40 PC/PVDF blends with MWNT and IL-MWNT. (c) A cartoon illustrating the effect of IL on the 3D network like structure of MWNTs. (d) AC electrical conductivity of various blends containing 2 wt % IL-MWNT wherein the BaFe nanoparticles are localized either in PVDF or in PC. (e) Selectively positioning IL-MWNTs in PVDF and BaFe nanoparticles in PC.

the conductivity of the blend has increased by several orders when MWNT content exceeds the critical concentration, known as percolation threshold. The percolation threshold in 60/40 PC/PVDF blend is in the range of 0−0.5 wt % of

MWNTs. Interestingly, PVDF-MWNT composites show insulating behavior at a similar MWNT content. So, by localizing the nanoparticles in the energetically favored phase decreases the percolation threshold significantly. Figure 6b 25456


Research Article


and 0.64 for IL modified MWNT at 2 wt %. This indicates that the charge transport is mainly through hopping in the former case and through tunnelling in the latter case. Magnetic and Dielectric Properties: Essential for Microwave Absorption. Electromagnetic radiation is a synchronized propagation of electric field and magnetic field perpendicular to each other. So to absorb such radiation, fence through dielectric and magnetic dipoles are required.63,64 Generally conducting nanoparticles are attenuate EM radiation through reflection due to lack of their dielectric and magnetic dipoles. Hence, magnetic and dielectric properties have been assessed here at room temperature. Figure 7a depicts that MWNTs shows negligible saturation magnetization (MS) of 0.34 emu g−1 which could be due to the presence of residual metal catalyst during synthesis. Very low remnant magnetization (Mr) and coercivity are observed (Figure 7b and Table 3). The saturation magnetization (MS) for BaFe is 7.7 emu gm−1(see Figure 7c). The effect of saturation magnetization is also directly related to permeability as explained in the equation μi = Ms2/(aK + bλξ), where constants a and b are determined by the material composition, λ is the magneto restriction constant, ξ is elastic strain parameter of the crystal, and K is the constant to decide the direction of easy axis.65 From the VSM plot, it is observed that saturation magnetization for BaFe is much higher than MWNTs, and hence, the permeability of the blend containing BaFe would be much higher. The remnant magnetization which is the measure of residual magnetization left after the external field is removed is higher in the case of BaFe (Figure 7d). The coercive field (Hc), which is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample, has been reached to saturation for BaFe is 2932 Oe. Generally, materials with higher Hc can increase the absorption of magnetic field associated with the incident EM waves through hysteresis magnetic losses. It is also envisaged that higher coercivity is mainly due to the magneto crystalline anisotropy, which mainly influences the resonance frequency of EM absorption.66 Further, the dielectric constant of BaFe nanoparticles was measured at room temperature as a function of frequency. Figure 7e shows that the BaFe nanoparticles possess a very high dielectric constant. So, with the addition of BaFe nanoparticles the probability of absorption is higher and will be discussed later on. Attenuation of EM Radiations through Compartmentalized Approach. The attenuation of EM radiation is analyzed by evaluating the total shielding effectiveness (SET) in X and Ku band. Generally, the SET is a quantitative measure of how much EM radiation is blocked by the shielding material. It is well-known that the material is capable to attenuate 99% of EM waves when the value of SET is 20 dB. As we discussed earlier, that material can shield by three different mechanisms, namely, shielding by absorption (SEA), shielding by reflection (SER), and shielding by multiple reflection (SEMR). So, SET is the combination of these three components. But due to the increasing absorption of reflected waves from the internal surface, the multiple reflections can be ignored when the shield thickness is greater than the skin depth.5 In this context SET can be expressed as,

depicted that, IL-MWNT showed higher electrical conductivity in contrast to MWNTs. Generally, the aspect ratio of the filler plays an essential role in determining the AC electrical conductivity in the blend. Though the aspect ratio of MWNTs is high, agglomeration due to van der Waals forces reduces the effective aspect ratio. To address this issue, surface functionalization of MWNTs by noncovalent modification is an efficient strategy that also preserves the structural integrity of MWNTs. The interconnected network of conducting pathway facilitates by IL is schematically described in Figure 6c. With the addition of BaFe nanoparticles, the conductivity of the blend decreased significantly. From the selective dissolution and EDS analysis, it was confirmed that BaFe nanoparticles are preferentially localized in the PVDF phase when all the constituents were mixed together. So, the presence of BaFe in PVDF impedes the conducting pathway between the nanotubes and presumably reduces the overall conductivity in the blends. Hence, we adopted a unique strategy wherein the BaFe nanoparticles were positioned in the PC phase via nucleophilic substitution reaction between the ester groups of PC and the NH2 functional groups in APTS modified BaFe nanoparticles. Hence, by selectively positioning IL-MWNTs in PVDF and BaFe nanoparticles in PC the electrical conductivity in the blends can be retained as can be seen from Figure 6d. This strategy is also schematically represented in Figure 6e. Further, addition of PMMA resulted in further increase in the overall electrical conductivity of the blends due to refinement in the co-continuous structure. This is an important result as we have observed earlier that PMMA also improves the stress transfer at the interface and results in better mechanical properties hence, introducing PMMA in PC/PVDF blend simultaneously improves both electrical as well as mechanical properties. Universal response of dielectric property in nanocomposites was established by Ngai at el.62 using a power law model. This is useful to understand the contribution of network formation of conducting nanoparticles which is responsible for various equivalent charge transfer phenomenon like capacitance and resistance. This quantitative analysis of dielectric behavior of various blends was further explored using the following equation, σ ′(ω) = σ(0) + σAC(ω) = σDC + Aωn

