Review of Polymer-Based Composites for Electromagnetic Shielding Application.

The rapid advancement of electronic communication technology has greatly aided human productivity and quality of life, but it has also resulted in significant electromagnetic pollution issues. Traditional metals and alloys are often used for electromagnetic interference (EMI) shielding due to their excellent electrical conductivity. However, they have drawbacks such as being heavy, expensive, and having low corrosion resistance, which limits their application in electromagnetic shielding. Therefore, it is crucial to develop novel EMI shielding materials. Polymers, being highly flexible, corrosion-resistant, and possessing high specific strength, are frequently employed in electromagnetic shielding materials. In this review, we firstly introduce the basic theory of electromagnetic shielding. Then, we outline the processing methods and recent developments of polymer-based electromagnetic shielding composites, including uniform-, foam-, layered-, and segregated structures. Lastly, we present the challenges and prospects for the field, aiming to provide direction and inspiration for the study of polymer-based electromagnetic shielding composite materials.

Heating and Strain Sensing Elements Based on Segregated Polyethylene/Carbon Black Composites in Polymer Welded Joints.

The development of easy and direct real-time monitoring of welded joint quality instead of surface damage analysis is crucial to improve the quality of industrial products. This work presents the results of high-density polyethylene (HDPE)-based composites with various carbon black (CB) content (from 20 to 30 vol.%) for use as a heating element and strain sensor in electrofusion-welded polymer joints. The pyroresistive heating process was used to determine the effect of generated Joule heat during welding on the structure and sensor properties of polymer-carbon composites. It is shown that the generation of Joule heat depends on the nanocarbon content and affects the crystallinity of the polymer matrix. The partial disruption of the conductive path of carbon black particles was observed and, as a result, a decrease in electrical conductivity for composites with lower CB content after welding was found. For the highest CB amount, conductivity increased, which is caused by smaller particle-to-particle distance for filler paths. Therefore, the best balance between pyroresistive and sensor properties was found.

Chemistry of Reduced Graphene Oxide: Implications for the Electrophysical Properties of Segregated Graphene-Polymer Composites.

Conductive polymer composites (CPCs) with nanocarbon fillers are at the high end of modern materials science, advancing current electronic applications. Herein, we establish the interplay between the chemistry and electrophysical properties of reduced graphene oxide (rGO), separately and as a filler for CPCs with the segregated structure conferred by the chemical composition of the initial graphene oxide (GO). A set of experimental methods, namely X-ray photoelectron spectroscopy (XPS), ultraviolet-visible spectroscopy, van der Paw and temperature-dependent sheet resistance measurements, along with dielectric spectroscopy, are employed to thoroughly examine the derived materials. The alterations in the composition of oxygen groups along with their beneficial effect on nitrogen doping upon GO reduction by hydrazine are tracked with the help of XPS. The slight defectiveness of the graphene network is found to boost the conductivity of the material due to facilitating the impact of the nitrogen lone-pair electrons in charge transport. In turn, a sharp drop in material conductivity is indicated upon further disruption of the π-conjugated network, predominantly governing the charge transport. Particularly, the transition from the Mott variable hopping transport mechanism to the Efros-Shklovsky one is signified. Finally, the impact of rGO chemistry and physics on the electrophysical properties of CPCs with the segregated structure is evaluated. Taken together, our results give a hint at how GO chemistry manifests the properties of rGO and the CPC derived from it, offering compelling opportunities for their practical applications.

Segregated Conductive Polymer Composite with Fe3O4-Decorated Graphite Nanoparticles for Microwave Shielding.

Graphite nanoplatelets (GNPs)-the segregated ultra-high molecular weight polyethylene (UHMWPE)-based composites with hybrid filler-decorated with Fe3O4 were developed. Using X-ray diffraction and scanning electron microscopy, it was shown that the decorated component has the shape of separate granules, or their clusters were distributed evenly over the GNPs surface. The individual Fe3O4 nanoparticles are predominantly rounded, with diameters of approximately 20-60 nm. The use of GNPs/Fe3O4 as a filler leads to significant decreases in the percolation limit φc, 0.97 vol% vs. 0.56 vol% for GNPs/UHMWPE- and (GNPs/Fe3O4)/UHMWPE segregated composite material (SCM), respectively. Modification of the GNP surface with Fe3O4 leads to an essential improvement in the electromagnetic interference shielding due to enhanced microwave absorption in the 26-37 GHz frequency range in its turn by abundant surface functional groups and lattice defects of GNPs/Fe3O4 nanoparticles.

