Migration mechanism of carbon nanotubes and matching viscosity-dependent morphology in Co-continuous Poly(lactic acid)/Poly(ε-caprolactone) blend: Towards electromagnetic shielding enhancement

Electromagnetic Interference Shielding of a Sequential Dual-Curing Thiol-Epoxy System Reinforced with GNPs with High Shape Memory

Modern electronics face several challenges during operation, such as interference of disruptive electromagnetic signals and high temperatures within a limited space. Both electromagnetic interference (EMI) and thermal management could be tackled simultaneously by employing smart efficient materials with high thermal and electrical conductivity. A dual-curing epoxy system, a new subset of adaptable materials, could potentially solve those challenges, with the proper selection of the reinforcement. Moreover, its manufacturing and synthesis process, which involves a sequential curing stage, constitute an attractive, selective, and fast methodology. The thiol-epoxy chemistry allows the synthesis of an epoxy system with high shape-memory capabilities while retaining optimal mechanical properties. Herein, dual-curing epoxy systems reinforced with graphene nanoplatelets (GNPs) are manufactured. The influence of the GNPs content is evaluated, which greatly increases upon loading while retaining a high shape-memory fixation and recovery rates (near 99%). A maximum EMI shielding efficiency of 24 dB is achieved for the higher GNPs content, which is endowed by the high electrical conductivity of the system. Moreover, a modelization of the near-field and far-field EMI shielding is reported, which agrees with experimental observation. This report shows the potential and multifunctional nature of dual-curing epoxy composites for EMI shielding and shape-memory-related application.

Excellent mechanical and electromagnetic interference shielding properties of polylactic acid/polycaprolactone/multiwalled carbon nanotube composites enabled by a multilayer structure design

In this work, a special multilayer structure consisting of polylactic acid (PLA) and a co-continuous PLA/polycaprolactone (PCL)/multiwalled carbon nanotube (MWCNT) (ALM) composite with a double-percolated conductive network was fabricated via layer-assembly coextrusion. It was revealed that PLA domains located at the layer interface could serve as rivets properly linking adjacent layers. Such a nacre-like structure with alternately stacked rigid PLA and flexible ALM increased the fracture strain to 354.4%, nearly quadruple that of the PLA/PCL/MWCNT conventional blending composite with the same composition, while maintaining an excellent strength above 46.0 MPa. In addition, the multilayer composites showed a special frequency-selective electromagnetic interference (EMI) shielding performance, with tunable shielding peak positions controlled by the layer number. Their maximum EMI shielding effectiveness almost contributed by absorption loss could reach 49.8 dB, which originated from two aspects: one was the high electrical conductivity offered by the double-percolated distribution of MWCNTs, and the other was the multiple wave attenuation effect that occurred at the interfaces between PLA and ALM layers and the blend interfaces in ALM layers. This effort paves a new way for developing composites with outstanding mechanical and EMI shielding properties that can be extended to other polymeric composite systems.

Efficient Synthesis of Fe3O4/PPy Double-Carbonized Core-Shell-like Composites for Broadband Electromagnetic Wave Absorption

Ever-increasing electromagnetic pollution largely affects human health, sensitive electronic equipment, and even military security, but current strategies used for developing functional attenuation materials cannot be achieved in a facile and cost-effective way. Here, a unique core-shell-like composite was successfully synthesized by a simple chemical approach and a rapid microwave-assisted carbonization process. The obtained composites show exceptional electromagnetic wave absorption (EMWA) properties, including a wide effective absorption band (EAB) of 4.64 GHz and a minimum reflection loss (RLmin) of -26 dB at 1.6 mm. The excellent performance can be attributed to the synergistic effects of conductive loss, dielectric loss, magnetic loss, and multiple reflection loss within the graphene-based core-shell-like composite. This work demonstrates a convenient, rapid, eco-friendly, and cost-effective method for synthesizing high-performance microwave absorption materials (MAMs).