(3)

where σDC is the direct electrical conductivity, ω is angular frequency, A is constant depends on only temperature and n is the exponent which depends on both frequency and temperature. Generally, the value of exponent is in the range of 0−1. Solid lines in Figure 6b demonstrate the respective power law fit and provide different parameters such as DC electrical conductivity (σDC), crossover frequency (ωc), and exponent n. It is clearly evident that σDC and ωc increased for the blends with 2 wt % IL-MWNTs, as compared to a similar fraction of MWNTs (Table 2). The exponent n is the measure of threedimensional (3D) network of capacitor or resistor. The value of exponent decreases when the dispersion of MWNTs facilitates in effective charge transport through 3D network. The exponent value in the case of blends with MWNTs is 0.78 Table 2. Parameters Obtained from Power Law Fitting −1

SE T = SEA + SE R

compositions

σDC (S cm )

ωc (Hz)

n

2 wt % MWNT 2 wt % IL-MWNT

5.57 × 10−5 1.1 × 10−3

3.36 × 10−6 8.4 × 10−6

0.78 0.64

(4)

From the vector network analyzer (VNA), the SET can be estimated using the following relation through scattering parameters: 25457


Research Article


Figure 7. (a) Magnetic hysteresis loop and (b) coercive field of MWNTs. (c) Magnetic hysteresis loop, (d) coercive field, and (e) dielectric constant of BaFe nanoparticles.

From the EMI theory, it is well understood that in order to attenuate EM radiation, materials should have sufficient electrical conductivity. In the case of polymer nanocomposites, the connectivity of the conducting nanoparticles is essential for achieving high electrical conductivity. A SET of −4 dB (at a frequency of 18 GHz) was obtained for the 60/40 PC/PVDF blend with 0.5 wt % MWNTs, while the neat PC/PVDF blend and PVDF-MWNT composite (containing 0.5 wt % MWNT) are transparent to EM radiation. By selectively localizing the MWNTs in the PVDF phase, the bulk electrical conductivity of the blends can be improved which eventually facilitates in EM attenuation. It is also observed that SET scales with increasing concentration of MWNTs. Additionally, because of large specific surface area, finer mesh like structure can also assist in multiple scattering inside the network which can also enhance the attenuation. A maximum SET of −24 dB was obtained at 18 GHz frequency for 2 wt % IL-MWNT (Figure 8a). As we discussed earlier, generally, the shielding mechanism can be classified in three different ways. Here, the shielding by

Table 3. Different Magnetization Values of Various Particles particles

saturation magnetization (MS), emu gm−1

remnant magnetization (Mr), emu gm−1

coercivity (HC), Oe

Mr/Ms

MWNT BaFe

0.344 7.7

0.10 2.7

234 2932

0.294 0.351

SE T(dB) = 10log

1 2

|S12 |

= 10log

1 |S21|2

(5)

where, S12 is a reverse transmission coefficient; S21 is a forward transmission coefficient. With the help of these coefficients, we can express the total reflection and absorption using the following equations: SE R = 10log10(1/(1 − S112 ))

(6)

SEA = 10log10((1 − S112)/S12 2)

(7)

where, S11 is the forward reflection coefficient. 25458


Research Article


Figure 8. (a) SET, (b) % absorption and % reflection, (c) shielding by absorption as a function of frequency, (d) shielding by reflection as a function of frequency for various blends.

