An efficient strategy to develop microwave shielding materials with enhanced attenuation constant

Shital Patangrao Pawar, Viraj Bhingardive, Ajinkya Jadhav and Suryasarathi Bose*
Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India. E-mail: sbose@materials.iisc.ernet.in

Received 31st August 2015 , Accepted 5th October 2015

First published on 6th October 2015


Abstract

A mutually miscible homopolymer (here polymethyl methacrylate; PMMA) was employed to tailor the interfacial properties of immiscible polycarbonate/styrene acrylonitrile (PC/SAN) blends. In order to design materials that can shield microwave radiation, one of the key properties i.e. electrical conductivity was targeted here using a conducting inclusion; multiwall carbon nanotubes (MWNTs). Owing to higher polarity, MWNTs prefer PC over SAN which though enhance the electrical conductivity of the blends, they don’t improve the interfacial properties and results in poor mechanical properties. Hence, an efficient strategy has been adopted here to simultaneously enhance the mechanical, electrical and microwave attenuation properties. Herein, the MWNTs were wrapped by PMMA via in situ polymerization of MMA (methyl methacrylate). This strategy resulted in the migration of PMMA modified MWNTs towards the blend’s interface and resulted in an effective stress transfer across the interface leading to improved mechanical and dynamic mechanical properties. Interestingly, the bulk electrical conductivity of the blends was also enhanced, manifesting the improved dispersion of the MWNTs. The state of dispersion of the MWNTs and the phase morphology were assessed using scanning electron microscopy. The microwave attenuation properties were evaluated using a vector network analyzer (VNA) in the X and Ku-band frequencies. The blends with PMMA wrapped MWNTs manifested a −21 dB of shielding effectiveness which suggests attenuation of more than 99% of the incoming microwave radiation. More interestingly, the attenuation constant could be tuned here employing this unique strategy. This study clearly opens a new tool box in designing materials that show improved mechanical, dynamic mechanical, electrical conductivity and microwave shielding properties.


Introduction

Over the past decades, various materials were designed based on polymer blends and were introduced for a wide range of applications. Since blending is the mixing of commercially available polymers and involves no synthesis of new materials, it is a well-established approach to design materials with synergistic properties and hence, has received a great deal of attention. In order to achieve a required set of properties and to design a commercially viable material, the blending of different engineered polymers has hence become an efficient practice. However, blending is not a straightforward task and it is greatly dependent on the nature of the interactions and compatibility between the components. The unfavourable enthalpy of mixing and high molecular weight often leads to macro phase separation. Apart from the intrinsic properties of the components, phase morphology plays an important role in deciding the final blend properties.1–6 Various types of phase structures such as matrix-droplet,7,8 co-continuous,9–11 lamellar12 and fibrillar13–15 can be designed in the case of immiscible polymer blends. In the case of immiscible blends, a weaker interface leads to poor mechanical properties, hence, manipulation of the interface is a key factor to realize the synergetic properties developed using blending.16–18 In this context, fundamental research to understand the effect of various parameters is an essential task. Over the years, it was realized that the compatibilization can be achieved by suppressing the droplet coalescence and minimizing the interfacial tension. The latter strategy turns out to be an efficient way of compatibilization in relation to binary blends.

Various types of approaches like reactive compatibilization,19–23 co-polymers24–27 and mutually soluble polymers28 have been extensively studied to manipulate the interfacial properties. In the case of the classical compatibilization approach, macromolecules act as an emulsifier which lowers the interfacial tension and stabilizes the phase morphology. Though the preferential adsorption of macromolecules at the blend interface minimizes the interfacial tension, it is greatly dependent on the concentration of the compatibilizer and often is addressed as critical micelle concentration (CMC). The CMC can be estimated using the following relationship:29

image file: c5ra17624g-t1.tif
where ϕCMC+ corresponds to the volume fraction of the compatibilizer and μCMC represents the chemical potential at the CMC. As the concentration of the compatibilizer increases the interfacial tension goes down, however beyond the CMC it migrates to one of the phases. This decreases the efficiency of compatibilization and eventually affects the mechanical properties of the blends.

The immiscible blend of PC (polycarbonate) and SAN (styrene acrylonitrile) is a classical system and has been studied extensively to understand the phase morphology and mechanical properties. Various compatibilizers have been reported in the literature to stabilize the phase morphology of the PC/SAN blends. In this context, Kim et al.30 reported that PC-b-PMMA and TMPC-b-PMMA are efficient compatibilizers for PC/SAN blends. This was also reported by Kang et al.31 In order to specifically adsorb the macromolecules at the interface, it should be neutral towards both phases. In this context, the appropriate selection of a macromolecule which is mutually soluble can eliminate the complexity in synthesizing suitable co-polymers. Apart from this approach, various nanoscopic particles like carbon nanotubes (CNTs), graphene and nanoclay were utilized for designing composites with enhanced mechanical strength. Since CNTs manifest a very low percolation threshold they has gained popularity for applications where both structural stability and conductivity is essential. Over the last few years, it was well understood that due to the nanoscopic size and π–π interactions between the tubes, agglomeration of the CNTs in the polymer matrices is inevitable and this further results in stress concentration points. At higher fractions of nanoparticles the mechanical performance of the composites deteriorates and limits their applications.