Enhanced Mechanical Property of Polyamide-6/Graphite Sheet Composites with Segregated 3D Network Binary Structure for High Thermal Conductivity.

Segregated conductive polymer composites exhibit excellent electrical properties with a low percolation threshold. However, the mechanical properties of the segregated conductive polymer composites were always poor because the conductive fillers at the interfaces hinder polymer chain diffusion and thus lead to weak interfacial interaction between the conductive fillers and the polymer matrix. In this paper, polyamide-6 and polyamide-612 microspheres were synthesized via the in situ anionic ring opening of caprolactam and laurolactam. Segregated graphite sheets/polyamide-6(GS/PA6) and polyamide-612(PA612) composites with good mechanical properties were realized via high-pressure solid-phase compression molding. The microstructures of the composite samples were observed by scanning electron microscopy, which showed that the formation of a GS-conductive network at the PA6 granule interfaces in the segregated conductive structures and the adopting of PA612 considerably improved the interfacial adhesion of the composites. A superior impact strength of 5.1 kJ/m2 was achieved with 50 wt% PA612 loading owing to improvements in the interface compatibility between PA6 and GS. The composites possessed an ultralow percolation threshold, which was ascribed to the segregated network structure being successfully constructed inside the material. As for GS/PA6 composites, the combination of segregated GS-conductive networks achieved an ultralow percolation of 2.8 vol%. The percolation of 80PA6/20PA612-GS composites was slightly higher, measuring up to 3.2 vol%. Moreover, the thermal conductivity of the 80PA6/20PA612-GS composites increased from 0.26 to around 0.5 W/(m·K), which was 1.9 times larger than the pure polyamide.

Development of Eco-Friendly and High-Strength Foam Sensors Based on Segregated Elastomer Composites with a Large Work Range and High Sensitivity.

Achieving a high-strength piezoresistive foam with high sensitivity and a large workable range remains a major challenge. To realize these goals, we developed a facile, novel, and eco-friendly strategy for constructing segregated microcellular structures fabricated using coating, heat compression molding, and supercritical CO2 (ScCO2) foaming. The segregated poly(ether block amide) (PEBA)/carbon nanostructure (CNS) composites were fabricated via compression molding. This effectively improved the foamability and cell morphology of PEBA/CNS composites. Moreover, compared with the randomly distributed structure, the segregated structure also endowed the foams with better conductivity and sensing capability. Subsequently, the ScCO2 foaming was employed to fabricate segregated PEBA/CNS composite foams. The foaming gave composites a large compressibility and reduced their percolation threshold. Under 1 wt % CNS loading, via tuning the expansion ratio of foam from ∼2.1 to 4.1, the compression stress at 50% compression strain of foam varied from ∼3.3 to 0.5 MPa, and the conductivity changed from 4.89 × 10-3 to 1.93 × 10-6 s/m, implying a tunable conductivity. Additionally, the adjustable conductivity enabled the sensitivity of segregated composite foams to be regulated. The segregated PEBA/CNS foam (FCNS1-4.1) exhibited a good combination of high sensitivity (GF = 3.5), large work range (80% strain), and high compression strength (∼0.5 MPa at 50% strain) as well as a stable, reproducible, and durable sensing response under a low CNS content (∼0.11 vol %). Furthermore, the ΔI/I0 of FCNS1-4.1 (75.6% porosity) reached a high value of ∼810 and exhibited an ultrahigh sensitivity of ∼3706 (ΔI/I0ε) from 60 to 80% strain. Moreover, the foam sensor could be used as a sensing function sole for monitoring diverse human motions. Therefore, the segregated PEBA/CNS composite foams with outstanding piezoresistive performances show promising potential applications in monitoring human motions as wearable electronics and provides a new design strategy for a new generation of foam sensors with high performance.

Development of Electromagnetic-Wave-Shielding Polyvinylidene Fluoride-Ti3C2Tx MXene-Carbon Nanotube Composites by Improving Impedance Matching and Conductivity.