reflection is prominent when MWNT alone is considered due to its poor magnetic properties. In the case of blends with MWNTs and IL-MWNTs, the attenuation is mainly via reflection (ca.78%), as observed from Figure 8b. The SET is increased by several orders with the incorporation of BaFe nanoparticles. But the order of increment is more when the BaFe nanoparticles are precisely localized in the PC phase. It is now well understood that selective localization of BaFe nanoparticles in PVDF impedes the charge transport through the 3D network of MWNTs. But when these nanoparticles are selectively localized in PC, the conductive network is not disrupted which eventually resulted in higher attenuation properties. Interestingly, the addition of PMMA further enhanced the total shielding efficiency. More interestingly, the addition of BaFe nanoparticles not only enhances the SET values but also alters the mechanism of attenuation. From Figure 8c,d, it is clear that the addition of BaFe nanoparticles enhances the shielding through absorption (ca. 60%) for blends containing 2 wt % IL-MWNT in PVDF phase, 5 wt % BaFe in PC phase and 10 wt % PMMA. Total shielding effectiveness and shielding by reflection and absorption are summarized in Table 4. The EM radiation is mostly reflected from the conducting shield surfaces whereas, radiation penetrated inside can be attenuate through absorption. In this context, to investigate the probable mechanism of shielding of these samples, we measured thecomplex (relative) permittivity (εr) and the complex relative permeability (μr) by well-established line theory. If we considered the air field coaxial line, the reflection coefficient can be expressed as

Table 4. Different Shielding Parameters of Various Blends SET (dB)

compositions 0.5 wt % MWNT 1 wt % MWNT 2 wt % MWNT 0.5 wt % IL-MWNT 1 wt % IL-MWNT 2 wt % IL-MWNT 5 wt % BaFe in PVDF 5 wt % BaFe in PC 2 wt % IL-MWNT + 5 wt % BaFe in PVDF 2 wt % IL-MWNT + 5 wt % BaFe in PC 2 wt % IL-MWNT + 5 wt % BaFe in PC + 10 wt % PMMA

−4 −9 −18 −6 −14 −24 −8 −7 −26 −34 −37

SEA (dB)

SER (dB)

−4

−14

−5

−19

−10 −21 −22

−16 −13 −15

⎞ ⎛ μ μ Γ = (Z − Z0 /Z + Z0) = ⎜⎜ r − 1/ r + 1⎟⎟ εr ⎠ ⎝ εr

(8)

with t thickness, the transmission coefficient can be expressed as, Z = e − jω

με t

ω

= e(−j( c )

μr εr t )

(9)

εr is the complex (relative) permittivity and μr is the complex relative permeability. Z is the new characteristic impedance of specimen mounting region and Z0 is the characteristic impedance through coaxial line. From VNA, scattering parameters were observed by using coaxial setup and their sum and difference are stated as below 25459


Research Article


Figure 9. (a) Total loss tangent for various blends as a function of frequency. (b) Total shielding effectiveness with respect to thickness. (c) Flexible film of the best blend structure designed here. (d) Cartoon illustrating the mechanism of EM attenuation for blends containing 2 wt % IL-MWNT in PVDF phase and 5 wt % BaFe in the PC phase.

V1 = S21 + S11

(10)

V2 = S21 − S11

(11)

increase the EM absorption properties of the blend. The dielectric tangent loss is represented by tan δε, which is the ratio of ε″ and ε′, and the magnetic tangent loss can be represented by tan δμ, which is the ratio of μ″ and μ′. So, for better comparisons of EM absorption properties we estimated the consolidated loss tangent values (tan δε + tan δμ) and represented as a function of frequency. From Figure 9a, it is clearly observed that addition of BaFe nanoparticles significantly increases the total loss. Hence, the combination of higher relative complex permittivity, relative complex permeability and consolidated loss tangent values, the absorption efficiency of the blend has increased from 22% for MWNT containing blend to 60% for both MWNTs and BaFe-containing blend. The shield thickness has a significant effect on EM attenuation. To analyze the effect of shield thickness, we evaluated SET on toroidal specimen with different thickness for various blends. From Figure 9b, it is clear that SET scales with shield thickness. The effect is comprehensively evident in all blends irrespective of the type of nanoparticles. The best blend structure designed here; 2 wt % IL-MWNT in PVDF phase and 5 wt % BaFe in PC phase along with 10 wt % PMMA represented the highest SE at any given thickness and moreover, the films (1 mm thick) were flexible, as seen from Figure 9c. This observation is essential from a practical application point of view. A cartoon (Figure 9d) explains the EM attenuation mechanism schematically. It is now understood that the incident EM radiation is reflected back (ca. 40%) due to conducting surface whereas, the penetrated EM radiation is absorbed by BaFe nanoparticles and through internal reflection from the MWNTs surfaces. Multiple scattering, which is different from multiple reflection from the shield, has been reported in the case of

The reflection coefficient can also be estimated using the scattering parameters Γ=X±

X2 − 1

(12)

where X = (1 − V1V2)/(V1 − V2). μr /εr = [(1 + Γ)/(1 − Γ)]2 = c1

(13)