Here, we have made an attempt to simultaneously enhance the mechanical/dynamic mechanical properties, electrical conductivity and electromagnetic interference (EMI) shielding performance of the PC/SAN blends. This was achieved by using PMMA wrapped MWNTs via in situ polymerization of MMA. Since, PMMA is a co-solvent for the PC/SAN blends, it acts as a vehicle to selectively localize the MWNTs at the blend interface which further enhances the mechanical properties and the bulk electrical conductivity. In order to realize the efficacy of PMMA wrapped MWNTs, a few compositions were also prepared wherein the PMMA and MWNTs were separately mixed with the blend components. The systematic study carried out here clearly demonstrates the fact that the PMMA wrapped MWNTs result in an extraordinary synergy which improves the structural, electrical and microwave shielding properties in the PC/SAN blends. Hence, this study shows new avenues in designing multifunctional properties using a relatively smaller fraction of MWNTs.

Experimental section

Materials

Polycarbonate (PC) Lexan-143 R with a MFI of 11 g per 10 min was procured from SABIC. PMMA with a weight average molecular weight (Mw) of 95[thin space (1/6-em)]000 g mol−1 (Atuglass V825T) was obtained from GSFC, India. Polystyrene-co-acrylonitrile (SAN, Mn of 165[thin space (1/6-em)]000 g mol−1), methyl methacrylate (MMA) and o-dichloro benzene (o-DCB) were procured from Sigma Aldrich. Pristine multiwall carbon nanotubes (MWNTs) (average diameter and average length of 9.5 nm and 1.5 μm, respectively) were obtained from Nanocyl SA, Belgium. Analytical grade sodium hydroxide (NaOH) and other solvents were obtained from commercial sources.

Preparation of the blends

Blends of PC and SAN were prepared by melt mixing using a twin-screw Haake minilab-ΙΙ micro-compounder under a controlled nitrogen atmosphere with a rotation speed of 60 rpm at 260 °C. A similar protocol was followed for the preparation of the PC/SAN blends with PMMA as a compatibilizer with varying concentrations of PMMA (3, 5 and 10 wt%). The given materials were pre-dried at 80 °C in a vacuum for 12 h before processing and PC was additionally vacuum dried for 4 h at 120 °C. In the case of the blends with the MWNTs and PMMA-MWNTs, in order to understand the individual effects of the MWNTs, the concentration of the MWNTs was fixed at 2 wt%. The as obtained melt mixed sample was compression moulded into the required shapes using a laboratory scale hydraulic hot press at 260 °C and then used for further characterization.

Synthesis of PMMA wrapped MWNTs

In order to wrap PMMA onto the surface of the MWNTs, in situ polymerization of MMA was accomplished in the presence of the MWNTs (Scheme 1). MMA was purified using an aqueous NaOH solution a priori. Subsequently, moisture and oxygen were removed from the round bottomed (RB) flask by preheating and by at least three freeze–pump–thaw cycles to keep the effective inert atmosphere during the reaction. The MWNTs (400 mg) were well dispersed using sonication in o-DCB for 30 min. Furthermore, 1 ml of MMA was subsequently added to this mixture. The reaction was carried out in the presence of AIBN as an initiator for 24 h in the preheated RB flask at 65 °C. The obtained slurry was vacuum dried and was used for further characterization. The final reaction product was referred to as PMMA-MWNTs.
image file: c5ra17624g-s1.tif
Scheme 1 Synthesis of PMMA wrapped MWNTs (PMMA-MWNTs).

Characterization

Fourier transform infrared (FTIR) spectra were obtained using a Perkin-Elmer GX in the range of 4000–400 cm−1 with a resolution of 4 cm−1. High resolution transmission electron micrographs were captured using a FEI Technai F30 S-TWIN transmission electron microscope (TEM). A scanning electron microscope (SEM) was employed for the morphological analyses of the various blends. The thermal properties were studied using a calibrated differential scanning calorimeter (DSC), Q2000 TA instruments with a controlled heating rate of 10 °C min−1. The glass transition was analysed from the second heating cycle to avoid the effect of processing history.

The tensile properties were measured using a universal tensile testing machine at room temperature. At least three dumbbell-shaped specimens were prepared for each composition by using a laboratory scale hydraulic press at 260 °C. The tensile tests were carried out at a cross head speed of 5 mm min−1 using a preload of 2 N. The thermo-mechanical analysis was carried out using a dynamic mechanical analyzer (DMA), Q 800 (TA instruments). The specimens required for analysis were prepared in the desired shape using a laboratory scale hydraulic hot press at 260 °C. The dynamic temperature spectra were recorded with tension loading with a vibrating frequency of 1 Hz in the linear viscoelastic region (LVR) which was estimated a priori using multi-strain experiments.

The viscoelastic properties were analysed by rheological experiments performed using a Discovery hybrid rheometer (TA instrument) with a parallel plate geometry of 25 mm diameter and 1 mm of experimental gap under a constant supply of nitrogen. All specimens were vacuum dried for 12 h before the experiments were carried out. The linear viscoelastic region (LVR) was determined a priori and all frequency sweep experiments were performed in the LVR.

The room temperature dielectric properties were studied using Novocontrol broadband dielectric spectroscopy.

