Sandwich composites of polyurethane reinforced with poly(3,4-ethylene dioxythiophene)-coated multiwalled carbon nanotubes with exceptional electromagnetic interference shielding properties

Abstract

Poly(3,4-ethylene dioxythiophene) (PEDOT)-coated multiwalled carbon nanotube (MWCNT) composite (PCNT) was synthesized by in situ emulsion polymerization and was used as a filler for the fabrication of polyurethane (PU) sandwich composites (PUPCNT) by a solution casting technique. Transmission electron microscopy (TEM) images revealed a coating of PEDOT over the MWCNTs, and scanning electron microscopy (SEM) micrographs showed uniform dispersion of PCNT filler in the fractured surfaces of PUPCNT films, which was further confirmed by X-ray diffraction (XRD) analysis. The tensile strengths of all the PUPCNT composites indicated that tensile strengths do not degrade on adding 10% and 20% PCNT filler in the PU matrix. Electrostatic charge dissipation (ESD) measurements of PEDOT filled PU composites showed a static decay time of 0.2 s, which indicates that they can be utilized for antistatic applications. The PUPCNT composites showed excellent electromagnetic interference shielding effectiveness (EMI SE) which increased with increasing filler loading in the PU matrix. The maximum EMI SE obtained was 45 dB with 30 wt% loading of PCNT filler in the frequency range of 12.4–18 GHz (Ku-band).

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1. Introduction

With the development of high technology systems, there has been an increase in the use of electronic and electrical equipments, which radiate and are affected by electromagnetic (EM) waves by a phenomenon called as electromagnetic interference (EMI). Thus, the shielding of both electronic instruments and radiation sources is essential to maintain their functionality and integrity for a longer span of time. The most common conventional materials that are mainly used for EMI shielding purposes include metals and metallic composites.^1–4 However, these materials are associated with several disadvantages such as heavy weight, rapid corrosion and expensive processing techniques. On the other hand, extrinsically conductive polymer composites and intrinsically conducting polymer (ICP) composites with various conducting fillers offer ¹superior properties over metal-based shielding materials. These composites are lightweight, anti-corrosive, flexible, and have processing advantages.^5,6 The EMI shielding effectiveness of a composite depends on the intrinsic conductivity, high aspect ratio and dispersion state of the conducting filler in the polymer matrix. ICPs represent a novel class of materials that are useful for EMI shielding⁶ applications. They possess a π-conjugated system that provides conductivity to the polymer. PEDOT, a derivative of polythiophene, represents a new member of the conducting polymer family. It possesses excellent environmental stability, low redox potential, good thermal stability, ease of synthesis, moderately high conductivity and is widely used for electro-chromic devices, organic solar cells, light emitting diodes and anti-static applications.^7–10