Absorption-dominated electromagnetic interference (EMI) shielding is attained by improving impedance matching and conductivity through structural design. Polyvinylidene fluoride (PVDF)-Ti3C2Tx MXene-single-walled carbon nanotubes (SWCNTs) composites with layered heterogeneous conductive fillers and segregated structures were prepared through electrostatic flocculation and hot pressing of the PVDF composite microsphere-coated MXene and SWCNTs in a layer-by-layer fashion. Results suggest that the heterogeneous fillers improve impedance matching and layered coating, and hot compression allows the MXene and SWCNTs to form a continuous conducting network at the PVDF interface, thereby conferring excellent conductivity to the composite. The PVDF-MXene-SWCNTs composite showed a conductivity of 2.75 S cm-1 at 2.5% MXene and 1% SWCNTs. The EMI shielding efficiency (SE) and contribution from absorption loss to the total EMI SE of PVDF-MXene-SWCNTs were 46.1 dB and 85.7%, respectively. Furthermore, the PVDF-MXene-SWCNTs composite exhibited excellent dielectric losses and impedance matching. Therefore, the layered heteroconductive fillers in a segregated structure optimize impedance matching, provide excellent conductivity, and improve absorption-dominated electromagnetic shielding.

Ni@CNTs/Al2O3 Ceramic Composites with Interfacial Solder Strengthen the Segregated Network for High Toughness and Excellent Electromagnetic Interference Shielding.

Ingenious microstructure design and appropriate multicomponent strategies are still challenging for advanced electromagnetic interference (EMI) shielding materials with excellent shielding effectiveness (SE) and reliable mechanical properties in harsh environments and low filling levels. In this study, nickel@multiwalled carbon nanotubes/alumina (Ni@CNTs/Al2O3) ceramic composites with segregated structures and electric/magnetic-coupling networks anchored by CNTs and magnetic Ni nanofillers were prepared by hot-press sintering. CNTs/Al2O3 ceramic composites exhibit a percolation threshold of only about 0.32013 vol %, which is lower than those of other reported CNTs/Al2O3 composites with segregated or uniformly dispersed structures. The electrical conductivity and EMI SE of 9CNTs/Al2O3 ceramic composites with 9 vol % (4.76 wt %) CNT content were 103.1 S/m and 33.6 dB, respectively. In addition, EMI SE and toughness were both enhanced by the synergistic effect of Ni nanoparticles and CNTs. In the unit of a segregated structure, a three-dimensional (3D) electric/magnetic-coupling network effectively captures and attenuates electromagnetic wave energy by electrical conduction, dielectric loss, and magnetic loss. On the other hand, the pull-out of CNTs and deflection of cracks distributed along the segregated structures synergistically enhance the fracture toughness of Ni@CNTs/Al2O3 ceramic composites. High-performance 3Ni@5CNTs/Al2O3 ceramic composites with 5 vol % (2.64 wt %) and 3 vol % (0.76 wt %) CNT contents have been achieved, whose EMI SE is 41.8 dB, density is 90.99%, flexural strength is 197.83 ± 18.62 MPa, and fracture toughness is 6.03 ± 0.23 MPa·m1/2. This efficient method provides a promising way to fabricate EMI shielding ceramic composites with high mechanical properties.

Constructing segregated polystyrene composites for excellent fire resistance and electromagnetic wave shielding.

Electroconductive polystyrene (PS) composites with ideal flame-retardant properties are considered as potential electromagnetic interference (EMI) shielding materials. In this work, PS/silicon wrapped ammonium polyphosphate/multi-wall carbon nanotubes (PS/SiAPP/MWCNT) composites with segregated structure were synthesized via the methods of balling mill and hot-pressing. The obtained results revealed that the SiAPP and MWCNT were successfully introduced onto PS spheres and showed uniform distribution on the PS surface. The thermogravimetric analysis showed that PS/SiAPP/MWCNT containing 7 wt% MWCNT exhibited excellent thermal stability. Furthermore, the results of cone calorimeter test indicated that the heat release rate and total heat release of the PS/SiAPP/MWCNT containing a loading of 7 wt% MWCNT were reduced by 60.5% and 33.9%, respectively. In addition, the EMI shielding performance could reach 11 dB. Above results implied that the synergistic effect between the MWCNT and SiAPP effectively enhanced the flame retardant performance of the PS by promoting the generation of dense and continuous char layer to protect the PS from burning. The multiple reflection and adsorption are responsible for improved EMI shielding effectiveness. Therefore, segregated PS/SiAPP/MWCNT hybrid is an up-and-coming candidate for satisfactory EMI shielding materials with exceptional fire retardancy for electronic devices.

