Lightweight flexible polyurethane/reduced ultralarge graphene oxide composite foams for electromagnetic interference shielding

Abstract

Multifunctional flexible polyurethane (PU)/reduced ultralarge graphene oxide (rUL-GO) composite foams with low density in the range of ∼53–92 kg m⁻³ were fabricated through the in situ polymerization of PU in the presence of rUL-GO. The incorporation of 1 wt% rUL-GO gave the insular flexible PU composite foams a high electrical conductivity of 4.04 S m⁻¹, and an excellent electromagnetic interference (EMI) shielding efficiency of ∼253 dB (g⁻¹ cm⁻³) at 8–12 GHz. Achieving such a high specific EMI shielding efficiency as well as a low percolation threshold, combined with the method of foam preparation, results in a uniform dispersion and very high aspect ratio (>20 000) for the rUL-GO nanosheets. Furthermore, by the introduction of rUL-GO, the Young’s modulus and tensile strength of the PU composite foams also improved significantly without reducing the flexibility. TGA experiments also indicated the enhanced thermal stability of the composite foams.

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1. Introduction

The optimum design of polymer composite materials is a crucial step towards the practical application of these classes of hybrid materials. A core concept is to mix two or more components together, which profits from the advantageous properties of each component and repairs the dysfunctionalities of the polymeric matrix. Therefore, the most favorable design depends very much on the final use of the composite materials. When it comes to using polymeric materials in electromagnetic interference (EMI) shielding, the main disadvantage of common polymers is their electrical insulating nature. Increasing the bulk electrical conductivity of polymer composites to the order of 1–10 S cm⁻¹ is often mandatory to achieve suitable and effective EMI shielding.

A common practice for making conductive composites with suitable EMI shielding efficiencies is to mix a polymer matrix with carbonaceous nanoparticles such as carbon black, carbon nanofibers, carbon nanotubes (CNTs), and graphene and its derivatives. A carbon fiber/polystyrene (PS) composite containing 15 vol% carbon fibers showed an EMI shielding efficiency of 73.9 dB in the frequency range of 8–12 GHz.¹ A PS/Fe₃O₄/thermally reduced graphene oxide (TRG) composite had an EMI shielding effectiveness of 25–35 dB in the range of 9.8–12.0 GHz with 2.24 vol% TRG.² An EMI shielding effectiveness of approximately 32 dB was obtained for a water-borne polyurethane (PU) composite containing 5 vol% non-covalent modified graphene nanosheets.³ Composite sheets consisting of phenolic resin filled with a mixture of 37 wt% reduced graphene oxide (rGO), 12 wt% γ-Fe₂O₃ and 1 wt% carbon fibers showed an EMI shielding effectiveness of 45.26 dB in the frequency range of 8.2–12.4 GHz.⁴ A multilayer graphene/EVA composite film prepared as paraffin-based sandwich structures of 350 μm thickness had a 27 dB EMI shielding efficiency with 69 vol% multilayer graphene nanosheets.⁵ The EMI shielding effectiveness of a rGO–SiO₂ composite containing 20 wt% rGO was found to be 38 dB at 473 K.⁶ The highly aligned rGO/epoxy composite with 2 wt% rGO exhibited an EMI shielding efficiency of 38 dB in 0.5–4 GHz.⁷ The porous graphene/PS composite containing 5.6 vol% graphene showed an EMI shielding effectiveness of 25–30 dB in the frequency range of 8–12 GHz.⁸ The EMI shielding effectiveness of a graphene/polyaniline composite was found to be 34.2 dB with 33 wt% graphene at 8–12 GHz.⁹ Yang et al.¹⁰ prepared PS/CNT composite foams using a chemical blowing agent. The prepared PS composite foam with 7 wt% CNTs exhibited an EMI shielding efficiency of ∼20 dB at 8–12 GHz. Zhang et al.¹¹ reported an EMI shielding efficiency of 13–19 dB for a polymethylmethacrylate/graphene composite foam with 5 wt% graphene in the frequency range of 8–12 GHz. The graphene–Fe₃O₄/polyetherimide (PEI) composite microcellular foam showed an EMI shielding efficiency of 14–18 dB with a graphene–Fe₃O₄ loading of 10 wt% at 8–12 GHz.¹² A prepared graphene/poly(dimethyl siloxane) composite foam containing 0.8 wt% graphene had a 20 dB EMI shielding effectiveness in the frequency range of 8–12 GHz.¹³