Al@SiO2 Core-Shell Fillers Enhance Dielectric Properties of Silicone Composites

Over the past decade, there has been significant interest in polysiloxane-based dielectric elastomers as promising soft electroactive materials. Nevertheless, the natural low permittivity of polydimethylsiloxane has limited its practical applications. In this study, we have developed silicone rubber/Al@SiO2 composites with a high dielectric constant, low dielectric loss, and high electrical breakdown strength by controlling the shell layer thickness and the content of the core-shell filler. We also investigated the dielectric behavior of the composites. The use of core-shell fillers has increased the Maxwell-Wagner-Sillars (MWS) relaxation process while reducing the dielectric loss of direct current conductance in silicone rubber composites. Moreover, the temperature dependence of the MWS relaxation time in the composites follows the Arrhenius equation. This strategy of increasing the permittivity of silicone composites through core-shell structural fillers can inspire the preparation of other high dielectric constant composites.

Multilayered Composites with Carbon Nanotubes for Electromagnetic Shielding Application

Bulk polylactic acid (PLA)/multiwall carbon nanotube (MWCNT) composites were prepared and investigated in wide frequency ranges (20 Hz-1 MHz and 24-40 GHz). It was determined that the percolation threshold in bulk PLA/MWCNT composites is close to 0.2 vol.% MWCNT. However, the best microwave dielectric properties and absorption were observed in composites with 3.0-5.0 vol.% MWCNT. Therefore, for future investigations, we selected layered (laminate) polymeric structures with gradual changes in MWCNT concentration from 0.2 to 8.0 vol.% MWCNT. Two approaches to laminate structure designs were examined and compared: a five-layer composite and a nine-layer composite that included four pure PLA middle layers. The addition of MWCNT enhanced the elastic modulus by up to 1.4-fold and tensile strength by up to 1.2-fold, with the best performance achieved at 5.0 vol.% loading. High microwave shielding was observed for these layered PLA/MWCNT structures with a gradient change in MWCNT concentration (up to 26 dB in both transmission and absorption coefficients) in the broad frequency range (from 24 to 40 GHz). Obtained structures are highly anisotropic, and the absorption coefficient is 2-5 dB higher in the direction of MWCNT concentration increase; however, the transmission coefficient is the same in both directions. The properties of microwave absorption are mainly unaffected by the additional polymeric layers. The absorption of the layered structure is greater than the absorption of single-layer composites with an optimal MWCNT concentration of the same thickness. The proposed laminate structure design is promising in the field of efficient electromagnetic shielding.

Influence of Polymer Processing on the Double Electrical Percolation Threshold in PLA/PCL/GNP Nanocomposites

For the first time, the double electrical percolation threshold was obtained in polylactide (PLA)/polycaprolactone (PCL)/graphene nanoplatelet (GNP) composite systems, prepared by compression moulding and fused filament fabrication (FFF). Using scanning electron microscopy (SEM) and atomic force microscopy (AFM), the localisation of the GNP, as well as the morphology of PLA and PCL phases, were evaluated and correlated with the electrical conductivity results estimated by the four-point probe method electrical measurements. The solvent extraction method was used to confirm and quantify the co-continuity in these samples. At 10 wt.% of the GNP, compression-moulded samples possessed a wide co-continuity range, varying from PLA55/PCL45 to PLA70/PCL30. The best electrical conductivity results were found for compression-moulded and 3D-printed PLA65/PCL35/GNP that have the fully co-continuous structure, based on the experimental and theoretical findings. This composite owns the highest storage modulus and complex viscosity at low angular frequency range, according to the melt shear rheology. Moreover, it exhibited the highest char formation and polymers degrees of crystallinity after the thermal investigation by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The effect of the GNP content, compression moulding time, and multiple twin-screw extrusion blending steps on the co-continuity were also evaluated. The results showed that increasing the GNP content decreased the continuity of the polymer phases. Therefore, this work concluded that polymer processing methods impact the electrical percolation threshold and that the 3D printing of polymer composites entails higher electrical resistance as compared to compression moulding.