The effect of molecular chain polarity on electric field-induced aligned conductive carbon nanotube network formation in polymer melt

Two ethylene–vinyl acetate (EVA) copolymers containing 10 and 25 wt.% vinyl acetate (EVA10 and EVA25) were utilized to explore the effect of molecular polarity on the formation of conductive carbon nanotube (CNT) network in EVA melt under an electric field. Because of the different interfacial energy, it was supposed to be stronger molecular chain-CNT interaction in CNT/EVA25 than that in CNT/EVA10. The critical time for conductive CNT network formation decreased with annealing temperature, filler loading and EVA polarity. The activation energy of conductive CNT network formation (93.9 kJ/mol) in CNT/EVA10 is lower than that (104.7 kJ/mol) in CNT/EVA25. By a thermodynamic percolation model, the percolation threshold at the equilibrium state was about 0.19 vol.% for CNT/EVA10, while it rose to 0.27 vol.% for CNT/EVA25. Morphological observation showed a high degree of CNT alignment in CNT/EVA10 compared to CNT/EVA25 after application of an electric field. The results suggested the strong CNT–EVA chain interaction and higher viscosity of polymer matrix limited the CNT alignment and the conductive network tended to form easily in EVA melt with a low chain polarity.     

Highly strong and conductive carbon nanotube/cellulose composite paper

Carbon nanotube (CNT)/cellulose composite materials were fabricated in a paper making process optimized for a CNT network to form on the cellulose fibers. The measured electric conductivity was from 0.05 to 671 S/m for 0.5–16.7 wt.% CNT content, higher than that for other polymer composites. The real permittivities were the highest in the microwave region. The unique CNT network structure is thought to be the reason for these high conductivity and permittivity values. Compared to other carbon materials, our carbon CNT/cellulose composite material had improved parameters without decreased mechanical strength. The near-field electromagnetic shielding effectiveness (EMI SE) measured by a microstrip line method depended on the sheet conductivity and qualitatively matched the results of electromagnetic field simulations using a finite-difference time-domain simulator. A high near-field EMI SE of 50-dB was achieved in the 5–10 GHz frequency region with 4.8 wt.% composite paper. The far-field EMI SE was measured by a free space method. Fairly good agreement was obtained between the measured and calculated results. Approximately 10 wt.% CNT is required to achieve composite paper with 20-dB far-field EMI SE.     

Fabrication and characterization of recyclable carbon nanotube/polyvinyl butyral composite fiber

A surface-draw method to fabricate recyclable carbon nanotube/polyvinyl butyral (CNT/PVB) composite fibers is reported. This method is effective for both single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube. The CNT mass content of CNT/PVB composite fibers can vary from 0 to 80 wt.%, which is higher than most CNT/polymer composites reported to date. The diameter of the composite fibers can be controlled in the range of 10-100 μm, with essentially unlimited draw length. The composite fibers with 7.4 wt.% SWCNTs showed optimal tensile properties. Compared with pure PVB fibers, the tensile strength, failure strain, and elastic modulus of the composite fiber have improved about 127%, 27%, and 73%, respectively. In addition, SWCNT/PVB composites with 66.7 wt.% SWCNTs have the highest conductivity of 42.9 S m⁻¹. More importantly, the major benefit is the “greenness” of the method, which involves environment friendly ethanol-water solvent with no functionalization of the nanotube required, and only simple apparatus are needed. The CNT/PVB composite fibers obtained can be dissolved in ethanol solution and reformed with the surface draw method without any additional treatment; and the material properties after recycle is comparable to those fabricated in the first round.     

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.     

