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.     

The effect of geometric factor of carbon nanofillers on the electrical conductivity and electromagnetic interference shielding properties of poly(trimethylene terephthalate) composites: a comparative study

In this study, the effects of filler geometry on the electrical conductivity and electromagnetic interference (EMI) shielding properties of poly(trimethylene terephthalate) (PTT) composites filled with graphene nanosheets (GNSs), carbon nanotubes (CNTs), and GNS–CNT hybrid nanofillers have been investigated. The GNSs, CNTs, and hybrid GNS–CNT were well dispersed in the PTT matrix using a simple coagulation process. GNSs were prepared from graphene oxide (GO) through hydrazine reduction, and thermal reduction of GO at two different temperatures of 1050 and 1500 °C. PTT filled with different aspect ratios and oxygen functional groups of GNS were also prepared in order to compare the electrical conductivity and EMI shielding properties. The aspect ratios of GNSs and CNTs were estimated by using an ellipsoid model. Percolation scaling laws were applied to the magnitudes of conductivity to reveal the percolation network and filler dispersion. The percolation exponent of the PTT/GNS composites was larger than that of the PTT/CNT composites. The percolated filler–filler network at which the percolation exponent changed was correlated with the filler geometric structure. GNS–CNT hybrid nanofillers formed a complex double brush structure in the PTT/GNS–CNT composites. The geometric structure, aspect ratio, and intrinsic conductivity of carbon nanofillers affected the electrical percolation threshold and EMI shielding efficiency of the composites.     

Micromechanics modeling of the electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites

A mixed micromechanics model was developed to predict the overall electrical conductivity of carbon nanotube (CNT)–polymer nanocomposites. Two electrical conductivity mechanisms, electron hopping and conductive networks, were incorporated into the model by introducing an interphase layer and considering the effective aspect ratio of CNTs. It was found that the modeling results agree well with the experimental data for both single-wall carbon nanotube and multi-wall carbon nanotube based nanocomposites. Simulation results suggest that both electron hopping and conductive networks contribute to the electrical conductivity of the nanocomposites, while conductive networks become dominant as CNT volume fraction increases. It was also indicated that the sizes of CNTs have significant effects on the percolation threshold and the overall electrical conductivity of the nanocomposites. This developed model is expected to provide a more accurate prediction on the electrical conductivity of CNT–polymer nanocomposites and useful guidelines for the design and optimization of conductive polymer nanocomposites.     

Dispersion assessment and studies on AC percolative conductivity in polymer-derived Si–C–N/CNT ceramic nanocomposites

Nanocomposites comprise polysilazane-derived SiCN ceramic charged with carbon nanotubes (CNTs) have been prepared by dispersion of multi-walled CNTs with a diameter of 80 nm in a cross-linked polysilazane (HTT 1800, Clariant) using a simple roll-mixer method. Subsequently, the composites were warm pressed and pyrolyzed in argon atmosphere. Scanning electron microscopy (SEM) and 3D Raman imaging techniques were used as major tools to assess the dispersion of CNTs throughout the ceramic matrix. Furthermore, studies on the effect of the volume fraction of CNTs in the nanocomposites on their electrical properties have been performed. The specific bulk conductivities of the materials were analyzed by AC impedance spectroscopy, revealing percolation thresholds (ρ_c) at CNT loadings lower than 1 vol%. Maximum conductivity amounted to 7.6 × 10⁻² S/cm was observed at 5 vol% CNT. The conductivity exponent in the SiCN/CNT composites was found equal to 1.71, indicating transport in three dimensions.     

Self-sensing and mechanical performance of CNT/GNP/UHMWPE biocompatible nanocomposites

