Simultaneous enhancement in mechanical strength

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Cite this: DOI: 10.1039/c5cp01452b

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Simultaneous enhancement in mechanical strength, electrical conductivity, and electromagnetic shielding properties in PVDF–ABS blends containing PMMA wrapped multiwall carbon nanotubes Goutam Prasanna Kar,† Sourav Biswas† and Suryasarathi Bose* A unique approach was adopted to drive the multiwall carbon nanotubes (MWNTs) to the interface of immiscible PVDF–ABS blends by wrapping the nanotubes with a mutually miscible homopolymer (PMMA). A tailor made interface with an improved stress transfer was achieved in the blends with PMMA wrapped MWNTs. This manifested in an impressive 108% increment in the tensile strength and 48% increment in the Young’s modulus with 3 wt% PMMA wrapped MWNTs in striking contrast to the neat blends. As the PMMA wrapped MWNTs localized at the interface of PVDF–ABS blends, the electrical conductivity could be tuned with respect to only MWNTs, which were selectively localized in the PVDF phase, driven by thermodynamics. The electromagnetic shielding properties were assessed using a vector network analyser in a broad range of frequency, X-band (8–12 GHz) and Ku-band (12–18 GHz). Interestingly,

Received 12th March 2015, Accepted 7th May 2015

enhanced EM shielding was achieved by this unique approach. The blends with only MWNTs shielded the

DOI: 10.1039/c5cp01452b

EM waves mostly by reflection however, the blends with PMMA wrapped MWNTs (3 wt%) shielded mostly by absorption (62%). This study opens new avenues in designing materials, which show simultaneous improve-

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ment in mechanical, electrical conductivity and EM shielding properties.

Introduction Materials with extraordinary combination of properties are obtained by blending two polymers.1–3 This class of materials are of great interest in the present era not only because they offer exciting mechanical, electrical, sensing or thermal behavior4 but also for their sustainability and recycling capabilities.5 Since the last few decades, work has been progressed to commercialize polymer blends with improved performance.6 However, the process of blending two polymers is not a straightforward approach because of a small gain in entropy of mixing often, leading to phase separation in the macro scale.7–10 Thermodynamically, the homogeneity in a polymeric mixture depends on the specific microscopic interactions such as H-bonding,11 dipole–dipole interaction,12 London dispersion force,13 p–p interaction14–16 etc. The interfacial tension between the polymers and the phase morphology can be fine-tuned with the aid of functional polymers.17 However, synthesizing polymers with particular chemical functionality is an expensive and industrially inefficient process. Interestingly, few other efficient ways to compatibilize polymer blend has been developed over Department of Materials Engineering, Indian Institute of Science, Bangalore-560012. E-mail: [email protected]; Tel: +91-80-2293-3407 † GPK and SB made equal contribution to this work.

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the years.18 Various strategies like addition of copolymers (block/graft),19 mutually miscible homopolymer,20 and very recently, nanoparticles21 and more specifically Janus-type nanoparticles22 have been explored to stabilize bi-phasic polymer blends. The compatibilizer lowers the interfacial tension,23 stabilizes the morphology under external stresses and enhances the molecular adhesion between the phases.24,25 According to the Flory–Huggins theory, the free energy of mixing in bi-phasic blends (A/B) in the presence of a compatibilizer (X) can be expressed as,26 DGmix = DGAX + DGBX DGAB

(1)

where the subscript refers to the interacting components. In general, for immiscible polymer blends (A/B), DGAB is positive. Thermodynamically stable structure is obtained when DGAX and DGBX are negative, manifesting in favorable interaction between the components. The preferential adsorption of X at the interface of A/B brings down the interfacial tension (nAB) expressed as,22 Dn ¼

1 5 KB T 3 3 S 2 2=3 2=3 ZCA ZA þ ZCB ZB 2 2 a 4 a

(2)

where ZCA and ZCB refer to the number of A and B monomeric segments, respectively, and Zc = ZCA + ZCB is the total number of