The microwave attenuation properties were analyzed using a vector network analyzer (VNA), (Anritsu, MS4642A) in the X and Ku-band frequency range. The experiments were performed using a Damaskos coax sample holder (M 07T) coupled with VNA. The toroidal shape specimens with a 5 mm thickness were moulded using lab scale compression moulding to fit in a coax set and the scattering parameters were recorded. In order to avoid the reflection and absorption losses generated due to the transmission line and sample holder, the measurement setup was calibrated using the two port short-open-load-through (SOLT) method a priori to the experiments.

Results and discussions

Characterization of PMMA wrapped MWNT

The in situ polymerization of MMA in the presence of the MWNTs was carried out to achieve PMMA wrapped MWNTs. The presence of PMMA on the surface of the MWNTs was confirmed using TEM and FTIR. Fig. 1a shows the TEM micrograph of the PMMA-MWNTs. The direct evidence of the PMMA wrapping onto the surface of the MWNTs is observed from the TEM micrograph and the layer is approximately 4–8 nm thick. The wrapping of the MWNTs with PMMA eventually manifested an increased diameter of the MWNTs and is clearly marked in the micrograph. Fig. 1b depicts the FTIR spectrum for the PMMA wrapped MWNTs and for reference, the FTIR spectrum of the MWNTs is also shown here as an inset. In the case of the PMMA-MWNTs, various vibrations were recorded at 1742, 2927 and 1442 cm−1 and were assigned to C[double bond, length as m-dash]O stretching, C–H stretching and C–H3 and C–H2 deformation, respectively. Both the FTIR and TEM analyses clearly show the presence of PMMA on the surface of the MWNTs.
image file: c5ra17624g-f1.tif
Fig. 1 (a) TEM micrograph of PMMA-MWNTs and (b) FTIR spectrum for PMMA-MWNTs and inset shows FTIR spectrum for MWNTs.

Tailored interface in PC/SAN blends in the presence of PMMA wrapped MWNTs

The phase morphology has a dominant effect on deciding the final properties of immiscible blends. The phase morphology was studied using SEM. Fig. 2 depicts the SEM micrographs of the PC/SAN blends with varying concentrations of PMMA and MWNTs. In order to improve the phase contrast; the PC phase was etched out by hydrolysis. From the SEM micrographs, it was realized that the PC/SAN (40/60) blends manifested a droplet-matrix type of microstructure where the PC droplets were dispersed in the SAN matrix. The morphology developed here is due to the high interfacial tension between the PC and SAN phases. Over the years, it was well appreciated that minimization of the interfacial tension can lead to finer structures.32,33 In our case, since the higher viscous phase (PC) is dispersed in the lower viscous SAN phase34 the resistance offered for coalescence by the matrix is rather small. Hence, we adopted a unique strategy to achieve a relatively finer structure by minimizing the interfacial tension. The addition of PMMA showed a significant effect on the droplet size. It was realized that 5 wt% PMMA resulted in an effective reduction in the droplet size. Whereas, further addition resulted in a coarser morphology. These observations suggest that the critical concentration of PMMA is 5 wt%. Fig. 2(e1) and (e2) show the SEM micrographs of the blends with the MWNTs. Interestingly, with the addition of 2 wt% MWNTs, the droplet size was observed to decreased when compared to the neat blends. It has been well studied that the localization of the MWNTs has a vital role in deciding the final morphology. The higher magnification SEM micrograph of the blends with the MWNTs clearly shows that the MWNTs are localized in the PC phase however a few MWNTs were also observed in the SAN phase. The localization of the MWNTs in the matrix phase may lead to smaller droplets due to the increased viscosity of the matrix phase thereby preventing coalescence. A similar morphology was observed in the case of the blends with 2 wt% MWNTs and 5 wt% PMMA (Fig. 2(f1) and (f2)). Interestingly, in the case of the blends with the PMMA-MWNTs, a significant refinement in the phase morphology was well realized (Fig. 2(g1)). This can be ascribed to the preferential localization of the PMMA-MWNTs at the blend interface. Fig. 2(g2) depicts that most MWNTs are adsorbed at the blend interface. This may further result in enhanced mechanical properties.
image file: c5ra17624g-f2.tif
Fig. 2 SEM micrographs of PC/SAN blends with (a) neat, (b) 3 wt% PMMA, (c) 5 wt% PMMA and (d) 10 wt% PMMA, (e1 and e2) 2 wt% MWNTs, (f1 and f2) 5 wt% PMMA and 2 wt% MWNTs and (g1 and g2) PMMA-MWNTs.

