PMMA

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Materials and Interfaces

Effect of Processing Techniques on EMI SE of Immiscible PS/PMMA Blends Containing MWCNT: Enhanced Intertube and Interphase Scattering S. M. Nourin Sultana, Shital Patangrao Pawar, and Uttandaraman Sundararaj Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05957 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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Industrial & Engineering Chemistry Research

Effect of Processing Techniques on EMI SE of Immiscible PS/PMMA Blends Containing MWCNT: Enhanced Intertube and Interphase Scattering S. M. Nourin Sultana, Shital Patangrao Pawar, Uttandaraman Sundararaj* Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Dr NW, Calgary, Canada *Corresponding Author’s Email: [email protected] Keywords: polystyrene (PS), poly methyl methacrylate (PMMA), immiscible polymer blend, morphology, processing technique, electrical conductivity, EMI shielding.

Abstract:

Polystyrene (PS)/ poly (methyl methacrylate) (PMMA)/ multi-walled carbon nanotube (MWCNT) blends are prepared using six different processing techniques to investigate the effect of processing techniques on electromagnetic interference shielding effectiveness (EMI SE) of immiscible polymer blend nanocomposites. Different processing strategies (e.g. sequence of addition of components) and blend compositions influence final morphology, selective 1 Environment ACS Paragon Plus

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localization and dispersion of MWCNT, which are studied by SEM and TEM analysis. We achieved increased EMI SE in polymer blend nanocomposites through multiple scattering (selective localization assisted interphase and better dispersion aided intertube scattering), unlike traditional metal shields, which mostly depend on reflection. PS/PMMA(20:80) and PS/PMMA(80:20) blend nanocomposites with 2.7 vol% MWCNT made by one step solution mixing exhibited 22 dB and 21 dB EMI SE at 0.8 mm thickness. EMI SE is found to increase with composite thickness. Immiscible polymer blend nanocomposites display higher rate of increase in EMI SE with increase in thickness than single polymer nanocomposites. This increased rate of EMI SE in blend nanocomposites is attributed to the synergistic effect of interphase scattering and intertube scattering; whereas, only intertube scattering plays a role in single polymer nanocomposite. PS/PMMA(80:20) blend nanocomposite with 2.7 vol% MWCNT shows 30 dB EMI SE at 2 mm thickness. This work shows the impact of multiple scattering on enhancing EMI SE in microphase-separated polymer blend nanocomposite.

Introduction Electromagnetic (EM) radiation from operating electronic devices may potentially hamper the efficacy and accuracy of other devices. Growing use of electronic devices is increasing the rate of EM pollution (or EMI smog), which needs to be controlled by designing microwave shielding materials1–4. Pre-tin plated carbon steel, copper and aluminum are the most popular metals used for EMI shielding5. However, these metals are heavy, expensive and susceptible to corrosion. Polymer/Carbon nanotube (CNT) nanocomposites6–12 are being studied extensively to overcome the problems involved with metals. CNT13 is one of the widely investigated carbonaceous conductive nanofillers to design nanocomposites with higher electrical properties14–20. It has been

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mentioned extensively that higher electrical conductivity introduces higher EMI shielding effectiveness (EMI SE) in metals21. However, appropriate mechanisms for EMI shielding are not well understood for complex materials like polymer nanocomposites. Studies have shown that immiscible polymer blend nanocomposites can utilize advanced properties of CNT even better through double percolation phenomenon22–24. By double percolation effect23, creating a selective continuous filled phase and increasing effective concentration of nanofiller in that phase alone, we can achieve higher electrical conductivity with higher EMI SE25. Most polymers are immiscible due to high molecular weight and thus, have less affinity to each other. Immiscible polymer blend nanocomposites are good options to design microwave shielding materials19 because microphase-separated nanocomposites26 prepared with microwave active fillers offer microphases with different electrical conductivity. When EM wave penetrates through a conductive microphase-separated section, waves keep scattering through interphases and intertubes27–29. Scattering imparts EMI attenuation30. Thus, multiphase conductive polymer blend nanocomposites exhibit promising EMI Shielding31,32 performance. A large degree of freedom is available to tune EMI attenuating properties of polymer blend nanocomposites such as morphology, nano localization and dispersion, which can be altered in blend systems using different processing33,34. Unlike traditional metal shields, enhanced absorption-driven EM attenuation can be achieved in immiscible polymer blend nanocomposites due to multiple scattering30. The extent of EMI SE attained by different mixing strategies, morphologies, quality of dispersion and level of MWCNT localization is discussed here. Scanning electron microscope (SEM) and transmission electron microscope (TEM) images are used to study and correlate with conductivity and EMI SE. It is observed that mixing technique has a great impact on dispersion