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segments in the copolymer and, ZA and ZB are the degree of polymerization in the homopolymers A and B, respectively, a is the monomer length, S is the interfacial area per copolymer joint. So, from the above equation, higher the absorption density (S/a2), lower the interfacial coefficient. However, this is dependent on the concentration of the compatibilizer. The critical micelle concentration (CMC) is expressed as,22


fCMC+ = e(mCMCfwABZc)

(3)

where fCMC+ is the volume fraction of the copolymer and mCMC is the chemical potential at CMC.22 Hence, interfacial tension, nAB, decreases when X is added to A/B till it reaches the CMC, above which the A/B interface is saturated and X starts migrating to one of the phases.22 Over the past few decades, industrially important immiscible polymer blends have been studied systematically in the presence and absence of a compatibilizer. For instance, Macosko and his co-workers studied 30/70 poly(methyl methacrylate) (PMMA)/ polystyrene (PS) blends compatibilized with symmetric P(S-b-MMA) di-block copolymers.27 They found that even with the addition of 1% co-polymer, the PMMA particle size was reduced significantly. Apart from stabilizing microstructures, addition of compatibilizers has also shown significant improvement in the structural properties. For instance, Na et al. showed significant improvement in the mechanical properties with the addition of PCL-b-PEG (poly(e-caprolactone)-b-poly(ethylene glycol)) in blends of PCL–PEG.28 The effect of nanoparticles was also studied with respect to compatibilizing binary polymer blends. For instance, Yong Wang et al. used organically modified clay to compatibilize immiscible PP–PS (poly(propylene)–polystyrene) blends.29 Significant reduction in the droplet size of PS was observed with the addition of silica nanoparticles in a 70/30 PP/PS blend.30 Recently, the effect of graphene oxides (GO) was also studied in immiscible polyamide/ polyphenylene oxide (PA/PPO, 90/10) blends and it was shown that even 0.5% of GO decreased the droplet size of the minor PPO phase significantly.5 In our earlier work, we achieved significant enhancement in mechanical properties in a 90/10 PVDF/ABS blend with PMMA-wrapped GO and a substantial decrease in the droplet size of ABS.20 Recently, a very interesting class of nanoparticles, i.e. Janus was synthesized.31,32 It is envisaged that as the Janus nanoparticles are amphiphilic in nature, they can compatibilize immiscible polymer blends. Walther and his co-worker were able to localize the Janus particles at the interface of an immiscible PS/PMMA blend even under extreme shear conditions.33 Similarly, Janus nanoparticles were added to compatibilize the PPE–SAN (poly(2,6-dimethyl-1,4-phenylene ether)– poly(styrene-co-acrylonitrile))blend.34 Over the years, it is well understood that the properties of the blends can be significantly improved with the incorporation of carbon nanotubes (CNTs) as they possess exceptional mechanical, electrical and sensing properties.35–39 Hence, the key role of the carbon nanotube as a compatibilizer has also been explored in polymer blends.40–43 However, hydrophobic CNTs tend to agglomerate in polymer blends due to poor interaction with the polymer limiting its use.3,44–46 Several strategies have been adopted to enhance the adhesion of CNTs and the polymer,


