Ultrafast Self-Healable Interfaces in Polyurethane Nanocomposites Designed Using Diels-Alder "Click" as an Efficient Microwave Absorber.

In the recent times, multifunctional materials have attracted immense interest. Self-healing polymers are in great demand in almost every coating application. With an increase in electromagnetic (EM) pollution, curbing the same has become an urgent necessity. Lightweight coatings and conducting polymeric materials are being highly researched upon in this regard, and combining these properties with self-healing systems would open new avenues in EM interference (EMI) shielding (specifically in the microwave frequency domain) applications. In the current study, a novel approach toward the development of microwave shielding materials capable of self-healing through microwave heating has been attempted. A covalently cross-linked material was developed using Diels-Alder (DA) chemistry, which shows self-healing properties when stimulated by heating. Herein, reduced graphene oxide grafted with magnetite nanoparticles (rGO/Fe3O4) was covalently cross-linked to thermoplastic polyurethane using DA chemistry. The addition of multiwalled carbon nanotubes into these nanocomposites led to exceptional EM wave shielding and self-healing properties through a synergistic effect. The synergism led to exceptional EMI shielding of -36 dB, primarily through absorption in the microwave region of the EM spectrum. When used in the form of thin coatings of about 1 mm in thickness, the shielding value reached -28 dB, manifesting in more than 99% attenuation of EM waves through absorption. The material was also found to be capable of healing scratches or cuts through microwave irradiation.

Introduction

The recent advancements in science and technology have led to pervasive use of electronic devices which emit electromagnetic (EM) radiations as their byproduct. The miniaturization of devices has led to various electric circuitries in a particular device to be placed in close proximity to each other. This invites its own repercussions, leading to interference among these devices causing device malfunctioning. This not only compromises the proper functioning of the devices but also has an adverse effect on human health.[1−3] Therefore, extensive research is being carried out to curb this menace of EM interference (EMI).

EMI can be prevented either by reducing the emission at the source or by immunizing the victim device. The immunization of the victim device is generally achieved by using EMI filters that are local devices used to protect limited number of sensitive elements. Filters pose certain disadvantages, one of which is that they only screen out conducted emissions having characteristics different from that of the device being protected. Therefore, to protect a device from interferences from a broad spectrum of EM waves, there is a pressing need to develop EMI shields. The basic requirements for a shield are good electrical conductivity to reflect EM waves, good magnetic permeability, and the presence of electric and magnetic dipoles to interact with the incoming EM waves and absorb them.

Metals, despite being the most prominent choice as shields, pose several drawbacks such as high density, cumbersome process ability, and poor corrosion resistance. Moreover, there is a demand for flexible and easily moldable materials, which has led to polymer nanocomposites becoming as emerging choices for shielding materials. Polymers are exceptional in the sense that they can be easily customized as per the requirement by adding certain fillers which impart EM shielding properties. Carbonaceous fillers such as carbon black, graphite, graphene, multiwalled carbon nanotubes (MWNTs), carbon fibers, and so forth are highly conducting in nature and can be dispersed in polymer matrices with the ease to develop an EMI shield. Carbonaceous fillers, being electrically conducting in nature, predominantly screen EM waves by reflection, which is not desirable. However, these fillers when used in conjugation with magnetic nanoparticles help in multiple scattering and absorption of EM waves. MWNTs and graphene derivatives are emerging choices because of their high aspect ratios and skin effect which leads to a high-frequency bandwidth.[4−9]

Graphene oxide (GO) is a versatile material that hosts a large number of functional groups which can be used to graft magnetic particles so that multifunctional materials can be developed for EMI shielding applications. GO is nonconducting in nature, but it can be made conducting by reducing it, forming reduced GO (rGO) which is partially conducting. The planar structure of graphene sheets makes them the ideal choices to graft nanoparticles, thus reducing the percolation concentration of nanoparticles. Grafting of magnetic nanoparticles such as Co, Fe3O4, and Ni or dielectric nanoparticles such as BaTiO3 and MnO2 on rGO sheets helps to improve microwave absorption due to enhanced charge transport and enhanced polarization losses.[10−12] In the light of the above-mentioned facts, we have attempted to conjugate rGO with magnetite nanoparticles using a facile, one-pot approach, solvothermal route. The magnetic nature of these heterostructures when combined with the high conductivity and aspect ratio of MWNTs and dispersed in a polymer matrix helps to develop an excellent EMI shielding material.[13−21]