με = −[(c/ω t)ln(1/z)]2 = c 2 r r

(14)

So, the complex permittivity can be expressed as εr = (c1/c2)1/2 and complex permeability μr= (c1c2)1/2. In general, such complex parameters mainly consist of polarization and loss. Storage ability of electric and magnetic energy is the source of polarization, whereas dissipated electric and magnetic energy is the source of loss. It is well observed that due to heterogeneous inclusion, the shielding efficiency can be altered.67−69 When EM radiation interacts with such heterogeneous structures, the resulting local field variation can have strong effect on energy of absorption on such boundaries of heterogeneous nucleation as the absorption depends quadratically on the electric field intensity. Mathematically, these complex parameters are directly related to determine the shielding by absorption or reflection. The equation is SEA = 8.68t (πfσμ)1/2, whereas μ = μ0μr and σ = 2πfε0ε″. So, the shielding by absorption increases with higher values of μr and εr where μr = μ′ − jμ″ and εr = ε′ − jε″. Further from EM shielding theory, it is well understood that enhanced dielectric and magnetic tangent losses can also 25460



■

network of CNTs, where the incoming radiation suffers multiple scattering within the CNT network. Hence, in our case, as the IL modification has resulted in exfoliation of CNTs, multiple scattering may be possible within the exfoliated network. In addition, the presence of ferrite can attenuate the incoming radiation by absorption. Taken together, IL-modified MWNT and the presence of ferrite can absorb the microwave radiation significantly (60%). Barium ferrite possesses high saturation magnetization, high Curie temperature, and very high chemical resistivity but exhibits low dielectric loss. The latter can be tailored by either functionalizing with conducting polymers or by adding conducting nanoparticles such as CNTs. It is envisaged that barium ferrite contributes to the magnetic hysteresis, domain− wall displacement, and eddy current loss and is possibly one of the main reasons behind attenuating 99.9% of the incoming radiation.

REFERENCES

(1) Gupta, T. K.; Singh, B. P.; Dhakate, S. R.; Singh, V. N.; Mathur, R. B. Improved nanoindentation and microwave shielding properties of modified MWCNT reinforced polyurethane composites. J. Mater. Chem. A 2013, 1 (32), 9138−9149. (2) Arjmand, M.; Mahmoodi, M.; Gelves, G. A.; Park, S.; Sundararaj, U. Electrical and electromagnetic interference shielding properties of flow-induced oriented carbon nanotubes in polycarbonate. Carbon 2011, 49 (11), 3430−3440. (3) Huang, Y.; Li, N.; Ma, Y.; Du, F.; Li, F.; He, X.; Lin, X.; Gao, H.; Chen, Y. The influence of single-walled carbon nanotube structure on the electromagnetic interference shielding efficiency of its epoxy composites. Carbon 2007, 45 (8), 1614−1621. (4) Kumar, A.; Singh, A.; Kumari, S.; Dutta, P.; Dhawan, S.; Dhar, A. Polyaromatic-hydrocarbon-based carbon copper composites for the suppression of electromagnetic pollution. J. Mater. Chem. A 2014, 2 (39), 16632−16639. (5) Al-Saleh, M. H.; Sundararaj, U. Electromagnetic interference shielding mechanisms of CNT/polymer composites. Carbon 2009, 47 (7), 1738−1746. (6) Saini, P.; Choudhary, V.; Vijayan, N.; Kotnala, R. Improved electromagnetic interference shielding response of poly (aniline)coated fabrics containing dielectric and magnetic nanoparticles. J. Phys. Chem. C 2012, 116 (24), 13403−13412. (7) Gupta, T. K.; Singh, B. P.; Singh, V. N.; Teotia, S.; Singh, A. P.; Elizabeth, I.; Dhakate, S. R.; Dhawan, S.; Mathur, R. MnO 2 decorated graphene nanoribbons with superior permittivity and excellent microwave shielding properties. J. Mater. Chem. A 2014, 2 (12), 4256−4263. (8) Fletcher, A.; Gupta, M. C.; Dudley, K. L.; Vedeler, E. Elastomer foam nanocomposites for electromagnetic dissipation and shielding applications. Compos. Sci. Technol. 2010, 70 (6), 953−958. (9) Wang, H.; Wang, G.; Li, W.; Wang, Q.; Wei, W.; Jiang, Z.; Zhang, S. A material with high electromagnetic radiation shielding effectiveness fabricated using multi-walled carbon nanotubes wrapped with poly (ether sulfone) in a poly (ether ether ketone) matrix. J. Mater. Chem. 2012, 22 (39), 21232−21237. (10) Kim, H.; Kim, K.; Lee, C.; Joo, J.; Cho, S.; Yoon, H.; Pejaković, D.; Yoo, J.-W.; Epstein, A. Electrical conductivity and electromagnetic interference shielding of multiwalled carbon nanotube composites containing Fe catalyst. Appl. Phys. Lett. 2004, 84 (4), 589−591. (11) Biswas, S.; Kar, G. P.; Bose, S. Microwave absorbers designed from PVDF/SAN blends containing multiwall carbon nanotubes anchored cobalt ferrite via a pyrene derivative. J. Mater. Chem. A 2015, 3 (23), 12413−12426. (12) Cao, M.-S.; Wang, X.-X.; Cao, W.-Q.; Yuan, J. Ultrathin graphene: electrical properties and highly efficient electromagnetic interference shielding. J. Mater. Chem. C 2015, 3 (26), 6589−6599. (13) Maiti, S.; Shrivastava, N. K.; Suin, S.; Khatua, B. Polystyrene/ MWCNT/graphite nanoplate nanocomposites: efficient electromagnetic interference shielding material through graphite nanoplate− MWCNT−graphite nanoplate networking. ACS Appl. Mater. Interfaces 2013, 5 (11), 4712−4724. (14) Kar, G. P.; Biswas, S.; Bose, S. Simultaneous enhancement in mechanical strength, electrical conductivity, and electromagnetic shielding properties in PVDF-ABS blends containing PMMA wrapped multiwall carbon nanotubes. Phys. Chem. Chem. Phys. 2015, 17 (22), 14856−14865. (15) Zhang, H.-B.; Yan, Q.; Zheng, W.-G.; He, Z.; Yu, Z.-Z. Tough graphene− polymer microcellular foams for electromagnetic interference shielding. ACS Appl. Mater. Interfaces 2011, 3 (3), 918−924. (16) Liang, J.; Wang, Y.; Huang, Y.; Ma, Y.; Liu, Z.; Cai, J.; Zhang, C.; Gao, H.; Chen, Y. Electromagnetic interference shielding of graphene/ epoxy composites. Carbon 2009, 47 (3), 922−925. (17) Thomassin, J.-M.; Jérôme, C.; Pardoen, T.; Bailly, C.; Huynen, I.; Detrembleur, C. Polymer/carbon based composites as electromagnetic interference (EMI) shielding materials. Mater. Sci. Eng., R 2013, 74 (7), 211−232.