such as introducing selective functional groups on the surface of CNTs.35 Grafting of polymer chains onto CNTs47 can also give excellent compatibility.7 CNTs can increase the electrical conductivity and electromagnetic interference shielding property of electrically insulating polymeric materials.48–50 PVDF (poly(vinylidene fluoride)) is an engineered polymer that exhibits excellent pyroelectric and piezoelectric properties. However, because of its poor mechanical properties, its use is restricted in structural applications. Therefore, blending with rubbery polymers can improve the mechanical properties significantly, especially the fracture toughness.22 ABS (acrylonitrile butadiene styrene) is a rubbery terpolymer. Hence, blending with ABS enhances the fracture toughness significantly besides improving the chemical resistance of PVDF. This opens new avenues in designing PVDF–ABS blends with a wide range of properties. In our earlier work51 we applied ionic-liquid modified MWNTs (IL–MWNTs) for dispersion of MWNTs in PVDF–ABS blends. Due to the p–p interaction between MWNTs and IL, the IL–MWNTs are more exfoliated in the polymer blend. Since the long (few mm) hairy CNTs have the end-to-end distance of the same order of magnitude as that of the radius of gyration of polymer chains,7 we adopted a unique method here to compatibilize immiscible polymer blends of PVDF–ABS. As PMMA is a co-solvent to both PVDF and ABS, it can localize at the interface of the blends and reduce the interfacial tension between the components. PMMA was wrapped on to MWNTs via in situ polymerization of MMA. The key role of PMMA wrapped MWNTs in PVDF–ABS blends was assessed here with respect to the quality of dispersion of MWNTs, morphology control and improved interface. In addition, the electrical conductivity and electromagnetic (EM) shielding property were evaluated and compared against neat blends and with different MWNTs. The localization of MWNTs was studied using electron microscopic techniques and solution-dissolution experiments. The pristine MWNTs (p-MWNTs) localized in the PVDF phase of the blends whereas, PMMA wrapped MWNTs (PMMA–MWNTs) are localized at the interface of co-continuous (50/50) PVDF/ABS blends thus generating ternary continuous structures. The improved dispersion and enhanced electrical conductivity in the presence of PMMA wrapped MWNTs motivated us to investigate in detail the electromagnetic interference (EMI) shielding properties in the blends. Intriguingly, PMMA wrapped MWNTs, which localized at the interface of the blends significantly improved the shielding properties besides improving the electrical conductivity and the structural properties. Hence, this study opens new avenues in designing advanced composites, which show simultaneous improvement in structural, electrical and EM shielding properties.

Experimental section Materials PVDF (Kynar-761, with Mw of 440 000 g mol1) was kindly provided by Arkema. ABS (Absolac 120, with a typical composition consisting of acrylonitrile: 24 wt% rubber content: 16.5 wt% and styrene: 59.5 wt%) was procured from Styrolution. PMMA (Atuglass V825T,


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with Mw of 95 000 g mol1 and polydispersity of 2.1) was obtained from GSFC, India. As received pristine MWNTs (p-MWNTs, average diameter and length of 9.5 nm and1.5 mm respectively) were procured from (Nanocyl SA (Belgium).). Methyl methacrylate (MMA), o-dichloro benzene (o-DCB) and azobisisobutyronitrile (AIBN) were procured from Sigma Aldrich. Inhibitor monomethyl ether hydroquinone in MA was removed by washing with 10% NaOH and then with distilled water. MA was dried over fused calcium chloride (CaCl2) before use. AIBN was recrystallized from a saturated methanol solution. Synthesis of PMMA wrapped MWNTs Initially, the pristine (p-MWNTs) was sonicated in o-DCB for 20 min. MMA and AIBN were subsequently added to the mixture of p-MWNT–o-DCB. The resultant mixture was then transferred to a round bottomed flask (RB) and the dissolved oxygen was then removed by a freeze–pump–thaw cycle and sealed.52 The RB was then placed in a preheated oil-bath under a N2 atmosphere for 24 h under constant stirring (Scheme 1). The black suspension was dispersed in chloroform and dried under vacuum. Blend preparation All the blends were prepared using a twin-screw microcompounder Haake minilab under a N2 atmosphere at 230 1C at a rotor speed of 60 rpm for 20 minutes. Blends with different concentrations of MWNTs and PMMA wrapped MWNTs were also prepared under the same extrusion conditions. Samples were pre-dried at 80 1C in a vacuum oven for 24 h prior to processing. Melt mixed samples were subsequently compression molded into specific shapes at 230 1C using a lab scale hydraulic press. Table 1 lists the various blends studied in this work. Characterization Fourier transform infrared (FTIR) spectroscopy was performed on a Perkin-Elmer GX in the range of 4000–400 cm1 using a resolution of 4 cm1. Morphological analysis was done using an ULTRA 55 scanning electron microscope (SEM) at an accelerating voltage of 10 kV. The flow characteristics of the blends were evaluated using a discovery hybrid rheometer (DHR-3) from TA-instruments under a nitrogen atmosphere to prevent any degradation of the samples. Parallel plate geometry of 25 mm diameter was used. Extruded strands were vacuum dried at 80 1C for 6 h before the rheological measurements. All the experiments were carried out under a linear viscoelastic region determined a priori. A universal tensile testing machine at room temperature was used for measuring the mechanical properties of the

Scheme 1

Synthesis of PMMA–MWNTs.


