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.     

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.     

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.     

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.     

A review and analysis of electrical percolation in carbon nanotube polymer composites

We review experimental and theoretical work on electrical percolation of carbon nanotubes (CNT) in polymer composites. We give a comprehensive survey of published data together with an attempt of systematization. Parameters like CNT type, synthesis method, treatment and dimensionality as well as polymer type and dispersion method are evaluated with respect to their impact on percolation threshold, scaling law exponent and maximum conductivity of the composite. Validity as well as limitations of commonly used statistical percolation theories are discussed, in particular with respect to the recently reported existence of a lower kinetic (allowing for re-aggregation) and a higher statistical percolation threshold.     

Nanocomposites based on MWCNT and styrene–butadiene–styrene block copolymers: Effect of the preparation method on dispersion and polymer–filler interactions

Composites of Kraton-D® 1102 BT (a styrene–butadiene–styrene block copolymer) and multi-walled carbon nanotubes (MWCNTs) were prepared by melt mixing. The composites were characterized by electrical conductivity measurements (Coleman’s method), mechanical properties (DMA and stress–strain tests), thermal stability (thermogravimetry) and morphology of dispersion (SEM). Finally, the resulting composites were compared with those made by the solution casting method. The results showed a strong influence of the preparation methodology on the final properties of the composites due to changes in morphology. Composites prepared by casting showed a higher electrical conductivity than extruded ones; the composites with 6 wt.% of MWCNT prepared by extrusion presented conductivity of the same order of magnitude as the composite with 1 wt.% of MWCNT prepared by casting – 10⁻³ to 10⁻⁴ S cm⁻¹. However, the extruded samples presented better mechanical properties than the casting ones.     

Melt mixed nano composites of PA12 with MWNTs: Influence of MWNT and matrix properties on macrodispersion and electrical properties

Nanocomposites containing four different polyamide 12 (PA12) types and three grades of multiwalled carbon nanotubes (MWNTs) were prepared via small-scale melt processing to study the effect of different MWNTs and the influence of polymer properties on the dispersion of the fillers and the electrical properties of the composites. Under the selected mixing conditions the lowest electrical percolation threshold of 0.7 wt.% was found for Nanocyl™ NC7000 in low viscous PA12. Moreover, big influences of the end group functionality (acid or amine excess) and the melt viscosity of the matrix were found. Composites of PA12 with acid excess showed lower percolation thresholds than those based on amine terminated materials. At constant end group ratio low viscous matrices resulted in lower percolation thresholds than high viscous materials. The best MWNT dispersion was obtained in both high viscous PA12 composites. In these systems the mixing speed was varied indicating an optimum concerning electrical conductivity at 150 rpm as compared to 50 and 250 rpm.     

Effects of carbon nanotubes and carbon fiber reinforcements on thermal conductivity and ablation properties of carbon/phenolic composites

The ablation properties and thermal conductivity of carbon nanotube (CNT) and carbon fiber (CF)/phenolic composites were evaluated for different filler types and structures. It was found that the mechanical and thermal properties of phenolic-polymer matrix composites were improved significantly by the addition of carbon materials as reinforcement. The concentrations of CF and CNT reinforcing materials used in this study were 30 vol% and 0.5 wt%, respectively. The thermal conductivity and thermal diffusion of the different composites were observed during ablation testing, using an oxygen–kerosene (1:1) flame torch. The thermal conductivity of CF mat/phenolic composites was higher than that of random CF/phenolic composites. Both CF mat and CNT/phenolic composites exhibited much better thermal conductivity and ablation properties than did neat phenolic resin. The more conductive carbon materials significantly enhanced the heat conduction and dissipation from the flame location, thereby minimizing local thermal damage.     

Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes

Aligned carbon nanotubes (CNTs) are implemented into alumina-fiber reinforced laminates, and enhanced mass-specific thermal and electrical conductivities are observed. Electrical conductivity enhancement is useful for electrostatic discharge and sensing applications, and is used here for both electromagnetic interference (EMI) shielding and deicing. CNTs were grown directly on individual fibers in woven cloth plies, and maintained their alignment during the polymer (epoxy) infiltration used to create laminates. Using multiple complementary methods, non-isotropic electrical and thermal conductivities of these hybrid composites were thoroughly characterized as a function of CNT volume/mass fraction. DC and AC electrical conductivity measurements demonstrate high electrical conductivity of >100 S/m (at 3% volume fraction, ∼1.5% weight fraction, of CNTs) that can be used for multifunctional applications such as de-icing and electromagnetic shielding. The thermal conductivity enhancement (∼1 W/m K) suggests that carbon-fiber based laminates can significantly benefit from aligned CNTs. Application of such new nano-engineered, multi-scale, multi-functional CNT composites can be extended to system health monitoring with electrical or thermal resistance change induced by damage, fire-resistant structures among other multifunctional attributes.     

