Comparison of rheological and electrical percolation phenomena in carbon black and carbon nanotube filled epoxy polymers

Epoxy nanocomposite suspensions including multi-wall carbon nanotubes (MWCNTs) and carbon black (CB) were produced and investigated by means of combined rheological and electrical analysis. The rheological percolation behaviour was compared to the electrical percolation behaviour. Due to similar dynamic agglomeration mechanisms the difference between the rheological and the electrical percolation threshold in the cured state is identical for MWCNT and CB filled systems. Non-covalent matrix–nanoparticle interactions in uncured epoxy suspensions are negligible since the onset of electrical and rheological percolation in the uncured state coincidence. Furthermore, the electrical percolation threshold in the cured state is always lower than in the uncured state because of the high tendency of CB and MWCNTs to form conductive networks during curing. The difference between rheological and electrical percolation threshold is dependent on the curing conditions. Thus, the rheological percolation threshold can be considered as an upper limit for the electrical percolation threshold in the cured state. Due to the formation of co-supporting networks multi-filler (MWCNTs and CB) suspensions exhibit a similar rheological behaviour as the binary MWCNT suspensions. For both types of suspensions a rheological percolation threshold of around 0.2 and 0.25 wt% was determined. Conversely, the binary CB nanocomposites exhibit a four-times higher percolation threshold of about 0.8 wt%. The difference between the binary MWCNT suspension and the ternary CB/MWCNT suspension in storage shear modulus at high filler concentrations (~0.8 wt%) turns out to be less than expected. Thus, synergistic effects in network formation are already present in the epoxy suspension and get more pronounced during curing.     

High-performance conductive materials based on the selective location of carbon black in poly(ether ether ketone)/polyimide matrix

A novel high performance conductive material with excellent comprehensive properties was prepared by melt-blending, and its performances were adjusted by controlling the selective location of carbon black (CB) in poly(ether ether ketone) (PEEK)/thermoplastic polyimide (TPI) matrix. With increasing the CB loadings, the morphology of PEEK/TPI blends changed from sea-island to co-continuous structure, which was owing to the selective location of CB in TPI phase. Notably, with the selective location of CB in the induced co-continuous PEEK/TPI matrix, the electrical percolation threshold was reduced to 5 wt%, which was significantly lower than that of binary PEEK/CB (9 wt%) and TPI/CB (10 wt%) composites. And the electrical conductivity of ternary PEEK/TPI/CB composites was 10⁴ to 10⁶ times higher than that of binary composites at identical 7.5 wt% CB loading, which was attributed to the double percolation effect. Moreover, the incorporation of CB could improve the thermal and mechanical properties effectively.     

Attaining high electrical conductivity and toughness in PA6 by combined addition of MWCNT and rubber

Tough polyamide 6 (PA6)/multi-walled carbon nanotube (MWCNT) nanocomposites modified by a maleated styrene/ethylene–butylene/styrene (mSEBS) were obtained in the melt state. The resistance calculations showed that both the conductivity values and the percolation threshold was not consequence of the physical contact among the nanotubes, and a double percolation phenomenon was observed after rubber addition. The addition of rubber did not affect the dispersion of the nanotubes, but the presence of MWCNT decreased the rubber particle size due to an increase in viscosity. Besides a high electrical conductivity and an elastic modulus similar to that of the PA6, the joint presence of 5% in weight (wt.%) MWCNT and 20 wt.% mSEBS was able to provide both a large impact strength and clear ductile behaviour. The matrix yielding of the carbon nanotube-based nanocomposites was less extensive than in nanoclay-based composites. However, they were efficient enough to lead to a clear multi-functionality.     

Thermally reduced graphene oxide acting as a trap for multiwall carbon nanotubes in bi-filler epoxy composites

The effect of thermally reduced graphene oxide (TRGO) on the electrical percolation threshold of multi wall carbon nanotube (MWCNT)/epoxy cured composites is studied along with their combined rheological/electrical behavior in their suspension state. In contrast to MWCNT and carbon black (CB) based epoxy composites, there is no prominent percolation threshold for the bi-filler (TRGO–MWCNT/epoxy) composite. Furthermore, the electrical conductivity of the bi-filler composite is two orders of magnitude lower (∼1 × 10⁻⁵ S/m) than the pristine MWCNT/epoxy composites (∼1 × 10⁻³ S/m). This result is primarily due to the strong interaction between TRGO and MWCNTs. Optical micrographs of the suspension and scanning electron micrographs of the cured composites indicate trapping of MWCNTs onto TRGO sheets. A morphological model describing this interaction is presented.     

