Low Percolation Threshold Research Articles

Dispersion of Carbon Nanotubes Improved by Ball Milling to Prepare Functional Epoxy Nanocomposites

There has been an increase in interest in developing functional polymer composites based on green chemistry principles. The purpose of this study was to investigate the preparation of functional epoxy/carbon nanotube nanocomposites using ball milling methods. In contrast to mechanical mixing, ball milling promoted good dispersion of CNTs within the epoxy matrix, thereby improving their mechanical properties and electrical conductivity. In epoxy nanocomposites with ball milling, Young’s modulus and tensile strength were increased by 653% and 150%, respectively, when CNT loading was 1.0 vol%. Additionally, the ball milling of CNTs improves their dispersion, resulting in a low percolation threshold at 0.67 vol%. The epoxy/CNT film sensor that was produced using the ball milling approach not only exhibited high reliability and sensitivity to mechanical strains and impact loads, but also possessed the ability to self-detect damage, such as cracks, and accurately locate them. This study marks a notable milestone in the advancement of functional epoxy/CNT composites through the ball milling approach.

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.

Ionic Liquids as Alternative Curing Agents for Conductive Epoxy/CNT Nanocomposites with Improved Adhesive Properties

Good dispersion of carbon nanotubes (CNTs) together with effective curing were obtained in epoxy/CNT nanocomposites (NCs) using three different ionic liquids (ILs). Compared to conventional amine-cured epoxy systems, lower electrical percolation thresholds were obtained in some of the IL-based epoxy systems. For example, the percolation threshold of the trihexyltetradecylphosphonium dicyanamide (IL-P-DCA)-based system was 0.001 wt.%. The addition of CNTs was not found to have any significant effect on the thermal or low-strain mechanical properties of the nanocomposites, but it did improve their adhesive properties considerably compared to the unfilled systems. This study demonstrates that ILs can be used to successfully replace traditional amine-based curing agents for the production of electrically conductive epoxy/CNT NCs and adhesives, as a similar or better balance of properties was achieved. This represents a step towards greater sustainability given that the vapor pressure of ILs is low, and the amount needed to effectively cure epoxy resins is significantly lower than any of their counterparts.

A flexible strain sensor of porous conductive silicone rubber composites prepared from high internal phase emulsion

AbstractIn this work, a stretchable and compressible strain sensor was fabricated from conductive porous carbon nanotubes/polydimethylsiloxane (CNT/PDMS‐P) high internal phase emulsion (HIPE), which was obtained by simple mechanical mixing of CNTs, PDMS, and water. Micropores were uniformly distributed in the PDMS matrix, with diameters mainly in the range of 3–9 μm and averaging 5.3 μm. Due to the confined dispersion of CNTs in the porous PDMS framework, the CNT/PDMS‐P achieved a lower percolation threshold of 0.29 wt%. Compared to the non‐porous composite (CNT/PDMS), its percolation threshold was reduced by 25%. Besides, the CNT/PDMS‐P showed excellent sensitivity and cyclic stability. The sensitivity of CNT/PDMS‐P was 50% higher than that of CNT/PDMS in the tensile strain range of 0%–10%. The stability tests for 1000 loading/unloading cycles showed that the maximum offset rate of CNT/PDMS‐P was 58% lower than that of CNT/PDMS. The sensitivity of CNT/PDMS‐P compression sensor was S = 0.00259 kPa−1, which could detect the stress up to 600 kPa and maintain the standard waveform even after 1000 compression cycles. Finally, the potential application scenarios of the sensor, such as perception of human body limb movement, facial movement, gesture transformation, and human‐computer interaction scenarios, were explored.

Porous liquid metal–elastomer composites with high leakage resistance and antimicrobial property for skin-interfaced bioelectronics

Liquid metal-elastomer composite is a promising soft conductor for skin-interfaced bioelectronics, soft robots, and others due to its large stretchability, ultrasoftness, high electrical conductivity, and mechanical-electrical decoupling. However, it often suffers from deformation-induced leakage, which can smear skin, deteriorate device performance, and cause circuit shorting. Besides, antimicrobial property is desirable in soft conductors to minimize microbial infections. Here, we report phase separation-based synthesis of porous liquid metal-elastomer composites with high leakage resistance and antimicrobial property, together with large stretchability, tissue-like compliance, high and stable electrical conductivity over deformation, high breathability, and magnetic resonance imaging compatibility. The porous structures can minimize leakage through damping effects and lower percolation thresholds to reduce liquid metal usage. In addition, epsilon polylysine is loaded into elastic matrices during phase separation to provide antimicrobial property. The enabled skin-interfaced bioelectronics can monitor cardiac electrical and mechanical activities and offer electrical stimulations in a mechanically imperceptible and electrically stable manner even during motions.

