Preparation, crystallization, electrical conductivity and thermal stability of syndiotactic polystyrene/carbon nanotube composites

A homogeneous dispersion of multi-walled carbon nanotubes (MWCNTs) in syndiotactic polystyrene (sPS) is obtained by a simple solution dispersion procedure. MWCNTs were dispersed in N-methyl-2-pyrrolidinone (NMP), and sPS/MWCNT composites are prepared by mixing sPS/NMP solution with MWCNT/NMP dispersion. The composite structure is characterized by scanning electron microscopy and transmission electron microscopy. The effect of MWCNTs on sPS crystallization and the composite properties are studied. The presence of MWCNTs increases the sPS crystallization temperature, broadens the crystallite size distribution and favors the formation of the thermodynamically stable β phase, whereas it has little effect on the sPS γ to α phase transition during heating. By adding only 1.0 wt.% pristine MWCNTs, the increase in the onset degradation temperature of the composite can reach 20 °C. The electrical conductivity is increased from 10^{−10∼−16} (neat sPS) to 0.135 S m⁻¹ (sPS/MWCNT composite with 3.0 wt.% MWCNT content). Our findings provide a simple and effective method for carbon nanotube dispersion in polymer matrix with dramatically increased electrical conductivity and thermal stability.     

Coupling activity of ionic liquids between diene elastomers and multi-walled carbon nanotubes

In order to ensure better rubber/multi-walled carbon nanotube (MWCNT) compatibility and to enhance the dispersibility, a series of ionic liquids has been tested in regard to an improved interaction between rubber and carbon nanotubes. We found that in the presence of especially one ionic liquid, namely, 1-allyl-3-methyl imidazolium chloride, for the blend of solution-styrene-butadiene and polybutadiene rubber, used as basic elastomer, a three fold increase of tensile strength was achieved with only ∼3 wt.% MWCNT loading. At this low concentration of MWCNTs the sample can be stretched up to 456% without mechanical failure. The use of this ionic liquid additionally results in higher electrical conductivity (10⁻² S cm⁻¹) at low concentration (<3 wt.%) of MWCNTs. Dynamic mechanical analysis confirmed the specific interaction of CNTs and diene rubber chains by showing an extra relaxation process at relatively higher temperature (∼T_g + 130 K) in the temperature sweep measurements. Raman spectroscopic analysis also supported the specific interaction between MWCNTs and rubber molecules with the help of 1-allyl-3-methyl imidazolium chloride. Transmission electron microscopic images confirm the good dispersion of the MWCNTs along with a ‘cellular’-like structure of the CNTs in the rubber matrix.     

Thermal and electrical conductivity of tall, vertically aligned carbon nanotube arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays up to ∼6 mm high with an array density of 0.06 g cm⁻³ have been grown by chemical vapor deposition. Thermal conductivities (κ) and electrical conductivities (σ) were determined from 5 K to 390 K. The range for κ at 300 K is 0.5-1.2 W m⁻¹ K⁻¹ along the tube growth direction, with the shortest array having the highest κ, and an order of magnitude lower in the direction perpendicular to the tubes. The same trends also were evident for electrical conductivity, i.e., decreasing values with increasing array height and conductivity an order of magnitude lower in the perpendicular direction. Values of σ ranged from 7 to 14 S cm⁻¹ along the array at 300 K. The Seebeck coefficient is ∼20 μV K⁻¹ at 300 K. The effective Lorentz number indicates that thermal conductivity in the carbon nanotube arrays is phonon dominated over the full temperature range.     

Influence of a cyclic butylene terephthalate oligomer on the processability and thermoelectric properties of polycarbonate/MWCNT nanocomposites