and selective localization of MWCNT. Furthermore, we have observed higher increment rate of EMI SE with thickness in immiscible polymer blend nanocomposite than single polymer nanocomposite. For instance, when thickness of samples is increased by 1.2 mm, PS/PMMA (80:20), PS/MWCNT and PMMA/MWCNT show an increment of 9 dB, 3.5 dB and 5 dB EMI SE, respectively. Multiple scattering effect in immiscible polymer blend nanocomposites is more effective to attenuate EMI than sole intertube scattering effect in single polymer nanocomposites. Intertube scattering and interphase scattering facilitate improved EMI SE in blend nanocomposite, because scattering always facilitates microwave absorption. Experimental Section Materials Polymer blend nanocomposites are prepared using PS Styron 666D from Americas Styrenic LLC, PMMA VM920 from Arkema Inc. and MWCNT (NC 7000) from Nanocyl S.A. (Sambreville, Belgium). According to supplier information, NC 7000 has 9.5 nm average diameter, 1.5 µm average length with 90 % purity and electrical conductivity of 104 S/cm. Chloroform is used to dissolve both of the polymers to formulate solution casted blend nanocomposites. Preparation Blend nanocomposites of three different blend compositions; PS/PMMA (20:80), (50:50) and (80:20) are prepared by six different processing strategies to investigate the effect on morphology evolution, level of selective localization, dispersion and distribution of MWCNT in the blends along with final electrical and EMI shielding properties. Different processing strategies to prepare blend nanocomposites are described in the Table 1:


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Table 1. Summary of different Processing Techniques

PS/PMMA-Solution:

Blend nanocomposites are prepared by solution mixing. First, 2.7 vol% MWCNT is dispersed in chloroform by 30 min of sonication and then transferred to the polymer solution. The mixture is further homogenized by 30 min of sonication. The nanocomposites are extracted from the solution by solvent evaporation and vacuum drying.

PS/PMMA-Melt:

PS and PMMA are melt blended initially for 3 min. MWCNT is added to neat PS/PMMA blend and melt blended for 15 min more.

PSMB(S)/PMMA:

Masterbatch (MB) of PS is prepared by solution mixing. The MB is further melt blended with PMMA for 5 min.

PSMB(M)/PMMA:

MB of PS is prepared by melt blending for 10 min. The MB is further melt blended with PMMA for 5 min.

PS/PMMAMB(S):

MB of PMMA is prepared by solution mixing. The MB is further melt blended with PS for 5 min.

PS/PMMAMB(M):

MB of PMMA is prepared by melt blending. The MB is further melt blended with PS for 5 min.

Melt blending was performed at 230 °C and 300 rpm in an Alberta Polymer Asymmetric minimixer (APAM)35. Blended nanocomposites were compression molded using a Carver compression molder (Carver Inc. Wabash. IN) into a rectangular shape (10 x 23 x 0.8 mm3) at 230 °C under pressure of 35 MPa for 10 min. At least three specimens were prepared for electrical conductivity and EMI shielding measurements.