Study on the semi-empirical model for the complex permittivity of carbon nanocomposite laminates in microwave frequency band

A semi-empirical model is proposed for the complex permittivity of composites containing electrical conductive carbon nanomaterials such as carbon black (CB), carbon nanofiber (CNF) and multi-walled carbon nanotube (CNT). The composites were fabricated with E-glass fabric/epoxy prepregs. The model is based on the percolation theory. The model is available for the composite of filler content over the percolation threshold and applicable within the high frequency band in which AC electrical conductivity of the composite is continuously proportional to the frequency. The proposed model is composed of the numerical equations of the scaling law in percolation theory and constants obtained from experiments to quantify the model. The model describes the complex permittivity as a function of frequency and filler content. The model was verified when compared with the measurements. The measurements for the complex permittivities of the composites were performed at the frequency band between 0.5 and 18.0 GHz using a vector network analyzer with a 7 mm coaxial air line.     

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.     

The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites

The sintering and grain growth behaviour of alumina + 2, 3.5 and 5 wt.% carbon nanotubes (CNTs) and alumina + 2 wt.% carbon black nanocomposites prepared by Spark Plasma Sintering (SPS) were studied. The addition of CNTs to ceramics produces a large reduction in the sintering temperature required for their complete densification and a significant grain size refinement by a previously unreported mechanism. The CNTs form a strong entangled network around the grains, which constrains the normal and abnormal grain growth. An alumina/alumina + 2 wt.% CNT/alumina laminate structure was prepared to demonstrate directly the large grain-growth retardation effect of CNTs. These effects open up the possibility of using CNTs as a sintering aid to control the sintering behaviour and microstructures of ceramics in bulk, laminate and functionally gradient (FGM) form.     

Selective location and conductive network formation of multiwalled carbon nanotubes in polycarbonate/poly(vinylidene fluoride) blends

Multiwalled carbon nanotubes (MWCNTs)-filled polycarbonate (PC), poly(vinylidene fluoride) (PVDF) and PC/PVDF conductive composites were fabricated using melt mixing, respectively. The dynamic process of MWCNTs conductive network formation in the composites was in situ traced by recording the variation of electrical resistivity with time during annealing treatments. As a result, the percolation threshold for the MWCNTs-filled PC/PVDF system was much lower than those of MWCNTs-filled individual polymers and the MWCNTs were selectively located in the PC phase of PC/PVDF composite, which had been verified by scanning electron microscopy measurements. The activation energy of conductive network formation for PC/PVDF/MWCNTs composite was close to that of the PC/MWCNTs system, which further confirmed that MWCNTs were dispersed mainly in the PC phase. Furthermore, the assembly velocity of MWCNTs in the polymer melt increased with annealing temperature.     

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.     

Poly(vinyl alcohol) reinforced with large-diameter carbon nanotubes via spray winding

For practical application of carbon nanotube (CNT)/polymer composites, it is critical to produce the composites at high speed and large scale. In this study, multi-walled carbon nanotubes (MWNTs) with large diameter (∼45 nm) and polyvinyl alcohol (PVA) were used to increase the processing speed of a recently developed spraying winding technique. The effect of the different winding speed and sprayed solution concentration to the performance of the composite films were investigated. The CNT/PVA composites exhibit tensile strength of up to 1 GPa, and modulus of up to 70 GPa, with a CNT weight fraction of 53%. In addition, an electrical conductivity of 747 S/cm was obtained for the CNT/PVA composites. The good mechanical and electrical properties are attributed to the uniform CNTs and PVA matrix integration and the high degree of tube alignment.     

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.     

Polyethersulfone-expanded graphite nanocomposites: Charge transport and impedance characteristics

Polyethersulfone (PES)-expanded graphite nanocomposites have been prepared by solution blending route after sonicating expanded graphite in dichloromethane. It has been observed that ultrasonication results in nanosheets formation leading to a low percolation threshold of 3 wt.%. At 5 wt.% filler loading the conductivity is of the order of 10⁻² S/cm. Hopping type of charge transport occurs at 3.2 wt.% expanded graphite in PES below which capacitive effects couple. The effective dielectric constant at low frequency increases with filler concentration. Impedance measurement has been carried out to evaluate interfacial capacitance which, for 3.2 wt.% expanded graphite addition in PES, increases to 110 pF from 32 pF for 1 wt.% expanded graphite in the polymer. DSC analysis shows an increment of 12 °C in the T_g of PES with 3 wt.% expanded graphite suggesting interaction between the polymer and filler.     