Electrical conductivity and mechanical properties of multiwalled carbon nanotube-reinforced polypropylene nanocomposites

In this paper, the electrical conductivity and mechanical properties such as elastic modulus of multiwalled carbon nanotubes (MWCNTs) reinforced polypropylene (PP) nanocomposites were investigated both experimentally and theoretically. MWCNT-PP nanocomposites samples were produced using injection mold at different injection velocities. The range of the CNT fillers is from 0 up to 12?wt%. The influence of the injection velocity and the volume fraction of CNTs on both electrical conductivity and mechanical properties of the nanocomposites were studied. The injection speed showed some effect on the electrical conductivity, but no significant influence on the mechanical properties such as elastic modulus and stress-strain relations of the composites under tensile loading. Parallel to the experimental investigation, for electrical conductivity, a percolation theory was applied to study the electrical conductivity of the nanocomposite system in terms of content of nanotubes. Both Kirkpatrick (Rev Mod Phys 45:574?C588, 1973) and McLachlan et?al. (J Polym Sci B 43:3273?C3287, 2005) models were used to determine the transition from low conductivity to high conductivity in which designates as percolation threshold. It was found that the percolation threshold of CNT/PP composites is close to 3.8?wt%. For mechanical properties of the system, several micromechanical models were applied to elucidate the elastic properties of the nanocomposites. The results indicate that the interphase between the CNT and the polymers plays an important role in determining the elastic modulus of the system.     

Electrical and thermal properties of polyamide 12 composites with hybrid fillers systems of multiwalled carbon nanotubes and carbon black

Hybrid filler systems of multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) were incorporated into two types of polyamide 12 (PA12) using small-scale melt mixing in order to identify potential synergistic effects on the interaction of these two electrical conductive fillers. Although no synergistic effects were observed regarding the electrical percolation threshold, at loadings well above the percolation threshold higher volume conductivities were obtained for samples containing both, MWCNT and CB, as compared to single fillers. This effect was more pronounced when using a higher viscous PA12 matrix. The formation of a co-supporting network can be assumed. The combined use of CB and MWCNTs improved the macrodispersion of MWCNT agglomerates, which can be assigned as a synergistic effect. DSC measurements indicated an effect of the nanofiller on crystallisation temperatures of PA12; however this was independent of the kind or amount of the carbon nanofiller.     

Percolation threshold and morphology of composites of conducting carbon black/polypropylene/EVA

Conducting carbon black (CB), one of the intrinsic semi-conductors, was added into matrix polypropylene (PP) to prepare conducting composites by means of the melt processing method. Another component EVA was mixed into the composites in order to lower the percolation threshold. The percolation threshold of the ternary CB/PP/EVA composites was merely 3.8 vol%, while it was up to 7.8 vol% for the binary CB/PP composites without EVA. The conductivity of the ternary CB/PP/EVA composites was up to 10^–2 S/cm when the CB percentage was 5 vol%, while that of the binary CB/PP was lower than 10^–2 S/cm when the CB percentage was up to 10 vol%. DSC thermograms of the CB/PP/EVA composites showed that the melting peak shifted to low temperature with increasing CB content. The addition of CB and EVA resulted in the decrease of the crystallinity of PP in the ternary composites. The mechanical properties are also discussed. SEM and TEM were employed to study the morphology of the blend system. The results indicated that CB existed in the form of aggregations in the blend system. The smallest unit that formed a percolation network was grape-like aggregates with some small branches, which consisted of some CB particles, rather than the individual particles. This distribution was very valuable for forming conducting paths and for lowering the percolation value.     

Electrical and dielectric properties of polypropylene nanocomposites based on carbon nanotubes and barium titanate nanoparticles

Functional polypropylene (PP) nanocomposites were prepared by melt compounding with multiwalled carbon nanotubes (MWNT) as the electrically conductive component and barium titanate (BT) spherical nanoparticles as the ferroelectric component. To make PP electrically conductive, more than 3 wt.% MWNT is required. Surface modification of either MWNT or BT with titanate coupling agent further improves the electrical conductivity of the PP/MWNT/BT ternary nanocomposites. Interestingly, by modifying both MWNT and BT, 2 wt.% MWNT are sufficient to make the ternary nanocomposite electrically conductive. In addition, the incorporation of MWNT greatly increases the dielectric permittivity of PP/BT nanocomposites. However, to retain a low dielectric loss, the MWNT loading should be slightly less than the percolation threshold of the nanocomposites. The improved electrical conductivity and dielectric properties make the ternary nanocomposites attractive in practical applications.     