Among the carbonaceous materials, the latest form, graphene, is attracting a great deal of attention not only due to its atomically thin nature and extraordinary properties but also because graphene is more eco-/environmentally friendly.^14,15 Bulk production of graphene and its derivatives is now easily possible using wet chemical methods and it is more cost effective than the tubular form of carbon i.e. CNTs. Despite the encouraging results on the EMI shielding of graphene-based polymer composites,¹⁶ one still wonders if the optimal performance of graphenic sheets has been obtained or not. In addition, there are the complicated functions of different parameters such as level dispersion and alignment, the intrinsic properties of the graphenic sheets, interaction with the polymer matrix etc. Let us simplify the problem and just imagine that the optimum performance of graphene for increasing electrical conductivity is the initial goal.

Adding graphene (and other conductive fillers) to a polymer matrix increases the electrical conductivity if an interconnected network of conductive graphenic sheets forms. As soon as this percolative network forms, electrons can pass through the matrix. The threshold for this increase in conductivity depends on the size of the graphenic sheets. In fact, the reason this network forms, is the excluded volume interaction among the sheets which is due to their 2D nature. The larger they are, the higher the chance a network will form. Therefore, it is preferred to use larger graphenic sheets. Another advantage of using large graphenic sheets is the lower resistance for passing electrons through the composites. There is a large resistance at the polymer–graphene interface. Using larger sheets substantially decreases the number density of these undesirable resistances. The remaining challenge is to find an effective way of dispersing giant graphenic sheets in the polymer matrix.

Lightweight PU foams are an excellent candidate as matrices for EMI shielding due to their superior mechanical properties, facile processability, very low density etc. In this study we used PU as the matrix for the fabrication of lightweight, conductive and flexible graphene-based composites. Graphene oxide with a very high lateral size (a few tens of microns, ultra-large graphene oxide, UL-GO) was synthesized according our previous work.¹⁷ Based on a novel method, UL-GO was incorporated into a PU matrix and its large size was preserved during the processing. In situ polymerization of PU monomers in the presence of rUL-GO was applied to prepare microcellular PU/rUL-GO composite foams, and rUL-GO was added, with a loading of up to 1 wt%. The properties of the PU/rUL-GO composite microcellular foams were investigated in terms of their microstructure, electrical conductivity, EMI shielding efficiency, thermal stability, and mechanical properties. To the best of our knowledge, few studies have investigated the preparation of lightweight microcellular PU composite foams for their application in electrical conducting and EMI shielding fields. This study provides a reproducible and scalable method which may develop the commercialization of such high performance, lightweight, and multifunctional foams.

2. Experimental

2.1 Preparation of reduced ultralarge graphene oxide (rUL-GO) nanosheets

UL-GO was synthesized based on the modified Hummers method using expanded graphite.^17–20 The obtained UL-GO nanosheets were diluted with DI water (3 mg mL⁻¹) and mildly sonicated for 20 min in a bath sonicator. The UL-GO nanosheets with a mean lateral size of ∼32.7 μm were obtained. To reduce the UL-GO, hydrazine solution was added to the mixture at a weight ratio of 3 : 1, and the resulting mixture was stirred at 80 °C for 24 h. The mixture was poured into flat molds and dried in an oven at 50 °C for 6 h to produce rUL-GO nanosheets.