Electrically conductive porous MXene-polymer composites with ultralow percolation threshold via Pickering high internal phase emulsion templating strategy.

HYPOTHESIS: Constructing a segregated network in electrically conductive polymer composites (ECPCs) is an effective method to lower the electrical percolation threshold. The segregated network structure can be formed naturally via polymerizing Pickering high internal phase emulsions (HIPEs) because solid particles are assembled at water-oil interfaces. However, most Pickering stabilizers show poor electrical conductivity. In this work, we propose a facile method to prepare lightweight ECPCs with well-controlled segregated structure via Ti3C2Tx-stabilized HIPE templating. EXPERIMENTS: Hydrophilic Ti3C2Tx flakes are delicately hydrophobized with a double-chain cation surfactant. The morphology of Ti3C2Tx flakes is investigated by transmission electron microscopy (TEM) and atom force microscopy (AFM). The surface properties of modified Ti3C2Tx are characterized by zeta potential and water contact angle tests. The stability of Ti3C2Tx-stabilized emulsions, and the structure of prepared ECPCs are systematically investigated. FINDINGS: Surface modified Ti3C2Tx flakes are used to stabilize water-in-oil (w/o) HIPEs for the first time. After the polymerization of continuous oil phase, ECPCs are successfully prepared with closed-cell porous structure. The pore size and size distribution of porous composites can be tailored by varying the content of Ti3C2Tx flakes. The Ti3C2Tx flakes are mainly immobilized at the water-oil interface and eventually form the segregated network in composites. Combining the unique segregated network and the outstanding metallic conductivity of Ti3C2Tx, the prepared porous polymer composites exhibit good conductivity even with ultralow Ti3C2Tx content of 0.016 vol%.

Polylactide/Carbon Black Segregated Composites for 3D Printing of Conductive Products.

One of the most important directions in the development of additive manufacturing or three-dimensional (3D) printing technologies is the creation of functional materials, which allow not only prototyping but also the manufacturing of products with functional properties. In this paper, poly-lactide acid (PLA) /carbon black (CB) composites with segregated (ordered) structure have been created. Computer simulation based on the Mamunya geometrical model showed that the CB content within φ = 2.5-5 vol.% in the polylactide matrix leads to the formation of a continuous electrically conductive phase with an increase of electrical conductivity σdc above the percolation threshold. The simulation results were experimentally confirmed by optical microscopy and studies of the electrical conductivity of the composites. It was found that increasing CB content from φ = 1 vol.% to φ = 7 vol.% in the composites causes insignificant (due to the segregated structure) phase changes in the polylactide matrix and improves the thermal properties of composites. Electrically conductive filaments for Fused Deposition 3D Printing (FDM) were developed from PLA/CB composites and then 3D printed. A correlation between the electrical conductivity σdc and the CB content φ for base composites, filaments produced from them, and final 3D samples, has been found. Conductivity varies within σdc = 3.1·10-11 - 10·10-3 S/cm for the filaments and σdc = 3.6·10-11 - 8.1·10-4 S/cm for the final 3D-products.

Segregated Structure Copolymer of Vinylidene Fluoride and Tetrafluoroethylene Composites Filled with rGO, SWCNTs and Their Mixtures.