■

SUMMARY A unique strategy adopted in this study has led to simultaneous improvement in mechanical, electrical, and EM attenuation in a co-continuous PC/PVDF blends. By positioning nanoparticles with different characteristic (like conducting or magnetic) in different phases, we achieved outstanding absorption of the incoming EM radiation. The conducting inclusion, (here, MWNTs) was noncovalently modified with a imidazoliumbased IL to improve the dispersion state and the overall conductivity of the blends. The IL-modified MWNTs localized in the PVDF phase, although it is not the thermodynamically favored phase. By introducing a magnetic phase (here, BaFe), the permeability of the blends was tuned; however, the overall electrical conductivity was observed to be strongly contingent on the location of BaFe in the blends. For instance, when BaFe is localized in the PVDF phase along with MWNTs, the overall conductivity dropped and the EM attenuation was poor. However, by introducing surface active groups on the BaFe nanoparticles, its position in the blends can be tailored. This strategy further led to high electrical conductivity and outstanding EM attenuation by absorption (>99.9%). It is often observed that introducing nanoparticles in a polymeric system though result in enhanced Young’s modulus but does not improve the tensile strength, as the latter depends strongly on the interfacial adhesion. Hence, to address this challenge PMMA, we employed a mutually soluble homopolymer in both PC and PVDF. Intriguingly, PMMA resulted in improved stress transfer at the interface besides retaining the electrical conductivity of the blends. This strategy has clearly demonstrated that by selectively positioning nanoparticles with different characteristic can result in materials with multifunctional properties.

■

Research Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +91-80-2293 3407. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from DST (India). 25461