Thermal properties: assessing through DMTA and DSC

The effect of the PMMA and MWNTs on the dynamic mechanical properties was studied here using DMTA. Fig. 3a shows the storage modulus as a function of temperature for the various blends. In the case of immiscible blends, the structural properties are mainly governed by phase morphology, interfacial adhesion and effective stress transfer. Since, all of the DMTA experiments were performed in the linear regime; the effect of stress transfer through the interface is well appreciated below and above the glass transition temperature. The neat PC/SAN blend manifested a storage modulus of 3.04 GPa in the glassy region (40 °C). The evident decrease in the storage modulus at ca. 125 °C manifests the Tg of SAN. It was difficult to determine the Tg of the PC phase as the samples were too soft to be measured hence, we carried out DSC to gain knowledge on the Tg of the PC phase and this will be discussed later on. The thermo-mechanical properties of the various PC/SAN blends are listed in Table 1. All of the blends showed the classical brittle to ductile transition at a higher temperature range. The compatibilization effect of PMMA on the interfacial properties was also realized from the DMTA where the storage modulus improved significantly, manifesting an enhanced phase compatibility. Over the years, it was well established that the incorporation of high aspect ratio nanoparticles generally results in an enhanced storage modulus. Herein, MWNTs were dispersed in the PC/SAN blends and this resulted in significantly enhanced storage modulus over the neat blends. This is due to the high aspect MWNTs which act as a reinforcement agent and improve the stress transfer. Similarly, the effect of improved interfacial tension by PMMA and the reinforcement effect of the MWNTs is realized in the blends with the simultaneous addition of 5 wt% PMMA and 2 wt% MWNTs. Interestingly, the highest storage modulus of 3.94 GPa was recorded in the blends with the PMMA-MWNTs. The striking enhancement in the storage modulus is due to the synergistic effect of PMMA on the enhanced phase compatibility and reinforcement effect of the well dispersed MWNTs. Fig. 3b illustrates the loss tangent (loss modulus/storage modulus) as a function of temperature for the various blends. The peak in the loss tangent represents the Tg of the blend component. The loss tangent peak centered at 135 °C corresponds to the Tg of SAN. The addition of PMMA showed no effect however a slight decrease in the Tg of SAN was recorded for the blends with 2 wt% MWNTs. A similar observation was recorded for the blends with 5 wt% PMMA and 2 wt% MWNTs where the Tg was further decreased. Interestingly, the addition of PMMA-MWNTs resulted in a significantly lower Tg for the SAN phase however, the Tg of PC is not affected as inferred from the DSC.
image file: c5ra17624g-f3.tif
Fig. 3 (a) Storage modulus and (b) tan delta as a function of temperature and (c) DSC thermograph for PC/SAN blends.
Table 1 Thermal properties of various PC/SAN blends
PC/SAN blends with Storage modulus (at 40 °C), MPa Glass transition temperature from tan[thin space (1/6-em)]δ Glass transition temperature of PC (from DSC) Glass transition temperature of SAN (from DSC)
Neat 3042 135.9 °C 140.5 °C 108.1 °C
5 wt% PMMA 3250 136 °C 140.2 °C 108.2 °C
2 wt% MWNTs 3463 132.4 °C 140.4 °C 108.7 °C
2 wt% MWNTs + 5 wt% PMMA 3665 129.5 °C 140.2 °C 108.7 °C
PMMA-MWNTs 3943 124 °C 140.5 °C 103.5 °C


The effect of the PMMA and MWNTs on the thermal transition properties was investigated here using DSC (Fig. 3c). In the case of immiscible polymer blends, in general, it is evident that the distinct thermal behavior corresponds to the blend component. The Tg of PC and SAN from the various PC/SAN blends are listed in Table 1. In the case of the neat blends, the transition at 108 and 140 °C corresponds to the glass transition temperature (Tg) of the SAN and PC phase, respectively. It was observed that the addition of 5 wt% PMMA or 2 wt% of MWNTs resulted in no significant change in the Tg of the components. From these observations it was understood that PMMA has a neutral affinity towards the blend components and localizes at the interface. However, the values of the Tg are significantly different in the DSC and DMTA due to the nature of signals that are measured being quite different. Interestingly, in the case of the blends with the PMMA-MWNTs, the Tg of the SAN phase decreased. This is due to the migration of the relatively smaller PMMA chains towards the SAN phase and this influences the chain mobility of SAN. In summary, the in situ synthesized PMMA with the given concentration of MWNTs resulted in an effective approach to enhance the mechanical performance of the blends along with other functional properties which will be discussed in the subsequent sections.

Viscoelastic behavior and network formation of MWNTs in PC/SAN blends

In order to get an insight into the viscoelastic properties and the polymer–particle interactions leading to a gel-like structure, frequency sweep experiments were carried out in the melt state. Fig. 4a illustrates the response of the storage modulus in the given frequency range of 0.1 to 100 rad s−1. It was realized that the addition of PMMA has no effect on the storage modulus of the blends. This suggests that PMMA is probably localized at the interface or the concentration of PMMA is not sufficient to alter the viscoelastic properties of the blends. In the case of the blends with the MWNTs, a pseudo network like structure is well evident from the secondary plateau of the storage modulus, especially at lower frequencies. The presence of the MWNTs hinders the macromolecular mobility resulting in confined motion. The blends with 2 wt% MWNTs exhibited increased storage modulus over the neat blends along with a frequency independent plateau at low frequencies which confirms the physical gelation and is often addressed as rheological percolation.35–37 This elastic characteristic of the blends can be attributed to the network formation of entangled polymer chains and flexible MWNTs which is a result of the physical interaction between the entities. Interestingly, in case of the blends with 2 wt% MWNTs further addition of PMMA resulted in a slightly enhanced storage modulus whereas, the blends with the PMMA-MWNTs manifested the highest storage modulus. This corresponds to the enhanced dispersion of the MWNTs facilitated by PMMA. It was well understood that in the case of CNTs wrapped with a layer of polymer, the layer of polymer prevented the agglomeration by counterbalancing the van der Waals’ interactions between the CNTs. This enhanced dispersion of MWNTs also resulted in a significant increase in the electrical conductivity and this will be discussed later.
image file: c5ra17624g-f4.tif
Fig. 4 (a) Storage modulus and (b) complex viscosity as a function of frequency for PC/SAN blends.