In an attempt to develop multifunctional materials with more than just one application, we have developed a self-healable EMI shield.[22−32] Self-healable materials are of great interest in this context because the EMI shields may be in the form of coatings which on being scratched can again recover through heating, without losing its shielding properties. These multifunctional materials were developed by cross-linking nanoparticles with the polymer matrix using Diels–Alder (DA) chemistry.[33−37] The nanocomposites developed in this study can be used in the form of thin self-healable coatings for EMI shielding applications. The proposed work represents a novel approach because there is no literature available on EMI shielding materials that are self-healable.

Characterizations

To characterize the nanoparticles, transmission electron microscopy (TEM) images were acquired using TEM-Titan Themis at 300 kV. The structural properties of the nanoparticles were analyzed using an X’pert Pro X-ray diffractometer using a Cu Kα source. The Fourier transform infrared (FTIR) spectra of the samples were recorded using PerkinElmer GX in the range of 4000–650 cm–1, in the ATR mode. The defect characterization of nanoparticles was carried out using a HORIBA LabRAM HR Raman spectrometer. The elemental state of nanoparticles was characterized by an X-ray photoelectron spectrometer (AXIS Ultra) using an Al monochromatic source (1.486 keV). The room-temperature electrical conductivity of the samples was measured on 10 mm compression molded discs using a TA Discovery HR-3 rheometer using a dielectric setup connected to an LCR meter in the frequency range of 20 Hz to 2 MHz. The EMI shielding properties of the as-prepared samples were studied in the X- and Ku-band frequency range using an Anritsu MS4642A vector network analyzer (VNA) using a coax (Damaskos M07T) setup on 5.8 mm thick toroidal specimens, obtained by compression molding. The samples had an outer diameter of 7 mm and inner diameter of 3 mm. The shielding effectiveness (SE) for thin films was measured on a 1 mm thin film using a Keykom waveguide. The healing ability of the nanocomposites was imaged using a ZEISS ULTRA 55 field emission scanning electron microscope with an accelerating voltage of 5 kV.

Results and Discussion

Characterization of rGO/Fe3O4, Furfuryl-Modified rGO/Fe3O4, and Furfuryl-Functionalized Polyurethane

Figure a represents the X-ray diffraction (XRD) pattern for neat GO and as-synthesised rGO/Fe3O4 nanoparticles. Neat GO shows its characteristic peak at 2θ = 10.5°, corresponding to its (001) reflection. After solvothermal functionalization of GO sheets with Fe3O4 nanoparticles, six new peaks at 30.4°, 35.7°, 43.4°, 53.7°, 57.4°, and 62.8° can be observed, which corresponds to the (200), (311), (400), (422), (511), and (440) reflections of a cubic spinel structure of pure Fe3O4, respectively (JCPDS no. 19-0629).[38] Apart from the characteristic reflections of Fe3O4, two broad peaks at 14.3° and 25.4° can be seen, which corresponds to that of the partially reduced GO.[39] Therefore, it can be said that the complete reduction of GO was prevented so that some functional groups can be made available for the functionalization of furfuryl amine (FA) onto GO.

Figure 1

(a) XRD pattern for neat GO and rGO/Fe3O4. (b) TEM image and the corresponding SAED pattern of rGO/Fe3O4. (c) Raman spectra of neat GO and rGO/Fe3O4. (d) FTIR spectrum of rGO/Fe3O4 and rGO/Fe3O4–FA. (e) FTIR spectrum of PU-FA and PU-SH. (f,g) SEM image and corresponding EDAX spectra of the rGO/Fe3O4 sheets. (h) Corresponding EDAX mapping of iron on the rGO sheets. (i) AFM topographic image of neat GO sheets. (j) AFM image of rGO/Fe3O4. (k) XPS survey spectra of rGO/Fe3O4 along with C 1s and Fe 2p spectra in the inset.

