Low Electrical Percolation Threshold Research Articles

Highly stretchable, sensitive and healable polyurethane-urea/graphene nanocomposite sensor for multifunctional applications

Flexible wearable electronic devices offer promising potential for monitoring physiological signals. However, creating a single sensor that integrates high tensile strength, sensitivity, self-healing capabilities and a wide working range presents a significant and multifaceted challenge. This study reports a novel nanocomposite consisting of polyurethane-urea elastomer (PUU) and graphene nanoplatelets (E-GNPs) that are mechanochemically modified with diethyltoluene diamine ethacure 100. A low electrical percolation threshold was observed at 4.17 vol% E-GNPs. The sensor based on the PUU nanocomposite at 7 vol% of E-GNPs has revealed a gauge factor up to 17.57 and a wide working range of 361.76 % with high tensile strength of 19.73 MPa. It can withstand 20,000 cycles at 50 % strain. The sensor exhibits negative temperature dependence at 20–100 °C, with a resolution of 0.01/°C at 36–40 °C. Treatments with solvents and heat enable a healing efficiency for sensitivity of up to 70.46 %. The healable sensor enables real-time monitoring of temperature and strain signals, making it ideal for wearable devices in human health and sports monitoring.

Effect of Polyvinylpyrrolidone on the Structure Development, Electrical, Thermal, and Wetting Properties of Polyvinylidene Fluoride-Expanded Graphite Nanocomposites.

Polyvinylidene fluoride (PVDF)-expanded graphite (ExGr) nanocomposites have been prepared by solution blending and melt processing methods. In the presence of polyvinylpyrrolidone (PVP), enhanced dispersion of graphite nanosheets (GNSs) in the PVDF matrix, as suggested by field emission scanning electron microscopy analysis, results in very low electrical percolation threshold (0.3 wt % ExGr). X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry (DSC) analyses confirm the coexistence of electroactive gamma and nonpolar alpha phases. Wrapping of PVP chains around GNSs reduces the crystallinity in PVDF-ExGr nanocomposites in comparison to that in neat PVDF films, as evidenced by DSC analysis. Thermogravimetric analysis confirms enhanced thermal stability of PVDF-ExGr nanocomposites above 500 °C mainly attributed to the PVP-assisted dispersion of GNSs. The water contact angle of solution-blended PVDF-ExGr nanocomposite films increases with and without PVP in comparison to that of the neat PVDF film. Compression-molded PVDF-ExGr nanocomposites also exhibit electroactive gamma and nonpolar alpha phases of PVDF with reduction in electrical conductivity compared to solvent-cast films.

Lowering the percolation threshold when filling polystyrene with surface-functionalized carbon nanohorns

Carbon nanohorns (CNHs) are highly porous material composed of spherical aggregates of short carbon tubes with closed ends. Given their significant potential for a variety of applications, including electromagnetic shielding, it is important to tune the morphology and surface functionality of CNHs. In this study, CNHs functionalized with –CHx groups were synthesized by arc discharge of a graphite rod with the addition of toluene vapor. The resulting material was used as a filler for polystyrene composites, whose electromagnetic properties were studied over the frequency range of 1 kHz–4 GHz. Our research showed that functionalization increases the electrostatic association of CNHs, thereby facilitating the formation of conduction paths. As a result, polymer composites containing functionalized CNHs exhibit improved electromagnetic response and lower electrical percolation threshold. The insights gathered from this study provide crucial information on the relation between charge transport and polarization phenomena, the structure of CNHs, their surface state, and their arrangement in the polymer matrix. This information could potentially aid in the further development of this exceptional material.

Polymer blend templated hierarchical porous composites with segregated structure and enhanced electromagnetic interference shielding performance