Table 1 codes

List of various compositions studied here and the respective

Compositions

PMMA (wt%)

MWNT (wt%)

50/50 (PVDF/ABS) neat blends 50/50/1 50/50/3 50/50/5 50/50/10 50/50 with 1 wt% p-MWNT 50/50 with 2 wt% p-MWNT 50/50 with 3 wt% p-MWNT 50/50/10 with 1 wt% p-MWNT 50/50/10 with 2 wt% p-MWNT 50/50/10 with 3 wt% p-MWNT 50/50 with 1 wt% PMMA–MWNT 50/50 with 1 wt% PMMA–MWNT 50/50 with 10 wt% PMMA + 3 wt% p-MWNT

— 1 3 5 10 — — — 10 10 10 10 10 10

— — — — — 1 2 3 1 2 3 1 3 3

dumbbell-shaped compression molded samples. At least 4 dumbbell-shaped specimens were prepared for each composition by compression molding at 230 1C. Tensile tests were carried out using a cross head speed of 5 mm min1 with a preload of 2 N. Prior to the measurements, all the specimens were vacuum dried at 80 1C for 6 h. The room temperature electrical conductivity of the blends was studied using an Alpha-N Analyser, Novocontrol (Germany) in the 0.1 Hz to 10 MHz frequency range. Uniformly polished compression molded disks were used as the specimen and the electrical conductivity was measured across the thickness. In order to study the electromagnetic interference SE in X and Ku-band frequency range, an Anritsu MS4642A vector network analyser (VNA) was coupled with a coax (Damaskos M07T) set up. The experimental set up was calibrated to avoid absorption and reflection losses due to the transmission line and the sample holder. Toroidal specimens (with length 5.6 mm and outer diameter and inner diameter 6.5 and 3 mm respectively) were compression molded and the S parameters (S11, S12, S22 and S21) were measured in a wide range of frequency.

Results and discussion Synthesis and characterization of PMMA wrapped MWNTs In situ polymerization of PMMA in the presence of MWNTs was adopted here to synthesize PMMA wrapped MWNTs (as shown in Scheme 1). For an efficient reaction, we removed the oxygen and the moisture from the RB by vacuum as discussed under the experimental section. The resultant black mass thus obtained was characterized by FT-IR (Fig. 1a) and 1H NMR (Fig. 1b). As shown in Fig. 1a, the –CH stretching vibration, CQO stretching vibration, CH3 and CH2 deformation vibration, and C–O–C stretching vibration are well evident from the spectra and appears at wavenumbers 2930, 1730, 1432 and 1126 cm1 respectively.20 The chemical shifts in the 1H are shown in Fig. 1b. The 1H in –COOCH3, methylene protons of –CH2 and the methylic protons of –CH3 appear at 3.6, 1.7 and 1.0–0.5 ppm respectively.53 We have compared the associated integration areas under the corresponding peaks with the commercial PMMA (not shown here)


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Fig. 1 (a) FT-IR spectrum of PMMA–MWNT and (b) 1H NMR spectrum of PMMA–MWNT in CDCl3, (c) TEM image of p-MWNTs (inset: showing a high resolution image of multiwall of p-MWNTs), (d) TEM image of PMMA–MWNTs (e) p-MWNT in DMF (i), PMMA–MWNT in DMF (ii).