The morphology of rGO/Fe3O4 nanoparticles was studied using TEM, as shown in Figure b. From the image, it can be seen that small Fe3O4 nanoparticles of about 5–10 nm size are dispersed uniformly over the GO sheet. The corresponding selected area electron diffraction (SAED) pattern clearly indicates the polycrystalline nature of these nanoparticles.[40]

The structural changes in the graphitic structure of GO before and after functionalization with Fe3O4 nanoparticles were studied using Raman spectroscopy. The ID/IG ratio of neat GO and rGO/Fe3O4 was found to be 1.07 and 1.70, respectively, as shown in Figure c. This shows that the defect concentration increased after solvothermal functionalization because of unpaired defects arising out of the removal of oxygen functional groups.

To determine the successful functionalization of rGO/Fe3O4 with FA, FTIR spectra were analyzed, as shown in Figure d. In the spectrum of rGO/Fe3O4, it can be seen that GO was not completely reduced by the solvothermal process. There are still some functional groups left, such as the peaks at 3220 and 1024 cm–1, corresponding to −OH– and −C–O– stretching vibrations of GO. In the spectrum of rGO/Fe3O4–FA, the peak at 3378 cm–1 and also the increase in peak broadness due to overlap of −NH– and −OH– stretching vibrations as compared to the spectrum of rGO/Fe3O4 shows the presence of the −NH– group from FA. The peaks at 1404 and 1336 cm–1 can be assigned to furfuryl, and the peak at 1050 cm–1 can be assigned to −C–O–C– of the furfuryl moiety of FA.[41]

The successful completion of the DA reaction and formation of self-healable structures was characterized using the corresponding FTIR spectrum. In the spectrum of PU-SH, the peaks at 1773 and 1708 cm–1 correspond to the C=C stretching of the DA adduct and the C=O stretching vibration of N,N′-(4,4′-diphenylmethane)bismaleimide (BMI), respectively. Additionally, the peak at 750 cm–1 for the furfuryl moiety of furfuryl-functionalized polyurethane (PU-FA) disappeared in the spectrum of PU-SH, which again confirms the successful formation of the DA adduct.

Figure f,g shows the scanning electron microscopy (SEM) micrograph and the corresponding energy dispersive spectroscopy (EDAX) mapping of the rGO/Fe3O4 nanoparticles. The percentage of Fe3O4 nanoparticles reduced onto rGO sheets was found to be 38%. Figure h shows the corresponding EDAX mapping of elemental Fe on the rGO sheets. It can be seen that Fe is distributed uniformly onto rGO sheets.

Figure i,j shows the atomic force microscopy (AFM) topographic images of neat GO and rGO/Fe3O4 nanoparticles, respectively. It can be observed that neat GO sheets are quite large in their lateral dimension. However, rGO/Fe3O4 nanoparticles on the other hand are comparatively smaller in their lateral dimension because of the high-temperature solvothermal treatment. The height of the neat GO sheet was measured to be about 10 nm, due to overlapping of GO sheets, whereas the height of the rGO/Fe3O4 sheets was measured to be about 30 nm. The increase in height may be due to grafting of Fe3O4 nanoparticles onto GO sheets.

To further confirm the grafting of Fe3O4 nanoparticles onto the rGO sheets, XPS spectra were analyzed. From Figure i, the XPS survey spectra show the presence of C 1s, O 1s, and Fe 2p peaks at ∼284, 529, and 710 eV, respectively. In the C 1s spectra (shown in the inset), three distinctive peaks at 284.7, 285.5, and 288.6 eV corresponding to C=C, C=O, and O–C=O functional groups, respectively, in GO can be realized. Similarly, in the Fe 2p spectra (shown in the inset), two distinctive peaks at 710.9 and 724.6 eV characteristic of Fe 2p3/2 and Fe 2p1/2, respectively, of Fe3O4 nanoparticles were realized. The absence of any peak at 719.9 eV confirms the absence of any traces of Fe2O3.[42,43]