AbstractConductive network built by less conductive filler is vital for high‐performance conductive polymer composites. Herein, porous ultrahigh molecular weight polyethylene (UHMWPE)/poly(vinylidene fluoride) (PVDF) blends filled with multi‐walled carbon nanotubes (MWCNT) were fabricated, where poly(lactic acid) (PLA) and polymethyl methacrylate (PMMA) were served as sacrificial template. An interconnecting conductive network is formed based on the UHMWPE segregated structure and PVDF co‐continuous structure in porous UHMWPE/PVDF/MWCNT (UFC) composites. The UFC composites demonstrate a low electrical percolation threshold (0.25 wt%) due to the segregated/co‐continuous structure. Porous UFCs exhibit higher electromagnetic interference shielding effectiveness (EMI SE) than their compact counterparts. The blend‐templated porous structure enhances the EMI SE of composites because a well‐dispersed and densely packed MWCNT network is formed around the polymer particles, facilitating the multiple reflections of electromagnetic waves inside the composites. This effort provides a facile way for preparing high‐performance shielding materials by controlling the hierarchical structure and conductive filler distribution.Highlights Hierarchical porous structure was constructed by blend sacrificial templating method. Carbon fillers are exclusively distributed at blend interface, forming the segregated structure. Porous composites exhibit better shielding performances than their solid counterparts.

Preparation and properties of double percolated co-continuous PC/PET/MWCNT/GNP engineering polymer blend nanocomposites

This study reports on the preparation and characterization of nanocomposites based on polycarbonate (PC)/poly ethylene terephthalate (PET) blend containing low amounts of hybrid of multiwalled carbon nanotube (MWCNTs) and graphene nanoplates (GNPs) by melt mixing. To use double percolation mechanism, at first a co-continuous polymer blend was prepared and the influence of nanofiller content on morphology, electrical conductivity, rheological and mechanical properties of co-continuous PC/PET blend was investigated. Blend containing 60 wt. % PET showed co-continuous morphology as observed by scanning electron microscopy and it was found that co-continuity remained unchanged by the addition of nanofillers. The direct current (DC) electrical conductivity results showed a low electrical percolation threshold around 0.5 phr. The results obtained from differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) showed an increase in thermal stability and crystallinity of nanocomposites. By increasing nanofiller amount, the storage modulus, loss modulus, and complex viscosity of samples increased as an indication of the strong network formation between nanofillers and polymer chains. The rheological percolation threshold was also found in the range of 2.5–3.5 phr of nanofillers. The tensile measurements results showed that the addition PC to PET increases the tensile strength of PET.

PMMA/SWCNT Composites with Very Low Electrical Percolation Threshold by Direct Incorporation and Masterbatch Dilution and Characterization of Electrical and Thermoelectrical Properties.

In the present study, Poly(methyl methacrylate) (PMMA)/single-walled carbon nanotubes (SWCNT) composites were prepared by melt mixing to achieve suitable SWCNT dispersion and distribution and low electrical resistivity, whereby the SWCNT direct incorporation method was compared with masterbatch dilution. An electrical percolation threshold of 0.05-0.075 wt% was found, the lowest threshold value for melt-mixed PMMA/SWCNT composites reported so far. The influence of rotation speed and method of SWCNT incorporation into the PMMA matrix on the electrical properties and the SWCNT macro dispersion was investigated. It was found that increasing rotation speed improved macro dispersion and electrical conductivity. The results showed that electrically conductive composites with a low percolation threshold could be prepared by direct incorporation using high rotation speed. The masterbatch approach leads to higher resistivity values compared to the direct incorporation of SWCNTs. In addition, the thermal behavior and thermoelectric properties of PMMA/SWCNT composites were studied. The Seebeck coefficients vary from 35.8 µV/K to 53.4 µV/K for composites up to 5 wt% SWCNT.

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.

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.

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.

Segregated MWCNT Structure Formation in Conductive Rubber Nanocomposites by Circular Recycling of Rubber Waste

The reclamation and devulcanization processes of rubber waste change its cross-link density and sol–gel proportions, which are essential in the reprocessing of reclaimed rubber (RR), especially in the dispersion of nanofillers throughout the RR matrix. In this work, conductive elastomers based on RR/carbon nanotubes (CNTs) were prepared. Proper control of the quality of RR is needed to ensure the effective formation of conductive pathways within the matrix. Therefore, this work aimed to control a segregated CNT network by manipulating how the RR is prepared. Three alternative devulcanization conditions were tested: (a) physical reclaiming, (b) thermomechanical reclaiming, and (c) thermochemical reclaiming. These techniques resulted in three types of RRs. Nanocomposite based on the physically RR presented the highest tensile strength, reaching approximately 15 MPa with addition of 2 phr of CNT. This was due to a comparatively lesser destruction of the rubber network and the formation of a segregated structure of CNT filler in the nanocomposite. Therefore, the conductive performance was significantly enhanced from 4.5 × 10–10 S/cm for unfilled to 1.25 × 10–6 S/cm with CNT dose as low as 1 vol %. The segregated CNT network, whose formation was assisted by gel fraction and excluded volume in RR, also resulted in a very low electrical percolation threshold φc, which was significantly reduced from 0.78 vol % for low cross-linked RR to 0.14 vol % for high cross-linked RR. This could reduce CNT filler use by 80% whereas 100% of pristine rubber would be replaced, and mechanical and electrical properties would improve. The approach described in this work will be helpful in manufacturing conductive rubber products efficiently yet sustainably with recycling of rubber waste.