Characterization Localization of MWCNT was analyzed by TEM. Thin sections of around 90 nm thickness were prepared by Leica EM UC6 from compression molded samples at room temperature. Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA) equipped with Gatal Ultrascan 4000 CCD Camera (Gatan, Pleasanton, California, USA) and Hitachi H-7650 were used to capture TEM images. Morphology of blends was investigated by SEM, using FEI XM30 (FEI Hillsboro OR, USA) at 20 kV accelerating voltage. DC electrical conductivity of the compression molded samples was measured by Loresta GP resistivity meter (MCP-T610 model, Mitsubishi Chemical Co., Japan). Measurement was done for three samples and average value has been reported with standard error bar. EMI SE was studied in X-band frequency range (i.e., 8.2 to 12.4 GHz). Network analyzer (ENA) (Model E5071C), connected to a rectangular wave guide WR-90 was used to measure scattering parameters. EMI SE was calculated using scattering parameters. Results and Discussion MWCNT Localization TEM images are assessed to know the dispersion of conductive MWCNT and to investigate selective localization and perfection towards selectivity of MWCNT in PS/PMMA blend nanocomposites with 2.7 vol% MWCNT. Figure 1 shows how MWCNT is dispersed, located and distributed in different PS/PMMA (20:80) blend nanocomposites, prepared by different processing methods.


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Figure 1. TEM images of PS/PMMA (20:80) vol% blend nanocomposites with 2.7 vol% MWCNT: (a) PS/PMMA-Solution, (b) PSMB(S)/PMMA, (c) PS/PMMAMB(S), (d) PS/PMMA-Melt, (e) PSMB(M)/PMMA, (f) PS/PMMAMB(M). (a1), (b1), (c1), (d1), (e1) and (f1) are high-resolution images of the respective nanocomposites, showing MWCNT with more clarity. Darker domains represent PS phase and lighter phase is PMMA.



All the nanocomposites show that conductive fillers are located mostly in PS phase with some of the MWCNTs in PMMA as well. However, the level of dispersion and level of selective localization are different in samples depending upon the processing strategy. Better dispersion of MWCNT in bigger PS droplets is observed in PS/PMMA-Solution sample but there is less selectivity of MWCNT towards PS. PS/PMMA-Melt mixing shows better selective localization of MWCNT in smaller PS droplets with very poor dispersion. PSMB(S)/PMMA and PSMB(M)/PMMA mixing techniques exhibit better selective localization of MWCNT in PS droplets (resided in PMMA), but dispersion is not good. Moreover, PS droplets are found smaller by PSMB(M)/PMMA strategy than PSMB(S)/PMMA. Migration of MWCNT from PMMA phase to PS phase is observed by PS/PMMAMB(S) and PS/PMMAMB(M) mixing techniques. Migration of MWCNT in PS phase induces partial continuity of minor PS phase in the blend nanocomposite prepared by PS/PMMAMB(S), which is absent in the sample processed by PS/PMMAMB(M) technique. This partial continuity of filled PS phase is expected to have a favorable effect on the electrical properties of PS/PMMAMB(S) processed PS/PMMA (20:80) blend nanocomposite. Figure 2 shows dispersion and localization of MWCNT in PS/PMMA-Solution samples with PS/PMMA (50:50) and PS/PMMA (80:20) blend composition. It is seen that dispersion of MWCNT in PS/PMMA (50:50) nanocomposite is better than all other samples explored in this work. Furthermore, for this sample, MWCNT is observed in both PS and PMMA phases. This phenomenon can be partially explained by composition dependent partial miscibility of PS/PMMA blend in presence of solvent, at particular PS/PMMA concentrations. Bui et al.36 reported partial miscibility of PS/PMMA in both tetrahydrofuran and chloroform and showed that partial miscibility can be influenced by molecular weight of polymers, blend composition,


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amount and type of solvent. Even slight miscibility of PS/PMMA in chloroform may cause MWCNT to disperse throughout the blend, and this may have occurred in PS/PMMA (50:50) blend nanocomposite. Consequently, volume exclusion37 effect is not attained in that sample. In contrast, PS/PMMA (80:20) shows selective localization of MWCNT in PS phase with some agglomerates. Although dispersion is not optimum in PS/PMMA (80:20), volume exclusion effect (i.e. unfilled PMMA droplets) increases the effective concentration of conductive MWCNT in PS phase.

