Low Percolation Threshold Research Articles

Structure-property relationships in the electrical conductivity of multilayer graphene sheet/epoxy nanocomposites

The relationship between the physicochemical properties of multilayer graphene sheets (GSs), their resulting state of dispersion, and the effective electrical conductivity of GS/epoxy nanocomposites is investigated. To construct such relationship, four types of GSs with dissimilar properties were physicochemically, structurally, and morphologically characterized, and correlated with the direct current and alternating current (AC) electrical responses of their polymeric nanocomposites. It was found that the most influential parameters that affect the electrical conductivity of the nanocomposites are the size and density of agglomerates formed at the mesoscale. Both parameters, in turn, are governed by the physicochemical properties of the individual GSs. At the nanostructure level, the lateral size of the GS arises as the most influential parameter, with larger GSs yielding higher conductivities and lower percolation thresholds. The carbon/oxygen ratio of the GSs also showed moderate influence. The nanocomposites showed a resistive (R)-capacitive (C) electrical response in AC, which fits to a two-element parallel RC model. Both, the power law and general effective media (GEM) models predict similar percolation thresholds, but the GEM model is more general and adequately reproduces the whole electrical curve for all filler concentrations.

Fe3O4 microplate filled PEI matrix composite with remarkable nonlinear conductive characteristics, dielectric property, and low percolation threshold

As the presence of a percolating network formed by filler is indispensable for field grading composite, particulate fillers often result in high filler content that can be unfavorable in some aspects. The utilization of fillers with high aspect ratio is an effective way of reducing percolation threshold. In this work, Fe3O4 microplate (FMP) was prepared by a PVP-assisted hydrothermal method and it was adopted to fabricate composite films with different filler content by using polyetherimide (PEI) as the matrix. The composite film exhibited a percolation threshold of approximately 8 phr. The nonlinear coefficient measured 6.28 at a filler content of 10 phr. The nonlinearity in the conductive behavior of the composites was attributed to tunneling effect and Schottky emission. The filling of the FMP into PEI resulted in increase in dielectric constant and the dielectric loss maintained low. This study suggests that the FMP is a promising filler of low-filler-content field grading composite.

Polymer Composites Based on Polylactide-Containing Various Kinds of Carbon Nanofillers

Filled nanocomposites of polylactide with graphite nanoplates (GNPs) and reduced grapheneoxide (RGO) are prepared by liquid-phase synthesis. A comparative study of the mechanical, electric, andthermophysical characteristics of the compositions depending on the nature of the nanofillers is carried out.An insignificant difference in the mechanical parameters of the compositions containing GNPs and RGO asfillers is established. At the same time, when studying the electrical properties, it is found that the use of RGOas a filler leads to the production of composites with a lower percolation threshold of the flow than in the caseof GNPs and increased conductivity. The differential scanning calorimetry (DSC) method shows that compositionscontaining GNPs as a filler have a higher degree of crystallinity in comparison with similar compositionscontaining RGO. This is caused by the structure of the filled compositions, which influences thenucleation rate of polylactide (PLA) crystallites on the surface of ordered planar nanoparticles of the GNPsand imperfect RGO particles. Thus, the use of different carbon nanofillers may promote the production ofcompositions that differ in their characteristics.

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.

Effect of carbon nanotube type and length on the electrical conductivity of carbon nanotube polymer nanocomposites

Carbon nanotube (CNT) type and length are two key factors that affect the electrical behavior of CNT/polymer nanocomposites. However, numerical studies that consider these two factors simultaneously are limited. This paper presented a stochastic multiscale numerical model to predict the electrical conductivity and percolation threshold of polymer nanocomposites containing single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The combined effects of CNT type and length on the electrical conductivity and percolation threshold of the polymer nanocomposites were investigated. The model predictions were validated against experimental data of commercially available CNTs. Our results showed that the effect of CNT type varied based on both the length and aspect ratio of the CNTs. Long SWCNTs exhibited the greatest enhancement of the polymer’s electrical conductivity with the lowest percolation threshold among all the CNT types studied.