AC Electrical Conductivity of Nanocomposites

It is envisioned that conductivity is one of the key factors that can enhance the EM wave shielding property of a material. Although conductivity is not the only the criterion for effective EMI shielding, EM wave attenuation can be escalated because of the well-connected MWNT network. MWNTs facilitate charge transport in a well-connected network either by hopping or by tunneling. Intertube distance plays a key role in the conductivity of MWNT-based nanocomposites. Intertube distance can be minimized by increasing the MWNT concentration, which facilitates charge transport through tunneling. In Figure a, it can be seen that MWNTs can percolate in the thermoplastic polyurethane matrix at a concentration of as low as 0.5 wt %. At higher loadings of the MWNT, the MWNTs can percolate very well and form effective charge transport networks. In Figure b, it can be seen that, when only 5 wt % rGO/Fe3O4–FA was added to PU-SH, the conductivity achieved is quite low. This can be attributed to lower conductivity of rGO compared to MWNTs. When both MWNTs and rGO/Fe3O4–FA were added together, an improvement in conductivity was again noticed. This can be perceived as an amplification in charge tunneling ability because of the presence of semiconducting MWNTs and partially conducting rGO sheets. The universal power law fitting onto frequency-dependent AC conductivity curves reveals the equivalent number of resistors and capacitive network taking part in the charge transport mechanism of these nanocomposites[44]

Figure 2

AC electrical conductivity for nanocomposites.

The value of the exponent “s” gives an indication of the extent of charge transfer that is taking place through tunneling. In Figure , a value of above 0.75 was achieved for the exponent “s”, which indicates 25 and 75% equivalent network of capacitors and resistances, respectively, for both PU-SH/3 wt % MWNT and PU-SH/3 wt % MWNT/5 wt % rGO/Fe3O4–FA. Capacitive networks create virtual connections between MWNTs at high frequencies, due to intertube polarization, thereby leading to amplification of conductivity at high frequencies, where DC conductivity has not yet reached.[45]

Microwave Shielding Ability of Nanocomposites

The EMI shielding efficiency of a material is quantified as the ratio of the intensities of the incident wave to that of the transmitted wave. The attenuation of the incident EM wave is manifested as the total SE (SET) and is expressed in decibels (dB). The total SE is a culmination of the contributions from shielding due to three different scattering mechanisms, namely, reflection from the rear surface of the shield, attenuation by absorption as the wave passes through the shield, and multiple reflections, which is neglected because it occurs only in materials with thickness less than its skin depth and whatever wave is reflected from the inner surface of the shield will eventually get absorbed. It is apparent that shielding by absorption is always the most preferred mechanism of shielding. Shielding by absorption is brought about by the magnetic and electric dipoles in the shield that can interact with the magnetic and electric fields, respectively, of the incident EM wave.

In a two-port VNA, SE is measured in terms of various scattering parameters (S11, S22, S12, and S21); therefore[46]

Figure a represents the total SE of nanocomposite samples of about 5.8 mm in thickness. Because SE scales with MWNT concentration, with only 3 wt % loading of conducting MWNTs, a SET of −27 dB was achieved at 18 GHz. MWNTs help in attenuating EM waves not only because of its conducting nature but also because they promote Ohmic losses through charge transport.[3] EMI shielding is enhanced by a very well-connected MWNT network. With the increase in the MWNT concentration, multiple scattering occurs within the various overlapping MWNT interfaces and eventually attenuation of the EM waves takes place because of their large specific area. This causes the EM wave shielding by MWNTs to be manifested as absorption. Absorption in the intrinsically conducting and nonmagnetic MWNTs may also be caused because of the presence of defects and intertube polarization (electric dipoles).[47]

Figure 3

Total SE (SET) for (a) 5.8 mm thick toroidal samples and (b) 1 mm thick films.

When 5 wt % of rGO/Fe3O4–FA was introduced, it was realized that the SET diminished to a value of −4 dB at 18 GHz. Therefore, it can be said that rGO/Fe3O4–FA alone is ineffective in attenuating EM waves because of its poor conductivity and inefficient charge transfer. However, when a 3 wt % MWNT was introduced along with 5 wt % of rGO/Fe3O4–FA, a synergistic effect was realized. The SET, increased to −36.7 dB at 18 GHz. This observed synergism was due to the nanoscopic nature of MWNTs and rGO/Fe3O4, leading to interparticle scattering and absorption. The presence of heterogeneities and macroscopic boundaries in the shield leads to nomadic charge transfer and charge trapping, thereby promoting EM wave absorption.[48]

Figure b represents the SET of 1 mm thin nanocomposite films. It was realized that even at such small thickness of the shield, the EM wave shielding of above 95% (SET of −15 dB) was achieved with only 3 wt % MWNTs. When both MWNTs and rGO/Fe3O4–FA were added, attenuation of above 99.7%, that is, SET of −28 dB was achieved. Therefore, the proposed shield can be very well used as an effective EM wave absorber and a self-healable coating.