Figure 2. TEM of (a) PS/PMMA (50:50) and (b) PS/PMMA (80:20) blend nanocomposites with 2.7 vol%

MWCNT by PS/PMMA-Solution processing technique. (a1) and (b1) are high-

resolution images of the respective nanocomposites, showing MWCNT with more clarity. Perfect selective localization and optimum dispersion of conductive fillers in polymer blend nanocomposites facilitate enhanced electrical conductivity. Detailed analysis of the TEM of the 9 Environment ACS Paragon Plus


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samples allows us to conclude that MWCNT is mostly located in the PS phase for PS/PMMA (20:80) and PS/PMMA (80:20) samples, which is similar to previously reported observations38. Moreover, migration of MWCNT to the minor PS phase from PMMA matrix is observed in PS/PMMAMB(S) and PS/PMMAMB(M) samples with (20:80) blend composition. That is, though the MWCNT was blended into the PMMA as a masterbatch, the MWCNT migrated completely to the PS from the PMMA during processing. Conversely, some authors observed MWCNT mostly in PMMA phase16,39 in PS/PMMA blend, which is the thermodynamically predicted phase per calculations shown in supporting information. It should be noted that the thermodynamic prediction shown in the supporting information is not always accurate, particularly if the surface tensions of the two polymers are similar, such as for PS and PMMA. Apart from thermodynamic effect; kinetics induced by viscosity ratio40, processing condition41,42, rheology of the polymer components40,43,44 and interaction in between π electrons of MWCNT and polymers are determining factors for the localization of MWCNT in blends and consequently final morphology of polymer blend system45. These important factors can direct nanofillers to a certain phase other than the thermodynamically predicted phase46,47. Another argument in the literature indicates that thermodynamic effect is dominant only when the viscosity ratio of the polymers at a given processing condition is close to one

43,44

. Moreover, it

has been shown in the literature that π-π interaction exists between PS and MWCNT while acid functionalization of MWCNT increases dipole-dipole interaction in between PMMA and MWCNT38. Morphology Evolution Figure 3 represents the effect of MWCNT in PS/PMMA (20:80) blend morphology. SEM images of neat PS/PMMA (20:80) blend exhibits droplet-matrix type of morphology, where PS


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droplets are dispersed in PMMA matrix. Incorporation of 0.3 vol% of MWCNT in the blend increases the domain size of PS droplets. Further increase of MWCNT concentration in the blend elongates the PS droplets, resulting in irregular interconnection among PS domains. Other authors reported that concentration of filler in the minor phase of a blend increases the selective phase continuity as the loading of the filler is increased48–50. It is evident from these findings that MWCNT has significant effect on morphology evolution of polymer blend nanocomposites.

Figure 3. Morphology of PS/PMMA (20:80) blend nanocomposites with (a) 0 vol%, (b) 0.3 vol% and, (c) 1 vol% MWCNT. PS Phase is etched by cyclohexane. Figure 4 shows how processing technique influences the morphology of polymer blend nanocomposites. Polymer blend nanocomposites, made by solution casting exhibit larger PS domains and higher interconnectivity than the samples which are fabricated by melt mixing. However, PS/PMMAMB(M) shows elongated domains and less interconnections. These



variations can be attributed to different shear rate, dispersion, distribution, mechanism of migration and localization of MWCNT during nanocomposites preparation.