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.

Graphene Nanocomposites for Electromagnetic Interference Shielding—Trends and Advancements

Electromagnetic interference is considered a serious threat to electrical devices, the environment, and human beings. In this regard, various shielding materials have been developed and investigated. Graphene is a two-dimensional, one-atom-thick nanocarbon nanomaterial. It possesses several remarkable structural and physical features, including transparency, electron conductivity, heat stability, mechanical properties, etc. Consequently, it has been used as an effective reinforcement to enhance electrical conductivity, dielectric properties, permittivity, and electromagnetic interference shielding characteristics. This is an overview of the utilization and efficacy of state-of-the-art graphene-derived nanocomposites for radiation shielding. The polymeric matrices discussed here include conducting polymers, thermoplastic polymers, as well as thermosets, for which the physical and electromagnetic interference shielding characteristics depend upon polymer/graphene interactions and interface formation. Improved graphene dispersion has been observed due to electrostatic, van der Waals, π-π stacking, or covalent interactions in the matrix nanofiller. Accordingly, low percolation thresholds and excellent electrical conductivity have been achieved with nanocomposites, offering enhanced shielding performance. Graphene has been filled in matrices like polyaniline, polythiophene, poly(methyl methacrylate), polyethylene, epoxy, and other polymers for the formation of radiation shielding nanocomposites. This process has been shown to improve the electromagnetic radiation shielding effectiveness. The future of graphene-based nanocomposites in this field relies on the design and facile processing of novel nanocomposites, as well as overcoming the remaining challenges in this field.

A highly stretchable carbon nanotube/reduced graphene oxide/ poly(dimethylsiloxane) composite with high thermal conductivity as a flexible strain sensor

Carbon nanotube (CNT) is an important filler for preparing poly(dimethylsiloxane) (PDMS)-based composite sensors with high thermal conductivity. However, CNT encounters challenges in terms of dispersion within the polymer matrix. In this work, a combined strategy was proposed to improve the dispersion of CNT, involving surface modification of CNT, CNT-reduced graphene oxide (rGO) hybridization and in-situ cross-linking of PDMS in a solution. The in-situ cross-linking of PDMS contributed to an increase in elongation at break, coupled with a reduction in the modulus of the resulting in-situ crosslinked PDMS/m-CNT@rGO (S-PDMS/m-CNT@rGO) composites. Through the implementation of this strategy, the dispersion of CNT within the PDMS matrix was enhanced, simultaneously facilitating the establishment of filler networks. At the filler content of 10.0 wt%, the thermal conductivity of the S-PDMS/m-CNT@rGO composite increased to 1.06 W/(m·K) and the electrical conductivity increased to 4.51 × 10−3 S/cm. The S-PDMS/m-CNT@rGO composite had a low percolation threshold which was 0.35 wt%. When utilized as a strain sensor, the S-PDMS/m-CNT@rGO composite exhibited high sensitivity, good repeatability and stability in monitoring human motion.

MWCNT-Coated Glass Fabric/Phenol Composite Heating Panel Fabricated by Resin Infusion Process.

MWCNTs (multiwalled carbon nanotubes) were applied to fiber-reinforced composite materials with phenolic resin having flame retardance for the composite heating panels of railroad vehicles. Instead of dispersing MWCNTs in the matrix, the surface of a pristine plain-weave glass fiber fabric was coated with MWCNTs through a series of dip-coating and drying processes, followed by the resin infusion of the phenolic resin to make the composite heating panel. Before and after the resin infusion process, low percolation thresholds of 0.00216 wt%MWCNT (weight percent of MWCNTs) and 0.001 wt%MWCNT, respectively, were achieved, as were very high electrical conductivities of 47.5 S/m at 0.210 wt%MWCNT and 26.7 S/m at 0.116 wt%, respectively. The low threshold and high conductivity can be attributed to the formation of electrical pathways directly onto the glass fabrics. It was confirmed that mechanical properties such as modulus, strength, and maximum strain were at the same level as those of the pristine glass fabric composite. The heating performance with temperature uniformity, as well as the electrical and mechanical properties, indicates that the resin-infused glass fabric composite having MWCNTs directly coated onto the fabric surface can be a solution for lightweight structural composite heating panels for railway vehicles.