2.2 Preparation of PU/rUL-GO composite foams

The required amounts of the resulting rUL-GO were mixed with the polyether based triol, Voranol 6150 (OH value: 27 mg KOH per g, Dow Plastics) in a reactor. The solution was diluted with a high amount of DI water. The reactor was immersed in an oil bath at 100 °C. To achieve a homogeneous dispersion of rUL-GO, a stirrer along with ultrasonic oscillators was used for 10 h while the water was continuously vaporized. The obtained mixture was dried in an oven at 100 °C for 12 h, and then cooled to room temperature. The prepared mixture (100 phr), along with dibutyltin dilaurate (FASCAT 4202, Arkema Inc.) as a tin catalyst (0.05 phr), TEDA-L33B (33% triethylenediamine in 1,4-butanediol) (0.25 phr) and NIAX catalyst E-A-1 (23% bis(2-dimethylaminoethyl) ether in dipropylene glycol) (0.1 phr) as tertiary amine catalysts, the polyol based triol, Voranol CP1421 (OH value: 31 mg KOH per g) (4 phr) as a cell-opener, DEOA (85 wt% diethanolamine in water) (0.8 phr) as a crosslink chain extender, SH 209 (0.4 phr) as a silicon surfactant and DI water (2.2 phr) as the blowing agent, was mixed with intensive stirring until a uniform mixture was obtained. Then, methylene diphenyl diisocyanate (MDI, Voranate M2940 (NCO content: 31.4 wt%), Dow Plastics) (43.4 phr) was added, and the mixture was stirred for an additional 30 s. Finally, the mixture was quickly poured into a mold with a lid. The foam expanded and filled the cavities. The obtained foams were post-cured in an oven for 4 h at 100 °C. The abbreviation which will be used for the foams is PU/rUL-GOw, where w denotes the weight fraction of the loaded rUL-GO. The procedure described above is schematically illustrated in Fig. 1.

Fig. 1 Preparation steps of PU/rUL-GO composite foam.

2.3 Characterization

Optical microscopy (OM) was carried out with an optical microscope (ZIESS, Primo Star iLED). Scanning electron microscope (SEM) images of freeze dried UL-GO samples and PU foams were taken with a JEOL-6390 SEM instrument at an acceleration voltage of 20 kV. Transmission electron microscopy (TEM) was performed with a JEM-2200 FS instrument operating at an accelerating voltage of 200 kV. For the TEM investigation, 0.2 mg mL⁻¹ of a UL-GO dispersion was prepared in DI water by ultrasonication for 5 min and drop-casted on a fresh carbon TEM grid. X-ray photoelectron spectroscopy (XPS) measurements of the powder samples were performed with an Omicron ESCA Probe (Omicron Nanotechnology, Taunusstein, Germany) using monochromatic Al K_α radiation (hν = 1486.6 eV). The X-ray diffraction (XRD) measurements were performed using a Philips 1825 instrument with Cu Kα radiation (λ = 0.154 nm), operating at 40 keV and with a cathode current of 20 mA. The layer-to-layer distance (d-spacing) of the graphite flacks, UL-GO, and rUL-GO samples was calculated according to Bragg’s law; d = nλ/2 sin θ, where n is an integer determined by the given order, and λ is the wavelength. Raman spectroscopy (Renishaw MicroRaman/Photoluminescence System) was used to analyze the samples using a 514 nm He–Ne laser. Fourier transform infrared spectra (FTIR) of the samples were obtained using an Equinox 25 Bruker instrument (Canada). The volume conductivity of the moderately conductive samples (>1 × 10⁻⁶ S m⁻¹) was measured using a standard four-probe method on a Physical Property Measurement System (Quantum Design, US). The samples with low volume conductivities (<1 × 10⁻⁶ S m⁻¹) were measured with a three-terminal fixture on an EST121 ultrahigh resistance and micro current meter (Beijing EST Science & Technology Co. Ltd.). Circular plates of 7 cm in diameter were fabricated for the conductivity measurements. The sample surfaces were coated with silver paste to reduce the contact resistance between the samples and the electrodes. The EMI shielding properties were measured using a WILTRON 54169A scalar measurement system in the frequency range of 8–12 GHz at room temperature. The samples were cut into rectangular plates with dimensions of 22.5 × 10.0 mm² to fit the waveguide sample holder. The thickness of the samples was about 2.5 mm. The tensile properties were measured on an Instron 5567 testing machine at a crosshead speed of 2.8 mm min⁻¹. At least three specimens were tested for each composition and the mean values were reported. The porosity of the foams was estimated using the following equation , where P is the porosity, ρ_app is the apparent density determined from the measured mass and the volume of the PU foam with a known size, and ρ_solid is the density of the solid polyurethane foam and is equal to 1200 kg m⁻³.²¹ Thermogravimetric analysis (TGA) was performed on a TGA Q500 instrument (TA Instruments) under a nitrogen environment at a heating rate of 10 °C min⁻¹ over a temperature range of 25–600 °C.