DC electrical conductivity of poly(methyl methacrylate)/carbon black composites at low temperatures

The study deals with the dc electrical conduction of poly(methyl methacrylate)/carbon black composites of different carbon black (CB) filler concentrations (2, 6, 12 wt%). The dc electrical conductivity was studied as a function of filler concentration, and temperature in the range (20–290 K). It was found that the composites exhibit negative temperature coefficient of resistivity (NTCR) at low temperatures and enhancement in the dc electrical conductivity with both temperature and CB concentration. The observed increase of conductivity with CB concentration was interpreted through the percolation theory. The dependence of the electrical conductivity of the composites in low temperatures was analyzed in term of a formula in consistence with Mott variable rang hopping (VRH) mechanism. The observed overall mechanism of electrical conduction has been related to the transfer of electrons through the carbon black aggregations distributed in the polymer matrix.     

Percolation behaviour of polypropylene glycol filled with multiwalled carbon nanotubes and Laponite

The percolation behaviour of the hybrid composites of polypropylene glycol (PPG) filled with multiwalled carbon nanotubes (MWCNTs) and Laponite RD (Lap), or with MWCNTs and organo-modified Laponite (LapO) was studied by wide angle X-ray diffraction (XRD), microscopic image analysis, and electrical conductivity measurements. Cetyltrimethylammoniumbromide (CTAB) was used as an organo-modifier of Laponite. The Lap and LapO were found to have rather different affinity to PPG. XRD data have evidenced finite PPG integration inside Lap and complete exfoliation of LapO stacks in a PPG matrix. In PPG + MWCNT composites containing no Lap or LapO, increase of MWCNT concentration above the critical value C_p ∼ 0.4 wt% resulted in percolation. The value of the percolation threshold, C_p, was practically the same for hybrid PPG + MWCNT + Lap composites. However, it noticeably decreased (C_p ∼ 0.2 wt%) in PPG + MWCNT + LapO materials. The observed behaviour of the percolation threshold may be attributed to the effects exerted by LapO on the size of MWCNT aggregates, state of their dispersion and homogeneity of their spatial distribution.     

Cost-efficient high performance polyetheretherketone/expanded graphite nanocomposites with high conductivity for EMI shielding application

The cost efficient expanded graphite (EG) filled polyetheretherketone (PEEK) nanocomposites were prepared by hot pressing, which exhibited an electrical conductivity percolation threshold of 1.5 wt%. The electrical conductivity of the 1.5 wt% nanocomposite increased approximately eleven orders of magnitude than that of pure PEEK. The conductivities of 5 wt% and 10 wt% nanocomposites were increased to about 3.24 S cm⁻¹ and 12.3 S cm⁻¹, respectively. Scanning electron microscope showed 3-dimensional conductive network of EG across the PEEK matrix. The significant increase in electrical conductivity of the nanocomposites leads to the tremendous increase in electromagnetic interference shielding effectiveness.     

Structure–property relationships in isotactic polypropylene/multi-walled carbon nanotubes nanocomposites

In this work, the influence of multi-walled carbon nanotubes (MWCNT) on electrical, thermal and mechanical properties of CNT reinforced isotactic polypropylene (iPP) nanocomposites is studied. The composites were obtained by diluting a masterbatch of 20 wt.% MWCNT with a low viscous iPP, using melt mixing. The morphology of the prepared samples was examined through SEM, Raman and XRD measurements. The effect of MWCNT addition on the thermal transitions of the iPP was investigated by differential scanning calorimetry (DSC) measurements. Significant changes are reported in the crystallization behavior of the matrix on addition of carbon nanotubes: increase of the degree of crystallinity, as well as appearance of a new crystallization peak (owing to trans-crystallinity). Dynamic mechanical analysis (DMA) studies revealed an enhancement of the storage modulus, in the glassy state, up to 86%. Furthermore, broadband dielectric relaxation spectroscopy (DRS) was employed to study the electrical and dielectric properties of the nanocomposites. The electrical percolation threshold was calculated 0.6–0.7 vol.% MWCNT from both dc conductivity and dielectric constant values. This value is lower than previous mentioned ones in literature in similar systems. In conclusion, this works provides a simple and quick way for the preparation of PP/MWCNT nanocomposites with low electrical percolation threshold and significantly enhanced mechanical properties.     