Conducting polymer-grafted carbon nanotube (CNT) composites^11–13 can be considered as a useful approach for the fabrication of EMI shielding materials. CNTs are advanced fillers that can improve the physical properties of polymers due to their excellent electrical and thermal conductivity, high aspect ratio, and high mechanical strength; these properties make them perfect candidates for the preparation of electrically conductive polymer composites. However, the major obstruction in the use of CNTs as a reinforcement in polymer matrices is their uneven dispersion in the matrix due to strong interfacial adhesion, which results in a high tendency to agglomerate. The development of efficient processes and chemical treatment methods has solved these problems to some extent; however, these processes may result in deterioration of the properties of the CNTs. Numerous efforts are being made by researchers to develop electrically conductive polymer composites using CNTs as fillers for EMI shielding applications. Recently, Junye Cheng et al. synthesized polyaniline grafted CNT composites by pre-treating CNTs with plasma and then coating the CNTs with polyaniline by in situ polymerization. The maximum reflection loss observed was 41.37 dB for 2 mm thickness in the frequency range of 2 to 18 GHz.¹⁴ Kotsilkova et al. studied the EMI shielding properties of amine grafted-MWCNTs/epoxy composites at a very low wt% loading of MWCNTs (0.3 wt%) and observed an EMI SE of 17 dB at 1 cm thickness.¹⁵ Mohammed Al Saleh prepared composites by placing CNTs at the external surface of ultrahigh molecular weight polyethylene powder by wet mixing and found an EMI shielding effectiveness of 50 dB for a 1.0 mm thick plate made with 10% CNT loading.¹⁶ Verma et al. blended MWCNT (4.6 vol%) in polypropylene random copolymers using a twin-screw extruder with a melt re-circulator and reported an EMI shielding of 47 dB in the X-band.¹⁷ Chen et al. successfully prepared flexible and conductive materials using quartz fibre cloth reinforced MWCNT-carbon aerogel and polydimethyl siloxane; the composite exhibited increased tensile strength and an EMI shielding of 16 dB at very low MWCNTs (1.6 wt%) loading.¹⁸ Gupta et al. reported the fabrication of acid modified MWCNTs reinforced PU composites by solution casting¹⁹ and observed an EMI SE of ∼29 dB with 10 wt% loading of MWCNTs in the X-band. Hoang dispersed MWCNTs in a PU matrix by grinding them in a planetary ball mill; an EMI SE of 20 dB was observed with 22 wt% MWCNTs in the frequency range of 8 to 12 GHz.²⁰ Ramoa et al. prepared thermoplastic PU/CNTs composites through melt blending and investigated their EMI SE as a function of filler loading.²¹ They reported a maximum EMI SE of 22 dB in the frequency range of 8 to 12 GHz. Bhattacharya et al. investigated the microwave absorption properties of thermoplastic PU incorporated with TiO₂ coated MWCNTs (15%) and magnetite Fe₂O₃ (15%) by solution blending and showed a maximum reflection loss of 42.53 dB at a frequency of 10.98 GHz.²² A detailed comparison of various polymer composites prepared for EMI shielding applications containing CNTs as fillers is given in Table 3.

Table 1 Compounding formulations of composites containing different wt% loading of PEDOT/PCNT filler in PU matrix^a

SI no.	Sample	PU	PEDOT	PCNT*
a PCNT*: PEDOT-coated MWCNTs (EDOT to MWCNT ratio was 1 : 0.25).
1	PU	100%	0	0
2	PUP1	90%	10%	0
3	PUP2	80%	20%	0
4	PUP3	0	30%	0
5	PUPCNT1	90%	0	10%
6	PUPCNT2	80%	0	20%
7	PUPCNT3	70%	0	30%

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Table 2 Variation of EMI SE (dB) of PU composites with room temperature electrical conductivity at 12.4 GHz

SI no.	Sample	Conductivity (S cm^−1)	SET (dB)
1	PU	1.53 × 10^−11	0.9353
2	PUP1	5.8 × 10^−8	4.396
3	PUP2	4.83 × 10^−8	6.322
4	PUP3	4.36 × 10^−7	6.667
5	PUPCNT1	2.75 × 10^−4	12.204
6	PUPCNT2	3.06 × 10^−1	23.232
7	PUPCNT3	2.7	40.535

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Table 3 EMI shielding performance of different conductive polymer composites containing CNTs as conducting filler

Matrix	Filler	Concentration	Frequency	SET (dB)	Reference
PU	f-MWCNTs	10 wt%	X-band	∼29	19
PU	SWCNTs	20 wt%	X-band	17	34
PU	TiO2 coated MWCNT	15 wt%	10.98 GHz	42.53	22
PU	MWCNT	22 wt%	X-band	20	20
PU	MWCNT	10 wt%	X-band	41.6	35
TPU	MWCNT	10 wt%	X-band	22	21
SCPU	MWCNT	5 wt%	8.8 GHz	22	36
PMMA	MWCNT	40 wt%	50 MHz–13.5 GHz	27	37
PMMA	MWCNT	10 vol%	X-band	40	38
Epoxy	SWCNT	15 wt%	500 MHz–1.5 GHz	15–20	39
Epoxy	SWCNT	15 wt%	X-band	20–30	40
PC	MWCNT	15 wt%	X-band	27	41
PVA	MWCNT	10 wt%	8 GHz	15	42
EMA	MWCNT	10 wt%	X-band	∼20	43
PTT	MWCNT	10 wt%	Ku-band	42	44
PU	PEDOT-coated MWCNT	30 wt%	12.4 GHz	45	This work