which are similar to Fig. 1(b).53 Hence, the presence of PMMA on the surface of MWNTs was confirmed using FTIR and NMR analysis. PMMA wrapping was further confirmed by an increase (3–5 nm) of the average diameter of MWNT (Fig. 1c and d). In addition, it is envisaged that wrapping of PMMA improves the dispersion of MWNTs in polar solvents. This was supported by the visual observation of the PMMA wrapped MWNT suspension in CHCl3 (Fig. 1e(ii)) which was stable even after several weeks whereas, the p-MWNTs (Fig. 1e(i)) quickly settled as manifested from the black solid precipitate at the bottom of the vial. Constructing ternary continuous structures using PMMA wrapped MWNTs Control of phase morphology during blending is a key parameter that dictates the ultimate properties in immiscible polymer blends. Fig. 2 shows the SEM morphologies of binary (PVDF– ABS) and ternary (PVDF–ABS–PMMA) blends with different concentrations of PMMA. It is important to note that in all the cases cryofractured samples were used. In order to improve the contrast between the phases the samples were etched with chloroform for 72 h at room temperature, which selectively removes the ABS and the PMMA phases from the blend. Hence, the voids represent the etched PMMA and the ABS phases in the SEM micrographs. It is observed that the neat blend shows co-continuous morphology at this composition. A significant refinement in the co-continuous structure is well noted with the addition of PMMA manifesting the key role of PMMA as an interfacial modifier in the blends.


The co-continuous structure was retained even after incorporation of p-MWNTs, which suggests only minor changes in the viscoelastic properties of the components and will be discussed latter on. The higher magnification SEM images reveal that the p-MWNTs are selectively localized in the PVDF phase. This is also supported by a selective dissolution method. It is important to note that the samples were etched with chloroform, which is a common solvent for both PMMA and ABS. Further confirmation comes from solution-dissolution experiments. Samples containing p-MWNTs were dissolved in chloroform for 15 minutes and then sonicated to monitor the change in color. The solution remained colorless manifesting that p-MWNTs are not localized either in ABS or PMMA phases however, the solution turned dark immediately after dissolving the samples in DMF which indicates that the p-MWNTs are located in the PVDF phase. However, when we dissolved the samples containing 3 wt% PMMA–MWNT in chloroform, the solution turned black which indicated that the MWNTs are either in ABS or at the interface. In order to gain more information about the position of MWNTs in the blend, the samples were dissolved in a glacial acetic acid solution. Interestingly, when the blend samples containing PMMA wrapped MWNTs were dissolved in glacial acetic acid, which selectively removes the PMMA phase without affecting PVDF and ABS, the solution turned dark after 10 minutes indicating that PMMA wrapped MWNTs are not localized in either of the phases. It is envisaged that PMMA wrapped MWNTs are localized at the interface of the blends (Fig. 3).


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Fig. 2 SEM morphology cryofractured and chloroform etched surfaces of (a) PVDF/ABS (50/50) (b) PVDF/ABS/PMMA (50/50/1) (c) PVDF/ABS/PMMA (50/50/3) (d) PVDF/ABS/PMMA (50/50/5) (e) PVDF/ABS/PMMA (50/50/10).

Tailored interface: effect of PMMA wrapped MWNTs on the structural properties of the blends Fig. 4 and 5 show the mechanical properties of different blends. The phase morphology and the interfacial adhesion between the constituents are the key factors that decide the mechanical properties in multicomponent systems. In this context, the elongation at break is an important parameter to judge the interfacial adhesion between the constituents and gives a measure of the fracture toughness. The interfacial adhesion is expected to be poor in the case of uncompatibilized blends because of its chemical incompatibility. The stress transfer at the interface thus is expected to be inefficient resulting in poor mechanical properties. The neat 50/50 (w/w) PVDF/ABS blends exhibit poor mechanical properties as compared to the constituent polymers. However, addition of PMMA significantly improved the mechanical properties and observed to scales with the concentration of PMMA in the blends. This clearly indicates the key role of PMMA in compatibilizing immiscible PVDF–ABS blends. As mentioned earlier, PMMA is mutually miscible to both PVDF and ABS thereby localizes at the interface of the blends. This helps in improved stress transfer at the interface resulting in enhanced mechanical properties. In addition, PMMA facilitates in refinement and stabilization of