This work is devoted to the formation and study of polymer composites with a segregated structure filled with single-walled carbon nanotubes (SWCNTs), reduced graphene oxide (rGO), and their mixtures. For the first time, polymer composites with a segregated structure filled with rGO/SWCNTs mixtures were obtained. A copolymer of vinylidene fluoride and tetrafluoroethylene (P(VDF-TFE)) was used as a polymer matrix. At a fixed value of the total mass fraction of carbon nanofillers (0.5, 1, and 1.5 wt%), the rGO/SWCNTs ratio was varied. The composites were examined using scanning electron microscopy, wide-range dielectric spectroscopy, and tested for the compression. The effect of the rGO/SWCNTs ratio on the electrical conductivity and mechanical properties of the composites was evaluated. It was shown that, with a decrease in the rGO/SWCNTs ratio, the electrical conductivity increased and reached the maximum at the 1 wt% filling, regardless of the samples' composition. The maximum value of electrical conductivity from the entire data set was 12.2 S/m. The maximum of elastic modulus was 378.7 ± 3.5 MPa for the sample with 1 wt% SWCNTs, which is 14% higher than the P(VDF-TFE) elastic modulus. The composite filled with a mixture of 0.5 wt% rGO and 0.5 wt% SWCNTs reflected 70% of the electromagnetic wave energy from the front boundary, which is 14% and 50% more than for composites with 1 wt% SWCNTs and with 1 wt% rGO, respectively. The lowest transmission coefficient of ultra-high frequencies waves was obtained for a composite sample with a mixture of 0.5 wt% rGO and 0.5 wt% SWCNTs and amounted to less than 1% for a 2 mm thickness sample.

Generic Method to Create Segregated Structures toward Robust, Flexible, Highly Conductive Elastomer Composites.

Electrically and thermally conductive polymer composites are extensively used in our daily life. It is of great significance to fulfill the conductivity requirement while maintaining desirable mechanical performance. An efficient solution to achieve this goal is to construct segregated structures in polymer composites by confining fillers into the interstitial areas among polymer domains. Thus far, it still remains a challenge to create segregated structures in cross-linked polymeric networks. Herein, we report a facile methodology to construct segregated structures in sulfur-cured rubbers using an industrially accessible process toward robust, flexible, highly conductive elastomer composites. Specifically, natural rubber granules (NR-RGs) with reactive di- and polysulfides on the surface are fabricated and then mixed with NR gum, carbon nanotubes (CNTs), and curing additives, followed by compression molding to yield two-phase separate composites. In the composites, CNTs are selectively dispersed in the continuous NR phase due to the volume exclusion effect caused by the separate NR-RG phase, leading to overwhelming electrical conductivity compared to the counterparts with randomly dispersed CNTs. In addition, NR-RGs can serve as novel reinforcement for NR, imparting the composites with remarkably improved modulus and retained stretchability. The simultaneously improved electrical conductivity and mechanical properties are due to the strong interfacial adhesion between the NR matrix and NR-RGs, as the di- and polysulfides on the surface of NR-RGs can participate in the cross-linking reactions of NR gum and enable the establishment of covalent bonding across the interfaces. The universality of this approach in preparing segregated composites with a combination of high conductivities and robust mechanical properties is demonstrated using other diene rubbers as the matrix and boron nitride as the filler.

Electro and Light-Active Actuators Based on Reversible Shape-Memory Polymer Composites with Segregated Conductive Networks.

Reversible shape-memory polymers (RSMPs) show great potential in actuating applications because of its repeatability among many other advantages. Indeed, in many cases, multiresponsive RSMPs are more expected, and the strategy to introduce functional fillers without deteriorating the reversible deformation performance is of great importance. Here, a facile strategy to balance the electro, photothermal performance, and molecular chain mobility is reported. Segregated conductive networks of carbon nanotubes (S-CNTs) are constructed in the poly(ethylene-co-octene) (POE) matrix at a relatively low filler loading, which renders the composite good electrical, photothermal, and actuating properties. A low percolation threshold of 0.25 vol % is achieved. The electrical conductivity is up to 0.046 S·cm-1 for the POE/S-CNT composites with 2 vol % CNT, and the absorption of light (760 nm) is above 90%. These characteristics guarantee that the actuator can be driven at low voltage (≤36 V) and suitable light intensity (250 mW·cm-2) with a good actuating performance. An electric gripper and a light-active crawling robot demonstrate the potential applications in multiresponsive robots. This work introduces a facile strategy to fabricate multiresponsive RSMPs by designing CNT network structures in polymer composites and holds great potential to enlarge the applications of RSMPs in many areas including artificial muscles and bionic robots.

Interface Design Strategy for the Fabrication of Highly Stretchable Strain Sensors.