Stretchable conductive nanocomposites of low electrical percolation threshold for washable high-performance-interconnects

We report a rationally designed stretchable conductive composite of low Ag percolation threshold concentration with the aid of boron nitride (BN) as a non-conductive auxiliary filler.

Percolative proton transport in hexagonal boron nitride membranes with edge-functionalization.

Two-dimensional layered materials have been used as matrices to study the structure and dynamics of trapped water and ions. Here, we demonstrate unique features of proton transport in layered hexagonal boron nitride membranes with edge-functionalization subject to hydration. The hydration-independent interlayer spacing indicates the absence of water intercalation between the h-BN sheets. An 18-fold increase in water sorption is observed upon amine functionalization of h-BN sheet edges. A 7-orders of magnitude increase in proton conductivity is observed with less than 5% water loading attributable to edge-conduction channels. The extremely low percolation threshold and non-universal critical exponents (2.90 ≤ α ≤ 4.43), are clear signatures of transport along the functionalized edges. Anomalous thickness dependence of conductivity is observed and its plausible origin is discussed.

Fabrication of novel strain sensors from green TPV nanocomposites based on poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/silicone rubber/silicon-modified graphene oxide.

In this study, a new thermoplastic vulcanizate (TPV) blend of silicone rubber (SR) and poly (3-hydroxybutyrate-co-3-hydroxy valerate) (PHBV) including silicon-modified graphene oxide (SMGO), is used to fabricate highly flexible and sensitive strain sensors. The sensors are built with an extremely low percolation threshold of 1.3 vol%. We investigated the effect of adding SMGO nanoparticles to strain-sensing applications. The findings demonstrated that increasing the SMGO concentration enhanced the composite's mechanical, rheological, morphological, dynamic mechanical, electrical, and strain-sensing capabilities. But too many SMGO particles can reduce elasticity and cause nanoparticle aggregation. The nanocomposite's gauge factor (GF) values were discovered to be 375, 163, and 38, with nanofiller contents of 5.0 wt%, 3.0 wt%, and 1.0 wt% respectively. Cyclic strain-sensing behavior showed their ability to recognize and classify various motions. Due to its superior strain-sensing capabilities, TPV5 was chosen to assess the repeatability and stability of this material when utilized as a strain sensor. The sensor's excellent stretchability, sensitivity (GF = 375), and remarkable repeatability during cyclic tensile testing allowed them to be stretched beyond 100% of the applied strain. This study offers a new and valuable method for building conductive networks in polymer composites, with potential uses in strain sensing, especially in biomedical applications. The study also emphasizes the potential of SMGO as a conductive filler for developing extremely sensitive and flexible TPEs with enhanced, environmentally friendly features.

High resistivity‐temperature effect of resistivity for economical and facile conductive polymer composites with low percolation threshold via self‐constructed dual continuous structure

AbstractIn this work, electrically conductive polyethylene/polypropylene‐flake graphite composites with different ratios (HDPE/PP‐FG) were prepared by simple mechanical blending (Mec) and melt blending (Mel) technologies, combining with the grinding technique. According to the effect of preparation technique, dual continuous microstructures for Mec‐HDPE/PP‐FG composites were self constructed by controlling specific molding temperature. As a result, the percolation threshold of 1.33 wt% of Mec‐HDPE1/PP4‐FG was achieved, much lower than that of Mel‐HDPE1/PP4‐FG (7.02 wt%) and the maximum in conductivity was over 10−3 S/m at about FG content of 6 wt%. However, Mel‐HDPE1/PP4‐FG only showed the electrical conductivity of about 8.0 × 10−11 S/m when FG content was 6 wt%. Meanwhile, Mec‐HDPE/PP‐FG composites presented obvious positive temperature coefficient (PTC) effect of electrical resistivity, without negative temperature coefficient (NTC) effect and PTC intensity was up to 6.57 orders of magnitude, far higher than those of Mel‐HDPE/PP‐FG composites. In addition, the excellent repeatability in PTC behaviors for Mec‐HDPE/PP‐FG was achieved after several run cycles. Therefore, the study on economical and facile polymer materials with high performance PTC characteristics had the great significance in practical application.

