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.     

Dielectric spectroscopy on melt processed polycarbonate—multiwalled carbon nanotube composites

Complex permittivity and related AC conductivity measurements in the frequency range between 10⁻⁴ and 10⁷ Hz are presented for composites of polycarbonate (PC) filled with different amounts of multiwalled carbon nanotubes (MWNT) varying in the range between 0.5 and 5 wt%. The composites were obtained by diluting a PC based masterbatch containing 15 wt% MWNT by melt mixing using a Micro Compounder. From DC conductivity measurements it was found that for samples processed at a mixing screw speed of 150 rpm for 5 min, the percolation occurs at a threshold concentration (p_c) between 1.0 and 1.5 wt% MWNT. For concentrations of MWNT near the percolation threshold, the processing conditions (screw speed and mixing time) were varied. The differences in the dispersion of the MWNT in the PC matrix could be detected in the complex permittivity and AC conductivity spectra, and have been explained by changes in p_c. The AC conductivity and permittivity spectra are discussed in terms of charge carrier diffusion on percolation clusters and resistor-capacitor composites.     

The influence of annealing on the crystallization and tribological behavior of MWNT/PEEK nanocomposites

In this study, annealing influence on crystallization and scratch behavior of neat and multi‐wall carbon nanotube (MWNT) reinforced poly(ether ether ketone) (PEEK) nanocomposites have been investigated. Crystallization behavior of normal and annealed samples was investigated by using differential scanning calorimeter (DSC). Scratch behavior of normal and annealed samples was investigated by using micro scratch tester. In DSC analysis, it was detected that, melting enthalpy of annealed neat PEEK was increased sharply when compared to neat PEEK. Melting enthalpies of annealed PEEK nanocomposites prepared with addition of up to 1 wt% MWNT were increased with a decreased trend. However, nanocomposites with higher contents of MWNTs (>1 wt%) were dramatically affected by annealing process and melting enthalpy decreased sharply. Friction coefficient values of “annealed MWNT reinforced PEEK composites” were found to be lower than “normal PEEK composites.” Annealing process affects scratch hardness of both annealed and MWNT reinforced PEEK. Annealed nanocomposites with various MWNT concentrations showed higher scratch hardness values than normal PEEK nanocomposites. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Rheological behavior of multiwalled carbon nanotube/polycarbonate composites

The rheological behavior of compression molded mixtures of polycarbonate containing between 0.5 and 15 wt% carbon nanotubes was investigated using oscillatory rheometry at 260 °C. The nanotubes have diameters between 10 and 15 nm and lengths ranging from 1 to 10 μm. The composites were obtained by diluting a masterbatch containing 15 wt% nanotubes using a twin-screw extruder. The increase in viscosity associated with the addition of nanotubes is much higher than viscosity changes reported for carbon nanofibers having larger diameters and for carbon black composites; this can be explained by the higher aspect ratio of the nanotubes. The viscosity increase is accompanied by an increase in the elastic melt properties, represented by the storage modulus G′, which is much higher than the increase in the loss modulus G″. The viscosity curves above 2 wt% nanotubes exhibit a larger decrease with frequency than samples containing lower nanotube loadings. Composites containing more than 2 wt% nanotubes exhibit non-Newtonian behavior at lower frequencies. A step increase at approximately 2 wt% nanotubes was observed in the viscosity-composition curves at low frequencies. This step change may be regarded as a rheological threshold. Ultimately, the rheological threshold coincides with the electrical conductivity percolation threshold which was found to be between 1 and 2 wt% nanotubes.     

Densified multiwalled carbon nanotubes–titanium nitride composites with enhanced thermal properties

Densified multiwalled carbon nanotube (MWNT)–TiN composites with various MWNTs contents were successfully obtained through a spark plasma sintering (SPS) method. The thermal conductivity k was found to increase with the MWNT amount and temperature. In the presence of 5 wt% MWNTs, there was a 97% enhancement in k at 703 K compared with that of TiN. The high thermal conductivity of MWNTs, a good interfacial combination and a homogeneous dispersion of MWNTs are key issues to enhance the thermal conductivity of MWNT–TiN composites.     