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To address the processing problem of pristine CNTs and to explore the combined effect of MWCNTs and the conducting polymer PEDOT, we have synthesized a PEDOT coated MWCNTs composite (PCNT). The combination of the good dispersion properties of dodecylbenzene sulfonic acid (DBSA) doped PEDOT in many organic solvents and the excellent electrical conductivity and mechanical properties of MWCNTs results in the formation of a unique PCNT composite with high processability and superior mechanical and electrical conductivity. In order to determine the practical applications of this PCNT composite, we have fabricated composite films with PCNT as a filler in a PU matrix by simple solution casting for EMI shielding applications. PU is known for its unique properties, such as good elasticity, high impact strength and elongation. These properties make PU a suitable candidate for the fabrication of flexible free standing films. The DBSA doped PEDOT imparts high dispersibility in the processing medium and prevents the agglomeration of the MWCNTs, resulting in the formation of uniformly distributed PCNT in the PU matrix.

The EMI shielding effectiveness of the resulting free standing PUPCNT films has been measured in the frequency range of 12.4 to 18 GHz (Ku-band). The EMI SE value increases remarkably with increasing wt% of PCNT in the PU matrix. This can be attributed to the enhanced conductive network formed due to the uniform dispersion of PCNT in the PU matrix. To the best of our knowledge, no studies have yet been reported on the fabrication of PEDOT coated MWCNTs as a filler in PU for EMI shielding applications.

2. Materials

The monomer 3,4-ethylene dioxythiophene (EDOT) was procured from TCI Chemicals, Japan. DBSA was purchased from Acros Organics (Belgium). Ammonium peroxydisulphate (APS) was procured from Merck (India). Isopropyl alcohol (IPA) and N,N-dimethylformamide (DMF) were obtained from Qualigens (India). Thermoplastic PU (Desmopan 385 S) was procured from Bayer (India). The MWCNTs were synthesized by a chemical vapour deposition (CVD) technique using toluene as the carbon source and ferrocene as the catalyst precursor. The as produced MWCNTs were 90% pure with a diameter range between 20 to 50 nm and with lengths between 50 to 100 μm.²³

3. Method

3.1 Synthesis of PEDOT

The PEDOT was synthesized by the oxidative emulsion polymerization of EDOT using DBSA and APS as the dopant and oxidant, respectively. In this method, 0.1 M EDOT and 0.3 M DBSA were emulsified in an aqueous medium at room temperature for an hour. The polymerization was initiated by the addition of 0.1 M APS solution drop-wise to the EDOT emulsion with constant stirring at −2 °C. After 12 hours of stirring, PEDOT was obtained as a bluish black precipitate and demulsified with IPA under vigorous stirring. After demulsification, the precipitate was washed with distilled water, filtered and dried in a vacuum oven for 24 h at 60 °C.

3.2 Synthesis of PCNT

The encapsulation of MWCNTs in the PEDOT was carried out by in situ polymerization. MWCNT and EDOT in a weight ratio of EDOT : MWCNT :: 1 : 0.25 were added to 0.3 M aqueous DBSA solution and homogenized at 12 000 rpm for 2 hours. 0.1 M EDOT was then added to the above solution and stirred for 2 hours; after that, 0.1 M APS was added drop-wise to this solution with continuous stirring at a temperature of −2 °C. The PCNT composite obtained after 12 h of continuous stirring was treated with IPA for demulsification. The resulting precipitate was filtered, washed with distilled water and then dried in a vacuum oven for 24 hours at 60 °C.