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Fig. 3 SEM morphology cryofractured and chloroform etched surfaces of (a) PVDF/ABS (50/50) with 3 wt% p-MWNTs; (b) PVDF/ABS/PMMA (50/50/10) with 3 wt% p-MWNTs; (c) PVDF/ABS (50/50) with 3 wt% PMMA–MWNT; (d) higher magnification SEM image of (a) and (e) higher magnification SEM image of (b) and (f) higher magnification SEM image of (c), (g) and (i) 50/50 with 3% p-MWNTs in DMF, (ii) 50/50 with 3% p-MWNTs in CHCl3 and (iii) 50/50 with 3 wt% PMMA–MWNTs in CHCl3, (h) in glacial acetic acid (i) 50/50 with 3 wt% p-MWNTs (ii) 50/50 with 3 wt% PMMA–MWNT.

the phases leading to improved interfacial adhesion. This can be appreciated by comparing the mechanical properties of the blends with 10 wt% PMMA. The tensile strength and the Young’s modulus is 91% and 7% higher, respectively, as compared to neat blends. An impressive increase of ca. 133% in the elongational properties was recorded in the blends upon addition of 10 wt% PMMA. The mechanical behaviour of polymer nanocomposites is directly related to their hierarchical microstructures. Therefore, the mechanical properties in the presence of nanoparticles are controlled by several microstructural parameters, such as property of the matrix, distribution of the filler, percolation threshold, and interfacial bonding. The reinforcing effect of p-MWNTs in 50/50 PVDF/ABS blends was evaluated by performing tensile tests at room temperature. The addition of p-MWNT only slightly improved the tensile strength; whereas a significant increment in Young’s modulus was observed. As the concentration of MWNTs increases in the blend (from 1 to 3 wt%), the interaction


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Fig. 5 Young’s modulus (a, c, e and g) and ultimate tensile strength (b, d, f and h) of different blends.

Fig. 4 Stress–strain behaviour of (a) neat PVDF/ABS blends; (b) blends with MWNTs and (c) PMMA wrapped MWNTs wherein the concentration of PMMA is fixed at 10 wt%.

between the MWNTs begins to dominate and enhance the Young’s modulus of the blends. However, the lack of interfacial adhesion with the constituent results in either a moderate or no significant increase in the tensile strength. This is also manifested from the elongational properties. The elongation at break is significantly sacrificed due to premature failure originating from either CNT aggregates, which act as stress concentrators or poor interfacial adhesion between the phases. It was only when the concentration of p-MWNTs reached the percolation threshold,


we observed a significant improvement in the structural properties. For instance, the tensile strength increased by 29% and Young’s modulus by 27% in the bends with 3 wt% p-MWNTs. It is now understood that 10 wt% PMMA in the blend can improve the interaction between the constituents and moreover, the stress transfer at the interface. This can be well appreciated by comparing the mechanical properties of blends with 10 wt% PMMA and the blends with only 3 wt% p-MWNTs. The latter results in poor mechanical properties in striking contrast to the blends with only PMMA. Hence, we prepared some compositions fixing the concentration of PMMA as 10 wt% and varying the concentration of p-MWNTs in the blends. The results clearly exhibit an impressive 50% increment in tensile strength and 36% increment in Young’s modulus with simultaneous addition of PMMA (10 wt%) and p-MWNTs (3 wt%). In all the cases the p-MWNTs are localized in the PVDF phase, as discussed earlier, and may not improve the interfacial interaction between the constituent polymers. It is now well understood that improved interfacial adhesion is the key parameter which can simultaneously enhance both the strength and the toughness. In order to address this, we adopted a unique strategy where PMMA, a mutually miscible polymer, was wrapped onto MWNTs by in situ polymerization. We expect that PMMA might drive MWNTs to the interface of the blends


