Improvement in EMI Shielding Properties of Silicone Rubber / POE Blends Containing ILs Modified with Carbon Black and MWCNTs

Silicone rubber (SR)/polyolefin elastomer (POE) blends containing ionic liquids modified with carbon blacks (CB-IL) and multi-walled carbon nanotubes (CNT-IL) were prepared by melt-blending and hot pressing. SR/POE/CB-IL and SR/POE/CB-CNT-IL composites showed co-continuous structural morphologies. The cation–π interactions between ILs and CNTs were stronger than those between ILs and CBs due to the large length and high surface area of CNTs, which promoted better dispersion of carbon fillers. SR/POE/CB-CNT-IL composites showed higher EMI SE than SR/POE/CB-IL composites containing identical filler contents because the CNTs with larger aspect ratios helped form more electrically-conductive networks.


Introduction
In the rapidly-growing information age, silicone rubber has seen extensive application in the electronic and electrical equipment industries due to its excellent chemical resistance, non-toxic nature, heat-resistance, freeze-resistance, and good flexibility [1].However, silicone rubber is electrically-insulating and transparent to electromagnetic radiation.As electronic information technology has developed and the use of electronic devices has become more widespread, electromagnetic interference (EMI), which interrupts the functionality of electronic devices and severely affects human organs, has become a serious problem [2][3][4].Therefore, conductive rubber composites containing carbon materials, such as carbon blacks and multi-walled carbon nanotubes, have excellent electrical conductivities and flexibility and have been widely used as EMI shielding materials [5,6].A typical approach is to add a single type of carbon material into a single kind of polymeric matrix to increase the shielding ability of the carbon-based polymer composites [7][8][9][10].Ionic liquids (ILs), which consist of a pair of soft cationic and anionic species, are used to improve the dispersion of carbon fillers in the polymer matrix since ILs and carbon fillers exhibit strong cation-π physical interactions [11][12][13][14].
In this study, carbon blacks (CBs) and carbon nanotubes (CNTs) were modified with 1-vinyl-3-ethylimidazolium bromide ILs that contained carbon double bonds.The synergistic effect of ILs modified with CBs and CNTs on the morphology and the EMI shielding properties of silicone rubber/polyolefin elastomer/carbon black-carbon nanotube-ionic liquid (SR/POE/CB-CNT-IL) composites were systematically investigated to reveal the mechanism of the enhanced EMI shielding performance.
The SR/POE/CB-CNT-IL composites were prepared as follows.First, CBs, CBs, and CNTs, ILs-modified CBs (CB-IL), ILs-modified CBs and CNTs by (CB-CNT-IL) were ground in an agate mortar.Then, the fillers were added to SR by a two-roll mill at 125 • C, and POE and DBPH were introduced and mixed for 15 min.Finally, the composites were vulcanized under 10 MPa pressure for 10 min at 180 • C, and then post-cured in a drying oven at 190 • C for 3 h.The amounts of components prepared for the composites are shown in Table 1.The morphology of the composites was observed by scanning electron microscopy (SEM, JEOL, JSM-6490LV, Japan) at an accelerating voltage of 20 kV and field-emission scanning electron microscopy (FESEM, JSM-6700F) at an accelerating voltage of 5 kV.The electrical conductivity was determined using a multi-function digital electric meter (Victor Tech, Victor 86-e).The EMI shielding effectiveness (SE) data were obtained by using nanocomposite slabs (22.86×10.16×0.6mm 3 ), which were fitted into a waveguide sample holder with a vector network analyser (Agilent Technologies, N5247A, America) at 8.0 to 12.0 GHz (X band).EMI shielding properties of the samples were evaluated from the S parameters, which could be calculated calculate SE T (total shielding effectiveness), SE R (reflection loss) and SE A (absorption loss) [15].