3.3 Fabrication of PUP and PUPCNT composite films

The PUP (polyurethane incorporated with PEDOT) and PUPCNT composite films were prepared by a simple solution casting method. The compounding formulations of the composites with different wt% loading of PEDOT and PCNT as fillers are given in Table 1. The schematic for the preparation of the PUPCNT composites is depicted in Fig. 1. Initially, PU granules were dissolved in DMF at 40 °C for 8 hours; meanwhile, PEDOT/PCNT filler was dispersed in another beaker in DMF under ultra-sonication for an hour. The dispersed PEDOT/PCNT filler in DMF and the dissolved PU in DMF were mixed thoroughly for 2 hours. The mixed solution was then poured into a Petri dish and then dried in an oven at 60 °C for 12 hours.

Fig. 1 Process for the fabrication of PUPCNT composite films by solution casting.

4. Material characterization

The morphological analyses were carried out by SEM (Zeiss EVO MA-10) at an accelerating voltage of 10 kV and TEM (Tecnai G20 S-TWIN, 300 kV instrument). XRD measurements were carried out with a D8 Advance XRD (Bruker) using Cu Kα radiation (λ = 1.54 Å) at a scanning rate of 0.02° s⁻¹ and a slit width of 0.1 mm. Fourier Transform Infrared (FTIR) spectra were recorded on a Nicolet 5700 instrument using the ATR (attenuated total reflectance) accessory. The tensile strength was measured with an ASTM D3039 dog bone die using an Instron Universal Testing Machine (model 1122) at the rate of 2 mm minute⁻¹. The electrical conductivity of the composite films was measured by a two probe technique using a Keithley current source (model 6221) and nano-voltmeter (model 2182 A) at room temperature. Electrostatic dissipation measurements were carried out using a JCI 155v5 charge decay test unit. Electromagnetic shielding measurements were carried out using an Agilent E8362B Vector Network Analyzer with a coaxial cable attached with a rectangular waveguide of the Ku-band (15.75 mm × 7.85 mm). The samples were cut into rectangular pellets of standard Ku-band dimensions and the thickness of the samples was maintained at ∼2.5 mm.

5. Results and discussion

5.1 Morphological analysis

The morphology and distribution of MWCNTs in the PU matrix was analysed by SEM and TEM. Fig. 2(a) shows an SEM micrograph of individual MWCNTs after dispersion in DMF, prepared on a silicon wafer. It can be observed that the MWCNTs are snarled with each other; however, some individual MWCNTs are easily distinguishable. Fig. 2(b) shows the granular/particulate amorphous nature of PEDOT, which is further confirmed by XRD (see Fig. 5). Fig. 2(c) and (d) show SEM images of the PCNT filler at low and high magnifications, respectively. The PCNT presents a homogenous structure and looks like rods of snowflakes. The nano-sized MWCNTs, having high surface area, serve as nucleation sites for the polymerization of EDOT monomers to form a coating over the MWCNTs. The images show that the PEDOT is uniformly coated over the MWCNTs, thereby forming a high aspect ratio filler, which is responsible for the good electrical conductivity of the PUPCNT composites.

Fig. 2 SEM micrographs of (a) MWCNTs and (b) pure PEDOT polymer; (c) and (d) low and high magnification images of PCNT filler, respectively, showing the uniform grafting of PEDOT over MWCNTs.

Fig. 3 SEM analyses of the fractured surface of PU-filled with PEDOT and PCNT filler: (a) PUP3 film and (b) PUPCNT1 showing the uniform distribution of the PCNT filler. (c) and (d) show that excess loading of PCNT filler on PUPCNT2 and PUPCNT3, respectively, leads to cluster formation.

Fig. 4 TEM images: (a) low magnification image of PCNT filler showing distinctly separated filler tubes, (b) high magnification image of PCNT showing the thickness of PEDOT coating over MWCNTs and presence of Fe catalyst inside MWCNTs, (c) PUPCNT1 film image showing PCNT filler incorporated in the PU matrix, and (d) HRTEM of PCNT filler showing the spacing between adjacent layers of carbon nanotube walls.

Fig. 5 XRD patterns of pristine MWCNTs, PU, PEDOT, PUP, and PUPCNT composites.