Underlying Mechanism of Microwave Shielding of Nanocomposites

In a two-port VNA, the different S-parameters generated help to determine the contribution of absorption and reflection toward the total SE. The SE due to reflection and absorption has been represented in Figure a,b, respectively. As mentioned earlier, when it comes to the mechanism of shielding by MWNTs, a small portion of the EM wave gets reflected at the front surface of the shield, but because of enhanced charge transport taking place in the MWNTs, the wave gets attenuated as it passes through the shield by multiple scattering and ohmic losses, which is interpreted by the VNA as absorption. In the case of nanocomposites, 3 wt % MWNTs, SER is about −1.5 dB and SEA is −25.5 dB. This proves that in a well-percolated MWNT network, absorption is the predominant mechanism of shielding. Nanocomposites containing only rGO/Fe3O4 show SER of only −0.1 dB and the corresponding SEA show −3.9 dB. This absorption may come from the magnetic Fe3O4 nanoparticles which interact with the magnetic component of the EM wave, causing attenuation, from the interconnected rGO network which is slightly conducting causing multiple scattering or due to the charge accumulation at the heterojunctions and interfaces.

Figure 4

SE due to (a) reflection (SER) and (b) absorption (SEA) for 5.8 mm thick toroidal samples.

As was discussed earlier, a combination of MWNTs and rGO/Fe3O4 leads to a synergistic effect which helps in attenuating EM waves by absorption through better impedance matching. From Figure , it can be realized that the synergism leads to enhanced EM wave absorption of about −35.4 dB and diminished reflection of only −1.3 dB.

From the above discussion, it can be concluded that the synergism achieved by loading both MWNTs and rGO/Fe3O4 is desirable to achieve the maximum possible EM wave absorption.[49,50] The mechanism of shielding can be further analyzed using complex microwave properties such as complex permeability and permittivity. These properties have been discussed in detail in the Supporting Information.

Microwave-Healing Ability of Nanocomposites

The microwave shielding nanocomposites developed in the current study were fabricated such that these materials could be used as self-healable coatings for electronic applications. As discussed earlier, furfuryl-linked rGO/Fe3O4 (rGO/Fe3O4–FA) and PU (PU-FA) were covalently cross-linked using BMI through the DA chemistry. This resulted in nanocomposites with self-healing properties using microwaves. The proposed material apart from being an excellent EMI shielding material also works as a self-healing material because of its microwave absorbing nature. The self-healing ability is triggered by the heat generated in the material as a result of microwave absorption. The absorbed microwave is dissipated across the material in the form of heat.

To demonstrate the self-healing ability of these nanocomposites, samples were made by molding the samples into thin films by compression molding. The film was then cut into two halves and then joined together with slight pressure. As shown in Figure a, a cut was made on the film. The joined film was allowed to heal by exposing it to waves from a domestic microwave having a wattage of 900 W for 10 min. After 10 min, the sample was removed from the microwave and observed under a microscope. As shown in Figure b, the cut healed completely. As opposed to the conventional hot-air oven, where healing of the cracks may take up to 24 h, healing using a microwave is a much faster and efficient process.[51]

Figure 5

SEM images for self-healing tests on molded samples showing (a) cut sample and (b) healed cut after heating in a microwave oven.

The mechanism of healing has been represented schematically in Scheme . The healing of the cut was brought about by the recombination of furan and maleimide groups to form the DA adduct. The DA bond being much weaker compared to other types of covalent bonds can break because of stretching or scratching of these bonds.[41,51] When the cut is heated using microwaves, the heat generated due to the absorption of microwaves by the microwave absorbers in the sample leads to fusing and recombination of these bonds, thereby healing the cut.