Simultaneously achieving high piezoresistive sensitivity, stretchability, and good electrical conductivity in conductive elastomer composites (CECs) with carbon nanofillers is crucial for stretchable strain sensor and electrode applications. Here, we report a facile and environmentally friendly strategy to realize these three goals at once by using branched carbon nanotubes, also known as the carbon nanostructure (CNS). Inspired by the brick-wall structure, a robust segregated conductive network of a CNS is formed in the thermoplastic polyurethane (TPU) matrix at a very low filler fraction, which renders the composite very good electrical, mechanical, and piezoresistive properties. An extremely low percolation threshold of 0.06 wt %, currently the lowest for TPU-based CECs, is achieved via this strategy. Meanwhile, the electrical conductivity is up to 1 and 40 S/m for the composites with 0.7 and 4 wt % CNS, respectively. Tunable piezoresistive sensitivity dependent on CNS content is obtained, and the composite with 0.7 wt % filler has a gauge factor up to 6861 at strain ε = 660% (elongation at break is 950%). In addition, this strategy also renders the composites' attractive tensile modulus. The composite with 3 wt % CNS shows 450% improvement in Young's modulus versus neat TPU. This work introduces a facile strategy to fabricate highly stretchable strain sensors by designing CNS network structures, advancing understanding of the effects of polymer-filler interfaces on the mechanical and electrical property enhancements for polymer nanocomposites.

Significant Enhancement of Thermal Conductivity in Polymer Composite via Constructing Macroscopic Segregated Filler Networks.

The low efficiency of thermal conductive filler is an unresolved issue in the area of thermal conductive polymer composites. Although it is known that minimizing phonon or electron interfacial scattering is the key for achieving high thermal conductivity, the enhancement is generally limited by preparation methods that can yield the ideal morphology and interfaces. Herein, low temperature expandable graphite (LTEG) is added into a commercial impact modifier (Elvaloy4170), which is then coated onto poly(butylene terephthalate) (PBT) particles with various sizes at millimeter scale between their melting temperatures. Thus, macroscopic segregated filler networks with several considerations are constructed: high LTEG loading leads to a short distance between fillers and a robust filler network; continuous Elvaloy-LTEG phase leads to a continuous filler network; and good interaction among filler and matrix leads to good interfacial interaction. More importantly, the rather large size of PBT particles provides the filler networks with low specific interfacial area, which minimizes the interfacial scattering of phonons or electrons. Relative to homogeneous composites with an identical composition, the thermal conductivity is enhanced from 6.2 to 17.8 W/mK. Such an enhancement span is the highest compared with results reported in the literature. Due to possible "shortcut" behavior, much higher effectiveness can be achieved for the current system than found in literature results when the Elvaloy-LTEG phase is considered as filler, with the effectiveness even exceeding the upper limit of theoretical calculation for highly loaded Elvaloy-LTEG phase with relatively large PBT particle sizes. This could provide some guidelines for the fabrication of highly thermal conductive polymer composites as well as multifunctional polymer composites.

Segregated Hybrid Poly(methyl methacrylate)/Graphene/Magnetite Nanocomposites for Electromagnetic Interference Shielding.

Nanocomposites of poly(methyl methacrylate)/reduced graphene oxide (PMMA/rGO) without and with decorated magnetite nanoparticles with a segregated structure were prepared using emulsifier-free emulsion polymerization. Various characterization techniques were employed to validate the presence of the nanofillers and the formation of the segregated structure within the nanocomposites. The percolation threshold of the nanocomposites was found to be 0.3 vol %, while a maximum electrical conductivity of 91.2 S·m-1 and electromagnetic interference shielding effectiveness (EMI SE) of 63.2 dB (2.9 mm thickness) were achieved for the PMMA/rGO nanocomposites at a loading of 2.6 vol % rGO. It was also observed that decorating rGO with magnetite nanoparticles (hybrid nanocomposites) led to a tremendous increase in EMI SE. For instance, 1.1 vol % PMMA/rGO nanocomposites indicated an EMI SE of 20.7 dB, while adding 0.5 vol % magnetite nanoparticles enhanced EMI SE to 29.3 dB. The excellent electrical properties obtained for these nanocomposites were ascribed to both superiorities of the segregated conductive structure and magnetic properties of the magnetite nanoparticles.