The fractured cross sectional surface of the composites was observed by SEM. Fig. 3(a) shows the fractured surface of PUP3, with indiscriminate distribution of the PEDOT filler in the PU matrix. In Fig. 3(b), the fractured surface of PUPCNT1 with uniformly distributed PCNT filler in the PU matrix can be distinctly observed. The PCNT filler appears as bright white lines embedded in the PU matrix, which are marked with red arrows in Fig. 3(b). The bright white lines increased with increasing PCNT loading in the PU matrix (see Fig. 3(c) and (d), respectively). It is interesting to note that the PCNT filler exhibits good dispersion, except for one or two clusters in the PUPCNT2 and PUPCNT3 composites due to excessive loading. It is very well known that the aggregation of fillers in the matrix is responsible for poor mechanical properties. This supports the reason for the improved tensile strength of PUPCNT composites compared to pure PU.

TEM images were further used to examine the core shell structure and thickness of the PEDOT coating over the MWCNTs. Fig. 4(a) shows a TEM image of the PCNT filler where individual PCNTs can be clearly observed. The Fig. 4(b) TEM image further confirms that MWCNTs are packed underneath the PEDOT layer. The diameters of the PCNT filler and the embedded MWCNT are about ∼150 and ∼46 nm, respectively, revealing the thick coating of the PEDOT layer (∼45 nm). The presence of Fe particles can also be observed inside the MWCNTs; these are remnants of the ferrocene catalyst used during the preparation of the MWCNTs. Fig. 4(c) shows the incorporation of the PCNT filler into the PU matrix, where the individual distribution of the PCNT filler can be clearly observed. Fig. 4(d) shows an HRTEM image of the PCNT filler, with the inset of the image showing a spacing of 0.34 nm between the concentric carbon nanotubes in the PEDOT matrix.

5.2 XRD

Fig. 5 shows the XRD patterns of the MWCNTs, PU, PEDOT, PUP and the PUPCNT composites. The XRD pattern of the MWCNTs shows two peaks at 2θ values of 26.1° and 43.2° for the (002) and (100) planes of the carbon atoms. These peaks imply interlayer spacings of 0.34 and 0.21 nm, respectively.²⁴ PU shows two peaks at 21.1° and 23.4°, which are attributed to crystals of polycaprolactone (PCL) corresponding to the (110) and (200) planes.²⁵ A broad diffraction peak at 24.5° is observed for PEDOT, due to its amorphous nature. Since PEDOT does not show any significant peaks, the PUP presents a much broader peak than PEDOT and PU. In the PUPCNT composites, the peaks for PCL and the MWCNTs are present at their respective 2θ values. Hence, this further confirms the co-existence of PCNT in the PU matrix.

5.3 FTIR

The FTIR spectra of PU, PUP3 and PUPCNT3, recorded using an ATR accessory, are shown in Fig. 6. All three samples have the characteristic peaks of PU. The characteristic peaks observed near 3320 and 1722 cm⁻¹ arise due to the stretching vibrations of the –NH and –C=O groups in PU.²⁶ Other peaks observed in the PU spectrum are assigned in the table given with Fig. 6. The spectrum of PEDOT shows bands at 678, 829, 917, and 967 cm⁻¹, which are due to the deformation modes of C–S–C in the thiophene ring; the peaks at 1052, 1084, 1134, and 1187 cm⁻¹ arise due to the bending vibrations of the C–O–C bonds of the ethylene group; and the peaks at 1312 and 1512 cm⁻¹ correspond to the C–C or C=C stretching of the quinoid structure and the ring stretching of the thiophene ring, respectively.²⁷ Despite some minor differences, PU, PUP3 and PUPCNT3 exhibit similar spectra due to the presence of significant amounts of PU in the matrix.

Fig. 6 FTIR spectra of PU, PUP3, PUPCNT3 and PEDOT samples with a table showing the assignment of bands for PU.