Polyethylene multiwalled carbon nanotube composites

Polyethylene (PE) multiwalled carbon nanotubes (MWCNTs) with weight fractions ranging from 0.1 to 10 wt% were prepared by melt blending using a mini-twin screw extruder. The morphology and degree of dispersion of the MWCNTs in the PE matrix at different length scales was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). Both individual and agglomerations of MWCNTs were evident. An up-shift of 17 cm⁻¹ for the G band and the evolution of a shoulder to this peak were obtained in the Raman spectra of the nanocomposites, probably due to compressive forces exerted on the MWCNTs by PE chains and indicating intercalation of PE into the MWCNT bundles. The electrical conductivity and linear viscoelastic behaviour of these nanocomposites were investigated. A percolation threshold of about 7.5 wt% was obtained and the electrical conductivity of PE was increased significantly, by 16 orders of magnitude, from 10⁻²⁰ to 10⁻⁴ S/cm. The storage modulus (G′) versus frequency curves approached a plateau above the percolation threshold with the formation of an interconnected nanotube structure, indicative of ‘pseudo-solid-like’ behaviour. The ultimate tensile strength and elongation at break of the nanocomposites decreased with addition of MWCNTs. The diminution of mechanical properties of the nanocomposites, though concomitant with a significant increase in electrical conductivity, implies the mechanism for mechanical reinforcement for PE/MWCNT composites is filler-matrix interfacial interactions and not filler percolation. The temperature of crystallisation (T_c) and fraction of PE that was crystalline (F_c) were modified by incorporating MWCNTs. The thermal decomposition temperature of PE was enhanced by 20 K on addition of 10 wt% MWCNT.     

Electrical and mechanical properties of polyimide-carbon nanotubes composites fabricated by in situ polymerization

Polyimide (PI)-carbon nanotubes composites were fabricated by in situ polymerization using multi wall carbon nanotubes (MWNT) as fillers. It suggested that in situ polymerization is an ideal technique to make a perfect dispersion of carbon nanotubes into matrixes. Besides it, the pre-treatment of carbon nanotubes in solvent to make the networks untied enough and to let solvent percolated into the networks is very important for forming uniform entanglements between carbon nanotubes and polymer molecular chains. The results imply that the percolation threshold for the electric conductivity of the resultant PI-MWNT composites was ca. 0.15 vol%. The electrical conductivity has been increased by more than 11 orders of magnitude to 10⁻⁴ S/cm at the percolation threshold. The mechanical properties of the polyimide composite were not improved significantly by addition of carbon nanotubes.     

Dispersion of multi‐walled carbon nanotubes in poly(aryl ether ketone) obtained by in situ polymerization

BACKGROUND: Recently, much work has focused on the efficient dispersion of carbon nanotubes (CNTs) throughout a polymer matrix for mechanical and/or electrical matrices. However, CNTs used as enhancement inclusions in a high‐performance polymer matrix, especially in poly(aryl ether ketone) (PAEK), have rarely been reported. Therefore, multi‐walled carbon nanotube (MWNT)‐modified PAEK nanocomposites were synthesized by in situ polymerization of monomers of interest in the presence of pre‐treated MWNTs. RESULTS: This process enabled a uniform dispersion of MWNT bundles in the polymer matrix. The resultant MWNT/PAEK nanocomposite films were optically transparent with significant mechanical enhancement at a very low MWNT loading (0.5 wt%). CONCLUSION: These MWNT/polymer nanocomposites are potentially useful in a variety of aerospace and terrestrial applications, due to the combination of excellent properties of MWNTs with PAEK. Copyright © 2009 Society of Chemical Industry     

Ultrahigh-shear processing for the preparation of polymer/carbon nanotube composites

Poly(vinylidene fluoride) (PVDF)/multiwalled carbon nanotube (MWCNT) composites were prepared using a novel ultrahigh-shear extruder by directly mixing MWCNT with PVDF in the molten state. A special feedback-type screw was used to obtain a high shear field and obtain a very uniform dispersion of the nanotubes in the polymer matrix under a higher screw rotation speed. Raman spectroscopy and scanning electron microscopy were used to determine the interaction and dispersion of nanotubes in the PVDF. The linear viscoelastic behavior and electrical conductivity of these composites were investigated. At low-frequencies, the storage shear modulus (G′) becomes almost independent of the frequency as nanotube loading increases, suggesting the onset of solid-like behavior in these composites. By plotting G′ vs. nanotube loading and fitting with a power-law function, we found that the rheological threshold of high-shear processed composites is about 0.96 wt% whereas that of low-shear processed composites is about 1.76 wt%. The electrical percolation threshold of high-shear processed composites is lower than that of low-shear processed composites.     