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and facilitate efficient stress transfer at the interface. The concentration of PMMA and MWNTs was kept constant in the batches and compared against the other strategies like in situ polymerization PMMA-wrapped MWNTs and simultaneous addition of PMMA and MWNTs during melt mixing. Few compositions were prepared where the concentration of PMMA was fixed at 10 wt% and that of the MWNTs was varied during the synthesis of PMMA wrapped MWNTs. We believe that PMMA chains wrapped onto MWNTs will drive the nanotubes to the interface of the blend. This can be well appreciated by comparing the results with the blends with either PMMA or combination of PMMA and MWNTs. The blends with 1 wt% in situ polymerized PMMA wrapped MWNTs resulted in 27% increment in Young’s modulus and 42% increment in the tensile strength with respect to the neat blend. More interestingly, the elongation at break increased by 66% manifesting in enhanced interfacial adhesion between the phases. In order to appreciate the key role of PMMA wrapped MWNTs in improving the mechanical property in the blends, we prepared few compositions wherein a master batch of PMMA and MWNTs was prepared and diluted in the subsequent process. In the master batch approach, though the Young’s modulus is increased but the ultimate tensile strength is more or less similar to that obtained in other approaches. So, it is well evident from SEM and tensile tests that selective localization of MWNTs aided by the wrapped PMMA chains shows simultaneous improvement in both strength and toughness which is not possible by the individual components. More interestingly, with the addition of PMMA wrapped MWNT a striking improvement in the tensile strength and Young’s modulus was observed. For instance, an impressive 108% increment in the tensile strength and 48% in the Young’s modulus was noted. Segregation of PMMA wrapped MWNT at the interface of the blend: effect on electrical conductivity The continuous conducting network of particles is the key factor in improving the electrical conductivity in polymeric systems.51,54,55 In general, electrical conductivity depends on size, shape, concentration, distribution, and surface treatment of the particles. In this context, the aspect ratio plays an important role in deciding the electrical percolation threshold. Most often, the surface functionalization by covalent treatments reduces the overall electrical conductivity in CNT based composites. Alternatively, by grafting polymeric chains or even wrapping can also impede the geometrical contact between the nanotubes. Fig. 6(a) shows the AC electrical conductivity in the blends with MWNTs. A clear jump in the bulk electrical conductivity was observed between 0.5 and 1 wt% of p-MWNTs manifesting in it the electrical percolation threshold in PVDF– ABS blends. As the concentration of MWNTs increases in the blends, the distance between two nanotubes reduces thereby allowing electron hopping or tunnelling. This is also manifested by comparing the critical frequency below which the ac electrical conductivity is independent of frequency. It is envisaged that above the percolation threshold a mesh like network of MWNTs forms and this mesh become finer and finer with increased filler loading and their ability to conduct electron increases through


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Fig. 6 AC conductivity plot of various PVDF–ABS blends (a) showing percolation threshold of the blend (b) with different concentrations of MWNTs.

the network. With addition of PMMA, the electrical conductivity is not affected much as the p-MWNTs are preferentially localized in the PVDF phase and retain the interconnections. However, interestingly, the percolation threshold reduced significantly in the case of PMMA–MWNT (0–0.5 wt%) manifesting in the fact that PMMA wrapping facilitated in the network like structure. It is well known that if three dimensional network structures are formed in polymer composites, the electrical conductivity pathway will be generated. We expect the PMMA wrapped MWNTs to localize at the interface of the blends and retain the three dimensional network. This also manifested in higher electrical conductivity at 3 wt% in contrast to the blends with p-MWNTs. The local concentration of MWNTs increases due to this interfacial segregation resulting in a more interconnected network as the interface is continuous.

EMI shielding effectiveness: effect of PMMA wrapped MWNTs in attenuating microwave radiations The shielding effectiveness (SE) is the amount of attenuation of the incident radiation by a particular material and is expressed in dB. There are three major mechanisms namely reflection (SER), absorption (SEA) and multiple reflections (SEMR). The latter can be


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incident EM radiation. EM radiation is attenuated by absorption when materials have both electric and magnetic dipoles. Hence, the total SE is expressed as SET = SEA + SER.