Tensile strength tests. Fig. 7 shows the effect of filler loading on the tensile strength of the PUP and PUPCNT composites. It is well known that pure conducting polymers are mechanically very weak. The blending of PEDOT filler in the PU matrix deteriorates the strength of the PUP composites consistently. This can be attributed to the weak interactions between the filler and the matrix. It is striking to note that addition of 10% PCNT filler significantly increases the tensile strength from 20 MPa to 26 MPa. This enhancement can be attributed to the homogeneous dispersion of PCNT and the effective load transfer from the matrix to the MWCNTs embedded in the PEDOT. However, with further increases in PCNT loading to 20 and 30 wt%, the tensile strength gradually decreased because of the plasticization effect on the mechanical properties; this is caused by the thick coating of the PEDOT layer, which has very poor mechanical strength. Hence, the tensile strength decreases with increasing loading of the PCNT filler, since the amount of PEDOT contributing to PCNT is greater than the amount of MWCNTs.

Fig. 7 Tensile strengths of PU, PUP1, PUP2, and PUP3; inset shows the tensile test results for PU, PUPCNT1, PUPCNT2, and PUPCNT3.

6. EMI shielding

An electromagnetic (EM) wave consists of two components: an electric field and a magnetic field, which are perpendicular to each other and change periodically. The EMI SE is described as the logarithmic ratio of the transmitted power (P_t) to the incident power (P_i) of the radiation and is measured in dB. The mechanism of attenuation of an EM wave by a shield depends on three factors: (1) reflection, occurring at the surface of the shield; (2) absorption, which arises when the EM wave travels inside the shield; and (3) multiple reflections of the EM wave at various interfaces (see Fig. 8(a)). Therefore, the total EMI SE is the sum of the effectiveness of the reflection (SE_R), absorption (SE_A) and multiple-reflection (SE_M) and can be expressed as:^6,28

SE_R = −log(1 − R) (3)

where R and T represent reflectance and transmittance, respectively.

Fig. 8 (a) Basic mechanism of EMI shielding by PUPCNT composites and (b) frequency dependence of the total shielding effectiveness (SE_T); losses due to the (c) reflection (SE_R) and (d) absorption (SE_A) of different samples of PU composites.

SE_M corresponds to the multiple reflections occurring at the various interfaces or surfaces within the shielding material. The absorption and reflection phenomena directly affect the SE_T; on the other hand, multiple reflection decreases the overall SE. If the thickness of the shield is greater than the skin depth (the distance of the shield at which the wave strength decreases to 1/e), the multiple reflection can be neglected. The skin depth (δ) is related to the angular frequency (ω), relative permeability (μ′) and ac conductivity (σ_ac) and can be expressed as . According to electromagnetic theory, when the thickness of the shield is greater than the skin depth (t > δ), the frequency (ω) dependence of the far field losses can be expressed in terms of the ac conductivity (σ_ac), real permeability (μ′), skin depth (δ), and thickness (t) of the shield material as:^29,30

SE_R(dB) = 10 log{σ_ac/16ωε₀μ′} (4)

The σ_ac and δ are related to the imaginary permittivity (ε′′) and real permeability (μ′) as σ_ac = ωε₀ε′′ and , which gives the absorption loss as:

It is known that SE_A becomes more dominant compared to the SE_R in the microwave region. This is due to the shallow skin depth (δ) and high conductivity (σ_ac) values at such high frequencies. The SE_T value for the PU, PUP and PUPCNT composites in the frequency range of 12.4 to 18 GHz is shown in Fig. 8(b). It is clearly visible from Fig. 8(b) that the PU film sample gives negligible attenuation to EM waves, whereas the solution mixing of PEDOT polymer in the PU matrix gives 3.98 dB, 5.75 dB and 7.41 dB of attenuation at a mid-frequency of 15.2 GHz for the PUP1, PUP2 and PUP3 composites, respectively. On the other hand, solution mixing of PCNT filler in the PU matrix results in a linear enhancement of the SE_T of the PUPCNT composites. The maximum EMI SE obtained with PUPCNT3 was 44.25 dB at 15.2 GHz which is due to the formation of conducting networks throughout the electrically insulating PU matrix by the addition of PCNT filler, which has a high aspect ratio. The EMI SE of electrically conductive polymer composites strongly depends upon the conductivity, dispersion, aspect ratio and loading of the conductive filler in the polymer matrix. To investigate the shielding mechanism, the total shielding effectiveness was further resolved into the reflection loss (SE_R) and absorption loss (SE_A) components, as shown in Fig. 8(c) and (d). From the experimental measurements, it is observed that the SE due to absorption and reflection (SE_A and SE_R) increases with increasing loading of the fillers (PEDOT and PCNT). This can be attributed to the increase in conductivity of the composites. It is interesting to note that the contribution of SE_A for the attenuation of EM waves is greater than that of SE_R, due to the increase in conductivity and dielectric losses as well as the magnetic permeability (entrapped Fe catalyst particles in the MWCNTs). Fig. 8(c) shows the anomalous behaviour of the SE_R of the samples in the applied frequency range. The SE_R of PU is almost constant throughout the frequency range due to its insulating nature, whereas the SE_R values for PUP3 and PUPCNT1 increase with frequency. This is because the samples with loadings of 30% of PEDOT and 10% PCNT in the PU matrix, respectively, were able to reach the percolation threshold (formation of a continuous electrically conducting network in the matrix) with uniform conductivity. The decreases in SE_R of PUP1 and PUP2 with increasing frequency can be attributed to the inability of the PEDOT fillers with 10% and 20% loading in the PU matrix, respectively, to form electrically conducting paths, whereas in the cases of PUPCNT2 and PUPCNT3, the decrease in SE_R is due to cluster formation due to excessive loading of the PCNT filler in the PU matrix which causes heterogeneity at the nano-level (non-uniform conductive network), thereby resulting in decreased SE_R values. The total shielding effectiveness of PUPCNT is greater than that of the PUP composites, and it increases systematically with increasing loading of PCNT.

We have also correlated various shielding parameters of the material with the ac conductivity; meanwhile, the attenuation constant was measured to determine the attenuation properties of the material.

6.1 Electrical conductivity and EMI shielding

The variations in electrical conductivity and EMI SE (at 12.4 GHz) of the PU, PUP, and PUPCNT composites at room temperature are shown in Fig. 9. The electrical conductivity of neat PU is 1.53 × 10⁻¹¹ S cm⁻¹. The loading of PEDOT (10, 20, and 30 wt%) in the PU matrix slightly increases the conductivity by 3 to 4 orders of magnitude. On the other hand, a loading of 10 wt% PCNT filler increases the conductivity of the composite by 7 to 8 orders of magnitude. This indicates the formation of an electrical percolation threshold in the PU matrix. The electrical conductivity of the PUPCNT composites increases with filler loading, and the value reaches up to 2.7 S cm⁻¹ from 1.53 × 10⁻¹¹ S cm⁻¹ of pure PU. This is due to the high aspect ratio of the PCNT filler, which helps establish it as an excellent conductive network between PU and PCNT. The SEM images of the fractured PUPCNT composites, showing good dispersion of the filler in the PU matrix, further confirm the formation of a conductive network throughout the matrix. It is a well-known fact that an increase in electrical conductivity enhances the EMI shielding values of composites (see Table 2); this is clearly depicted in Fig. 9.

Fig. 9 Variation of EMI SE (dB) with increase in room temperature dc electrical conductivity of different samples.