Polyvinylidene fluoride‐poly(vinyl acetate)‐natural graphite blend nanocomposites: Investigations of electroactive phase formation, electrical, thermal, and wetting properties

AbstractSolution blended polyvinylidene fluoride (PVDF)‐20 wt% poly(vinyl acetate) (PVAc)‐natural graphite (NtGr) blend nanocomposites exhibit a very low electrical percolation threshold of less than 0.5 wt% NtGr due to the formation of graphite nanosheets. The electroactive gamma phase induced with and without NtGr in PVDF‐20 wt% PVAc blend film is confirmed by X‐ray diffraction, fourier transform infrared spectroscopy, and differential scanning calorimetry analyzes. The onset degradation temperature corresponding to PVDF in PVDF‐20 wt% PVAc blend nanocomposites is increased by 27°C when 1 wt% NtGr is incorporated into the blend, suggesting enhanced thermal stability of blend nanocomposites. Due to an increase in surface roughness of PVDF‐20 wt% PVAc‐3 wt% NtGr as evidence by field emission scanning electron microscopy analysis, the water contact angle increases to 141.6° from 83.44° for PVDF‐20 wt% PVAc blend. AC conductivity analysis suggests hopping conduction in PVDF‐20 wt% PVAc‐x wt% NtGr (x = 1, 3, 5) blend nanocomposites. The interjunction capacitance increases with graphite loading in the blend.

Flexible 3D Printed Acrylic Composites based on Polyaniline/Multiwalled Carbon Nanotubes for Piezoresistive Pressure Sensors

AbstractThe development of tunable UV‐curable polymeric composites for functional applications, taking into consideration environmental issues and additive manufacturing technologies, is a research topic with relevant challenges yet to be solved. Herein, acrylic composites filled with 0–3 wt.%. polyaniline/ multiwalled carbon nanotubes (PANI/MWCNT) are prepared by Digital Light Processing (DLP) in order to tailor morphology, thermal, mechanical, and electromechanical properties. Viscosity, real‐time infrared spectroscopy, and cure depth tests allow optimizing resin composition for suitable DLP printing. 2 wt.% is the maximum filler content reproducibly embedded in the polymer matrix. The advantages of PANI/MWCNT (50/50 wt.%) compared with single‐component composites include safety issues, enhanced printability, increased electrical conductivity and thermal stability, and lower electrical percolation threshold (0.83 wt.%). Above this threshold the composites display excellent piezoresistive response, no hysteresis, and stability for over 400 compression cycles. The pressure sensibility (PS) of 2 wt.% composites decreases with applied pressure from PS ≈ 15 to 0.8 Mpa−1 for maximum pressures of 0.02 and 0.57 MPa, respectively. A proof‐of‐concept of the functionality of the novel materials is developed in the form of a tactile sensor, demonstrating their potential for pressure sensing applications as cost‐effective, sustainable, and flexible materials for printed electronics.

Influence of Carbon Nanotubes Purity on the Properties of Carbon Nanotubes/Low-Density Polyethylene Composites

The influence of carbon nanotubes (CNTs) purity on the viscosity, thermal properties, impedance characteristics, and conductivity of CNT/low-density polyethylene (LDPE) nanocomposites was investigated. Two grades of CNT of similar aspect ratios but with different levels of carbon purities were investigated. CNTs with 90% ( and 95% ( carbon purity were used. The impedance analysis showed a lower electrical percolation threshold (EPT) of the compared to the . Also, at the same nanofiller loading, the characteristic frequency () of the composites was higher than that of the composites. For example, at 7 wt% CNT, the of the and composites were 102 and 1.0 Hz, respectively. This finding reveals better dispersion of the compared to the in the LDPE. The better dispersion of CNT90 compared to CNT95 in the LDPE is supported by the processing behavior analysis that showed higher viscosity and mixing torque for the composites compared to the composites.