In situ polymerized nanocomposites: Polystyrene/CNT and Poly(methyl methacrylate)/CNT composites

Carbon nanotubes (CNTs) were incorporated into polystyrene (PS) and poly(methyl methacrylate) (PMMA) matrices via in situ emulsion and emulsion/suspension polymerization methods. The polymerizations were carried out using various initiators, surfactants, and carbon nanotubes to determine their influence on polymerization and on the properties of the composites. The loading of CNTs in the composites varied from 0 to 15 wt.%, depending on the CNTs used. Morphology and dispersion of the CNTs were analyzed by transmission and scanning electron microscopy techniques. The dispersion of multi-walled carbon nanotubes (MWCNT) in the composites was excellent, even at high CNT loading. The mechanical properties, and electrical and thermal conductivities, of the composites were also analyzed. Both electrical and thermal conductivities were improved.     

Enhancement of tensile and thermal properties of epoxy nanocomposites through chemical hybridization of carbon nanotubes and alumina

This paper presents the properties of epoxy nanocomposites, prepared using a synthesized hybrid carbon nanotube–alumina (CNT–Al₂O₃) filler, via chemical vapour deposition and a physically mixed CNT–Al₂O₃ filler, at various filler loadings (i.e., 1–5%). The tensile and thermal properties of both nanocomposites were investigated at different weight percentages of filler loading. The CNT–Al₂O₃ hybrid epoxy composites showed higher tensile and thermal properties than the CNT–Al₂O₃ physically mixed epoxy composites. This increase was associated with the homogenous dispersion of CNT–Al₂O₃ particle filler; as observed under a field emission scanning electron microscope. It was demonstrated that the CNT–Al₂O₃ hybrid epoxy composites are capable of increasing tensile strength by up to 30%, giving a tensile modulus of 39%, thermal conductivity of 20%, and a glass transition temperature value of 25%, when compared to a neat epoxy composite.     

Mechanical, thermal and electrical properties of aluminum nitride/polyetherimide composites

The mechanical, thermal and electrical properties of modified AlN/polyetherimide (PEI) composites were investigated. It revealed that the surface of AlN modified by silane could effectively increase the adhesion with matrix, which was beneficial for AlN to reinforce the polyetherimide matrix. After silane modification, the AlN showed good dispersion and wetibility in the polyetherimide matrix and imparted excellent mechanical, electrical and thermal properties. The tensile strength, modulus, electrical and thermal stability were improved with the increasing of AlN content. The tensile strength of AlN/PEI composites increased by 27% when 12.6 vol.% AlN was added to neat polyetherimide. The thermal conductivity of the 57.4 vol.% AlN/PEI composites increased three times compared with neat polyetherimide. Test results indicate that the silane grafted AlN incorporated into the polyetehetimide matrix effectively enhance the thermal stability, thermal conductivity and mechanical properties of the polyetherimide composites.     

Hybrid network structure and thermal conductive properties in poly(vinylidene fluoride) composites based on carbon nanotubes and graphene nanoplatelets

A small quantity of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) were introduced into the poly(vinylidene fluoride) (PVDF)/GNP and PVDF/CNT composites, respectively, to prepare the corresponding ternary PVDF/CNT/GNP and PVDF/GNP/CNT composites. The results demonstrated that adding CNTs into the PVDF/GNP composites greatly promoted the formation of the hybrid network structure of fillers. This was much different from the scenario that adding GNPs into the PVDF/CNT composites. GNPs and CNTs exhibited excellent nucleation effects for the crystallization of PVDF matrix; however, the variation of the PVDF crystallinity was small. Adding CNTs into the PVDF/GNP composites greatly enhanced the electrical conductivity of the PVDF/CNT/GNP composites. This was also different from the scenario of the PVDF/GNP/CNT composites. Furthermore, the PVDF/CNT/GNP composites exhibit higher thermal conductivity and higher synergistic efficiency compared with the PVDF/GNP/CNT composites. The conductive mechanisms and the synergistic effects of the ternary composites were then analyzed.     