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.     

Widely dispersed PEI-based nanocomposites with multi-wall carbon nanotubes by blending with a masterbatch

High performance poly(etherimide) (PEI)-based nanocomposites (PNs) with multi-walled carbon nanotubes (MWCNT) were obtained via melt mixing. To achieve this, PEI was mixed with a well-dispersed commercial poly(butylene terephthalate) (PBT)/MWCNT master-batch in an attempt to transfer the dispersed MWCNTs to a PEI matrix. A broad and homogeneous dispersion of MWCNTs throughout the PEI-based matrix was obtained. The electrical percolation threshold (p_c) was reached at only 1 wt.% MWCNT. This p_c showed a power law dependence of conductivity on filler concentration, with a critical exponent of 1.92, which indicates that a three dimensional percolated structure was achieved. The glass transition temperature and the pressure at the output end of the extruder decreased when the master-batch was added, indicating that the processability of PEI was improved. In addition to this, the modified PEI-based PNs presented ductile behaviour and an ameliorated (12% with 5 wt.% MWCNT) elastic modulus compared with pure PEI.     

Effect of fabrication process on electrical properties of polymer/multi-wall carbon nanotube nanocomposites

Polymer/carbon nanotubes nanocomposites were fabricated by an in situ polymerization process using multi-wall carbon nanotubes (MWNT) as filler in an epoxy polymer. Effects of curing process, mixing speed, mixing time, addition of ethanol, timing of hardener addition, etc., in the fabrication process on the electrical properties of nanocomposites have been investigated. In the fabrication process, the effective formation of macroscopic conducting network in matrix is most important to enhance the electrical properties of nanocomposites. It was found that the curing temperature and the mixing conditions are key factors in the fabrication process, which influence the formation of conducting network significantly. Therefore, careful design of these factors in the fabrication process is required to achieve high electrical performances of nanocomposites. The experimental percolation threshold of the resultant nanocomposites was around 0.1 wt%. Moreover, a statistical percolation model was built up to numerically investigate the percolation threshold. The experimental electrical conductivity increases from the percolation threshold following a percolation-like power law with the identified critical exponent t as 1.75.     

炭黑碳纤维填充双组分聚合物体系的导电网络及其阻温特性

以2种不同形态尺寸的导电填料炭黑(CB)、 碳纤维(CF)填充双组分聚合物体系高密度聚乙烯(HDPE)聚丙烯(PP), 制备了四元导电复合材料。研究了导电网络的结构形态及其对材料阻温特性的影响。光学显微镜及SEM 观察表明: 炭黑选择性地分布于HDPE中, 体系中HDPE与PP呈双连续相分布, 形成双渗流导电网络结构。而具有较高长径比的碳纤维在两相基体中均匀分布并贯通多个相区, HDPE导电相区的碳纤维相互桥接形成导电网络。电性能测试结果表明: 体系的体积电阻率与CB/HDPEPP及CBCF/HDPE三元复合体系相比下降了1~5个数量级。同时, 双渗流导电网络的存在也有效抑制了负温度系数(NTC)效应, 提高了循环稳定性。与CBCF/HDPE体系相比, CBCF/HDPEPP体系的NTC效应从2个数量级下降到0.6个数量级, 电阻特征弛豫时间从951s增加到了2370s。      

Electrical conductivity and shielding effectiveness of poly(trimethylene terephthalate)/multiwalled carbon nanotube composites

Poly(trimethylene terephthalate) (PTT)/multiwalled carbon nanotube (MWCNT) composites have been fabricated to evaluate the potential of PTT composites as electromagnetic interference (EMI) shielding material. The room temperature electrical conductivity, complex permittivity, and shielding effectiveness (SE) of PTT/MWCNT composites were studied in the frequency range of 8.2–12.4 GHz (X-band). The dc conductivity (σ) of composites increased with increasing MWCNT loading and a typical percolation behavior was observed at 0.48 vol% MWCNT loading. The highest EMI SE of PTT/MWCNT composites was ~23 decibel (dB) at 4.76 vol% MWCNT loading which suggest that these composites can be used as light weight EMI shielding materials. The correlation among the SE, complex permittivity, and electrical conductivity was also studied. The EMI shielding mechanism of PTT/MWCNT composites was studied by resolving the total EMI SE into absorption and reflection loss.     