Research Article

ACS Applied Materials & Interfaces (18) Kodgire, P. V.; Bhattacharyya, A. R.; Bose, S.; Gupta, N.; Kulkarni, A. R.; Misra, A. Control of multiwall carbon nanotubes dispersion in polyamide6 matrix: An assessment through electrical conductivity. Chem. Phys. Lett. 2006, 432 (4), 480−485. (19) Rath, S. K.; Dubey, S.; Kumar, G. S.; Kumar, S.; Patra, A.; Bahadur, J.; Singh, A.; Harikrishnan, G.; Patro, T. U. Multi-walled CNT-induced phase behaviour of poly (vinylidene fluoride) and its electro-mechanical properties. J. Mater. Sci. 2014, 49 (1), 103−113. (20) Yang, Y.; Gupta, M. C.; Dudley, K. L.; Lawrence, R. W. Novel carbon nanotube-polystyrene foam composites for electromagnetic interference shielding. Nano Lett. 2005, 5 (11), 2131−2134. (21) Ren, F.; Yu, H.; Wang, L.; Saleem, M.; Tian, Z.; Ren, P. Current progress on the modification of carbon nanotubes and their application in electromagnetic wave absorption. RSC Adv. 2014, 4 (28), 14419−14431. (22) Biswas, S.; Kar, G. P.; Bose, S. Engineering nanostructured polymer blends with controlled nanoparticle location for excellent microwave absorption: a compartmentalized approach. Nanoscale 2015, 7 (26), 11334−11351. (23) Bilalis, P.; Katsigiannopoulos, D.; Avgeropoulos, A.; Sakellariou, G. Non-covalent functionalization of carbon nanotubes with polymers. RSC Adv. 2014, 4 (6), 2911−2934. (24) Ponnamma, D.; Sadasivuni, K. K.; Grohens, Y.; Guo, Q.; Thomas, S. Carbon nanotube based elastomer composites−an approach towards multifunctional materials. J. Mater. Chem. C 2014, 2 (40), 8446−8485. (25) Bose, S.; Khare, R. A.; Moldenaers, P. Assessing the strengths and weaknesses of various types of pre-treatments of carbon nanotubes on the properties of polymer/carbon nanotubes composites: A critical review. Polymer 2010, 51 (5), 975−993. (26) Pawar, S. P.; Marathe, D. A.; Pattabhi, K.; Bose, S. Electromagnetic interference shielding through MWNT grafted Fe3O4 nanoparticles in PC/SAN blends. J. Mater. Chem. A 2015, 3 (2), 656−669. (27) Hekmatara, H.; Seifi, M.; Forooraghi, K.; Mirzaee, S. Synthesis and microwave absorption characterization of SiO2 coated Fe3O4− MWCNT composites. Phys. Chem. Chem. Phys. 2014, 16 (43), 24069− 24075. (28) Saini, P.; Arora, M.; Gupta, G.; Gupta, B. K.; Singh, V. N.; Choudhary, V. High permittivity polyaniline−barium titanate nanocomposites with excellent electromagnetic interference shielding response. Nanoscale 2013, 5 (10), 4330−4336. (29) Zhu, J.; Wei, S.; Haldolaarachchige, N.; Young, D. P.; Guo, Z. Electromagnetic field shielding polyurethane nanocomposites reinforced with core−shell Fe−silica nanoparticles. J. Phys. Chem. C 2011, 115 (31), 15304−15310. (30) Roh, J.-S.; Chi, Y.-S.; Kang, T. J.; Nam, S.-w. Electromagnetic shielding effectiveness of multifunctional metal composite fabrics. Text. Res. J. 2008, 78 (9), 825−835. (31) Shen, B.; Zhai, W.; Tao, M.; Ling, J.; Zheng, W. Lightweight, multifunctional polyetherimide/graphene@ Fe3O4 composite foams for shielding of electromagnetic pollution. ACS Appl. Mater. Interfaces 2013, 5 (21), 11383−11391. (32) Kar, G. P.; Biswas, S.; Rohini, R.; Bose, S. Tailoring the dispersion of multiwall carbon nanotubes in co-continuous PVDF/ ABS blends to design materials with enhanced electromagnetic interference shielding. J. Mater. Chem. A 2015, 3 (15), 7974−7985. (33) Ren, Y.-L.; Wu, H.-Y.; Lu, M.-M.; Chen, Y.-J.; Zhu, C.-L.; Gao, P.; Cao, M.-S.; Li, C.-Y.; Ouyang, Q.-Y. Quaternary nanocomposites consisting of graphene, Fe3O4@ Fe core@ shell, and ZnO nanoparticles: synthesis and excellent electromagnetic absorption properties. ACS Appl. Mater. Interfaces 2012, 4 (12), 6436−6442. (34) Tung, T. T.; Feller, J. F.; Kim, T.; Kim, H.; Yang, W. S.; Suh, K. S. Electromagnetic properties of Fe3O4-functionalized graphene and its composites with a conducting polymer. J. Polym. Sci., Part A: Polym. Chem. 2012, 50 (5), 927−935. (35) Cao, M.-S.; Yang, J.; Song, W.-L.; Zhang, D.-Q.; Wen, B.; Jin, H.-B.; Hou, Z.-L.; Yuan, J. Ferroferric oxide/multiwalled carbon nanotube vs polyaniline/ferroferric oxide/multiwalled carbon nano-