3. Results and discussion

3.1 Characterization of the UL-GO nanosheets

Fig. 2a shows the OM micrograph of the prepared UL-GO nanosheets confirming a lateral size of tens of micrometers. The corresponding SEM micrograph presented in Fig. 2b further supports the OM micrograph observation. As can be seen, the UL-GO nanosheets mainly have a lateral size of more than 20 μm, while some of them are even between 70 and 100 μm. However, a few smaller GO nanosheets with diameters in the range of 5–10 μm are also observed, which is due to breakages of GO nanosheets during the exfoliation process.^17,20 Moreover, from both the OM and SEM results, the formation of single layered GO can be confirmed to be homogeneous in contrast to GO sheets on a Si/SiO₂ substrate.^22–24 However, exfoliation of GO down to a single layer is also confirmed using TEM. Fig. 2c shows the TEM micrograph of the exfoliated monolayer UL-GO nanosheets. The TEM image indicates that the UL-GO nanosheets are made up of single layers, which is in accordance with the OM and SEM observations. These observations suggest exfoliated monolayered UL-GO nanosheets were obtained.

Fig. 2 (a) OM and (b) SEM images of UL-GO deposited on a Si/SiO₂ substrate, and (c) TEM micrographs of the as-produced UL-GO nanosheets.

Fig. 3 shows the XRD patterns of the natural graphite flakes, UL-GO, and rUL-GO samples. A sharp reflection, 2θ = 26.5°, in the XRD scattering pattern of pristine graphite originates from the interlayer (002) spacing (d-spacing = 0.337 nm). Upon oxidation, neighboring layers are ∼0.87 nm apart (2θ = 10.1°) because of the intercalation of oxygen groups and moisture. Moreover, in the case of UL-GO, peak broadening and a shift to a lower angle takes place owing to the disorder introduced by a reduced average crystalline size. For rUL-GO, the characteristic reflections are absent, implying high exfoliation via the chemical reduction of UL-GO.^17,25

Fig. 3 XRD patterns of (a) natural graphite flakes, (b) UL-GO, and (c) rUL-GO samples. The d-spacing is represented by d.

The results from the XPS analysis are shown in Fig. 4. The C1s spectra of UL-GO and rUL-GO are compared. The spectra were deconvoluted into three peaks corresponding to the following functional groups: carbon sp² (C=C and C–C), carbon sp³, epoxy/hydroxyl (C–O), and carbonyl (C=O), and the summary is presented in Table 1. The C1s XPS spectra of UL-GO indicate a considerable degree of oxidation which indicates the presence of different oxygen functional groups in the UL-GO structure (e.g., carbonyl, epoxy, hydroxyl groups).^17,26,27 The peaks corresponding to the covalent bonds of carbon and oxygen atoms are more intense for UL-GO than for rUL-GO. Therefore, it can be said that the reduction process was successful in eliminating oxidized carbon in the form of carbonyl and epoxy/hydroxyl groups.

Fig. 4 C1s XPS spectra of (a) UL-GO and (b) rUL-GO.