Results and Discussion
Figure 1 shows the SEM images of the fractured surfaces of SR/POE/CB-IL and SR/POE/CB-CNT-IL composites.The samples were first fractured in liquid nitrogen, and then the fractured surfaces were immersed into n-hexane at 90 • C for 12 h to remove the POE phase.All samples display typical co-continuous structures, and the collapsed surface morphology is attributed to the dissolution of the POE phase.Carbon fillers modified by ILs were premixed with SR and then mixed with POE.Most fillers were initially dispersed in the SR matrix because the localization of fillers is controlled by flow properties and dynamic factors.When CNTs were added, the viscosity of the SR phase increased significantly, and the volume fraction of the SR phase decreased significantly due to the large length and high surface area of CNTs.This caused the SR/POE/CB-CNT-IL composites to form regular and orderly co-continuous phase structures [16].
Figure 1 shows the SEM images of the fractured surfaces of SR/POE/CB-IL and SR/POE/CB-CNT-IL composites.The samples were first fractured in liquid nitrogen, and then the fractured surfaces were immersed into n-hexane at 90 °C for 12 h to remove the POE phase.All samples display typical co-continuous structures, and the collapsed surface morphology is attributed to the dissolution of the POE phase.Carbon fillers modified by ILs were premixed with SR and then mixed with POE.Most fillers were initially dispersed in the SR matrix because the localization of fillers is controlled by flow properties and dynamic factors.When CNTs were added, the viscosity of the SR phase increased significantly, and the volume fraction of the SR phase decreased significantly due to the large length and high surface area of CNTs.This caused the SR/POE/CB-CNT-IL composites to form regular and orderly co-continuous phase structures [16].Figure 2 shows the FESEM images of the fracture surfaces of the SR/POE/CB-IL, SR/POE/CB, SR/POE/CB-CNT-IL and SR/POE/CB-CNT composites.When the ILs were added, the dispersion of carbon filler was improved significantly because the cation-π interactions between ILs and carbon fillers promoted the further dispersion of fillers and reduced agglomeration.In addition, when the CNTs were added, the cation-π interactions between ILs and CNTs were stronger than those between ILs and CBs due to the large length and high surface area of CNTs [17].Figure 2 shows the FESEM images of the fracture surfaces of the SR/POE/CB-IL, SR/POE/CB, SR/POE/CB-CNT-IL and SR/POE/CB-CNT composites.When the ILs were added, the dispersion of carbon filler was improved significantly because the cation-π interactions between ILs and carbon fillers promoted the further dispersion of fillers and reduced agglomeration.In addition, when the CNTs were added, the cation-π interactions between ILs and CNTs were stronger than those between ILs and CBs due to the large length and high surface area of CNTs [17].As the CB-IL content increased, the electrical conductivity of the SR/POE/15CB-IL and SR/POE/20CB-IL composites was 3.6 × 10 −1 and 8.9 × 10 −1 S/m, respectively.For the SR/POE/CB-CNT-IL composites, as the CB-CNT-IL content increased, the conductivity of the SR/POE/15CB-CNT-IL and SR/POE/20CB-CNT-IL composites was 6.7 × 10 −1 and 3.5 S/m, respectively.CB-CNT-IL can regulate and control the aggregation, dispersion and uniformity of carbon fillers to form three-dimensional conductive networks.This improves the electrical conductivity, and the EMI SE values increased as the amount of fillers in the composites increased.Figure 3a shows the EMI shielding effectiveness (SE) of the composites in the frequency range of 8.0 to 12.0 GHz.SR/POE/CB-CNT-IL composites have higher EMI SE values than the SR/POE/CB-IL composites at identical filler loadings [18].The CNTs with larger aspect ratios helped form a more conductive network and improved the EMI performance of the SR/POE/CB-CNT-IL composites.Specifically, the shielding effectiveness of the SR/POE/20CB-IL and SR/POE/20CB-CNT-IL composites was 29.4 dB and 36.5 dB, over the frequency range of 8.0 to 12.0 GHz, respectively.This gives the SR/POE/20CB-CNT-IL composite a novel potential electromagnetic interference that shields materials by eliminating electromagnetic pollution over a wide frequency range.To correlate the EMI shielding properties with the composite structures, both the average SE T , SE A , and SE R values of SR/POE/CB-IL and SR/POE/CB-CNT-IL composites at 9.0 GHz were plotted with filler content as shown in Figure 3b.The SE A and SE R showed an increase as the filler content increased due to the synergistic effect between ILs and carbon fillers, which enhanced the electrical conductivity.Furthermore, the contribution of absorption to SE T is greater than that of the reflection because network conductivity supports abundant interfaces which form multiple reflections and attenuate the incident EM waves [19].The filler content in the SR/POE/CB-CNT-IL composites should be well above their percolation thresholds.Herein, the excellent EMI SE of the SR/POE/CB-CNT-IL composites is attributed to the presence of highly conductive networks, which effectively reflect, multi-scatter and adsorb the incident radiations.Figure 4a  in Figure 4b.The excellent EMI SE in this work can be attributed to the highly assembled carbon nano-fillers conductive networks that effectively reflect, multiple scatter and adsorb the incident radiation.[20].