Fig. 4b shows the complex viscosity behavior of the blends as a function of frequency. Neat PC/SAN (40/60) blends exhibited a Newtonian behavior over the measured frequency range wherein the complex viscosity is almost frequency independent. The effect of the filler on the flow properties is most prominent at lower frequencies where sufficient time is available for molecular relaxation. Similar observations are evident in the case of the blends with 2 wt% MWNTs which showed a drastic increase in complex viscosity along with a strong shear thinning behaviour especially at lower frequencies. The enhanced dispersion of MWNTs is also realized from the higher complex viscosity of the blends in the presence of the PMMA-MWNTs.

The effect of tailored interface on structural properties of PC/SAN blends

Fig. 5 depicts the tensile properties of the various PC/SAN blends. In the case of multicomponent systems, structural properties are greatly influenced by the phase morphology and interfacial tension between the immiscible phases. Hence, in order to tailor the interfacial properties, we have chosen a unique strategy wherein a mutually soluble homopolymer is used which preferentially adsorbs at the interface. It is well established that the effect of the modified interface can be realized from the % elongation at break and the increased elongation suggests enhanced adhesion or compatibility of the blend component. The PC/SAN blends are chemically incompatible and form immiscible phases with poor adhesion strength which further results in poor mechanical properties due to inefficient stress transfer. The effect of PMMA on phase adhesion was well evident here and the % elongation scaled with the concentration of PMMA (see Fig. 5d). The enhanced mechanical properties confirm the key role of PMMA in compatibilizing the PC/SAN blends. In general, this corresponds to the effective stress transfer at the phase interface due to the improved interfacial tension. Moreover, the refined phase structure resulted in improved phase adhesion which eventually led to the enhanced mechanical properties. This can be perceived by comparing the mechanical properties of the neat blends and the blends with 5 wt% PMMA (Fig. 5a). In the latter case, a significant enhancement was realized in the tensile strength (Fig. 5b) and Young’s modulus (Fig. 5c) as well. Interestingly, at a higher concentration of PMMA (10 wt%), the % elongation significantly decreased which suggests that the critical concentration of PMMA is ca. 5 wt% for the PC/SAN (40/60) blends. This observation is also supported by the phase morphology studied using SEM.
image file: c5ra17624g-f5.tif
Fig. 5 (a) Stress as a function of strain, (b) tensile strength, (c) Young’s modulus and (d) % strain for various PC/SAN blends. In case of a–c, neat blends (indicated as sample 1), blends with 3, 5 and 10 wt% PMMA (indicated as sample 2, 3 and 4 respectively), blends with 2 wt% MWNTs (indicated as sample 5), blends with PMMA-MWNTs (indicated as sample 6) and blends with 5 wt% PMMA and 2 wt% MWNTs (indicated as sample 7).

In the case of the polymer blends with dispersed nanoparticles, the structural properties are governed by different parameters like matrix properties, state of nanoparticle dispersion, network formation (percolation threshold) and the specific interaction of the nanoparticles with the host matrix. In general, the use of nanoparticles (in this case CNTs) leads to enhanced mechanical properties however, at higher concentrations, the lack of polymer–filler interactions often leads to poor mechanical properties. In the case of the blends with 2 wt% MWNTs, it was realized that the Young’s modulus was significantly enhanced however; the tensile strength almost remained unchanged when compared to the neat blends. This is often ascribed to the agglomerated network formation of the MWNTs due to the inter-tube π–π interactions. The lower % elongation suggests poor polymer–filler interactions and the agglomeration of the MWNTs which act as a stress concentrator. One can enhance the stress transfer by improving dispersion and developing better polymer–filler interactions. Here, the PMMA wrapped MWNTs resulted in enhanced dispersion by minimizing the inter-tube π–π interactions and resulted in improved stress transfer. This can be realized from the enhanced tensile strength (Fig. 5b) and significantly high Young’s modulus (Fig. 5c) in contrast to the neat blends. Similarly, blends with 5 wt% PMMA and 2 wt% MWNTs manifested significantly enhanced tensile strength and % elongation (Fig. 5d) over the blends with only MWNTs. Herein, the use of mutually soluble PMMA as a compatibilizer is well justified, however the unique strategy of PMMA-MWNTs is well appreciated here for enhancing the mechanical properties of the blends along with the enhanced multifunctional properties designed using MWNTs.