Figure 4. Effect of processing condition on the morphology of PS/PMMA (20:80) vol% blend nanocomposites with 2.7 vol% MWCNT: (a) PS/PMMA-Solution, (b) PSMB(S)/PMMA, (c) PS/PMMAMB(S), (d) PS/PMMA-Melt, (e) PSMB(M)/PMMA, (f) PS/PMMAMB(M). PS Phase is etched by cyclohexane. In this context, in the case of solution mixed samples, the viscosity of polymer solution is very low, which facilitates effective migration and better dispersion of MWCNTs in the polymer matrix. It has been well established that the localization and dispersion quality of fillers greatly influence the phase morphology. Moreover, solution mixing involves very small shear rate compared to melt mixed samples. This leads to significantly larger phase domains in the solution mixed samples compared to melt mixed samples.


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DC Electrical Conductivity (σDC) Figure 5 shows the effect of processing condition and blend composition on DC electrical conductivity (σDC) of nanocomposites containing 2.7 vol% MWCNT. Solution mixing strategy achieves better electrical conductivity at all three compositions of PS/PMMA/MWCNT blend nanocomposites, which is reasonable due to the better MWCNT dispersion confirmed by TEM. Additionally, PSMB(S)/PMMA is found to be a useful technique to attain comparable electrical conductivity in PS/PMMA (50:50) and PS/PMMA (80:20) blend nanocomposites, but this strategy gives lower electrical conductivity in PS/PMMA (20:80) blend nanocomposite. An opposite trend is observed in the case of PS/PMMAMB(S) technique, where comparatively higher electrical conductivity in PS/PMMA (20:80) and very low conductivity in PS/PMMA (80:20) blend nanocomposite is observed. It is noted that lower electrical conductivity is observed for the samples fabricated solely by melt mixing (i.e. no solution mixing used for either masterbatch or nanocomposite).



Figure 5. DC electrical conductivity (σDC) of single polymer nanocomposites and blend nanocomposites with 2.7 vol% MWCNT at different processing techniques. Solution method introduces better electrical conductivity in the blend nanocomposites than melt mixing, which can be partly attributed to better dispersion of MWCNT attained by solution casting. Moreover, unlike melt mixing, where higher shear stresses are involved; solution mixing preserves the aspect ratio of MWCNT to a great extent during processing39. The polymer/MWCNT MB technique depicts another interesting point to analyze; introduction of MWCNT MB to a continuous component of the blend shows better conductivity than introducing it as the minor component/MWCNT MB. This phenomenon can be explained with


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the help of TEM provided in Figure 1 for PS/PMMA (20:80) blend nanocomposites prepared by PSMB(S)/PMMA and PS/PMMAMB(S). PSMB(S)/PMMA and PSMB(M)/PMMA provide poor dispersion of MWCNT in PS droplets, dispersed in PMMA matrix. As mentioned before, PS/PMMAMB(S) and PS/PMMAMB(M) techniques show that MWCNT has migrated into narrower and thin PS domains from PMMAMB. PS/PMMAMB(S) mixing technique show better electrical conductivity than PS/PMMAMB(M) technique. This is attributed to the partial continuity of filled PS phase in the blend nanocomposite prepared by PS/PMMAMB(S) technique. On the other hand, higher electrical conductivity in PS/PMMA (80:20) blend nanocomposite prepared by PSMB(S)/PMMA results due to better dispersion of MWCNT in PS phase by extra net shear applied during dilution of PSMB with PMMA. Likewise, incomplete migration of MWCNT from PMMAMB to PS and agglomeration of MWCNT are potential reasons behind poor electrical conductivity in PS/PMMA (80:20) blend nanocomposite prepared by PS/PMMAMB(S) technique. EMI SE SET of a blend nanocomposite is the summation of three components: shielding by absorption (SEA), reflection (SER), and multiple reflection (SEMR). It has been reported that the overall EMI shielding correlates well with the electrical conductivity of the material. In this context, immiscible polymer blend nanocomposites offer the flexibility to tune phase microstructure and dispersion of conductive nanofillers by changing blend composition and processing conditions. Moreover, Pawar et al.