The thermoelectric properties of melt-processed nanocomposites consisting of a polycarbonate (PC) thermoplastic matrix filled with commercially available carboxyl (–COOH) functionalized multi-walled carbon nanotubes (MWCNTs) were evaluated. MWCNTs carrying carboxylic acid moieties (MWCNT-COOH) were used due the p-doping that the carboxyl groups facilitate, via electron withdrawing from the electron-rich π-conjugated system. Preliminary thermogravimetric analysis (TGA) of MWCNT-COOH revealed that the melt-mixing was limited at low temperatures due to thermal decomposition of the MWCNT functional groups. Therefore, PC was mixed with 2.5 wt% MWCNT-COOH (PC/MWCNT-COOH) at 240 °C and 270 °C. In order to reduce the polymer melt viscosity, a cyclic butylene terephthalate (CBT) oligomer was utilized as an additive, improving additionally the electrical conductivity of the nanocomposites. The melt rheological characterization of neat PC and PC/CBT blends demonstrated a significant decrease of the complex viscosity by the addition of CBT (10 wt%). Optical and transmission electron microscopy (OM, TEM) depicted an improved MWCNT dispersion in the PC/CBT polymer blend. The electrical conductivity was remarkably higher for the PC/MWCNT-COOH/CBT composites compared to the PC/MWCNT-COOH ones. Namely, the PC/MWCNT-COOH/CBT processed at 270 °C exhibited the best values with electrical conductivity; σ = 0.05 S/m, Seebeck coefficient; S = 13.55 μV/K, power factor; PF = 7.60 × 10⁻⁶μW/m K⁻², and thermoelectric figure of merit; ZT = 7.94 × 10⁻⁹. The PC/MWCNT-COOH/CBT nanocomposites could be ideal candidates for large-scale thermal energy harvesting, even though the presently obtained ZT values are still too low for commercial applications.     

Moderate anisotropy in the electrical conductivity of bulk MWCNT/epoxy composites

Bulk aligned multi-walled carbon nanotube films and their epoxy composites were prepared and their DC and AC conductivity studied. Nanotube films of up to 2 mm thickness were grown by catalytic chemical vapor deposition. Composites of nanotubes were made by infiltrating the films with a commercial epoxy. DC electrical resistivities in the axial direction of as-grown and purified films were found to be ∼1.2 Ωmm and ∼3.4 Ωmm, respectively. For the transverse direction the resistivity values were higher only with a factor of ∼2. In the case of composites, anisotropy is more pronounced showing more than an order of magnitude higher resistivity in the transverse direction (∼71.4 Ωmm) as compared to the axial value (∼4.2 Ωmm). AC behavior of the films investigated between 1 MHz and 3 GHz shows the presence of inductive and capacitive components at frequencies above ∼100 MHz. The moderate anisotropy for both DC and AC electrical properties are explained on the basis of the films’ structure combined with percolation theory and equivalent circuit models.     

Cellular structures of carbon nanotubes in a polymer matrix improve properties relative to composites with dispersed nanotubes

A new processing method has been developed to combine a polymer and single wall carbon nanotubes (SWCNTs) to form electrically conductive composites with desirable rheological and mechanical properties. The process involves coating polystyrene (PS) pellets with SWCNTs and then hot pressing to make a contiguous, cellular SWCNT structure. By this method, the electrical percolation threshold decreases and the electrical conductivity increases significantly as compared to composites with well-dispersed SWCNTs. For example, a SWCNT/PS composite with 0.5 wt% nanotubes made by this coated particle process (CPP) has an electrical conductivity of ∼3 × 10⁻⁴ S/cm, while a well-dispersed composite made by a coagulation method with the same SWCNT amount has an electrical conductivity of only ∼10⁻⁸ S/cm. The rheological properties of the composite with a macroscopic cellular SWCNT structure are comparable to PS, while the well-dispersed composite exhibits a solid-like behavior, indicating that the composites made by this new CPP are more processable. In addition, the mechanical properties of the CPP-made composite decrease only slightly, as compared with PS. Relative to the common approach of seeking better dispersion, this new fabrication method provides an important alternative means to higher electrical conductivity in SWCNT/polymer composites. Our straightforward particle coating and pressing method avoids organic solvents and is suitable for large-scale, inexpensive processing using a wide variety of polymers and nanoparticles.     

Effect of surface modified porous inorganic-organic hybrid polyphosphazene nanotubes on the properties of polyethylene oxide based solid polymer electrolytes

2-(2-methyloxyethoxy)ethanol modified poly (cyclotriphosphazene-co-4,4′-sufonyldiphenol) (PZS) nanotubes were synthesized and solid composite polymer electrolytes based on the surface modified polyphosphazene nanotubes added to PEO/LiClO₄ model system were prepared. Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM) were used to investigate the characteristics of the composite polymer electrolytes (CPE). The ionic conductivity, lithium ion transference number and electrochemical stability window can be enhanced after the addition of surface modified PZS nanotubes. The electrochemical investigation shows that the solid composite polymer electrolytes incorporated with PZS nanotubes have higher ionic conductivity and lithium ion transference number than the filler SiO₂. Maximum ionic conductivity values of 4.95 × 10⁻⁵ S cm⁻¹ at ambient temperature and 1.64 × 10⁻³ S cm⁻¹ at 80 °C with 10 wt % content of surface modified PZS nanotubes were obtained and the lithium ion transference number was 0.41. The good chemical properties of the solid state composite polymer electrolytes suggested that the inorganic-organic hybrid polyphosphazene nanotubes had a promising use as fillers in solid composite polymer electrolytes and the PEO₁₀-LiClO₄-PZS nanotubes solid composite polymer electrolyte can be used as a candidate material for lithium polymer batteries.     