Enhanced electrical conductivity: effect of PMMA wrapped MWNTs

The AC electrical conductivity of various PC/SAN blends at room temperature was analyzed by employing broadband dielectric spectroscopy. Fig. 6 shows the frequency dependent electrical conductivity in the presence of PMMA as a compatibilizer and the MWNTs as a conducting inclusion. Owing to the insulating characteristics, electrical conductivity of the neat blends scaled with frequency (not shown here for clarity). The addition of 2 wt% MWNTs manifested a strikingly enhanced electrical conductivity (8.5 × 10−6 S cm−1) along with a frequency independent plateau at lower frequencies. This observation supports the network formation of MWNTs in the blends and often is addressed as electrical percolation.37–40 Furthermore, no significant effect was recorded in the electrical conductivity for the blends with the simultaneous addition of PMMA (5 wt%) and MWNTs (2 wt%) over the blends with only MWNTs. This suggests that the simultaneous addition of PMMA and MWNTs has no effect on the network formation of the MWNTs. Interestingly, the electrical conductivity further improved in the blends in the presence of the PMMA-MWNTs. From recent literature it was realized that in general the in situ synthesis of polymers in the presence of nanoparticles results in enhanced dispersion by counterbalancing the inter-tube π–π interactions due to the wrapping of the thin polymer layer. Moreover, since PMMA is mutually soluble in PC and SAN it may drive the MWNTs to the interface. This presumably explains the enhanced electrical conductivity which is not achieved by the simultaneous addition of PMMA and MWNTs.
image file: c5ra17624g-f6.tif
Fig. 6 Room temperature AC electrical conductivity as a function of frequency.

Enhanced EMI shielding effectiveness and attenuation constant: effect of PMMA-MWNTs

In general, EMI shielding theory suggests that the conductivity is an essential property for the attenuation of microwave radiation.41–43 Since polymers are insulating in nature they are transparent to microwave radiation. Hence, dispersion of conducting inclusions in host polymer matrices is an effective practice to achieve enhanced conductivity.40,44,45 Here, MWNTs are chosen as a conducting inclusion due to their high aspect ratio and low percolation threshold.40 Apart from the electrical conductivity, network formation and electrical dipoles are part of how CNTs achieve the enhanced attenuation of microwave radiation. It has been well accepted that microwave attenuation is governed by three different mechanisms: reflection, absorption and multiple reflection.46–49 The total shielding effectiveness (SET) is a summation of shielding effectiveness through reflection (SER), absorption (SEA) and multiple reflection (SEMR) and expressed as SET = SER + SEA + SEMR. The SET is the amount of incident microwave radiation blocked by the shield and is presented in dB. In general, values of −10 and −20 dB for SET correspond to the 90 and 99% attenuation of incoming radiation. The SET can be estimated using incident and transmitted power balance and expressed as,
 
image file: c5ra17624g-t2.tif(1)
where E corresponds to the electric, H represents the magnetic and P is the electromagnetic power. The subscripts I and T correspond to the incident and transmission parts of the respective powers mentioned above. Here, we have employed the VNA to record the scattering parameters on toroidal specimens. The various scattering parameters thus obtained using the VNA represent the absorbed, reflected and transmitted power. The SET can be estimated using scattering parameters from the following relationship:
 
image file: c5ra17624g-t3.tif(2)
where S21 and S12 correspond to the coefficients of forward transmission and reverse transmission respectively.

Fig. 7a illustrates a cartoon depicting the attenuation of microwave radiation in the PC/SAN blends. The cartoon provides a clear understanding about the reflection, absorption and transmission of radiation through the shield thickness. Fig. 7b depicts the SET as a function of frequency. Since, the neat blends are insulating in nature it manifested negligible microwave attenuation. The addition of 2 wt% MWNTs manifested a SET of −17 dB at a reference frequency of 18 GHz. It was observed that the SET increases with frequency which is also realized from the EMI shielding theory. The blends with 5 wt% PMMA and 2 wt% MWNTs manifested an enhanced SET in contrast to the blends with only MWNTs. Interestingly, the blends with PMMA-MWNTs manifested a significantly enhanced SET over the blends with MWNTs at the given concentration of MWNTs. The highest SET of −21 dB was recorded at a 18 GHz reference frequency which suggests more than 99% of the incoming microwave radiation is blocked by the shield. The enhanced microwave attenuation is in line with electrical conductivity of the blends. Moreover, the enhanced network formation of the well dispersed PMMA-MWNTs may result in a higher SET due to multiple scattering within the network of the CNTs.


image file: c5ra17624g-f7.tif
Fig. 7 (a) Cartoon illustrating microwave attenuation, (b) SET, (c) dielectric loss tangent (tan[thin space (1/6-em)]δε) and (d) attenuation constant as a function of frequency.

We took a closer look into the mechanism of microwave attenuation by evaluating the dielectric loss tangent and attenuation constant in the X and Ku-band. Fig. 7c illustrates the dielectric loss tangent (tan[thin space (1/6-em)]δε) as a function of frequency for the blends studied here. The dielectric loss observed in the blends represents the attenuation of the electric field associated with the microwave radiation through dielectric polarization and electrical conductivity. The blends with PMMA and 2 wt% MWNTs showed an enhanced tan[thin space (1/6-em)]δε over the blends with only MWNTs. Interestingly, the blends with the PMMA-MWNTs manifested significantly high lossy characteristics in the X and Ku-band. This observation can be correlated with the SET. The highest total attenuation in the case of the blends with PMMA-MWNTs is due to the enhanced electrical conductivity and dielectric loss. Apart from dielectric loss, the attenuation constant also suggests a mechanism of shielding. The attenuation constant provides a clear understanding about the ability of a shielding material to attenuate microwave radiation. The attenuation constant was estimated using the complex microwave properties obtained using the Nicolson and Ross method using the following relationship.

 
image file: c5ra17624g-t4.tif(3)