Ultra-high molecular weight polyethylene (UHMWPE)-based conductive nanocomposites with reduced percolation and tunable piezoresistive behavior were prepared via solution mixing followed by compression molding using carbon nanotubes (CNT) and graphene nanoplatelets (GNP). The effect of varying wt% of GNP with fixed CNT content (0.1 wt%) on the mechanical, electrical, thermal and piezoresistive properties of UHMWPE nanocomposites was evaluated. The combination of CNT and GNP enhanced the dispersion in UHMWPE matrix and lowered the probability of CNT aggregation as GNP acted as a spacer to separate the entanglement of CNT with each other. This has allowed the formation of an effective conductive path between GNP and CNT in UHMWPE matrix. The thermal conductivity, degree of crystallinity and degradation temperature of the nanocomposites increased with increasing GNP content. The elastic modulus and yield strength of the nanocomposites were improved by 37% and 33%, respectively, for 0.1/0.3 wt% of CNT/GNP compared to neat UHMWPE. The electrical conductivity was measured using four-probe method, and the lowest electrical percolation threshold was achieved at 0.1/0.1 wt% of CNT/GNP forming a nearly two-dimensional conductive network (critical value, t = 1.20). Such improvements in mechanical and electrical properties are attributed to the synergistic effect of the two-dimensional GNP and one-dimensional CNT which limits aggregation of CNTs enabling a more efficient conductive network at low wt% of fillers. These hybrid nanocomposites exhibited strong piezoresistive response with sensitivity factor of 6.2, 15.93 and 557.44 in the linear elastic, inelastic I and inelastic II regimes, respectively, for 0.1/0.5 wt% of CNT/GNP. This study demonstrates the fabrication method and the self-sensing performance of CNT/GNP/UHMWPE nanocomposites with improved properties useful for orthopedic implants.     

Modeling electrical conductivities of nanocomposites with aligned carbon nanotubes

We have developed an improved three-dimensional (3D) percolation model to investigate the effect of the alignment of carbon nanotubes (CNTs) on the electrical conductivity of nanocomposites. In this model, both intrinsic and contact resistances are considered, and a new method of resistor network recognition that employs periodically connective paths is developed. This method leads to a reduction in the size effect of the representative cuboid in our Monte Carlo simulations. With this new technique, we were able to effectively analyze the effects of the CNT alignment upon the electrical conductivity of nanocomposites. Our model predicted that the peak value of the conductivity occurs for partially aligned rather than perfectly aligned CNTs. It has also identified the value of the peak and the corresponding alignment for different volume fractions of CNTs. Our model works well for both multi-wall CNTs (MWCNTs) and single-wall CNTs (SWCNTs), and the numerical results show a quantitative agreement with existing experimental observations.     

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.     

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.     

Preparation and properties of poly (p-phenylene sulfide)/multiwall carbon nanotube composites obtained by melt compounding

In this paper, electrical and mechanical properties of Poly (p-phenylene sulfide) (PPS)/multi-wall carbon nanotubes (MWNTs) nanocomposites were reported. The composites were obtained just by simply melt mixing PPS with raw MWNTs without any pre-treatment. The dispersion of MWNTs and interfacial interaction were investigated through SEM &TEM and Raman spectra. The rheological test and crystallization behavior were also investigated to study the effects of MWNTs concentration on the structure and chain mobility of the prepared composites. Though raw MWNTs without any pre-treatment were used, a good dispersion and interaction between PPS and MWNTs have been evidenced, resulting in a great improvement of electrical properties and mechanical properties of the composites. Raman spectra showed a remarkable decrease of G band intensity and a shift of D bond, demonstrating a strong filler–matrix interaction, which was considered as due to π–π stacking between PPS and MWNTs. The storage modulus (G′) versus frequency curve presented a plateau above the percolation threshold of about 2–3 wt% with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behavior. Meanwhile, a conductive percolation threshold of 5 wt% was achieved and the conductivity of nanocomposites increased sharply by several orders of magnitude. The difference between electrical and rheological percolation threshold, and the effect of critical percolation on the chain mobility, especially on crystallization behavior of PPS, were discussed. In summary, our work provides a simple and fast way to prepare PPS/MWNTs nanocomposites with good dispersion and improved properties.     

Advanced elastomer nano-composites based on CNT-hybrid filler systems

Different techniques to disperse multiwalled carbon nanotubes (CNT) in elastomers using an internal mixer are applied and physical properties of the composites are evaluated: stress–strain behavior, dynamic-mechanical, thermal diffusivity, dielectric and fracture mechanical properties. The electrical percolation threshold is found to decrease by using ethanol as dispersion agent, compared to “dry” mixing, correlating with improved optical dispersion. The effect of nanoscopic gaps between adjacent CNTs on the electrical and thermal conductivity of the composites and the missing percolation behavior of the thermal conductivity are discussed. We have found some technically promising synergetic effects of the hybrid filler systems. For all systems one observes significantly steeper stress–strain curves by addition of 1.6 vol.% CNT to the systems with conventional fillers. In natural rubber the fatigue crack propagation resistance, tensile strength and electrical conductivity is found to be improved also for dry mixed CNT-silica hybrid systems.     