Multifunctional properties of high volume fraction aligned carbon nanotube polymer composites with controlled morphology

Advanced composites, such as those used in aerospace applications, employ a high volume fraction of aligned stiff fibers embedded in high-performance polymers. Unlike advanced composites, polymer nanocomposites (PNCs) employ low volume fraction filler-like concepts with randomly-oriented and poorly controlled morphologies due to difficult issues such as dispersion and alignment of the nanostructures. Here, novel fabrication techniques yield controlled-morphology aligned carbon nanotube (CNT) composites with measured non-isotropic properties and trends consistent with standard composites theories. Modulus and electrical conductivity are maximal along the CNT axis, and are the highest reported in the literature due to the continuous aligned-CNTs and use of an unmodified aerospace-grade structural epoxy. Rule-of-mixtures predictions are brought into agreement with the measured moduli when CNT waviness is incorporated. Waviness yields a large (10×) reduction in modulus, and therefore control of CNT collimation is seen as the primary limiting factor in CNT reinforcement of composites for stiffness. Anisotropic electron transport (conductivity and current-carrying capacity) follows expected trends, with enhanced conductivity and Joule heating observed at high current densities.     

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.     

Simultaneous global and local strain sensing in SWCNT-epoxy composites by Raman and impedance spectroscopy

Polymer composites can be benefited in many ways through the addition of carbon nanotubes (CNT). For instance, CNT can build up a percolated network within the polymer matrix, which results in a composite material with electrical conductivity and piezoresistive characteristics. This has very important implications for the realization of self-stress sensing structural composites. Moreover, the remarkable optical and transport properties of CNT permit to obtain information about the stress state of the composite at different scales. In the present work, the local and global stress response of SWCNT-epoxy composites is characterised by simultaneous Raman spectroscopic and electrical measurements on nanocomposite specimens submitted to different levels of surface strain. Both the Raman G′-band resonance frequency and the electrical resistance of the composite are found to change monotonically with strain until an inflection point is reached at ∼1.5% strain. Increased sensitivity of the piezoresistive network and lower load transfer efficiency occur beyond this strain level, and are considered to be the result of CNT slippage from the polymer. The reversibility of the stress sensitivity of the composites is verified by performing cyclic loading tests. Hysteresis loop are found to develop earlier on the Raman curves as in the resistance curves, which indicates that even at low strain levels, permanent damage is induced in the vicinity of carbon nanotubes. The use of Raman spectroscopy in combination with electrical methods provides a further insight on the stress sensing capabilities of CNT and the factors which affect the sensitivity and reproducibility of this behaviour.     

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.     

A multi-scale model for coupled heat conduction and deformations of viscoelastic functionally graded materials

An integrated micromechanical-structural framework is presented to analyze coupled heat conduction and deformations of functionally graded materials (FGM) having temperature and stress dependent viscoelastic constituents. A through-thickness continuous variation of the thermal and mechanical properties of the FGM is approximated as an assembly of homogeneous layers. Average thermo-mechanical properties in each homogeneous medium are computed using a simplified micromechanical model for particle reinforced composites. This micromechanical model consists of two isotropic constituents. The mechanical properties of each constituent are time–stress–temperature dependent. The thermal properties (coefficient of thermal expansion and thermal conductivity) of each constituent are allowed to vary with temperature. Sequentially coupled heat transfer and displacement analyses are performed, which allow analyzing stress/strain behaviors of FGM having time and temperature dependent material properties. The thermo-mechanical responses of the homogenized FGM obtained from micromechanical model are compared with experimental data and the results obtained from finite element (FE) analysis of FGMs having microstructural details. The present micromechanical-modeling approach is computationally efficient and shows good agreement with experiments in predicting time-dependent responses of FGMs. Our analysis forecasts a better design for creep resistant materials using particulate FGM composites.     

Growth of carbon nanotubes on carbon fibre substrates to produce hybrid/phenolic composites with improved mechanical properties

Carbon nanotubes were grown by chemical vapor deposition (CVD) on different carbon fibre substrates namely, unidirectional (UD) carbon fibre tows, bi-directional (2D) carbon fibre cloth and three dimensional (3D) carbon fibre felt. These substrates were used as the reinforcement in phenolic resin matrix to develop hybrid CF–CNT composites. The growth morphology and other characteristics of the as grown tubes were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal gravimetry (TGA) which confirmed a copious growth of multiwalled carbon nanotubes (MWNTs) on these substrates. The mechanical properties of the hybrid composites was found to increase with the increasing amount of deposited carbon nanotubes. The flexural strength (FS) improved by 20% for UD, 75% for 2D and 66% for 3D hybrid composites as compared to that prepared by neat reinforcements (without CNT growth) under identical conditions. Flexural modulus (FM) of these composites also improved by 28%, 54% and 46%, respectively.