25

demonstrated that the interphase scattering and intertube scattering of

penetrated EM waves in polymer blend nanocomposites impart additional EMI attenuation by absorption mechanism. Heterogeneous microstructures with different electrical conductivities inside the immiscible polymer blend nanocomposites initiate impedance mismatch at the micro



level. Penetrated EMI radiation gets scattered due to impedance mismatch at conductive filler carrying-insulative medium interphase (interphase scattering) and conductive filler network (intertube scattering). Increased scattering leads to improved EMI attenuation via absorption mechanism of polymer blend nanocomposites14,25,31. This is anticipated to increased demand for polymer blend nanocomposites over heavier metals for EMI shielding purposes. Moreover, the higher internal scattering of electromagnetic waves inside polymer blend nanocomposites and reduced shielding by reflection of polymer nanocomposite will eventually reduce the amount of EMI pollution since absorbed EMI will be transformed to heat while reflection results in additional EMI smog18,25. In this study, we have recorded different electrical conductivity in samples, prepared by different processing techniques and blend compositions. Furthermore, altered phase morphology and dispersion quality of MWCNT are observed. Therefore, the effect of various processing strategies on final EMI shielding properties of blend nanocomposites is studied. Figure 6 shows total EMI SE (SET) of the samples, prepared by different processing strategy as a function of frequency. EMI SE of PS/PMMA blend nanocomposites with 2.7 vol% MWCNT is assessed over X-band frequency range which is important for commercial applications. As mentioned in previous sections, different preparation approaches and blend compositions alter the level of selective localization, nanodispersion and phase morphology. Consequently, EMI SE of the blend nanocomposites is varied. Similar to conductivity, PS/PMMA-Solution technique shows better EMI attenuation and correlated to better dispersion observed from the analysis of TEM images.


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Figure 6. EMI SE of (a) PS/PMMA (20:80), (b) PS/PMMA (50:50) and (c) PS/PMMA (80:20) vol% blend nanocomposites with 2.7 vol% MWCNT at different processing techniques. PS/PMMA (50:50) shows lowest EMI SE among the samples prepared by PS/PMMA-Solution approach and even lower than PS/MWCNT-Solution composite (shown in next figure 7), which is unusual in case of co-continuous morphology. It has been well established32 that co-continuous blends show enhanced EMI SE if there are selective localization and adequate dispersion of MWNTs in the blends. In this work, PS/PMMA (50:50) by PS/PMMA-Solution strategy attains



better dispersion but really poor selective localization, which results in lower EMI SE. On the other hand, both PS/PMMA (20:80) and PS/PMMA (80:20) blend nanocomposites exhibit an average EMI SE higher than 20 dB for only 0.8 mm thickness (20 dB is commercially viable). EMI SE of 20 dB represents 99 % attenuation of incident EM waves. In case of PS/PMMA (80:20) blend nanocomposite, good EMI SE is seen because of interconnections of MWCNT in continuous PS phase and increased effective concentration of MWCNT in PS due to volume exclusion by unfilled PMMA droplets (see TEM micrographs in figure 2(b) ). Similar phenomenon is observed by Pawar et al.30 in melt mixed PC/PMMA/3 wt % MWCNT nanocomposite. Higher EMI SE of PS/PMMA (20:80) can be attributed to the bridging effect of MWCNT in PMMA phase among the filled PS droplets. Furthermore, increased interphase due to enlarged and impregnated PS droplets increase internal scattering effect, allowing us to attain higher EMI SE30. Figure 7 compares average EMI SE attained by 0.8 mm thick samples of PS/MWCNT, PS/PMMA (20:80), PS/PMMA (50:50), PS/PMMA (80:20) and PMMA/MWCNT blend nanocomposites with 2.7 vol% MWCNT at different processing conditions. PS/PMMA (20:80) and PS/PMMA (80:20) nanocomposites processed by PS/PMMA-Solution method along with PS/PMMA (80:20) nanocomposite prepared by PSMB(S)/PMMA-Melt approach show better EMI SE than PS/MWCNT-Solution nanocomposite. This is mainly due to the higher electrical conductivity of the blend nanocomposites. Certain processing techniques lead to better dispersion and enhanced interconnected networks of MWCNTs in a given matrix, which act as microwave-absorbing screen. Therefore, it can be concluded that the various processing techniques adopted here show a great influence on the dispersion quality of MWCNTs, and hence, altering EMI shielding properties.