6.2 Electromagnetic attributes

To investigate the observed values of SE, the electromagnetic attributes, i.e. complex permittivity (ε′ − jε′′) and permeability (μ′ − jμ′′), were measured. These parameters were calculated from experimental scattering parameters (S₁₁ and S₂₂) using theoretical calculations given in the Nicolson–Ross–Weir algorithm.³¹ It is known that the ε′ and ε′′ values of the electromagnetic attributes are due to the amount of polarization occurring in the material. The dielectric values of the material depend on its ionic, electronic, orientation (arising due to the presence of bound charges) and space charge polarization (due to the heterogeneity in the system). In addition, interfacial polarization (also known as Maxwell–Wagner polarization³²), which is due to the different dielectric constants and conductivities of the individual components in the heterogeneous system, further enhances the electromagnetic attributes. In the case of the PUPCNT composites, PEDOT-coated MWCNTs were incorporated in the PU matrix; these have different dielectric constants and conductivities, and hence these composites show interfacial polarization in the applied frequency range. The presence of the conductive filler PCNT in the insulating PU matrix leads to the formation of various interfaces and a heterogeneous system; the space charge accumulating at these interfaces results in higher microwave absorption.³³ ICPs consists of polarons (radical cations) and bipolarons (biradical cations) which are free to move along the polymer backbone. ICPs also possess bound charges (dipoles) which have restricted mobility, leading to strong polarization in the material. Hence, the orientation polarization and dipolar polarization probably contribute to the dielectric permittivity. Fig. 10 shows the dielectric constants and dielectric losses of the PU, PUP3, and PUPCNT3 composites in the 12.4 to 18 GHz frequency range. It is observed that PUPCNT3 has higher dielectric loss and dielectric constant values (ε′ ∼ 26.08 to 23.5 and ε′′ ∼ 17.75 to 12.9) than PUP3 (ε′ ∼ 6.5 to 6.24 and ε′′ ∼ 2.06 to 1.82), which are much higher than those of neat PU (ε′ ∼ 2.21 to 2.04 and ε′′ ∼ 0.41 to 0.32). The synergistic effect of the PCNT filler in the PU matrix leads to an increased amount of polarization, which contributes to the high dielectric attributes of the PUPCNT3 composite.

Fig. 10 Behaviour of the dielectric constants (ε′) and dielectric losses (ε′′) of PU (a), PUP3 (b), and PUPCNT (c) samples as a function of frequency.

6.3 Electrostatic dissipation studies

ESD is critical for highly sensitive electronics and industrial applications. An electrostatic dissipative material prevents unwanted charge build-up that could otherwise transfer to sensitive electronic components. This is useful in the grounding of potentially hazardous charges. For ESD, typically, a surface resistivity of 10⁶–10⁹ Ω sq⁻¹ is required. The room temperature DC conductivity test of the samples showed that the PUP composites can be effectively used for ESD applications. Since the PUP composites failed to reach the minimum EMI SE (20 dB) required for most commercial applications in EMI measurements, we carried out ESD measurements of the PUP composites using a JCI 155v5 charge decay test unit. Fig. 11 shows the static decay times of the PUP1, PUP2 and PUP3 samples measured by applying a positive voltage of 5000 V. From these measurements, it is observed that the PUP1, PUP2 and PUP3 composites show a decay time of less than 0.2 seconds for the initial peak voltage to reach 1/e (about 37%), which meets the required time limit of less than half a second in the 1/e criterion. We also measured the 10% criterion (initial peak voltage to 10% within 2 seconds), and it is observed that all the samples show an excellent 10% criterion decay time of less than 0.8 seconds. Hence, the ESD results indicate that the loading of PEDOT in PU increases the conductivity of composites required for static charge dissipation. Thus, it is preferable to use the PUP composites for ESD rather than EMI shielding applications.

Fig. 11 Static charge decay times of (a) PUP1, (b) PUP2, and (c) PUP3 samples at a charging voltage of 5.0 kV.

7. Conclusion

Specially designed sandwich composites were prepared by blending PEDOT modified MWCNTs in a PU matrix by solution casting. The composites showed improved mechanical, electrical and EMI SE properties. The PEDOT grafted MWCNTs exhibited enhanced dispersion in the PU matrix, which resulted in improved tensile strengths of the composites with 10 and 20 wt% loading of the filler. Moreover, the EMI SE of the PUPCNT composites increased significantly with increasing filler loading, due to the formation of an enhanced conductive network in the PU matrix. A maximum SE of 45 dB was observed with 30 wt% loading of PCNT filler in the PU matrix, which is the highest compared to other CNT composites made for EMI shielding applications. An absorption dominated shielding mechanism was observed. Hence, the EMI shielding results suggest that these brand new flexible conducting materials can effectively be used as next generation shielding materials.

Acknowledgements

The authors are grateful to the Director CSIR-National Physical Laboratory, New Delhi for his kind support and encouragement. The authors also wish to thank Mr K. N. Sood, Mr Dinesh Singh and Mrs Shweta Sharma for the SEM, TEM and tensile strength characterizations, respectively.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14105b

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