Parasitic reaction driven facile preparation of segregated-MXene/polycarbonate nanocomposites for efficient electromagnetic interference shielding

Conductive polymer composites with segregated structures (s-CPCs) would have great potential in electromagnetic interference (EMI) shielding applications owing to their superior electrical conductivity along with a low percolation threshold. This study investigated a facile manufacturing approach for synthesizing s-CPCs by introducing parasitic reaction of MXene flakes into positively charged polycarbonate microspheres simply prepared by a cationic surfactant. As a result, an electrostatic assembly of MXene flakes and PC microspheres was successfully formed, resulting in unprecedented efficient and effective manufacturing process. Given with the fabrication approach investigated in this study, it was experimentally found that the resultant s-CPCs exhibited the considerably low percolation threshold of 0.15 vol% as well as outstanding EMI shielding performance of 38.7 ± 1.2 dB with only 2.75 vol% of MXene. This study can show, if properly optimized, the great potential of the use of s-CPCs in a wide range of EMI shielding applications.

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.

A mechanical, thermal and electrical properties study of novel vulcanised blends of poly(epichlorohydrin) elastomer with polyaniline dodecylbenzenesulfonate

With good mechanical and thermal properties, elastomers having appreciable electrical conductivity may potentially be developed as smart materials for applications such as strain sensors, artificial muscles or flexible biosensors. With such applications in mind, the purpose of this study was to investigate the structure-properties relationships of new vulcanised blends of a poly (epichlorohydrin) elastomer (PECH) and electrically-conducting polyaniline dodecylbenzenesulfonate salt (PAni.DBSA). PAni.DBSA, synthesised by a published method, was blended with PECH and vulcanised with a commercial sulfur cross-linking agent in an internal mixer. The morphological, mechanical, thermal and electrical properties were examined as a function of the amount of PAni.DBSA in the blends. The electrical conductivities increased with the proportion of polyaniline, showing a low percolation threshold of about 1 wt. % (1.07 vol %) PAni.DBSA (from about 10−12 to 10−10 S cm−1), and a second stage of percolation around 5 wt.%, ultimately reaching around 3 × 10−8 S cm−1. The results from microscopy and other techniques indicated that a mixture of micro- and nano-sized PAni.DBSA particles was dispersed in the elastomer matrix at compositions above 5 wt. % PAni.DBSA. The infrared spectra of vulcanised PECH-PAni.DBSA blends showed features of the pure polymers, with some notable peak shifts due to intermolecular interactions between the constituents. Thermal properties of the conductive blends were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The thermal stabilities of the blends were strongly dependent on the ratio of PAni.DBSA to PECH, and the two components inhibited the thermal decomposition of each other. The elastomer’s glass transition temperature (Tg) was determined by thermomechanical analysis (TMA); each blend showed only one such transition, at temperatures that increased monotonically with the proportion of PAni.DBSA present, indicating a significant degree of molecular interaction between the two polymers.

A facile and large-scale approach to prepare macroscopic segregated polyether block amides/carbon nanostructures composites with a gradient structure for absorption-dominated electromagnetic shielding with ultra-low reflection

Conductive polymer composites have been utilized commonly as efficient electromagnetic interference (EMI) shielding materials. However, they inevitably bring distinctly secondary electromagnetic radiation. Moreover, current EMI shielding materials with low reflection also face problem of complex preparation. Therefore, it is urgent to obtain EMI shielding materials with ultra-low reflection via a simple and large-scale method. Herein, a design strategy for combining segregated structure with gradient structure was proposed to obtain EMI shielding material of ultra-low reflection. A facile, eco-friendly and large-scale approach was employed to prepare macroscopic segregated polyether block amides (PEBA)/carbon nanostructures (CNS) composites consisting of large-size PEBA beads (∼3.4 mm) via direct hot compression. The obtained nanocomposites exhibited a low percolation threshold of 0.024 wt%, good electrical heating performance with 113 °C saturation temperature and absorption-dominated EMI shielding of ∼22.9–55.3 dB with 0.1–0.5 wt% CNS, along with low R coefficient of ∼0.28–0.42. To further reduced R coefficient, the gradient structure was prepared via compression molding, which endowed gradient PEBA/CNS composites with ultra-low R coefficient of 0.071 with EMI shielding of ∼33.0 dB at 0.2 wt% CNS content. The design strategy provided an eco-friendly and large-scale method to obtain EMI shielding material with ultra-low reflection.