tube multiheterostructures for highly effective microwave absorption. ACS Appl. Mater. Interfaces 2012, 4 (12), 6949−6956. (36) Sumita, M.; Sakata, K.; Asai, S.; Miyasaka, K.; Nakagawa, H. Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black. Polym. Bull. 1991, 25 (2), 265−271. (37) Rohini, R.; Bose, S. Electromagnetic Interference Shielding Materials Derived from Gelation of Multiwall Carbon Nanotubes in Polystyrene/Poly(methyl methacrylate) Blends. ACS Appl. Mater. Interfaces 2014, 6 (14), 11302−11310. (38) Maiti, P.; Nandi, A. K. Influence of chain structure on the miscibility of poly (vinylidene fluoride) with poly (methyl acrylate). Macromolecules 1995, 28 (25), 8511−8516. (39) Singh, A.; Mishra, R.; Prakash, R.; Maiti, P.; Singh, A. K.; Pandey, D. Specific interactions in partially miscible polycarbonate (PC)/poly (methyl methacrylate) (PMMA) blends. Chem. Phys. Lett. 2010, 486 (1), 32−36. (40) Moussaif, N.; Maréchal, P.; Jérôme, R. Ability of PMMA to improve the PC/PVDF interfacial adhesion. Macromolecules 1997, 30 (3), 658−659. (41) Verma, M.; Singh, A. P.; Sambyal, P.; Singh, B. P.; Dhawan, S. K.; Choudhary, V. Barium ferrite decorated reduced graphene oxide nanocomposite for effective electromagnetic interference shielding. Phys. Chem. Chem. Phys. 2015, 17 (3), 1610−1618. (42) Pötschke, P.; Fornes, T.; Paul, D. Rheological behavior of multiwalled carbon nanotube/polycarbonate composites. Polymer 2002, 43 (11), 3247−3255. (43) Bose, S.; Bhattacharyya, A. R.; Kulkarni, A. R.; Pötschke, P. Electrical, rheological and morphological studies in co-continuous blends of polyamide 6 and acrylonitrile−butadiene−styrene with multiwall carbon nanotubes prepared by melt blending. Compos. Sci. Technol. 2009, 69 (3), 365−372. (44) Kar, G. P.; Biswas, S.; Bose, S. Tailoring the interface of an immiscible polymer blend by a mutually miscible homopolymer grafted onto graphene oxide: outstanding mechanical properties. Phys. Chem. Chem. Phys. 2015, 17 (3), 1811−1821. (45) Biswas, S.; Kar, G. P.; Arora, D.; Bose, S. A unique strategy towards high dielectric constant and low loss with multiwall carbon nanotubes anchored onto graphene oxide sheets. RSC Adv. 2015, 5 (31), 24132−24138. (46) Kar, G. P.; Begam, N.; Basu, J. K.; Bose, S. Thermally Induced Demixing in an LCST Mixture in the Presence of Densely Grafted Nanoparticles: Tuning the Graft Chain Length To Induce Thermodynamic Miscibility. Macromolecules 2014, 47 (21), 7525− 7532. (47) Kar, G. P.; Bharati, A.; Xavier, P.; Madras, G.; Bose, S. The key role of polymer grafted nanoparticles in the phase miscibility of an LCST mixture. Phys. Chem. Chem. Phys. 2015, 17 (2), 868−877. (48) Zhao, X.; Cao, J.-P.; Zhao, J.; Hu, G.-H.; Dang, Z.-M. Advanced dielectric polymer nanocomposites by constructing a ternary continuous structure in polymer blends containing poly (methyl methacrylate) (PMMA) modified carbon nanotubes. J. Mater. Chem. A 2014, 2 (27), 10614−10622. (49) Vo, L. T.; Giannelis, E. P. Compatibilizing poly (vinylidene fluoride)/nylon-6 blends with nanoclay. Macromolecules 2007, 40 (23), 8271−8276. (50) Joseph, K.; Thomas, S.; Pavithran, C. Effect of chemical treatment on the tensile properties of short sisal fibre-reinforced polyethylene composites. Polymer 1996, 37 (23), 5139−5149. (51) Aravind, I.; Ahn, K. H.; Ranganathaiah, C.; Thomas, S. Rheology, morphology, mechanical properties and free volume of poly (trimethylene terephthalate)/polycarbonate blends. Ind. Eng. Chem. Res. 2009, 48 (22), 9942−9951. (52) Thomas, R.; Yumei, D.; Yuelong, H.; Le, Y.; Moldenaers, P.; Weimin, Y.; Czigany, T.; Thomas, S. Miscibility, morphology, thermal, and mechanical properties of a DGEBA based epoxy resin toughened with a liquid rubber. Polymer 2008, 49 (1), 278−294. (53) Wang, Y.; Zhang, Q.; Fu, Q. Compatibilization of immiscible poly (propylene)/polystyrene blends using clay. Macromol. Rapid Commun. 2003, 24 (3), 231−235. 25462