Improved capacitance characteristics during electrochemical charging of carbon nanotubes modified with polyoxometallate monolayers

By modification of surfaces of multi-walled carbon nanotubes with ultra-thin monolayer-type films of phosphododecamolybdic acid, H₃PMo₁₂O₄₀, an electrode material with improved capacitance properties is produced. It is apparent from three distinct test experiments (based on cyclic voltammetry, galavanostatic charging-discharging and AC impedance) that capacitors utilizing H₃PMo₁₂O₄₀-modified carbon nanotubes are characterized by specific capacitances and energy densities on the levels of 40 F g⁻¹ and 1.3 Wh kg⁻¹, whereas the respective values for the systems built from bare carbon nanotubes are lower, 22 F g⁻¹ and 0.7 Wh kg⁻¹. It is reasonable to expect that fast and reversible multi-electron transfers of the Keggin-type H₃PMo₁₂O₄₀ account for the pseudocapacitance effect and significantly contribute to the observed overall capacitance.     

Epoxy-based carbon films with high electrical conductivity attached to an alumina substrate

Carbon-based films (0.8-13 μm thick) with good bonding to the substrate and high processability were produced at 650 °C on an alumina substrate, using SU 2.5 bisphenol-A type novolac epoxy (plus triethyleneteramine curing agent) as the carbon precursor. This precursor gave crack-free and scratch resistant carbon films. Interconnected filamentary nickel nanoparticles were more effective for conductivity enhancement than silver nanoparticles or multiwalled carbon nanotubes at 5 vol.% or below, in spite of the high conductivity of silver and the high aspect ratio of nanotubes. The carbon film with 2.5 vol.% nickel showed resistivity 6 × 10⁻³ Ω cm.     

Modified carbon nanotube composites with high dielectric constant, low dielectric loss and large energy density

Flexible dielectric polystyrene based composites containing multi-walled carbon nanotubes (MWCNTs) were reported. The MWCNTs were coated with polypyrrole (PPy) by an inverse microemulsion polymerization. Transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy indicated that the MWCNTs were coated with PPy. Our composites presented a stable high dielectric constant (∼44), rather low loss (<0.07), and large energy density (up to 4.95 J cm⁻³). The largely-enhanced dielectric performance originates from the organic shell PPy, which not only ensure good dispersion of MWCNTs in the polymer matrix but also screen charge movement to shut off leakage current. Such MWCNT composites can be used to store charge and electrical energy and play a key role in modern electronics and electric power systems.     

High electrical conductivity and n-type thermopower from double-/single-wall carbon nanotubes by manipulating charge interactions between nanotubes and organic/inorganic nanomaterials

Single- and double-wall carbon nanotubes were decorated with organic or inorganic nanomaterials in order to obtain desired electrical transport properties such as a high electrical conductivity or an n-type thermopower. For instance, the electrical conductivity of double-wall carbon nanotubes (DWCNTs) decorated with tetrafluoro-tetracyanoquinodimethane (F₄TCNQ) was increased up to 5.9 × 10⁵ S/m, and single-wall carbon nanotubes (SWCNTs) were converted from p-type to n-type with a large thermopower (−58 μV/K) by using polyethyleneimine without vacuum or controlled environment. When inorganic nanoparticles made of Fe and Cu were used for decorating nanotubes, the electrical conductance of the nanotube films was decreased with an enlarged thermopower. On the other hand, Au decorations yielded higher electrical conductances with lower thermopowers. The thermoelectric power factors were improved by ∼180% with F₄TCNQ on DWCNTs and ∼140% with Fe on SWCNTs. We believe these transport property changes can be attributed to charge interactions resulted from the difference between the work functions/reduction potentials of nanotubes and nanomaterials. This study shows a first step toward the synthesis of both n-type and p-type conductors with carbon nanotubes, which are essential to thermoelectric energy conversion applications.     