Effects of Graphene Morphology on Properties of Carbon Nanotube/Polyurethane Film Strain Sensors

The film flexible sensors can convert tiny changes in size or force into electrical signals. They are key components of intelligent devices and wearable devices, and are widely used in human-computer interaction, electronic skin, health monitoring, implantable diagnosis, and other fields. This kind of sensor is generally composed of polymer matrix and conductive components, while carbon nanotubes (CNT) and graphene (GN), as typical one-dimensional and two-dimensional conductive carbon nano-materials, respectively, have been used to build film flexible sensors. In order to explore the relationship between the GN size and thickness, and the performance of film sensors, the GN-CNT/PU composite film sensors were prepared by in situ polymerization of polyurethane (PU) in the presence of GN and CNT. A highly sensitive GN-CNT/PU flexible film sensor was prepared with a high gauge factor (GF) up to 13.15 in a strain range of 0–20%; an exceptionally low percolation threshold of GN is about 0.04 vol% when the CNT content is fixed at 0.2 vol%, which is below the percolation threshold of CNT/PU nanocomposites. The size of the GN layer affects the GFs of the flexible film sensors; a GN with a smaller size can achieve a greater GF. This study paves the way for the better application of different qualities of GN in flexible sensors.

Highly sensitive flexible strain sensor based on carbon nanotube/styrene butadiene styrene@ thermoplastic polyurethane fiber with a double percolated structure

AbstractThe combination of a high sensitivity and a wide strain detection range in conductive polymer composites‐based flexible strain sensors is still challenging to achieve. Herein, a double‐percolation structural fiber strain sensor based on carbon nanotubes (CNT)/styrene butadiene styrene (SBS)@thermoplastic polyurethane (TPU) composite was fabricated by a simple melt mixing and fused filament fabrication strategy, in which the CNT/SBS and TPU were the conductive and insulating phases, respectively. Compared with the sensor without the double percolated structure, the CNT/SBS@TPU sensor achieved a lower percolation threshold (from 2.0 to 0.5 wt%, a reduction of 75%), and better electrical and sensing performance. It is shown that the strain detection range of the CNT/SBS@TPU sensor increases with increasing CNT loading. An opposite trend was observed for the sensitivity. The 1%‐CNT/SBS@TPU sensor exhibited a high conductivity (1.08 × 10−3 S/m), high sensitivity (gauge factor of 2.65 × 106 at 92% strain), wide strain detection range (0.2%–92% strain), high degree of linearity (R2 = 0.954 at 0–10% strain), broad monitoring frequencies (0.05–0.5 Hz), and excellent stability (2000 cycles). Moreover, the CNT/SBS@TPU sensor was shown to successfully monitor a range of human physiological activities and to be capable of tactile perception and weight distribution sensing.

Explosive percolation yields highly-conductive polymer nanocomposites

Explosive percolation is an experimentally-elusive phenomenon where network connectivity coincides with onset of an additional modification of the system; materials with correlated localisation of percolating particles and emergent conductive paths can realise sharp transitions and high conductivities characteristic of the explosively-grown network. Nanocomposites present a structurally- and chemically-varied playground to realise explosive percolation in practically-applicable systems but this is yet to be exploited by design. Herein, we demonstrate composites of graphene oxide and synthetic polymer latex which form segregated networks, leading to low percolation threshold and localisation of conductive pathways. In situ reduction of the graphene oxide at temperatures of <150 °C drives chemical modification of the polymer matrix to produce species with phenolic groups, which are known crosslinking agents. This leads to conductivities exceeding those of dense-packed networks of reduced graphene oxide, illustrating the potential of explosive percolation by design to realise low-loading composites with dramatically-enhanced electrical transport properties.

3D printing of flexible strain sensor based on MWCNTs/flexible resin composite

The flexible strain sensor is an indispensable part in flexible integrated electronic systems and an important intermediate in external mechanical signal acquisition. The 3D printing technology provides a fast and cheap way to manufacture flexible strain sensors. In this paper, a MWCNTs/flexible resin composite for photocuring 3D printing was prepared using mechanical mixing method. The composite has a low percolation threshold (1.2% ωt). Based on the composite material, a flexible strain sensor with high performance was fabricated using digital light processing technology. The sensor has a GF of 8.98 under strain conditions ranging between 0% and 40% and a high elongation at break (48%). The sensor presents mechanical hysteresis under cyclic loading. With the increase of the strain amplitude, the mechanical hysteresis becomes more obvious. At the same time, the resistance response signal of the sensor shows double peaks during the unloading process, which is caused by the competition of disconnection and reconstruction of conductive network in the composite material. The test results show that the sensor has different response signals to different types of loads. Finally, its practicability is verified by applying it to balloon pressure detection.