Research Article

ACS Applied Materials & Interfaces (54) Bhattacharyya, A. R.; Ghosh, A. K.; Misra, A.; Eichhorn, K.-J. Reactively compatibilised polyamide6/ethylene-co-vinyl acetate blends: mechanical properties and morphology. Polymer 2005, 46 (5), 1661−1674. (55) Bose, S.; Bhattacharyya, A. R.; Kodgire, P. V.; Misra, A.; Pötschke, P. Rheology, morphology, and crystallization behavior of melt-mixed blends of polyamide6 and acrylonitrile-butadiene-styrene: Influence of reactive compatibilizer premixed with multiwall carbon nanotubes. J. Appl. Polym. Sci. 2007, 106 (5), 3394−3408. (56) Meincke, O.; Kaempfer, D.; Weickmann, H.; Friedrich, C.; Vathauer, M.; Warth, H. Mechanical properties and electrical conductivity of carbon-nanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene. Polymer 2004, 45 (3), 739−748. (57) Knauert, S. T.; Douglas, J. F.; Starr, F. W. The effect of nanoparticle shape on polymer-nanocomposite rheology and tensile strength. J. Polym. Sci., Part B: Polym. Phys. 2007, 45 (14), 1882−1897. (58) Maiti, S.; Suin, S.; Shrivastava, N. K.; Khatua, B. A strategy to achieve high electromagnetic interference shielding and ultra low percolation in multiwall carbon nanotube−polycarbonate composites through selective localization of carbon nanotubes. RSC Adv. 2014, 4 (16), 7979−7990. (59) Al-Saleh, M. H.; Sundararaj, U. A review of vapor grown carbon nanofiber/polymer conductive composites. Carbon 2009, 47 (1), 2− 22. (60) Dey, P.; Naskar, K.; Dash, B.; Nair, S.; Unnikrishnan, G.; Nando, G. B. Selective dispersion of carbon fillers into dynamically vulcanized rubber/plastic blends: a thermodynamic approach to evaluate polymer reinforcement and conductivity enhancement. RSC Adv. 2015, 5 (40), 31886−31900. (61) Mao, C.; Zhu, Y.; Jiang, W. Design of electrical conductive composites: Tuning the morphology to improve the electrical properties of graphene filled immiscible polymer blends. ACS Appl. Mater. Interfaces 2012, 4 (10), 5281−5286. (62) Ngai, K.; White, C.; Jonscher, A. On the origin of the universal dielectric response in condensed matter. Nature 1979, 277, 185−189. (63) Verma, M.; Singh, A. P.; Sambyal, P.; Singh, B. P.; Dhawan, S.; Choudhary, V. Barium ferrite decorated reduced graphene oxide nanocomposite for effective electromagnetic interference shielding. Phys. Chem. Chem. Phys. 2015, 17 (3), 1610−1618. (64) Sun, G.; Dong, B.; Cao, M.; Wei, B.; Hu, C. Hierarchical dendrite-like magnetic materials of Fe3O4, γ-Fe2O3, and Fe with high performance of microwave absorption. Chem. Mater. 2011, 23 (6), 1587−1593. (65) Lv, R.; Cao, A.; Kang, F.; Wang, W.; Wei, J.; Gu, J.; Wang, K.; Wu, D. Single-crystalline permalloy nanowires in carbon nanotubes: Enhanced encapsulation and magnetization. J. Phys. Chem. C 2007, 111 (30), 11475−11479. (66) Cho, S.; Kang, D.; Oh, J. Relationship between magnetic properties and microwave-absorbing characteristics of NiZnCo ferrite composites. J. Mater. Sci. 1996, 31 (17), 4719−4722. (67) Kumar, A.; Alegaonkar, P. S. Impressive Transmission Mode Electromagnetic Interference Shielding Parameters of Graphene-like Nanocarbon/Polyurethane Nanocomposites for Short Range Tracking Countermeasures. ACS Appl. Mater. Interfaces 2015, 7 (27), 14833− 14842. (68) Bose, S.; Kar, G. P.; Biswas, S. Unique strategy towards simultaneous improvement in structural properties and microwave shielding. ChemNanoMat 2015, DOI: 10.1002/cnma.201500159. (69) Biswas, S.; Kar, G. P.; Bose, S. Attenuating microwave radiation by absorption through controlled nanoparticle localization in PC/ PVDF blends. Phys. Chem. Chem. Phys. 2015, 17 (41), 27698−27712.

25463


Tailor-Made Distribution of Nanoparticles in Blend Structure toward

Recommend Documents