Conclusion
In summary, this study has investigated the morphology and EMI shielding properties of SR/POE/CB-CNT-IL composites, which were fabricated by a conventional two-roll mill mixing process.The cation-π interactions between ILs and CNTs were stronger than those between ILs and CBs due to the large length and surface areas of CNTs which promoted further dispersion of carbon fillers.The conductive networks play an important role in reflecting, scattering multiplication and absorption of the incident radiation.The SR/POE/CB-CNT-IL composites exhibited excellent EMI shielding properties due to the synergistic effect of ILs-modified CBs and CNTs.The EMI SE of the SR/POE/20CB-CNT-IL composite could reach 36.5 dB at 9.0 GHz.This gives the SR/POE/20CB-CNT-IL composite a novel potential electromagnetic interference that shields materials by eliminating electromagnetic pollution over a wide frequency range.
presents the SE T , SE A , and SE R of SR/POE/CB, SR/POE/CB-CNT, SR/POE/CB-IL and SR/POE/CB-CNT-IL composites at 9.0 GHz.The EMI shielding performance of SR/POE/CB and SR/POE/CB-CNT composites obviously decreased compared with that of SR/POE/CB-IL and SR/POE/CB-CNT-IL composites.The results reveal that ILs promote the dispersion of carbon fillers due to the cation-π interactions between ILs and carbon fillers.Compared to SR/POE/CB-IL composites, the SR/POE/CB-CNT-IL composites with co-continuous structures have enhanced carrier mobility due to the presence of denser conductive networks and more interfacial structures formed by the uniformly-dispersed CB-CNT-IL.The schematic illustration of EMI shielding mechanisms for the SR/POE/CB-CNT-IL composites is shown in Figure 4b.The excellent EMI SE in this work can be attributed to the highly assembled carbon nano-fillers conductive networks that effectively reflect, multiple scatter and adsorb the incident radiation [20].Appl.Sci.2019, 9, x FOR PEER REVIEW 5 of 7

Figure 3 .
Figure 3. (a) Electromagnetic interference shielding effectiveness (EMI SE) of the SR/POE/CB-IL and SR/POE/CB-CNT-IL composites in the frequency range of 8.0 to 12.0 GHz, and (b) comparison of SET, SEA, and SER of the SR/POE/CB-IL and SR/POE/CB-CNT-IL composites at 9.0 GHz.

Figure 3 .
Figure 3. (a) Electromagnetic interference shielding effectiveness (EMI SE) of the SR/POE/CB-IL and SR/POE/CB-CNT-IL composites in the frequency range of 8.0 to 12.0 GHz, and (b) comparison of SE T , SE A , and SE R of the SR/POE/CB-IL and SR/POE/CB-CNT-IL composites at 9.0 GHz.

Figure 3 .
Figure 3. (a) Electromagnetic interference shielding effectiveness (EMI SE) of the SR/POE/CB-IL and SR/POE/CB-CNT-IL composites in the frequency range of 8.0 to 12.0 GHz, and (b) comparison of SET, SEA, and SER of the SR/POE/CB-IL and SR/POE/CB-CNT-IL composites at 9.0 GHz.

Figure 4 .
Figure 4. (a) Comparison of SET, SEA, and SER of the SR/POE/CB, SR/POE/CB-IL, SR/POE/CB-CNT-IL, and SR/POE/CB-CNT composites at 9.0 GHz, and (b) Schematic illustration of EMI shielding mechanisms for the SR/POE/CB-CNT-IL composites

Figure 4 .
Figure 4. (a) Comparison of SE T , SE A , and SE R of the SR/POE/CB, SR/POE/CB-IL, SR/POE/CB-CNT-IL, and SR/POE/CB-CNT composites at 9.0 GHz, and (b) Schematic illustration of EMI shielding mechanisms for the SR/POE/CB-CNT-IL composites.
In summary, this study has investigated the morphology and EMI shielding properties of SR/POE/CB-CNT-IL composites, which were fabricated by a conventional two-roll mill mixing process.The cation-π interactions between ILs and CNTs were stronger than those between ILs and CBs due to the large length and surface areas of CNTs which promoted further dispersion of carbon fillers.The conductive networks play an important role in reflecting, scattering multiplication and absorption of the incident radiation.The SR/POE/CB-CNT-IL composites exhibited excellent EMI shielding properties due to the synergistic effect of ILs-modified CBs and CNTs.The EMI SE of the SR/POE/20CB-CNT-IL composite could reach 36.5 dB at 9.0 GHz.This gives the SR/POE/20CB-CNT-IL composite a novel potential electromagnetic interference that shields materials by eliminating electromagnetic pollution over a wide frequency range.