Table 1 Summary of the relative percentages of carbon and their assignation

Binding energy (eV) and assignation	284.6 eV, C–C and C=C	286.6–287.0 eV, C–O	287.8–288.2 eV, C=O
UL-GO	48.6	44.8	6.6
rUL-GO	70.0	17.5	12.5

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The FTIR spectra of UL-GO before and after reduction by hydrazine are shown in Fig. S1.† UL-GO displayed the typical absorption bands at 1717 (carboxyl C=O), 1633 (C=C), 1398 (carboxy C–O), 1225 (epoxy C–O), and 1055 cm⁻¹ (alkoxy/alkoxide C–O). Also, a broad band centered at 3437 cm⁻¹ was seen in the spectrum which is ascribed to the hydroxyl groups located on the surface of UL-GO.²⁸ After the reduction, the intensities of all absorption bands related to the oxygenous groups decreased dramatically, indicating that UL-GO had been almost completely reduced to rUL-GO.

The Raman spectra for UL-GO and rUL-GO were also characterized as shown in Fig. 5. For both samples, distinctive peaks for the D bands and G bands could be observed. It was observed that the D to G band intensity ratio, I_D/I_G, increased from 0.78 (UL-GO) to 0.83 (rUL-GO) and the G peak position red shifted upon reduction to rUL-GO. The decrease in the size of the newly formed graphene-like sp² domains is also partly responsible for the increase in the intensity ratio, I_D/I_G.^17,29 Moreover, a prominent D peak is a reflection of disorder in the carbon structure. The above observations are consistent with previous reports on similar rUL-GOs.^30,31

Fig. 5 Raman spectra of UL-GO and rUL-GO.

3.2 Characterization of PU/rUL-GO composite foams

The dispersion and chemical interaction of rUL-GOs in the PU matrix are critical issues in the development of meaningful materials for various applications.³² Therefore, XRD and FTIR analyses were respectively performed to evaluate the dispersion state and chemical interaction of rUL-GO with the PU matrix (Fig. 6 and 7). The XRD pattern of the PU/rUL-GO1 foam was almost the same as that of the pure PU foam, without the presence of the graphite or GO layer structure diffraction peaks at 26° and 10.1° (shown in Fig. 6). These results clearly indicate that rUL-GO is fully exfoliated into individual graphene nanosheets in the PU matrix.^33,34 The XRD patterns of other PU/rUL-GO composite foams also showed the same results (data not shown).

Fig. 6 XRD patterns of rUL-GO nanosheets, pure PU, and PU/rUL-GO1 foams.

Fig. 7 FTIR spectra of pure PU and PU/rUL-GO1 foams; the highlighted region in the original spectrum is magnified in the right-hand image.

Fig. 7 shows the FTIR spectra of the pure PU and PU/rUL-GO1 foams. Since the rUL-GO nanosheets derived by chemical synthesis possess remnant oxygenated groups, these active sites may interact with the urethane, carboxyl and isocyanate groups of the PU chains to give strong interfacial adhesion.^18,35–37 The pure PU foam showed its characteristic bands at: 3427 and 3520 cm⁻¹ (as a shoulder), which are attributed to the stretching vibrations of hydrogen bonded and free N–H groups, respectively. The absorption bands related to the non-symmetric and symmetric stretching vibrations of CH₂ groups were observed at 2930 and 2866 cm⁻¹, respectively. The peaks at 1722, 1228 and 1072 cm⁻¹ are attributed to the stretching vibration of carbonyl (C=O) groups, the stretching vibration of aromatic C–O groups, and the non-symmetric stretching vibration of C–O–C groups, respectively. The combination of N–H deformation and C–N stretching vibrations occurs at 1533 and 1228 cm⁻¹, respectively. By comparing the FTIR spectra of pure PU and PU/rUL-GO1 foams, two significant changes are noted. First, the shoulder at 3520 cm⁻¹ completely disappeared for the PU/rUL-GO1 foam. Second, the peak related to the C=O groups of PU was shifted from 1722 to 1699 cm⁻¹. These results indicate that both the free N–H and the C=O groups of PU become involved in hydrogen bonding interactions with the remaining oxygenated groups of the rUL-GO nanosheets.^35,37 Besides this, the bands at 1379, 1228 and 1072 cm⁻¹ completely disappeared after the addition of rUL-GO, further corroborating the presence of strong interfacial adhesion and chemical interactions between the PU chains and the rUL-GO nanosheets.