Construction of 1D biomass-derived tubular carbon fiber/Ni nanoparticles composite for broadband and lightweight microwave absorbers

The novel electromagnetic wave (EMW) absorber puts forward higher requirements for broadband and lightweight. Compared with other options, the construction of system for low-dimensional composite material is an effective way to realize this aspiration. In this research, making using of Typha's fluff as a template, 1D tubular carbon fiber/Ni nanoparticles composite (TN) was rationally constructed to achieve the remarkable electromagnetic wave absorption (EMA) properties through green and simple synthesis process. The low percolation threshold of the carbon fiber ensures sufficient conduction loss effects of TN at a low filling ratio to achieve the goal of lightweight. The design of the magnetic-dielectric synergistic system is more conducive to the acquisition of excellent impedance matching. The uniform distribution of in-situ grown nickel nanoparticles provides a magnetic coupling network and forms a rich heterogeneous interface with the carbon fiber to enhance interfacial polarization. These facilitate the broadband absorption behavior of composite. Accordingly, TN exhibits impressive EMA properties with an EAB of 8.6 GHz and a RL of −56.4 dB with a filling ratio of only 10 wt%. These inspiring achievements light the way to the progress of the novel EMW absorbers, including the high targets of ultra-light and ultra-wide. • 1D tubular carbon fiber/Ni nanoparticles composites were obtained via carbonization process and in-situ hydrothermal method. • A minimum RL of up to −56.4 dB and an EAB of 8.6 GHz with a filling ratio of only 10 wt% was achieved. • The characteristic tubular carbon skeleton of Typha's fluff guarantees the formation of a well-conducting network of TN composites at a low over-permeation threshold.

Silver-Sandwiched Natural Rubber/St-LDH/MWCNT Hybrid Bio-Nano-Composite System as a High-Performing Multimedia Laminated Electromagnetic Interference Shield Through a Tripling Mechanism

High-performance electromagnetic interference (EMI) shields are extremely important to protect human beings and sophisticated electronic devices from harmful EM radiation emissions in this digital age. Herein, a novel multimedia laminated system based on silver nanosheet-sandwiched natural rubber/stearic acid-modified layered double hydroxide/multiwalled carbon nanotube hybrid bio-nano-composites [NR/St-LDH/MWCNT]-Ag-[NR/St-LDH/MWCNT] has been fabricated successfully. The fabricated nanocomposite presents a remarkably high electromagnetic interference shielding efficiency (EMI-SE) of −28 dB in the absence of a silver sheet and an enhanced EMI-SE of −84 dB in the presence of a silver nanosheet. The very low percolation threshold of MWCNT (0.009) and the maximum dielectric strength of the NR/St-LDH/MWCNT hybrid biocomposite (15,556) satisfied the criteria for the composite to act as an EMI shield. The mechanism behind this significant enhancement that is tripling in the EMI-SE value of this novel hybrid material is analyzed in detail. An EM absorption process through an innovative tripling mechanism is proposed for the first time by interpreting experimental results. In the tripling mechanism, the first hybrid nanocomposite sheet absorbs radiation and the light thence transmitted from the first sheet is reflected back to the same sheet by the sandwiched silver sheet for further absorption and the residual light transmitted from the silver sheet is absorbed by the second hybrid nanocomposite sheet. The absorbing efficiency of the silver-sandwiched system was found to be 99.999%, which is a shred of solid evidence for the tripling mechanism. Further, detailed physico-chemical and EMI shielding efficiency measurements indicate that the proposed hybrid material can be used as a very smart, lightweight shielding material for future EMI shielding applications.

Room-temperature self-healing graphene/rubber-based supramolecular elastomers utilized by dynamic boroxines and hydrogen bonds for human motion detection

Flexible electronic sensors with self-healing capabilities exhibit distinctive fascination in human motion detection. However, it remains challenging in the mechanically robustness, highly healing efficiency, mildly healing condition, and steady electrical performance. Here, a self-healing nanostructured graphene/rubber-based supramolecular elastomer (GRSE) with a covalent/non-covalent system is rationally designed. The 1-pyrenamine-modified graphene nanosheets (GPA) disperse uniformly on the modified epoxy natural rubber (S-ENR), leading to good electrical conductivity (0.0029 S−1 m), low percolation threshold (2.73 wt%), a fast-response time (250 ms), and a low stress-strain detection range (1 %). Based on the synergistic effect of dynamic boroxines and interfacial hydrogen binding, GRSE-based sensor exhibits excellent mechanical strength (3.46 MPa) and high-performance healing efficiency (η = 91.1 %) at room temperature. The GRSE-based sensor can still detect precise and stable sensing signals accurately in human movements (including breath, speech, facial expression, and joint motion) after healing, implying a promising route to develop an efficient and robust self-healing flexible sensor for the more durable human motion detection in practice.