Effect of different carbon fillers and dopant acids on electrical properties of polyaniline nanocomposites

Electrically conducting nanocomposites of polyaniline (PANI) with carbon-based fillers have evinced considerable interest for various applications such as rechargeable batteries, microelectronics, sensors, electrochromic displays and light-emitting and photovoltaic devices. The nature of both the carbon filler and the dopant acid can significantly influence the conductivity of these nanocomposites. This paper describes the effects of carbon fillers like carbon black (CB), graphite (GR) and muti-walled carbon nanotubes (MWCNT) and of dopant acids like methane sulfonic acid (MSA), camphor sulfonic acid (CSA), hydrochloric acid (HCl) and sulfuric acid (H₂SO₄) on the electrical conductivity of PANI. The morphological, structural and electrical properties of neat PANI and carbon–PANI nanocomposites were studied using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT–IR), UV–Vis spectroscopy and the four-point probe technique, respectively. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) studies were also conducted for different PANI composites. The results show that PANI and carbon–PANI composites with organic acid dopants show good thermal stability and higher electrical conductivity than those with inorganic acid dopants. Also, carbon–PANI composites generally show higher electrical conductivity than neat PANI, with highest conductivities for PANI–CNT composites. Thus, in essence, PANI–CNT composites prepared using organic acid dopants are most suitable for conducting applications.     

Evaluation and visualization of the percolating networks in multi-wall carbon nanotube/epoxy composites

In this study, epoxy-based nanocomposites containing multi-wall carbon nanotubes (CNTs) were produced by a calendering approach. The electrical conductivities of these composites were investigated as a function of CNT content. The conductivity was found to obey a percolation-like power law with a percolation threshold below 0.05 vol.%. The electrical conductivity of the neat epoxy resin could be enhanced by nine orders of magnitude, with the addition of only 0.6 vol.% CNTs, suggesting the formation of a well-conducting network by the CNTs throughout the insulating polymer matrix. To characterize the dispersion and the morphology of CNTs in epoxy matrix, different microscopic techniques were applied to characterize the dispersion and the morphology of CNTs in epoxy matrix, such as atomic force microscopy, transmission electron microscopy, and scanning electron microscopy (SEM). In particular, the charge contrast imaging in SEM allows a visualization of the overall distribution of CNTs at a micro-scale, as well as the identification of CNT bundles at a nano-scale. On the basis of microscopic investigation, the electrical conduction mechanism of CNT/epoxy composites is discussed.     

Nanoinfiltration behavior of carbon nanotube based nanocomposites with enhanced mechanical and electrical properties

In this work,carbon nanotube (CNT) based nanocomposites with high mass fraction are proposed by in-situ bridging carbon matrix into CNT paper through optimized chemical vapor infiltration (CVI).Nanoinfiltration behavior of CNTs is basically investigated under the CVI process.The contact between each CNT can be strengthened and the conductive pathways can be established,resulting in the better mechanical and electrical properties.Compared with the pristine CNT paper,the CNT/C composite after pyrolysis process confirms a remarkable advance in tensile strength (up to 310 ± 13 MPa) and Young's modulus (up to 2.4 ± 0.1 GPa).Besides,a notable feature of electrical conductivity also shows an improvement up to 8.5 S/cm,which can be attributed to the mass fraction of CNT (41 wt％) breaking the limits of percolation thresholds and the efficient densification of this sample to establish the conductive pathways.This study has a broad application in the development of the multi-functional electrical and engineering materials.     

Effects of carbon nanotube alignment on electrical and mechanical properties of epoxy nanocomposites

This paper reports the alignment of multi-walled carbon nanotubes (MWCNTs) in an epoxy matrix as a result of DC electric fields applied during composite curing. Optical microscopy and polarized Raman spectroscopy are used to confirm the CNT alignment. The alignment of CNTs gives rise to much improved electrical conductivity, elastic modulus and quasi-static fracture toughness compared to those with CNTs of random orientation. An extraordinarily low electrical percolation threshold of about 0.0031 vol% is achieved when measured along the alignment, which is more than one order of magnitude lower than 0.034 vol% with random orientation or that measured perpendicular to the aligned CNTs. The examination of the fracture surfaces identifies pertinent toughening mechanisms in aligned CNT composites, namely crack tip deflection and CNT pullout. The significance of this paper is that the technique employed here can tailor the physical, mechanical and fracture properties of bulk nanocomposites even at a very low CNT concentration.     

Numerical investigation of mechanisms affecting the piezoresistive properties of CNT-doped polymers using multi-scale models

Carbon nanotube polymer nanocomposites exhibit conductive behavior due to the formation of conductive nanotube networks inside the polymer. Their electrical resistance is known to vary with strain. Two mechanisms that affect the conductivity and piezoresistive response of CNT nanocomposites are investigated using models at two discrete material scales: (a) nanoscale models to analyze the electromechanical response of carbon nanotubes and (b) nanotube percolation models to investigate the composites electrical resistance at microscale. Numerical studies determine the impact of each mechanism on the macroscopic response of the nanocomposite. Results suggest that the variation of nanotube resistance with strain is the dominant mechanism.     