Impedance characteristics and electrical modelling of multi-walled carbon nanotube/silicone rubber composites

Multi-walled carbon nanotube (MWCNT)-filled silicone rubber (SR) composites were prepared by solvent evaporation method, with different MWCNT concentrations from 0.5 wt% to 6.5 wt%. Alternating current (AC) electrical properties of samples with interdigital electrodes were measured in the frequency range from 20 Hz to 1 MHz. Impedance spectroscopy analysis reveals a frequency-independent percolation transition between 2.0 wt% and 2.9 wt%. Samples above the percolation threshold exhibit more regular variations: the magnitude of impedance decreases gradually with frequency in the low-frequency range, and then decreases as a power law beyond a critical frequency, with the exponent in a limited range indicating the AC universality of disordered solids; the plots of real and imaginary parts of impedance fit semicircles well in the complex plane, implying semiconductive behaviours. Over the concentration range tested, a multi-stage circuit model consisting of resistor–capacitor (RC) networks is proposed to simulate the electrical responses of samples. The validity of the modelling approach is verified by comparing simulation results to experimental results, and is further supported by the analysis of the characteristic frequency. The use of equivalent circuits in modelling provides a further insight into the conducting network inside nanocomposites and more valuable guidance for the design of correlative devices.     

Complementary percolation characteristics of carbon fillers based electrically percolative thermoplastic elastomer composites

Electrically percolative composites of thermoplastic elastomers (TPE) filled with different concentrations of carbon nanotubes (CNT), carbon black (CB) and (CNT–CB) hybrid fillers were fabricated by melt blending. The effects of filler type and composition on the electrical properties of the percolative TPE composites were studied. Percolation threshold for CB-, CNT- and (CNT–CB)-based composites was found to be 0.06, 0.07 and 0.07 volume fraction respectively. Compared to CB-based composites and earlier reported results, CNT- and (CNT–CB)-based ones revealed an unexpectedly high percolation threshold, which otherwise considered an unwelcome phenomenon, lead to distinct and rare percolation characteristics of CNT filled percolative composites like per-percolation conductivity and a relatively steep percolation curves. CB-based composites showed a comparatively sharp insulator–conductor transition curve complementing the percolation characteristics CNT- and (CNT–CB)-based composites. Percolation threshold conductivity of the fillers was in the order of CB > CNT > (CNT–CB), while maximum attained conductivities followed the order of CNT > (CNT–CB) > CB. Conductivity order of fillers not only denied much reported synergic effect in (CNT–CB) filler but also highlighted the effect of percolation characteristics on the outcome of conductivity values. Results obtained were of theoretical as well as practical importance and were explained in the context of filler morphology and different dispersion characteristics of the carbon based fillers.     

Monitoring the mechanical behaviour of electrically conductive polymer nanocomposites under ramp and creep conditions

Various amounts of carbon black (CB) and carbon nanofibres (CNF) were dispersed in an epoxy resin to prepare nanocomposites whose mechanical behaviour, under ramp and creep conditions, was monitored by electrical measurements. The electrical resistivity of the epoxy resin was dramatically reduced by both nanofillers after the percolation threshold (1 wt% for CB and 0.5 wt% for CNF), reaching values in the range of 10(3)-10(4) omega . cm for filler loadings higher than 2 wt%. Due to the synergistic effects between the nanofillers, an epoxy system containing a total nanofiller amount of 2 wt%, with a relative CB/CNF ratio of 90/10 was selected for the specific applications. A direct correlation between the tensile strain and the increase of the electrical resistance was observed over the whole experimental range, and also the final failure of the samples was clearly detected. Creep tests confirmed the possibility to monitor the various deformational stages under constant loads, with a strong dependency from the temperature and the applied stress. The obtained results are encouraging for a possible application of nanomodified epoxy resin as a matrix for the preparation of structural composites with sensing (i.e., damage-monitoring) capabilities.