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Figure 7. EMI SE of single polymer nanocomposites and blend nanocomposites with 2.7 vol% MWCNT at different processing techniques. Effect of Shield Thickness: Enhanced Intertube and Interphase Scattering Figure 8 shows SET as a function of the concentration of PS phase in the blend nanocomposites with different thicknesses. EMI SE value scales with the thickness of the shield. The increment rate of EMI SE in blend nanocomposite is higher than PS/MWCNT or PMMA/MWCNT nanocomposite.



Figure 8. EMI SE of single polymer nanocomposites and blend nanocomposites at different thickness with 2.7 vol% MWCNT, prepared by solution mixing technique. Multiple scattering (interphase and intertube) effect in polymer blend nanocomposites makes polymer blend nanocomposites more effective than single polymer nanocomposites, where only intertube scattering is in effect. Hence, the more heterogeneity in the material, the higher the microwave scattering, and enhanced microwave screening. Therefore, blends with fine morphology (i.e., more interfaces) and excellent selective localization show a higher increase in microwave shielding as we increase thickness. Figure 9 is a schematic representation of intertube scattering based EMI shielding mechanism in single polymer/MWCNT and multiple scattering (intertube and interphase scattering) based EMI shielding mechanism in immiscible polymer blend/MWCNT nanocomposite.


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Figure 9. Schematic representation of EMI shielding mechanism in single polymer/MWCNT nanocomposite and immiscible polymer blend/MWCNT nanocomposite. Regardless of blend composition, all the blends depict enhanced SET at higher thickness. However, the extent to which SET scales with thickness is controlled by the blend morphology, dispersion quality of MWCNT and perfectness of selective localization of MWCNT in the blends. The maximum extent to which SET increases at higher thickness is seen for PS/PMMASolution blends with (50:50) and (80:20) compositions. In the case of PS/PMMA-Solution (50:50) blends, enhanced SET is mainly due to better dispersion of MWCNT, which leads to increased MWCNT interphases for microwave scattering, thus enhanced intertube scattering (see Figure 2). On the other hand, imperfect selective localization of MWCNT limits the interphase scattering. In the case of PS/PMMA-Solution (80:20) blends, high level of selective localization of MWCNT leads to maximum interphase scattering (see Figure 2(b) ), thus maximum microwave shielding. Therefore, it can be concluded that the enhanced interphase scattering is not only controlled by phase morphology but also governed by perfectness of selective localization of fillers in the



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immiscible blends. Therefore, it is important to understand the effect of nanocomposite fabrication techniques on selective localization of MWCNT in the blends. From a detailed analysis of processing techniques, immiscible blends, dispersion quality of MWCNT and perfectness of MWCNT localization, we determined that the solution mixing technique provides better dispersion (i.e., enhanced intertube scattering) and better localization of MWCNT in one of the phases of the blends (i.e., enhanced interphase scattering). However, the perfectness of selective localization of MWCNT is also controlled by the blend composition. Hence, taken together, immiscible blends show great potential as microwave absorbing materials where intertube and additional interphase scattering trap microwaves inside the material, and eventually eliminate EMI through absorption. In Table 2, the result of this work is compared with the EMI SE of some immiscible polymer blend nanocomposites reported in the literature. Table 2. EMI SE of polymer based nanocomposites. Blend system

Fillers and concentrations

Frequency (GHz)

SET (dB)

Thickness Ref. (mm)

PC/PVDF

MWCNT (2 wt%)

8–18

11

0.9

51

PC/PVDF

MWCNT–AHB (2 wt%)

8–18

14

0.9

51

PP/PA6

MWCNT (3 phr)

7.3

0.2

52

PC/SAN

MWCNT–PMMA (3 wt%)