Highly stretchable, sensitive and wide linear responsive fabric-based strain sensors with a self-segregated carbon nanotube (CNT)/Polydimethylsiloxane (PDMS) coating

Flexible strain sensors have received increasing attention with the development of wearable electronic devices. However, integrating wide strain detection range, high sensitivity while maintaining relatively wide linear response range for such sensor system still remain a challenge. A fabric based flexible sensor (S-CNT/PDMS-F) was designed and fabricated, and the sensor can simultaneously achieve high sensitivity, wide linear response and strain detection range by combining self-segregated carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composites and elastic medical bandage. It has been observed that this new sensor system can achieve a high sensitivity with a gauge factor of 615, a large linear responsive range of 0–100% strain (R2 ​= ​0.993) and a wide strain detection range of ∼ 200%, which is superior to almost all the reported CNT/PDMS flexible strain sensors. Compared to the similar fabric based strain sensor system deploying non self-segregated structure (C-CNT/PDMS-F), our S-CNT/PDMS-F demonstrates higher electrical conductivity and lower electrical percolation threshold and response time of 55 ​ms, as well as more stable and repeatable performance under cyclic loading conditions. The capability of the sensors in monitoring physiological activities and weight distribution has also been demonstrated.

Electrically conductive nanocomposites based on poly(lactic acid)/flexible copolyester blends with multiwalled carbon nanotubes

AbstractNanocomposites of poly(lactic acid) (PLA)/poly(butylene adipate‐co‐terephthalate) (PBAT) blends with multiwalled carbon nanotubes (MWCNTs) were prepared and their morphology, as well as electrical, mechanical, and thermal properties were investigated. The motivation of this work is to prepare electrically conductive and environmentally benign polymer nanocomposites using biodegradable PLA/PBAT blends. The composites were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning and transmission electron microscopy, tensile and microindentation tests, and thermogravimetric analyses (TGA). Volume resistivity and resistance to tensile deformation were measured for electrical characterization. The nanocomposites films were integrated into an electrical circuit to confirm their electrical conductivity. The FTIR spectra revealed the physical mixing between the polymer matrix and the filler. TEM micrographs suggested selective localization of MWCNTs in the PBAT phase with partial agglomeration forming a co‐continuous morphology. TGA and derivative thermogravimetric curves suggested the overall decreasing thermal stability of composites than pure polymer blends regardless of the effects on individual blend components. A relatively low electrical percolation threshold (around 1 wt% of the fillers) compared to the literature works was achieved. Increasing electrical resistance of nanocomposites upon tensile deformation suggested their possibility of piezoresistive properties. Furthermore, the overall mechanical performance (i.e., elastic modulus, tensile strength, and hardness) of the materials was found to improve with increasing filler content.

Multifunctional wax based conductive and piezoresistive nanocomposites for sensing applications

Carbon nanotube (CNT) and reduced graphene oxide (rGO) filled wax (W) composites were developed and proven to be suitable for screen-printing and molding techniques, allowing simple manufacturing of conductive and multifunctional materials. A homogeneous filler dispersion is essential to obtain smaller filler clusters within the wax. Low electrical percolation thresholds were obtained for CNT/W and rGO/W composites, near 0.3 and 3 wt% filler content, respectively, with maximum electrical conductivity of 2.5 × 10−4 S/m for both fillers. Further, good filler dispersion and mechanical properties have been obtained for all samples. The composites were evaluated as sensing material with good linearity between force and resistance variation. The piezoresistive sensibility is GF < 11 (5 mm of bending) and PS < 0.17 MPa−1 (50 N of pressure), being suitable for the development of sensing applications. Further, the piezoresistive response is stable under cycling solicitations.Finally, the applicability was demonstrated by developing a screen-printed sensor, allowing to detect bending movements, connected to a mobile device through an electronic system.