Fig. 8 depicts the electrical conductivity of the PU composite foams plotted as a function of the rUL-GO content. The conductivity of pure PU (∼10⁻¹¹ S cm⁻¹) is in good agreement with the values reported in the literature.¹⁸ On the other hand, the conductivity of thermally exfoliated graphene has been reported to be ∼80 S cm⁻¹.²⁵ As can be seen, the electrical conductivity of the PU composite foams exponentially increased at low rUL-GO contents, followed by a slow increase at higher contents. Thanks to the homogeneous dispersion of monolayer rUL-GO nanosheets in the PU matrix, the electrical conductivity was sharply raised by nearly 7 orders of magnitude when 0.4 wt% rUL-GO was incorporated. Above 1 wt% of rUL-GO, the electrical conductivity increased at a lower rate, then reached a plateau. It is noteworthy to mention that an electrical conductivity of 4.04–9.34 S m⁻¹ was achieved for a rUL-GO content in the range of 1–2 wt%, making these composite foams very promising for consideration in the electronic industry for areas such as electromagnetic interference shielding, electrostatic discharge protection, actuators and photoconductors.

Fig. 8 Electrical conductivity of PU/rUL-GO composite foams as a function of rUL-GO content (wt%); inset: log σ plotted against log(φ − φ_c).

It should be added that, generally, as the amount of conductive filler gradually increases, the foam undergoes a rapid insulator-to-conductor transition at a certain nanofiller content, referred to as the percolation threshold. The percolation threshold, φ_c, was calculated based on the power law equation:^38,39

σ_c = σ_f(φ − φ_c)ⁿ (1)

where σ_c, σ_f, and φ are the conductivity of the composite foam, filler conductivity, and filler content, respectively. From the log–log plot of the equation, the percolation threshold was obtained as φ_c = 0.06 wt% (shown in the inset of Fig. 8). The percolation threshold value in terms of the volume fraction was obtained as about 0.029 vol% by considering the densities of PU (1.05 g cm⁻³) and graphene (2.2 g cm⁻³). It should be highlighted that the percolation threshold values of foams are generally lower than those of their corresponding bulk composites. For example, Yousefi et al.³⁹ achieved the percolation threshold of 0.16 wt% in PU bulk composites using nearly the same size rUL-GO. In general, obtaining a lower percolation threshold value for polymer foams stems from two effects; one is that cell formation followed by in situ generated strong extensional flow pushes nanosheets together, and considerably facilitates the orientation of nanoparticles in the cell wall.⁴⁰ The other effect of the foaming process is the volume expansion, which increases the distance between adjacent nanoparticles.⁴¹

To the best of the authors’ knowledge, up until now, the obtained percolation threshold value is one of the lowest reported in the literature, not only for polymer/graphene composites foams but also for bulk composites. The lowest percolation threshold values of composite foams filled with graphene for different polymer matrices are summarized as follows; PMMA (φ_c ∼ 0.66 wt%),¹¹ polyvinylidene fluoride (φ_c ∼ 0.5 wt%),⁴² and PEI (φ_c ∼ 0.63 wt%).⁴¹ The ultralow percolation threshold value obtained in this work is ascribed to the following features of the PU composite foams: first, the inherently high electrical conductivity of rUL-GO; second, the good state of dispersion; and last but not the least, the ultralarge size of the monolayer graphene nanosheets.

Kim et al.⁴³ proposed an improved analytical model based on the average interparticle distance concept, predicting the percolation threshold of polymer composites filled with nanoplatelets or disk-shaped fillers. According to the results of this model, a percolation threshold of 0.029 vol% is equivalent to an aspect ratio in the order of 10⁴. Assuming that all of the rUL-GO nanosheets were exfoliated in the PU matrix as monolayer sheets (i.e., a thickness of about 1 nm), an aspect ratio of 10⁴ would be equivalent to an average lateral size of 10 μm. This estimation is in the same region as the size of GO sheets used in this study, verifying the molecular-level dispersion of individual rGO nanosheets in the PU matrix.³⁹ The lower estimation of the aspect ratio compared to what was observed experimentally (Fig. 2 and ref. 17) might be due to mere breakage of the nanosheets, the inherent flexibility of them (especially at such high aspect ratio) and also imperfect dispersion. However, these findings further emphasize the high potential of using rUL-GO nanosheets as highly efficient ECNs for the production of highly electrically conductive materials.

