Ultrathin structures derived from interfacially modified polymeric nanocomposites to curb electromagnetic pollution

The use of electronic devices and wireless networks is increasing rapidly, and electromagnetic (EM) pollution remediation remains a challenge. We employed a unique approach to fabricate two ultrathin (approx. 53 μm) multilayered assemblies to address this. By sequentially stacking thin films of polyvinylidene difluoride (PVDF) and polycarbonate (PC) nanocomposites and interfacially locking them with a mutually miscible polymer (PMMA, polymethyl methacrylate), materials with enhanced structural properties and electromagnetic interference (EMI) shielding performance can be designed. Utilizing reduced graphene oxide (rGO) and molybdenum disulfide (MoS2) as a template, ferrite was grown on the surface to design two different nanohybrid structures (rGO–Fe3O4 and MoS2–Fe3O4). PVDF was composited with either rGO–Fe3O4 or MoS2–Fe3O4, and multiwall carbon nanotubes (CNTs) were dispersed in the PC component. As PC and PVDF are immiscible, their poor interface would result in inferior structural properties, which can be challenging in designing EMI shielding materials due to cyclic thermal fatigue. Hence, PMMA is sandwiched to interfacially stitch the components (PC and PVDF) and improve interfacial adhesion. This was confirmed using SEM/EDS and Raman mapping/imaging. The mechanical stability of the multilayered assemblies was characterized using a dynamic mechanical analyzer (DMA), and the storage modulus was found to be as high as 2767 MPa at 40 °C (@constant frequency and strain amplitude), for the multilayered film with rGO–Fe3O4 in PVDF, PMMA as a sandwich layer and CNTs in PC. A typical assembly of 9 multilayers (∼480 μm) with rGO–Fe3O4 in PVDF, and CNTs in PC, and interfacially stitched with PMMA gave rise to a high EMI shield effectiveness (SET) of −26.3 dB @ 26.5 GHz. This unique arrangement of a multilayered assembly suppressed EMI primarily by absorption.


1) The conceptual basis for the choice of polymer and solvent
For a preliminary test of miscibility, neat films and multilayered films were cast using a doctor blade setup and observed using digital imaging, as shown in figure S1. Before casting, the individual polymeric solutions/dispersions were prepared. The solvent used for PVDF was dimethylformamide (DMF), and PMMA and PC were chloroform (CHCl 3 ). As already mentioned, PMMA is used as the interfacial layer for better stitching of the other two layers.
PMMA is soluble in both CHCl 3 and DMF, and hence diffusion of the PMMA layer into both the phases of the multilayered assembly can be expected.
A couple of trial experiments were performed by dissolving polymers in various solvents such as dimethylformamide (DMF), tetrahydrofuran (THF), chloroform (CHCl 3 ), and N-Methyl-2pyrrolidone (NMP). The combinations of polymer-solvents that were attempted are shown in table S1a. The ratio of solvent to polymer was decided manually based on the viscosity of the resulting solutions, which have to be optimum for the film preparation. The ones with the tick mark in table S1a denote the systems that were practically (visually) found to dissolve at least 1g of polymer in 10 ml solvent (temperature was kept 40 °C for low boiling point solvents such as Electronic Supplementary Material (ESI) for Nanoscale Advances. This journal is © The Royal Society of Chemistry 2021 2 CHCl 3 and THF and 100 °C for high boiling point solvents such as DMF and NMP). The multilayered films cast using these solutions were visually inspected, and only a few of the combinations were found to mechanically stable. These combinations include PVDF-DMF/PMMA-DMF/PC-CHCl 3 , PVDF-DMF/PMMA-CHCl 3 /PC-CHCl 3 , PVDF-DMF/PMMA-THF/PC-THF. The idea was to choose a solvent such that PMMA can diffuse towards both the PVDF and PC sides for better stitching. The final selection of the pairs, PVDF-DMF, PMMA-CHCl 3 , and PC-CHCl 3 , served the purpose. The multilayered film obtained using these pairs is shown in figure S1f.
Thus, finally, the PVDF solution was prepared by dissolving 2 g PVDF in 10 ml DMF, the PMMA solution was prepared by dissolving 3 g PMMA in 10 ml CHCl 3 , and the PC solution was prepared by dissolving 2.5 g PC in 10 ml CHCl 3 . The films of individual layers of PC, PMMA, and PVDF were cast using a 300 µm doctor blade, and they were sufficiently dried before immersing it in cold water to peel them off from the glass substrate. The films were further dried in a vacuum oven to evaporate the water, and the final films are shown in figures S1a, S1b, and S1c. Similarly, PVDF/ PMMA and PMMA/ PC films were sequentially cast using 200 µm and 300 µm blades, and the final films are shown in figures S1d and S1e, respectively. Figure S1f shows the final film obtained by the sequential casting of the PVDF/PMMA/PC layer using 100 µm, 200 µm, and 300 µm blade (slit size). Figure S1g and S1h show the sequential stacking of PVDF followed by PC (or PVDF/PC) and PC followed by PVDF (or PC/PVDF), respectively. Doctor blades with 200 µm and 300 µm slit sizes were used to sequentially stacking these layers. The PMMA layer was purposefully not added in these two cases (figure S1g and S1h) to justify its importance as a mid-layer.
As expected, PC and PMMA films were transparent, as seen in the digital image. PVDF was opaque in appearance. As seen in the images S1d, S1e, and S1f, the films are also transparent.
However, figures S1g and S1h showing both combinations of PC and PVDF indicate opaque films. Thus, the miscibility of PMMA with both PC and PVDF has played its role in enhancing the transparency of the multilayered assembly of PVDF/PMMA/PC. The films obtained finally for PVDF/PMMA, PVDF/PC, and PC/PVDF were too brittle for mechanical analysis.
where refers to the energy from dispersion forces between molecules, refers to the energy from the dipolar intermolecular force between molecules, refers to the energy from hydrogen ℎ bonds between molecules, subscript 1 and 2 stands for solvent and solute and refers to the 0 interaction parameter of the polymer.
If RED < 1 → polymer will dissolve RED = 1 → polymer will partially dissolve RED > 1 → polymer will not dissolve The theoretical prediction holds for most cases except the PVDF-NMP system, which was practically found to be soluble (1g PVDF was soluble in 10 ml NMP), as is also evident from existing literature 2 . Moreover, the PC-DMF system was practically poorly/sparingly soluble for concentrations as low as 1 g PC in 10 ml DMF.

2) Characterization of polymers
This section includes the characterization of neat polymer for reference. Figure S2 shows the Fourier transform infrared (FTIR) spectra of commercial PVDF, PMMA and PC. The peaks corresponding to signature functional groups are highlighted in the respective polymers' FTIR spectra, based on the existing literature [3][4][5][6][7][8][9] . The observed spectra confirm that the materials under test are neat PVDF, PMMA and PC. The XRD of commercial PVDF used in this work can be obtained from the existing article published by Bose et al. 10 . The DSC thermogram of commercial PVDF and PC used in this work is already reported by Bose et al. 9,10 . DSC was performed for PMMA and the glass transition temperature was observed to be ~100 °C.   does not show a visible wear, it was structurally inferior for mechanical testing because of PC composite's brittle nature at such low thickness.    The percentage absorption for [PC(CNT)] 1 is higher compared to [PC(CNT)] 9 . As CNTs amount is 9-fold when using [PC(CNT)] 9 , reflection-based shielding overpowered the absorption-based shielding 11 . However, this is not the case for the multilayered assemblies of PVDF(rGO-