Melt rheology of nanocomposites based on acrylic copolymer and cellulose whiskers

Nanocomposites based on poly(styrene-co-hexylacrylate) copolymer and cellulose whiskers as the nanosize filler were prepared by in situ miniemulsion polymerization and their melt rheological behaviours were investigated under dynamic shear conditions. The effects of γ-methacryloxypropyl triethoxysilane (MPS) content along with the whisker loading were explored. In the absence of whiskers, a transition from a liquid- to a solid-like behaviour was observed when the polymer was synthesized in the presence of MPS. When cellulose nanofiller was added, the storage modulus G′ and the dynamic viscosities η of the nanocomposites increased monotonically with whisker content and the resulting materials displayed a solid-like behaviour. Above 2 wt.%. loading, a percolated interconnected whisker-whisker network is built up, producing a jump in the storage modulus and strong shear-thinning behaviour of the viscosity. However, as the nanocomposites were prepared in the presence of 3% of MPS, no enhancement nor in the storage modulus nor in the viscosity was observed up to 5 wt.%. of whisker loading. Such a phenomenon was ascribed to inhibition of build-up of the whisker network. The non-linear viscoelastic behaviour of the nanocomposites was also investigated and analysed in terms of the breakdown of different networks, namely the filler-filler and the polymer-filler networks.     

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.     

Electric field effects on CNTs/vinyl ester suspensions and the resulting electrical and thermal composite properties

In this study, electrical conductivity of a vinyl ester based composite containing low content (0.05, 0.1 and 0.3 wt.%) of double and multi-walled carbon nanotubes with and without amine functional groups (DWCNTs, MWCNTs, DWCNT-NH₂ and MWCNT-NH₂) was investigated. The composite with pristine MWCNTs was found to exhibit the highest electrical conductivity. Experiments aimed to induce an aligned conductive network with application of an alternating current (AC) electric field during cure were carried out on the resin suspensions with MWCNTs. Formation of electric anisotropy within the composite was verified. Light microscopy (LM), scanning electron (SEM) and transmission electron microscopy (TEM) were conducted to visualize dispersion state and the extent of alignment of MWCNTs within the polymer cured with and without application of the electric field. To gain a better understanding of electric field induced effects, glass transition temperature (T_g) of the composites was measured via Differential Scanning Calorimetry (DSC). It was determined that at 0.05 wt.% loading rate of MWCNTs, the composites, cured with application of the AC electric field, possessed a higher T_g than the composites cured without application of the AC electric field.     

Fabrication of transparent conductive carbon nanotubes/polyurethane-urea composite films by solvent evaporation-induced self-assembly (EISA)

Transparent and conductive carbon nanotubes (CNTs)/polyurethane-urea (PUU) composite films were prepared by solvent evaporation-induced self-assembly (EISA). Pristine CNTs were treated with acids (H₂SO₄/HNO₃ = 3:1, v:v), acylated with thionyl chloride, and purified after filtration. These acylated CNTs (0.05 wt.% in dimethylformamide, DMF) were deposited onto the 3-aminopropyl triethoxysilane (APTES)-modified glass substrate by DMF EISA at 100 °C with the withdrawal rate of 3 cm/h. The CNT layers of 200–400 nm thicknesses were transferred to the PUU films by solution casting or resin transfer molding (RTM) at ambient temperature. Optical transmittances of the composite films were 60–75% at 550 nm wavelength and their sheet resistances were 5.2 × 10⁰–2.4 × 10³ kΩ/square, and which varied significantly with type of CNTs and the transferring methods of CNT layers.     