8–18

21

5

53

PC/PVDF

PMMA–MWCNT (10 wt%)

8–18

32

5

54

PC/PVDF

MWCNT–OH (3 wt%)

8–18

21

5

54

PVDF/ABS

MWCNT (3 wt%)

8–18

23

5

17

PVDF/ABS

PMMA–MWCNT (3 wt%)

2–18

32

5

55

PVDF/SAN

MWCNT (2 wt%)

2–18

16

5

56

PVDF/SAN

MWCNT–PBA (2 wt%)

2–18

19

5

56


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PS/PMMA

MWCNT–NH2 (1 wt%)

8–18

24

5

16

PS/PVME

MWCNT (2 wt%)

8.2–18

28

5

57

PE/PEO

MWCNT (3 wt%)

8–18

16.3

6

58

PVDF/SMA

MWCNT (5 wt%)

8–18

15.6

5.5

59

PC/ABS

MWCNT (1.5 wt %)

5-12

2

2

60

PP/PE

MWCNT (5 vol%)

8-12

25

1

15

PC/PMMA

MWCNT (3 wt %)

8-18

27

5

30

(PLLA+PDLA)/PCL MWCNT (0.8 wt %)

8-12

17

1.5

34

PS/PMMA

8.2-12.4

30

2

This work

MWCNT (2.7 vol%)

PC - Polycarbonate, PVDF - Polyvinylidene fluoride, PP - Polypropylene, PA6 - Polyamide 6, SAN – Poly (styrene-co-acrylonitrile), ABS - Acrylonitrile butadiene styrene, PVME - Polyvinyl methyl ether, PE - Polyethylene, PEO – Poly (ethylene oxide), SMA - Styrene maleic anhydride, PS - Polystyrene, PMMA - Poly (methyl methacrylate), PLLA – Poly (L-lactide), PDLA - Poly (D-lactide), PCL - Poly(e-caprolactone). Conclusions The effect of various processing techniques on EMI shielding performance and the underlying mechanism of microwave absorption in immiscible PS/PMMA blends is studied systematically. Unlike traditional metal-based EMI shields, for immiscible polymer blends with selectively localized MWCNT in one of the phases, multiple scattering (i.e., intertube and interphase scattering) of microwaves occur inside polymer blend nanocomposites. This effect is often neglected in microwave absorbers, but here, it plays a vital role in microwave screening. The processing techniques greatly influence blend morphology, dispersion quality, and level of selective localization of MWCNT in PS/PMMA blends. It is observed that the improved dispersion of MWCNT leads to enhanced intertube scattering; whereas higher level of selective localization of MWCNT leads to enhanced interphase scattering. Blends with almost perfectively



localized MWCNT in one of the phases are the ones that depicts better EMI shielding performance. Due to multiple scattering effect, the rate of incrase of EMI SE with thickness is also higher in immiscible polymer blend nanocomposites than in single polymer nanocomposites. In single polymer nanocomposite, only intertube scattering is effective. In summary, blend processing techniques can be effectively used to create immiscible blends with finer morphologies, and to achieve improved dispersion and higher level of localization of MWCNT in one of the phases of the blends. Thus, we can use processing to optimize the intertube and interphase scattering of microwaves in the material, leading to enhanced microwave shielding through absorption. Associated Content Supporting Information This section contains detail calculation of wetting co-efficient and interfacial tension at both room temperature and processing condition. Author Information Corresponding Author *E-mail: [email protected] Author Contributions The manuscript is prepared via contributions of all authors. The authors have approved the final version of the manuscript.

Notes The authors declare no competing financial interest.


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Acknowledgement This research work is funded by Natural Sciences and Engineering Research Council of Canada. Authors would like to acknowledge Microscopy and Imaging Facility (MIF) at the University of Calgary for their support in TEM and SEM imaging.



Page 26 of 35

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TOC 271x203mm (300 x 300 DPI)

ACS Paragon Plus Environment

PMMA

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