The EMI shielding of the PU/rUL-GO composite foams with a thickness of 2.5 mm was determined in the microwave frequency range of 8–12 GHz (Fig. 9). The measured shielding efficiency is defined as

where P_in is the incident power and P_out is the power transmitted through a shielding material, and it is directly related with its conductivity. As shown in Fig. 9a, the EMI shielding of the PU/rUL-GO composite foams presented a weak frequency dependency at the X-band. The EMI shielding of the PU/rUL-GO composite foams increased gradually up to ∼23 dB at 1 wt% rUL-GO loading, which indicates that the EMI shielding properties of these foams are enhanced with the increase of the rUL-GO loading. Fig. 9b summarizes the specific EMI shielding of polymer composite foams reported by other researchers.^{10,11,41,44,45} It is worth noting that our PU/rUL-GO composite foam with 1 wt% rUL-GO presented a higher specific EMI shielding than other foaming systems, resulting possibly from the large surface area of the as-prepared rUL-GO and the good dispersion of the ultralarge monolayer graphene nanosheets in the cell walls.^35,39,41 These results strongly suggest that such microcellular PU/rUL-GO composite foams are very promising for use as lightweight and high-performance EMI shielding materials with strong microwave absorption.

Fig. 9 (a) EMI shielding efficiency of PU/rUL-GO composite foam at different frequencies. (b) Comparison of the specific EMI shielding efficiency of our material with other reported materials.

The mechanical strength of the composite foams is of great importance, especially when they are generally exposed to high stress and strain fields. The interfacial interactions, orientation, aspect ratio, and surface functional groups of the nanofiller are some important factors which play a key role in determining the mechanical properties.^46–48 The mechanical properties of pure PU and PU composite foams (Fig. S2†) are summarized in Table 2. Even though the changes in the mechanical characteristics of the samples were remarkable, they still exhibited their typical elastomeric behavior. Interestingly, a content of as low as 0.1 wt% of rUL-GO increased the Young’s modulus and tensile strength from 5.5 to 42.6 MPa and from 15.3 to 30.2 MPa, respectively. The elongation at break of the composite foams was lower than that of the pure PU foam, but the obtained values are still in the range of high-elongation-at-break foams. The notable improvement of the modulus in the PU composite foams is due to the well exfoliated rUL-GO nanosheets as well as strong interfacial bonding between the PU chains and rUL-GOs. As demonstrated in the FTIR section, the remaining oxygenated functional groups on the surface of rUL-GO interact with the urethane groups of PU. Such interactions lead to well dispersed and exfoliated monolayer rUL-GO nanosheets, and consequently result in strong interfacial adhesion between PU and the nanosheets.^18,37,49 However, these interactions increase the molecular restrictions of the PU chains, and in turn decrease the elongation at break of the composite foams as compared to that of the pure PU foam.

Table 2 Mechanical properties, density, and porosity of flexible pure PU and PU/rUL-GO composite foams

Sample	Tensile strength (MPa)	Young’s modulus (MPa)	Density (kg m^−3)	Porosity (%)
Pure PU	15.3 ± 0.05	5 ± 0.1	53 ± 6	95.58 ± 0.50
PU/rUL-GO0.1	30.2 ± 0.06	42.6 ± 0.3	58 ± 5	95.16 ± 0.41
PU/rUL-GO0.5	38.4 ± 0.04	62.9 ± 0.1	72 ± 7	94.00 ± 0.59
PU/rUL-GO1	54.2 ± 0.2	75.2 ± 0.2	92 ± 9	92.33 ± 0.75

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The surface morphology of the pure PU and PU/rUL-GO1 foams is shown in Fig. 10. The PU foam exhibits a 3D-hierarchical mesoporous structure with the pore size ranging from 500 to 1200 μm which results from the high expansion ratio of the foaming. The density of the PU composite foams increased with the increasing rUL-GO content which is ascribed to an increase of the polyol viscosity^50,51 (Table 2). The increase in the density of the PU composites not only decreased the porosity, but also led to more polyhedral open cell structures. Despite this porosity reduction, the composite foams are still considered as highly porous materials, and therefore possess good compressibility and large internal surface areas. Moreover, the smooth inner and outer layers of these polyhedral open cell structures are free from the interjunctions that enhance the superior electrical conducting path.