Comparison of nanotubes produced by fixed bed and aerosol-CVD methods and their electrical percolation behaviour in melt mixed polyamide 6.6 composites

The electrical percolation behaviour of five different kinds of carbon nanotubes (CNTs) synthesised by two CVD techniques was investigated on melt mixed composites based on an insulating polyamide 6.6 matrix. The electrical percolation behaviour was found to be strongly dependent on the properties of CNTs which varied with the synthesis conditions. The lowest electrical percolation threshold (0.04 wt.%) was determined for as grown multi-walled carbon nanotubes without any purification or chemical treatment. Such carbon nanotubes were synthesised by the aerosol method using acetonitrile as ferrocene containing solvent and show relatively low oxygen content near the surface, high aspect ratio, and good dispersability. Similar properties could be found for nanotubes produced by the aerosol method using cyclohexane, whereas CNTs produced by the fixed bed method using different iron contents in the catalyst material showed much higher electrical percolation thresholds between 0.35 and 1.02 wt.%.     

Electrically conductive carbon nanotube/polypropylene nanocomposite with improved mechanical properties

Conductive polymer nanocomposites based on carbon nanotubes (CNTs) have wide range of applications in the electronics and energy sectors. For many of these applications, such as the electromagnetic interference (EMI) shielding, high nanofiller loading is typically needed to achieve the desired properties. The high nanofiller concentration deteriorates the composite's tensile strength due to the increase in nanofiller aggregation. In this work, highly conductive CNT/polypropylene (PP) nanocomposite with improved tensile strength was prepared by melt mixing. The effects of CNT content on the processing behavior, microstructure, mechanical and electrical properties of the nanocomposite were investigated. Scanning electron microscopy was used to investigate the composite microstructure. Good level of CNT dispersion with remarkable adhesion at the CNT/PP interface was observed. Based on a theoretical model, the interfacial strength was estimated to be in the range of 36–58 MPa. As a result of this microstructure, significant enhancement in ultimate tensile strength was reported with the increase of CNT content. The tensile strength of the 20 wt.% CNT/PP nanocomposite was 80% higher than that of the unfilled PP. Moreover, and due to the good dispersion of CNT particles, an electrical percolation threshold concentration of 0.93 wt.% (0.5 vol.%) was obtained.     

Influence of dry grinding in a ball mill on the length of multiwalled carbon nanotubes and their dispersion and percolation behaviour in melt mixed polycarbonate composites

Ball milling of carbon nanotubes (CNTs) in the dry state is a common way to produce tailored CNT materials for composite applications, especially to adjust nanotube lengths. For Nanocyl^TM NC7000 nanotube material before and after milling for 5 and 10 h the length distributions were quantified using TEM analysis, showing decreases of the mean length to 54% and 35%, respectively. With increasing ball milling time in addition a decrease of agglomerate size and an increase of packing density took place resulting in a worse dispersability in aqueous surfactant solutions. In melt mixed CNT/polycarbonate composites produced using masterbatch dilution step, the electrical properties, the nanotube length distribution after processing, and the nano- and macrodispersion of the nanotubes were studied. The slight increase in the electrical percolation threshold in the melt mixed composites with ball milling time of CNTs can be assigned to lower nanotube lengths as well as the worse dispersability of the ball milled nanotubes. After melt compounding, the mean CNT lengths were shortened to 31%, 50%, and 66% of the initial lengths of NC7000, NC7000-5 h, and NC7000-10 h, respectively.     

Influence of screw configuration,residence time,and specific mechanical energy in twin-screw extrusion of polycaprolactone/multi-walled carbon nanotube composites

Melt processing of thermoplastic-based nanocomposites is the favoured route to produce electrically conductive or electrostatic dissipative polymer composites containing carbon nanotubes (CNT). As these properties are desired at low filler fractions, a high degree of dispersion is required in order to benefit from the intrinsic CNT properties. This study discusses the influence of screw configuration, rotation speed, and throughput on the residence time and specific mechanical energy (SME) and the resulting macroscopic CNT dispersion in polycaprolactone (PCL) based masterbatches containing 7.5 wt.% multi-walled carbon nanotubes (MWNT) using an intermeshing co-rotating twin-screw extruder Berstorff ZE25.