Synthesis and Electrical Percolation of Highly Amorphous Polyvinyl Alcohol/Reduced Graphene Oxide Nanocomposite.

Polyvinyl alcohol is the most commercially water-soluble biodegradable polymer, and it is in use for a wide range of applications. It shows good compatibility with most inorganic/organic fillers, and enhanced composites may be prepared without the need to introduce coupling agents and interfacial modifiers. The patented high amorphous polyvinyl alcohol (HAVOH), commercialized with the trade name G-Polymer, can be easily dispersed in water and melt processed. HAVOH is particularly suitable for extrusion and can be used as a matrix to disperse nanocomposites with different properties. In this work, the optimization of the synthesis and characterization of HAVOH/reduced graphene oxide (rGO) nanocomposite obtained by the solution blending process of HAVOH and Graphene Oxide (GO) water solutions and 'in situ' reduction of GO is studied. The produced nanocomposite presents a low percolation threshold (~1.7 wt%) and high electrical conductivity (up to 11 S/m) due to the uniform dispersion in the polymer matrix as a result of the solution blending process and the good reduction level of GO. In consideration of HAVOH processability, the conductivity obtained by using rGO as filler, and the low percolation threshold, the nanocomposite presented here is a good candidate for the 3D printing of a conductive structure.

The role of conductive fillers on the rheological behavior and electrical conductivity of multi-functional shear thickening fluids (M-STFs)

Multi-functional shear thickening fluids (M-STFs) with both shear thickening behavior and electrical conductivity have a great potential for usage in a variety of applications ranging from intelligent anti-impact and vibration damping structures to effective electric mechanical platforms. However, the influences of conductive fillers on the rheological behavior and electrical conductivity of M-STFs remained unclear. In this study, the role of conductive fillers including multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs), and mixtures of MWCNT/CNF was investigated through the response surface methodology (RSM) in the temperature range of 0 °C to 60 °C. The individual and combined effects of filler content, temperature, and type of fillers on the electrical resistance and rheological behavior of M-STFs were studied. The results revealed the significant role of conductive fillers on the rheological properties and electrical conductivity of M-STFs. It is found that the initial viscosity of M-STF increases with increasing the filler content. Moreover, the M-STFs containing CNF exhibits higher electrical conductivity and lower percolation threshold (0.4 wt%). The results of this work provide new insights for the development of novel STF-based systems with multi-functional properties.

Phase Morphology and Conductive Properties of PBT/POE-g-GMA/PP/CNT Nanocomposites with a Tri-Continuous Structure via Thermal Annealing

In order to improve the conductivity of polymer composites and elucidate the relationship between the microstructure and conductivity, a series of PBT/POE-g-GMA/PP/CNT nanocomposites with a tri-continuous structure and low conductive percolation threshold were prepared by melt blending. Compared to PBT/PP/CNT composites, the PBT/POE-g-GMA/PP/CNT nanocomposites show better conductivity owing to the selective distribution of carbon nanotubes (CNTs) in the POE-g-GMA phase with the same CNT content. The increasing content of the CNTs and POE-g-GMA refines phase morphology, thus influencing the CNTs’ conductive path. Composites containing 10 wt % POE-g-GMA display the lowest conductive percolation threshold at 1.0 phr. In addition, the influencing mechanism of thermal annealing on microstructure and conductivity was methodically investigated. Thermal annealing can effectively enhance the rebuilding of CNT conductive networks and continuity of the POE-g-GMA phase on the premise of the original phase structure, decreasing the volume resistivity of nanocomposites. The nanocomposites obtained in this study possess excellent electrical properties alongside the potential to be a candidate material in fields of sensors, electromagnetic interference shielding, and so forth.