Fig. 10 SEM micrographs of (a) pure PU and (c) PU/rUL-GO1 foams. (b) and (d) are the corresponding higher magnification images of (a) and (c), respectively.

The thermal stability of the pure PU and PU/rUL-GO1 foams is shown in Fig. 11. It has widely been reported that PU has a three-stage degradation process at 370–400, 400–440, and 470–550 °C. These three characteristic temperatures are referred to as T_1st, T_2nd, and T_3rd, respectively. The first stage is attributed to the dimerization and trimerization reaction of isocyanates;^18,52 the second step is due to the de-polymerization of PU to form isocyanates, primary or secondary amines, polyols, olefins, and carbon dioxide; and the last degradation step is assigned to the decomposition of the hard segment.^53,54 It is important to note that the slight variations, though negligible, seen in the thermogram curves of the pure PU foam are due to the presence of foaming additive components. Similarly, the typical three-stage degradation process is seen in the PU/rUL-GO1 thermogram curve. The effect of 1 wt% rUL-GO on the degradation steps of the PU foams, in terms of the characteristic temperatures, is shown in Table 3. The addition of 1 wt% rUL-GO, increases T_1st, T_2nd, and T_3rd by about 14, 27, and 37 °C, respectively. Moreover, as can be seen, the addition of 1 wt% rUL-GO improved the heat dissipation as well as the thermal stability of the PU/rUL-GO1 foam as compared to the pure PU one. There are several factors responsible for such a conspicuous thermal stability improvement. First, the interaction of the remaining oxygenated functional groups of rUL-GO with the urethane groups of PU, mentioned in the FTIR section, enhance the interfacial adhesion between the PU chains and rUL-GO.^55,56 Well dispersed and monolayer exfoliated rUL-GO nanosheets provide a high surface for direct covalent bonding with the PU matrix. These results continue to support the conclusion that the rUL-GO nanosheets can act as a barrier to reduce the diffusion of volatile compounds and hence an enhancement of the thermal stability is observed.¹⁸

Fig. 11 TGA trace of pure PU and PU/rUL-GO1 foams under a nitrogen atmosphere; inset: DTG trace.

Table 3 Degradation temperatures of flexible pure PU and PU/rUL-GO1 foams

Sample	T1st	T2nd	T3rd
Pure PU foam	382	420	521
PU/rUL-GO1 foam	396	447	557

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4. Conclusion

Lightweight flexible PU/rUL-GO composite foams with high electrical conductivity and specific EMI shielding efficiency have been fabricated by the in situ polymerization of PU in the presence of rUL-GO. The OM, SEM, and TEM micrographs showed the ultralarge size and monolayer structure of the as-prepared GO. Interestingly, thanks to the ultralarge size of rGO nanosheets, the PU composite foams exhibited an ultralow percolation concentration of 0.06 wt% and a high electrical conductivity of 9.34 S m⁻¹ with a low rUL-GO loading of 2 wt%, as well as an excellent specific EMI shielding efficiency of ∼253 dB in the presence of 1 wt% rUL-GO. In addition, the PU/rUL-GO composite foams exhibited not only improved mechanical properties, but also a good thermal stability. Overall, it has been demonstrated that such flexible conductive PU/rUL-GO composite foams with improved properties exhibit the potential for use as novel EMI shielding materials.

Acknowledgements

The first two authors wish to express their sincerest thanks to Mr Jalal Nasrollah Gavgani for his generous financial supports throughout this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra25374h

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