Carbon nanotube/epoxy composites with low percolation threshold and negative dielectric constant

Carbon nanotube/epoxy composites with low percolation threshold and negative dielectric constant

Ferric Ions Crosslinked Epoxidized Natural Rubber Filled with Carbon Nanotubes and Conductive Carbon Black Hybrid Fillers.

Natural rubber with 50 mol % epoxidation (ENR-50) was filled with carbon nanotubes (CNTs) and conductive carbon black (CCB) hybrid fillers with various CCB loadings of 2.5, 5.0, 7.0, 10.0 and 15.0 phr, and the compounds were mixed with ferric ion (Fe3+) as a crosslinking agent. The ENRs filled exclusively with CNTs, and CNT–CCB hybrid fillers exhibited typical curing curves at different CCB loadings, i.e., increasing torque with time and thus crosslinked networks. Furthermore, the incorporation of CNT–CCB hybrid fillers and increasing CCB loadings caused an enhancement of tensile properties (modulus and tensile strength) and crosslink densities, which are indicated by the increasing torque difference and the crosslink densities. The crosslink densities are determined by swelling and temperature scanning stress relaxation (TSSR). Increasing CCB loadings also caused a significant improvement in bound rubber content, filler–rubber interactions, thermal resistance, glass transition temperature (Tg) and electrical conductivity. A combination of 7 phr CNT and CCB with loading higher than 2.5 phr gave superior properties to ENR vulcanizates. Furthermore, the secondary CCB filler contributes to the improvement of CNT dispersion in the ENR matrix by networking the CNT capsules and forming CNT–CCB–CNT pathways and thus strong CNT–CCB networks, indicating the improvement in the tensile properties, bound rubber content and dynamic properties of the ENR composites. Moreover, higher electrical conductivity with a comparatively low percolation threshold of the hybrid composites was found as compared to the ENR filled with CNTs without CCB composite. The superior mechanical and other properties are due to the finer dispersion and even distribution of CNT–CCB hybrid fillers in the ENR matrix.

Multiwalled Carbon Nanotubes Polylactide Composites for Electrical Engineering—Fabrication and Electrical Properties

In the article, both the processes of manufacturing highly homogeneous polylactide resin/multiwalled carbon nanotubes (CNTs) and their properties are described. Regarding the application of carbon nanotubes polymer composites, one of the most important problems to solve is obtaining good dispersion of the filler in the polymer matrix. Preparation of polylactide/multiwall CNTs composites by quick polymer solidification and freezing the state of the dispersion of the nanotubes in the polymer solution is described. The method we used employs an increase in viscosity (carried out rapidly) of the sonicated polymer solution containing the CNTs by spray deposition. Good dispersion of the nanotubes is confirmed by electron microscopy. The obtained nanocomposites exhibit a low percolation threshold for electrical conductivity (above 0.25% by weight). The described method leads to obtaining an electrical conductive surface on virtually any material and reduces the small amount of an expensive filler (CNTs) needed to achieve good electrical conductivity. Furthermore, the carbon nanotubes used in the fabrication process of the composites were obtained using the liquid-source chemical-vapor deposition (LSCVD) synthesis method.

Facile production of binary polymer/carbonic nanofiller‐based biodegradable electromagnetic interference shield films with low electrical percolation threshold

AbstractInterference of electromagnetic (EM) radiation generated from different electronic gadgets produces noises diminishing their performance. So, shielding of EM interferences (EMI) is necessary to achieve noise‐free performance from electronic gadgets. In the present work, we have produced thermoplastic polyurethane (TPU)/polybutylene adipate‐co‐terephthalate (PBAT) based biodegradable binary polymeric blend and incorporated different carbonic nanofillers such as Vulcan XC‐72 conductive carbon black (VCB) and multiwalled carbon nanotube (MWCNT) into it employing solvent mixing and casting technique and studied the morphology of polymer blends, EMI shielding effectiveness in X‐band of the EM spectrum as well as mechanical performance, electrical performance, and biodegradability of polymer nanocomposites. The selective nanofiller distribution in the low viscous PBAT phase of the “co‐continuous” binary blend is found to achieve a “lower electrical percolation threshold” compared to neat polymer nanocomposites. For each of the TPU/PBAT/VCB and TPU/PBAT/MWCNT nanocomposites, electrical percolation threshold of around 12.5 and 5 wt% respectively has been achieved successfully. Nanocomposite sheets with a thickness of 1.0 mm and loading of 30 wt% of VCB and 15 wt% of MWCNT gave EMI shielding values of −25 dB and −28 dB respectively at a frequency of 8.4 GHz.