A review concerning the main factors that interfere in the electrical percolation threshold content of polymeric antistatic packaging with carbon fillers as antistatic agent

AbstractThe use of high contents of carbon fillers in polymeric composites may decrease the mechanical properties of the polymeric matrices, as well as reduce their processability and increase the production costs of antistatic packaging used in the electronic industry. Therefore, it is of great technological interest the research on alternative approaches to produce polymer composites with low electrical percolation threshold. In this way, this review article focuses on the discussion of the main factors that interfere in the electrical percolation threshold of electrically conductive polymer composites, such as the aspect ratio of the carbon fillers and its particle size, the compatibility between the composite phases, the crystallinity degree of the polymeric matrix, the processing route and the location of fillers in multi‐phase polymer blends. Additionally, the review article reports the latest studies related to the obtainment of polymer composites with low percolation threshold contents and produced with different types of carbon fillers.

The Entangled Conductive Structure of CB/PA6/PP MFCs and Their Electromechanical Properties.

In this work, carbon black (CB)/polyamide 6 (PA6)/polypropylene (PP) microfibrillar composites (MFCs) were fabricated through an extrusion (hot stretching) heat treatment process. The CB-coated conductive PA6 microfibrils with high aspect ratio were in situ generated as a result of the selective accumulation of CB at the interface. At the proper temperature, a 3D entangled conductive structure was constructed in the PP matrix, due to topological entanglement between these conductive microfibrils. This unique conductive structure provided the PP composites with a low electrical conductivity percolation threshold. Moreover, the electromechanical properties of conductive MFCs were investigated for the first time. A great stability, a high sensitivity and a nice reproducibility were achieved simultaneously for CB/PA6/PP MFCs. This work provides a universal and low-cost method for the conductive polymer composites’ (CPCs) fabrication as sensing materials.

Dispersibility-tailored conductive epoxy nanocomposites with silica nanoparticle-embedded silver nanowires

We prepared epoxy/silver nanowire (AgNW) nanocomposites with excellent dispersibility and electrical conductivity by optimally embedding silica nanoparticles (SNPs) on AgNW surface. Through the use of tailored SNP-embedded content, enhanced electrical properties, such as high electrical conductivity (~101 S/m) and low electrical percolation threshold (<0.5 wt%) of epoxy nanocomposites were realized due to superb dispersion. Interfacial interactions between the epoxy matrix and SNP-AgNWs were improved by the combination of physical attraction and chemical reactions between the matrix and silanol moieties of silica, which led to the uniform dispersion of the nanowires in the matrix. The nanocomposites with well-chosen surface modification (i.e., embedding SNPs (2 h-reaction) on AgNW surface) exhibited results (~101 S/m) superior to those with untreated (~10−1 S/m) and highly treated surfaces (24 h-reaction; ~10−2 S/m). Rheological and electrical properties of the nanocomposites were primarily probed based on dispersion of nanowires influenced by the SNP coating on AgNW. Moreover, the morphological, compositional, and thermal properties of the nanocomposites were examined.

Insights to low electrical percolation thresholds of carbon-based polypropylene nanocomposites

Electrically conductive polypropylene nanocomposites filled with carbon-based nanofillers such as carbon nanotubes, graphene and carbon black are widely being used in different applications such as electromagnetic interference shielding, wearable electronics, sensors, soft actuators, structural health monitoring and energy storage capacitors. The electrical conductivity of polypropylene nanocomposites depends on multiple factors, and can be tailored by using different processing and pre/post-processing techniques. There has been a lot of research in the past decade or so to maintain the electrical conductivity of polypropylene nanocomposites at the lowest possible filler concentration. Low percolation threshold is critically important for several technical applications as a key factor to well tailor mechanical properties, maintain optical transparency, and bring cost-effectiveness in manufacturing. This review article presents a comprehensive insight of the methodologies that have been used by different researchers to achieve low/ultralow electrical percolation thresholds in carbon-based polypropylene nanocomposites. Most importantly, different parameters that directly influence the percolation threshold and electrical conductivity such as the type, size, aspect ratio, functionalization and nucleating ability of carbon nanofillers, inter-particle interactions, density, crystallinity, viscosity of matrices, nanocomposite morphology, interaction of nanofillers with the polymer matrix, as well as post-processing techniques like annealing have also been discussed in detail. Percolation threshold and its theory, as well as the conduction phenomena such as the tunneling effects are also mentioned. The research has been characterized based on carbon filler types and critical comparisons have been made between the studies using similar methodologies.