In a vector network analyser, the EMI SE is represented in terms of scattering parameters which are S11, S12, and S21.The total EMI SE can be evaluated from the S parameter using the following equation, SET ðdBÞ ¼ 10 log

1 jS12 j

2

¼ 10 log

1 jS21 j2

SE due to different mechanisms, reflection and absorption mentioned above, can be evaluated using the following equations, SER = 10 log10(1/(1 S112)) SEA = 10 log10((1 S112)/S122)

Fig. 7 (a) Total shielding effectiveness of various blends studied here; (b) absorption and reflection component of (1) 50/50 blends with 3 wt% MWNT (2) 50/50 blends with 10 wt% PMMA and 3 wt% MWNT (3) 50/50 blends with 3 wt% PMMA wrapped MWNTs.

neglected when the shield thickness is more than the skin depth or when the total shielding effectiveness is greater than 15 dB. It is envisaged that the reflection mechanism is prominent in the case of materials containing free mobile charge carriers like metals. In order to reflect EM radiation, the material should have high electric conductivity and mobile charge carriers that can interact with the

where scattering parameters S11, S12 and S21 are termed as forward reflection coefficient, reverse transmission coefficient and forward transmission coefficient respectively. Conductivity and network formation of MWNTs are the key requirements to achieve high attenuation of EM radiation. It is well established that by selectively localizing MWNTs in a given phase of the blend, the network like structure of CNTs can be facilitated. The neat blends are transparent to the EM waves and are a poor shielding material (Fig. 7a). A SE of 24 dB (at 18 GHz frequency) was obtained for 50/50 blends with 3 wt% MWNTs. In the case of blends with both PMMA and MWNTs (separately added) the SE was slightly decreased (20 dB). However, with the addition of 3 wt% PMMA wrapped MWNT the SE improved56 significantly (32 dB) which clearly indicates an efficient three dimensional network of MWNTs at the interface. A critical insight into the mechanism of shielding can be gained by comparing the reflection and the absorption components (Fig. 7b). In the case of blends with 3 wt% p-MWNT the SE is only by reflection (83%) whereas, in the case of blends with MWNTs and PMMA (separately added) the absorption increased with respect to the blends with only MWNTs. However, the

Fig. 8 (a) A cartoon illustrating the mechanism of shielding in 50/50 PVDF/ABS blends in the presence of MWNTs and PMMA wrapped MWNTs; (b) flexible films of PVDF–ABS blends with PMMA wrapped MWNTs.



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dominant mechanism was mostly by reflection. Interestingly, in the blends with PMMA wrapped MWNTs, the SE were mostly dominated by absorption (62%). This possibly may be due to the high dielectric constant material (PMMA) wrapped on the MWNTs, attenuating the charge for longer time scales. Overall, the material was capable of absorbing the EM radiations. This study clearly demonstrates that this unique approach has led to simultaneous improvement in mechanical, electrical conductivity and EM shielding properties. To get a clearer picture, a cartoon further illustrates how the PMMA wrapped MWNTs at the interface of PVDF–ABS blends can shield EM radiations. The concentration of MWNTs is a key factor that decides the flexibility of the polymer based composites. This study also shows (Fig. 8b) that the flexibility of the composites can be retained and lightweight composites can be designed by tailoring the interface of an immiscible polymer blend.

Conclusions In summary, a mutually miscible homopolymer can improve the compatibility of an immiscible polymer blend and facilitate better stress transfer at the interface under tensile loading. PMMA, a co-solvent to both PVDF and ABS, was employed to drive the MWNTs to the interface of the blends by modifying p-MWNTs with PMMA via insitu polymerization of MMA. The PMMA wrapped MWNTs resulted in an impressive improvement in the mechanical properties of the blends. We attribute the efficient load transfer between the PMMA–MWNT and PVDF–ABS blend to this improvement. In addition, the localization of the MWNTs can be tuned in the blends by wrapping it with PMMA. This resulted in improved electrical conductivity in the blends. Interestingly, the blends with PMMA wrapped MWNTs shielded EM radiation mostly by absorption as against blends with MWNTs where the dominant mechanism was reflection. This was addressed to high dielectric constant materials (like PMMA) on the surface of MWNTs. This study opens new avenues in designing materials that can show simultaneous improvement in mechanical, electrical conductivity and shielding EM radiations.

Acknowledgements The authors gratefully acknowledge the financial support from CSIR (India).

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