Scheme 1

Self-Healing Process of Nanocomposites after Heating to 65 °C Using a Microwave Oven (the Inset Illustrates the DA Click Chemistry)

Conclusions

The current study was focused on developing self-healable EMI shielding materials. The nanocomposites developed by cross-linking furfuryl-functionalized rGO/Fe3O4 with furfuryl-functionalized PU using BMI and DA chemistry showed self-healing properties when heated using microwave irradiation. The microwave absorbers in the nanocomposite helped in absorbing most of the incoming microwave radiations, and this manifested in exceptional microwave shielding properties. The material when used in the form of 1 mm thin coatings/films provided above 99% microwave screening ability. Substantially, this current work not only verifies the microwave shielding ability of the developed nanocomposites but also verifies that the absorbed microwave when dissipated as heat can be used to heal cuts and scratches in the material.

Experimental Section

Materials

Polyethylene glycol (PEG, Mn = 4000 g mol–1) was procured from Merck. 4,4-Diphenylmethane diisocyanate (MDI, 99%), FA, and (BMI) were purchased from Sigma-Aldrich. Pristine MWNTs were purchased from Nanocyl. GO was purchased from BT Corp. Other chemicals and solvents used were obtained from commercial sources.

Preparation of rGO/Fe3O4 Nanoparticles by the Solvothermal Method

The reduction of magnetic Fe3O4 nanoparticles onto rGO sheets was achieved using a one-pot solvothermal method.[52] In a typical procedure, 100 mg of GO was first dispersed in 30 mL ethylene glycol (EG), by probe and bath sonication. 200 mg of FeCl3·6H2O was dissolved in 10 mL of EG. To this solution, predetermined amount of urea was added and dissolved under magnetic stirring. The as-prepared GO solution was added subsequently to the above solution followed by stirring for 20 min. To this solution, 2 mL of hydrazine hydrate was added and stirred for another 20 min. This solution was transferred to a Teflon-lined autoclave, and solvothermal reduction was carried out at 180 °C for 10 h. After that, the solution was allowed to cool to room temperature, and rGO/Fe3O4 was magnetically separated and washed thoroughly with ethanol, followed by drying at 60 °C for 12 h.

Synthesis of FA-Modified rGO/Fe3O4 (rGO/Fe3O4–FA) Nanoparticles

As shown in Scheme , 100 mg of rGO/Fe3O4 was dispersed in 100 mL water. pH was adjusted to 1 by adding few drops of HCl. Then, 1 g of FA was added, and the reaction was carried out at 80 °C for 12 h. The obtained product was washed thoroughly with water to remove redundant HCl and unreacted FA and dried under vacuum at 60 °C for 12 h.[41]

Scheme 2

Synthesis of rGO/Fe3O4–FA

Synthesis of PU-FA

PU-FA was synthesized by a two-step process.[51] The first step is the synthesis of PU. In a typical process, 1 mol PEG and 2 mol MDI were dissolved in dimethylformamide (DMF) separately. The two solutions were mixed together and stirred at room temperature for 1 h. The temperature was then increased to 80 °C, and the reaction was carried out for 4 h under nitrogen atmosphere. To this solution, 1 mL of FA was added, and the reaction was carried out for another 4 h. The obtained solution was dried first in air and then under vacuum at 80 °C for 24 h (Scheme ).

Scheme 3

(a) DA Reaction between FA and BMI. (b) Synthesis of PU-FA. (c) Fabrication of PU-SH

Fabrication of Microwave Shielding Self-Healing PU Nanocomposites

Self-healable PU nanocomposites, capable of microwave shielding, was fabricated by adding 5 wt % of rGO/Fe3O4–FA to the PU-FA solution in DMF followed by bath sonication for 30 min. The solution was subjected to mechanical mixing along with sonication for 2 h. To this solution, 1 g of BMI was added, and mechanical stirring was continued at 65 °C for 3 h. To further improve the EMI shielding performance of these nanocomposites, 3 wt % MWNTs was added to these nanocomposites. The solution was then poured into a Teflon-lined Petri plate and dried in air for 24 h followed by vacuum drying for 48 h. Unfilled self-healable PU (PU-SH), PU-SH with 3 wt % MWNT and PU-SH with 5 wt % rGO/Fe3O4–FA was prepared as per the above protocol as control samples.