Fig. 7d depicts the attenuation constant as a function of frequency. It is realized that the attenuation constant was significantly increased in the blends with PMMA and MWNTs over the blends with only MWNTs. Interestingly, the blends with PMMA-MWNTs manifested the highest attenuation constant at the given concentration of MWNTs. One can easily correlate the enhanced attenuation constant with dielectric losses and SET. The enhanced attenuation constant is due to the increased electrical conductivity and network-like structure of the MWNTs. Moreover, the high dielectric constant for PMMA also results in improved complex microwave properties and facilitates in enhancing the attenuation constant. Taken together, the enhanced dielectric losses and attenuation constant can provide a synergistic enhancement in the EMI shielding properties. Here, the microwave properties like the dielectric loss tangent and attenuation constant were tuned using the PMMA wrapped MWNTs and enhanced microwave attenuation was achieved at a lower fraction of MWNTs. The PC/SAN blends with the PMMA-MWNTs showed attenuation of more than 99% of the incoming microwave radiation with significantly enhanced mechanical properties. This study clearly demonstrates that a multifunctional material with a wide set of properties can be designed and be explored for numerous commercial applications where structural stability and EMI shielding are essential criteria.

Conclusions

In summary, the interfacial properties of immiscible polycarbonate/styrene acrylonitrile (PC/SAN) blends were tailored using a mutually miscible homopolymer (here poly(methyl methacrylate); PMMA). The mechanism behind the phase compatibility is realized as decreased interfacial tension between the phases. In order to design materials that can shield microwave radiation, electrical conductivity was targeted here using a high aspect conducting inclusion (multiwall carbon nanotubes). Since the PMMA is a co-solvent for PC and SAN, a unique strategy was adopted here to drive the MWNTs to the interface using PMMA and this resulted in enhanced electrical conductivity. The PMMA was uniformly coated on to the surface of MWNTs by in situ polymerization of MMA in the presence of the MWNTs. Form the SEM studies it was realized that the MWNTs were at the interface. This eventually resulted in a finer phase morphology. The enhanced mechanical properties were realized at a critical concentration of 5 wt% of PMMA. The PMMA-MWNTs manifested significantly improved mechanical properties over the blends with the MWNTs at the given concentration of MWNTs. Moreover, the highest storage modulus was realized in the blends with the PMMA-MWNTs from the DMTA analysis. These observations reveal that the mechanical performance of the composites is enhanced significantly by the addition of the PMMA-MWNTs. While the interfacial tension was tailored using the specific absorption of the PMMA-MWNTs at the interface the electrical conductivity was significantly enhanced from the synergistic combination of the PMMA and MWNTs. Moreover, enhanced microwave attenuation was realized through the PMMA-MWNTs by tuning the dielectric properties and the attenuation constant in the X and Ku-band. This study clearly opens a new tool box in designing materials that show unique synergy in mechanical, dynamic mechanical, electrical conductivity and microwave shielding properties.

Acknowledgements

The authors would like to acknowledge the Department of Science and technology (DST), and CSIR, India, for providing the financial aid.