Highly conductive and transparent thin films fabricated with predominantly semiconducting single-walled carbon nanotubes

A predominantly semiconducting single-walled carbon nanotube-based thin conductive film was fabricated on a flexible poly(ethylene terephthalate) substrate. The structural features of the nanotubes and careful experimental scrutinization consistently yielded the films with very low surface resistance (59 Ω sq⁻¹) and high transparency (80%). The morphological studies of these films before and after acid treatment revealed the self orientation of nanotubes clustered at favorable centers.     

Synthesis and electrochemical capacitance of core-shell poly (3,4-ethylenedioxythiophene)/poly (sodium 4-styrenesulfonate)-modified multiwalled carbon nanotube nanocomposites

A noncovalent method was used to functionalize multiwalled carbon nanotubes with poly (sodium 4-styrene sulfonate). And then, the core-shell poly (3,4-ethylenedioxythiophene)/functionalized multiwalled carbon nanotubes (PEDOT/PSS-CNTs) nanocomposite was successfully realized via in situ polymerization under the hydrothermal condition. In the process, PSS served for not only solubilizing and dispersing CNTs well into an aqueous solution, but also tethering EDOT monomer onto the surface of CNTs to facilitate the formation of a uniform PEDOT coating. Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM) were used to characterize the resultant PEDOT/PSS-CNTs. In addition, the PEDOT/PSS-CNTs nanocomposite (50 wt.% PEDOT) had a specific capacitance (SC) of 198.2 F g⁻¹ at a current density of 0.5 A g⁻¹ and a capacitance degradation of 26.9% after 2000 cycles, much better than those of pristine PEDOT and PEDOT/CNTs (50 wt.% PEDOT). The enhanced electrochemical performance of the PEDOT/PSS-CNTs nanocomposite (50 wt.% PEDOT) should be attributed to the high uniform system of the nanocomposite, resulting in the large surface easily contacted by abundant electrolyte ions through the three-dimensional conducting matrix.     

The development and characterisation of polyaniline—single walled carbon nanotube composite fibres using 2-acrylamido-2 methyl-1-propane sulfonic acid (AMPSA) through one step wet spinning process

High strength, flexible and conductive polyaniline (PANi)-carbon nanotube (SWNT) composite fibres have been produced using wet spinning. The use of dichloroacetic acid (DCAA) containing 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPSA) has been shown to act as an excellent dispersing medium for carbon nanotubes and for dissolution of polyaniline. The viscosity of DCAA-AMPSA solution undergoes a transition from Newtonian to non-Newtonian viscoelastic behaviour upon addition of carbon nanotubes. The ultimate tensile strength and elastic modulus of PANi-AMPSA fibres were increased by 50 and 120%, respectively, upon addition of 0.76% (w/w) carbon nanotubes. The elongation at break decreased from 11 to 4% upon addition of carbon nanotubes, however, reasonable flexibility was retained. An electronic conductivity percolation threshold of ∼0.3% (w/w) carbon nanotubes was determined with fibres possessing electronic conductivity up to ∼750 S cm⁻¹. Raman spectroscopic evidence confirmed the presence of carbon nanotubes in the polyaniline and also the interaction of the quinoid ring with the nanotubes to provide a doping effect.     

Spark plasma sintering of double-walled carbon nanotubes

Bulk samples of double-walled carbon nanotubes are prepared for the first time. The best spark plasma sintering conditions are (1100 °C, 100 MPa). Raman spectroscopy and scanning electron microscopy show that the nanotubes are undamaged. The density is equal to 1.29 g cm⁻³ and the pores are all below 6 nm in diameter. The electrical conductivity is equal to 1650 S cm⁻¹. The transverse fracture strength is equal to 47 MPa.     