Double percolation behavior through the preferential distribution of conductive black in polymer blends to boost electrical properties and EMI shielding effectiveness

The preferential distribution of the filler in the polymer blends leads to a lower percolation threshold in comparison to a single polymer matrix with random filler distribution. Poly(ethylene methyl acrylate) (EMA) and carboxylated acrylonitrile butadiene rubber (XNBR) composites of EMA-XNBR (EX) with a variable amount of conductive carbon black (Vulcan XC 72; VXC) has been prepared by conventional melt blending. These composites have superior electromagnetic interference (EMI) shielding effectiveness to attenuate the hazardous effects and save society from proliferating electromagnetic radiation. The suitable distribution of carbon black lowers the electrical percolation threshold of 12.5 phr by the formation of interconnected architecture following the two-step percolation phenomenon and results in an improved EMI SE of 33.5 dB. The morphological analysis demonstrates that the formation of a polymer-filler network of VXC in an EX technologically compatible blend increases the mechanical and thermal stability of composites by retaining their flexibility, which facilitates the application of EXV composites in techno-commercial fields.

Electromagnetic Shielding Enhancement of Butyl Rubber/Single-Walled Carbon Nanotube Composites via Water-Induced Modification.

Electromagnetic properties of polymer composites strongly depend on the loading amount and the completeness of the filler's dispersive structure. Improving the compatibility of single-walled carbon nanotubes (SWCNTs) with isobutylene butyl rubber (IIR) is a good solution to mitigate aggregation. The change in configuration of poly-oxyethylene octyl phenol ether (OP-10) was induced using water as the exposed hydrophilic groups linking with water molecules. The SWCNT and IIR/SWCNT composites were then prepared via wetly-melt mixing at a relatively high temperature to remove water, and they were then mixed with other agents after vacuum drying and cured. The SWCNTs were dispersed uniformly to form a good network for a lower percolation threshold of the wave-absorbing property to 2 phr from 8 phr. With 8 phr SWCNTs, the tensile strength of the material improved significantly from 7.1 MPa to 15.1 MPa, and the total electromagnetic shielding effectiveness of the material was enhanced to 23.8 dB, a 3-fold increase compared to the melt-mixed material. It was demonstrated that water-induced modification achieved good dispersion of SWCNTs for electromagnetic shielding enhancement while maintaining a wide damping temperature range from -55 °C to 40 °C with a damping factor over 0.2.

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.

Evaluation of Piezoresistive and Electrical Properties of Conductive Nanocomposite Based on Castor-Oil Polyurethane Filled with MWCNT and Carbon Black.

Flexible films of a conductive polymer nanocomposite-based castor oil polyurethane (PUR), filled with different concentrations of carbon black (CB) nanoparticles or multiwall carbon nanotubes (MWCNTs), were obtained by a casting method. The piezoresistive, electrical, and dielectric properties of the PUR/MWCNT and PUR/CB composites were compared. The dc electrical conductivity of both PUR/MWCNT and PUR/CB nanocomposites exhibited strong dependences on the concentration of conducting nanofillers. Their percolation thresholds were 1.56 and 1.5 mass%, respectively. Above the threshold percolation level, the electrical conductivity value increased from 1.65 × 10-12 for the matrix PUR to 2.3 × 10-3 and 1.24 × 10-5 S/m for PUR/MWCNT and PUR/CB samples, respectively. Due to the better CB dispersion in the PUR matrix, the PUR/CB nanocomposite exhibited a lower percolation threshold value, corroborated by scanning electron microscopy images. The real part of the alternating conductivity of the nanocomposites was in accordance with Jonscher's law, indicating that conduction occurred by hopping between states in the conducting nanofillers. The piezoresistive properties were investigated under tensile cycles. The nanocomposites exhibited piezoresistive responses and, thus, could be used as piezoresistive sensors.