References

  1. S. Jose, A. Aprem, B. Francis, M. Chandy, P. Werner, V. Alstaedt and S. Thomas, Eur. Polym. J., 2004, 40, 2105–2115 CrossRef CAS PubMed.
  2. S. Cimmino, L. D’orazio, R. Greco, G. Maglio, M. Malinconico, C. Mancarella, E. Martuscelli, R. Palumbo and G. Ragosta, Polym. Eng. Sci., 1984, 24, 48–56 CAS.
  3. M. A. Huneault and H. Li, Polymer, 2007, 48, 270–280 CrossRef CAS PubMed.
  4. C. C. Chen and J. L. White, Polym. Eng. Sci., 1993, 33, 923–930 CAS.
  5. B. Majumdar, H. Keskkula and D. Paul, Polymer, 1994, 35, 5453–5467 CrossRef CAS.
  6. R. Dell’Erba, G. Groeninckx, G. Maglio, M. Malinconico and A. Migliozzi, Polymer, 2001, 42, 7831–7840 CrossRef.
  7. J. Vermant, S. Vandebril, C. Dewitte and P. Moldenaers, Rheol. Acta, 2008, 47, 835–839 CrossRef CAS.
  8. P. Martin, P. Carreau, B. Favis and R. Jérôme, J. Rheol., 2000, 44, 569–583 CrossRef CAS.
  9. P. Sarazin and B. D. Favis, Biomacromolecules, 2003, 4, 1669–1679 CrossRef CAS PubMed.
  10. R. Tol, G. Groeninckx, I. Vinckier, P. Moldenaers and J. Mewis, Polymer, 2004, 45, 2587–2601 CrossRef CAS PubMed.
  11. P. Pötschke and D. Paul, J. Macromol. Sci., Polym. Rev., 2003, 43, 87–141 CrossRef PubMed.
  12. D. Wu, L. Wu, M. Zhang, W. Zhou and Y. Zhang, J. Polym. Sci., Part B: Polym. Phys., 2008, 46, 1265–1279 CrossRef CAS PubMed.
  13. R. Shields, D. Bhattacharyya and S. Fakirov, J. Mater. Sci., 2008, 43, 6758–6770 CrossRef CAS.
  14. R. Willemse, E. Ramaker, J. van Dam and A. P. de Boer, Polymer, 1999, 40, 6651–6659 CrossRef CAS.
  15. M. Afshari, R. Kotek, M. H. Kish, H. N. Dast and B. S. Gupta, Polymer, 2002, 43, 1331–1341 CrossRef CAS.
  16. D. Paul, Polym. Blends, 1978, 2, 35–62 CAS.
  17. S. S. Ray, S. Pouliot, M. Bousmina and L. A. Utracki, Polymer, 2004, 45, 8403–8413 CrossRef CAS PubMed.
  18. G. Guerrica-Echevarrıa, J. Eguiazábal and J. Nazabal, Polym. Test., 2000, 19, 849–854 CrossRef.
  19. N. Liu and W. Baker, Adv. Polym. Technol., 1992, 11, 249–262 CrossRef CAS PubMed.
  20. Y. Pietrasanta, J. J. Robin, N. Torres and B. Boutevin, Macromol. Chem. Phys., 1999, 200, 142–149 CrossRef CAS.
  21. M. Xanthos and S. Dagli, Polym. Eng. Sci., 1991, 31, 929–935 CAS.
  22. N. Liu, H. Xie and W. Baker, Polymer, 1993, 34, 4680–4687 CrossRef CAS.
  23. C. Tselios, D. Bikiaris, V. Maslis and C. Panayiotou, Polymer, 1998, 39, 6807–6817 CrossRef CAS.
  24. C. Auschra and R. Stadler, Macromolecules, 1993, 26, 6364–6377 CrossRef CAS.
  25. S. H. Anastasiadis, I. Gancarz and J. T. Koberstein, Macromolecules, 1989, 22, 1449–1453 CrossRef CAS.
  26. I. Aravind, P. Albert, C. Ranganathaiah, J. Kurian and S. Thomas, Polymer, 2004, 45, 4925–4937 CrossRef CAS PubMed.
  27. C.-R. Chiang and F.-C. Chang, J. Appl. Polym. Sci., 1996, 61, 2411–2421 CrossRef CAS.
  28. G. P. Kar, S. Biswas and S. Bose, Phys. Chem. Chem. Phys., 2015, 17, 14856–14865 RSC.
  29. L. A. Utracki, Polymer blends handbook, Kluwer Academic Publishers, Dordrecht, The Netherlands, 2002 Search PubMed.
  30. J. Kim and C. Kim, J. Appl. Polym. Sci., 2003, 89, 2649–2656 CrossRef CAS PubMed.
  31. E. Kang, J. Kim, C. Kim, S. Oh and H. Rhee, Polym. Eng. Sci., 2000, 40, 2374–2384 CAS.
  32. B. Favis, Polymer, 1994, 35, 1552–1555 CrossRef CAS.
  33. P. Macaubas and N. Demarquette, Polymer, 2001, 42, 2543–2554 CrossRef CAS.
  34. S. P. Pawar, K. Pattabhi and S. Bose, RSC Adv., 2014, 4, 18842–18852 RSC.
  35. P. Pötschke, T. Fornes and D. Paul, Polymer, 2002, 43, 3247–3255 CrossRef.
  36. A. K. Kota, B. H. Cipriano, M. K. Duesterberg, A. L. Gershon, D. Powell, S. R. Raghavan and H. A. Bruck, Macromolecules, 2007, 40, 7400–7406 CrossRef CAS.
  37. C. McClory, T. McNally, M. Baxendale, P. Pötschke, W. Blau and M. Ruether, Eur. Polym. J., 2010, 46, 854–868 CrossRef CAS PubMed.
  38. W. Lu, T.-W. Chou and E. T. Thostenson, Appl. Phys. Lett., 2010, 96, 223106 CrossRef PubMed.
  39. G. Hu, C. Zhao, S. Zhang, M. Yang and Z. Wang, Polymer, 2006, 47, 480–488 CrossRef CAS PubMed.
  40. W. Bauhofer and J. Z. Kovacs, Compos. Sci. Technol., 2009, 69, 1486–1498 CrossRef CAS PubMed.
  41. D. Chung, J. Mater. Eng. Perform., 2000, 9, 350–354 CrossRef CAS.
  42. S. P. Pawar, S. Kumar, A. Misra, S. Deshmukh, K. Chatterjee and S. Bose, RSC Adv., 2015, 5, 17716–17725 RSC.
  43. S. P. Pawar, S. Stephen, S. Bose and V. Mittal, Phys. Chem. Chem. Phys., 2015, 17, 14922–14930 RSC.
  44. R. Sengupta, M. Bhattacharya, S. Bandyopadhyay and A. K. Bhowmick, Prog. Polym. Sci., 2011, 36, 638–670 CrossRef CAS PubMed.
  45. O. Breuer and U. Sundararaj, Polym. Compos., 2004, 25, 630–645 CrossRef CAS PubMed.
  46. M. H. Al-Saleh, W. H. Saadeh and U. Sundararaj, Carbon, 2013, 60, 146–156 CrossRef CAS PubMed.
  47. S. Geetha, K. Satheesh Kumar, C. R. Rao, M. Vijayan and D. Trivedi, J. Appl. Polym. Sci., 2009, 112, 2073–2086 CrossRef CAS PubMed.
  48. D. Chung, Carbon, 2001, 39, 279–285 CrossRef CAS.
  49. S. P. Pawar, D. A. Marathe, K. Pattabhi and S. Bose, J. Mater. Chem. A, 2015, 3, 656–669 CAS.

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