Buckypaper-based catalytic electrodes for improving platinum utilization and PEMFC's performance

Platinum (Pt) catalytic electrode was developed by using carbon nanotube films (buckypaper) as supporting medium and electrodeposition method to deposit Pt catalyst. Buckypapers are free-standing thin films consisting of single-walled carbon nanotubes (SWNTs), multi-walled carbon nanotubes (MWNTs) and/or carbon nanofibers (CNFs) held together by van der Waals forces without any chemical binders. Special mixed buckypapers was developed by layered microstructures with a dense and high-conducting SWNT networks at the surface, as well as large porous structures of CNF networks as back supports. This unique microstructure can lead to improve Pt catalyst accessibility and mass exchange properties. Pt particles of about 6 nm were uniformly deposited in porous buckypapers. A promising electrochemical surface area of ∼40 m²/g was obtained from these electrodes. A Pt utilization as low as 0.28 g_Pt/kW was achieved for the cathode electrode at 80 °C. Pt utilization efficiency can be further improved by optimization of the electrodeposition condition in order to reduce the Pt particle size.     

Latex technology as a simple route to improve the thermal conductivity of a carbon nanotube/polymer composite

A novel route was revealed to reduce the interfacial phonon scattering that was considered as the bottleneck for carbon nanotubes (CNTs) to highly improve the thermal conductivity of CNT/polymer composites. Semicrystalline polyurethane (PU) dispersions were used as latex host to accommodate multi-walled carbon nanotubes (MWCNTs) following the colloidal physics method. The thermal conductivity increased from 0.15 W m⁻¹ K⁻¹ to 0.47 W m⁻¹ K⁻¹, by ∼210%, as the addition of the MWCNTs increases to 3 wt%. The morphology of the composites that was characterized by optical microscopy, scanning electron microscopy and differential scanning calorimeter suggested that the continuous nanotube-rich phase existing in the interstitial space among the latex particles and the crystallites nucleated at the nanotube–polymer interface were the main factors for the effective reduction of interfacial phonon scattering.     

Structure and conductivity of multi-walled carbon nanotube/poly(3-hexylthiophene) composite films

This work reports a structure-property investigation of a conjugated polymer nanocomposite with enhanced conductivity. Regioregular poly(3-hexylthiophene) (rrP3HT) was used to prepare composites with thin, short, multi-walled carbon nanotube (MWNT) addition over a wide range of concentrations. Scanning and transmission electron microscopies demonstrated an excellent dispersion and good wetting properties within the carbon nanotube composites. Coated MWNTs showed superstructures of P3HT self-organized on nanotube surfaces. Changes in the long range order and on the self-ordered mesophase of the bulk material were investigated by infrared and Raman spectroscopies, differential scanning calorimetry and X-ray diffraction. Interplay between charge transport through the semiconducting polymer and carbon nanotube network increased the composite's conductivity after percolation to values close to 10⁻² S cm⁻¹.     

Synthesis of Pt nanocatalyst with micelle-encapsulated multi-walled carbon nanotubes as support for proton exchange membrane fuel cells

Micelle-encapsulated multi-walled carbon nanotubes (MWCNTs) with sodium dodecyl sulfate (SDS) were used as catalyst support to deposit platinum nanoparticles. High resolution transmission electron microscopy (HRTEM) images reveal the crystalline nature of Pt nanoparticles with a diameter of ∼4 nm on the surface of MWCNTs. A single proton exchange membrane fuel cell (PEMFC) with total catalyst loading of 0.2 mg Pt cm⁻² (anode 0.1 and cathode 0.1 mg Pt cm⁻², respectively) has been evaluated at 80 °C with H₂ and O₂ gases using Nafion-212 electrolyte. Pt/MWCNTs synthesized by using modified SDS-MWCNTs with high temperature treatment (250 °C) showed a peak power density of 950 mW cm⁻². Accelerated durability evaluation was carried out by conducting 1500 potential cycles between 0.1 and 1.2 V with 50 mV s⁻¹ scan rate, H₂/N₂ at 80 °C. The membrane electrode assembly (MEA) with Pt/MWCNTs showed superior performance stability with a power density degradation of only ∼30% compared to commercial Pt/C (70%) after potential cycles.     

Enhanced optical absorption cross-section characteristics of multi-wall carbon nanotubes

The optical absorption anisotropy of multi-walled carbon nanotubes (MWCNTs) has been quantitatively characterized through the determination of the absorbance and the degree of linear polarization. A model considering the orientation of the MWCNTs and the sensitivity to both co-polarized and cross-polarized radiation, through electric field depolarization effects, was used to understand the experimental results. The MWCNT optical absorption cross-sections for both the co-polarized radiation (∼0.1 Å²/atom) and the cross-polarized radiation (∼0.05 Å²/atom) were found to be much larger than for single-walled carbon nanotubes. Our results indicate the promise of MWCNTs for applications involving radiation absorption.