Percolation threshold of carbon nanotubes filled unsaturated polyesters

This paper reports on the development of electrically conductive nanocomposites containing multi-walled carbon nanotubes in an unsaturated polyester matrix. The resistivity of the liquid suspension during processing is used to evaluate the quality of the filler dispersion, which is also studied using optical microscopy. The electrical properties of the cured composites are analysed by AC impedance spectroscopy and DC conductivity measurements. The conductivity of the cured nanocomposite follows a statistical percolation model, with percolation threshold at 0.026 wt.% loading of nanotubes. The results obtained show that unsaturated polyesters are a matrix suitable for the preparation of electrically conductive thermosetting nanocomposites at low nanotube concentrations. The effect of carbon nanotubes reaggregation on the electrical properties of the spatial structure generated is discussed.     

Percolation model of reinforcement efficiency for carbon nanotubes dispersed in thermoplastics

Considerable experimental work on carbon nanotube-reinforced composites has shown that the reinforcement efficiency of carbon nanotubes (CNTs) becomes lower than the theoretical expectation when CNT content reaches a critical value. This critical volume fraction (percolation threshold) is considered related to the formation of percolating network. In this work, a percolation model is proposed to describe the observed sharp decrease in the reinforcement efficiency of multiwalled CNTs (MWCNTs) dispersed in thermoplastics when the CNT content exceeds the percolation threshold. The percolation threshold is estimated via a numerical simulation of randomly curved CNTs according to the statistics on geometrical features of real CNTs. The percolation model, integrated into the Halpin–Tsai equations, is verified using the experimental data of various thermoplastic composites reinforced with MWCNTs. The developed mechanical model achieves a good agreement with the measured moduli of nanocomposites, and demonstrates an excellent prediction capability over a wide range of CNT content.     

Improvement of toughness and electrical properties of epoxy composites with carbon nanotubes prepared by industrially relevant processes

The addition of carbon nanotubes (CNTs) to polymeric matrices or master batches has the potential to provide composites with novel properties. However, composites with a uniform dispersion of CNTs have proved to be difficult to manufacture, especially at an industrial scale. This paper reports on processing methods that overcome problems related to the control and reproducibility of dispersions. By using a high pressure homogenizer and a three-roll calendaring mill in combination, CNT reinforced epoxies were fabricated by mould casting with a well dispersed nanofiller content from 0.1 to 2 wt%. The influence of the nano-carbon reinforcements on toughness and electrical properties of the CNT/epoxies was studied. A substantial increase of all mechanical properties already appeared at the lowest CNT content of 0.1 wt%, but further raising the nanofiller concentration only led to moderate further changes. The most significant enhancement was obtained for fracture toughness, reaching up to 82%. The low percolation thresholds were confirmed by electrical conductivity measurements on the same composites yielding a threshold value of only about 0.01 wt%. As corroborated by a thorough microscopic analysis of the composites, mechanical and electrical enhancement points to the formation of an interconnected network of agglomerated CNTs.     

Effect of carbon nanotube type and functionalization on the electrical,thermal, mechanical and electromechanical properties of carbon nanotube/styrene–butadiene–styrene composites for large strain sensor applications

Thermoplastic elastomer tri-block copolymer, namely styrene–butadiene–styrene (SBS) composites filled with carbon nanotubes (CNT) are characterized with the main goal of obtaining electro-mechanical composites suitable for large deformation sensor applications. CNT/SBS composites with different filler contents and filler functionalizations are studied by morphological, thermal, mechanical and electrical analyses. It is shown that the different dispersion levels of CNT in the SBS matrix are achieved for pristine or functionalized CNT with strong influence in the electrical properties of the composites. In particular covalently functionalized CNTs show percolation thresholds higher than 8 weight percentage (wt%) whereas pristine CNT show percolation threshold smaller than 1 wt%. On the other hand, CNT functionalization does not alter the conduction mechanism which is related to hopping between the CNT for concentrations higher than the percolation threshold.Pristine single and multiwall CNT within the SBS matrix allow the preparation of composites with electro-mechanical properties appropriate for strain sensors for deformations up to 5% of strain, the gauge factor varying between 2 and 8. Composites close to the percolation threshold show larger values of the gauge factor.