Effect of dihydroxydiphenyl ether on poly(ether ether ketone)

A series of modified poly(ether ether ketone) (PEEK) polymers were synthesized by introduction of addition ether groups from dihydroxydiphenyl ether (DHDE) into the PEEK structure. The inherent viscosity of the DHDE-modified PEEK increased with reaction time at 320 °C. DSC thermograms showed the melting points of the obtained PEEK decreased with the increase of the DHDE content in the backbone. The degradation temperature (T_d) was slightly decreased by the introduction of DHDE. The crystallinity as measured via the X-ray diffraction (XRD) increases with the introduction of DHDE into the modified PEEK. The crystalline structure was identified as an orthorhombic structure with lattice constants a = 7.72 Å, b = 5.86 Å, and c = 10.24 Å. Due to the glass transition temperature (T_g) and the melting temperature (T_m) decreasing with the increase of the DHDE content in the reaction system. the processability of the resultant PEEK could be improved through this DHDE modification.     

Low percolation thresholds of electrical conductivity and rheology in poly(ethylene terephthalate) through the networks of multi-walled carbon nanotubes

Coagulation method was first used to prepare nanocomposites of multi-wall carbon nanotubes (MWNT) and poly(ethylene terephthalate) (PET). The morphology of nanocomposites is characterized using transmission electronic microscopy and scanning electronic microscopy. A coating on MWNT by PET chains is observed by comparison of micrographs of purified MWNT and MWNT encapsulated by PET chains in the nanocomposites, and this coating is considered as evidence of interfacial interaction between MWNT and PET chains. Both electrical conductivity and rheological properties have been well characterized. With increasing MWNT loading, the nanocomposites undergo transition from electrically insulative to conductive at room temperature, while the melts show transition from liquid-like to solid-like viscoelasticity. The percolation threshold of 0.6 wt% (based on viscosity) for rheological property and 0.9 wt% for electrical conductivity has been found. The low percolation threshold results from homogeneous dispersion of MWNT in PET matrix and high aspect ratio of MWNT. The less rheological percolation threshold than electrical percolation threshold is mainly attributed to the fact that a denser MWNT network is required for electrical conductivity, while a less dense MWNT network sufficiently impedes PET chain mobility related to the rheological percolation threshold.     

Sulfonated poly(ether ether ketone) membranes for electric double layer capacitors

Sulfonated poly(ether ether ketone) (S-PEEK) with different degree of sulfonation (DS) has been prepared and evaluated as a proton conducting membrane for electric double layer capacitor (EDLC). The polymer electrolytes prepared with S-PEEK membrane exhibited ionic conductivities about 1.2 × 10⁻³-4.5 × 10⁻³ S cm⁻¹ at room temperature, which depended on both soaking solvent and degree of sulfonation. The quasi-solid-state EDLCs consisted of activated carbon electrodes and S-PEEK membrane were assembled, and their electrochemical characteristics were studied by cyclic voltammetry and charge-discharge cycle tests. The effect of DS on the electrochemical performances of EDLCs has been investigated.     

Conductivity spectroscopy on melt processed polypropylene-multiwalled carbon nanotube composites: Recovery after shear and crystallization

Frequency dependent investigations of conductivity and dielectric permittivity have been performed on composites of polypropylene (PP) containing different amounts of 2, 3.5, and 5 wt% of multiwalled carbon nanotubes (MWNTs) in the melt and during crystallization. The experiments were performed in a measurement slit die containing two dielectric sensors in plate-plate geometry, which was flanged to the outlet of a single screw laboratory extruder. AC conductivity and the related complex permittivity were measured in the frequency range from 20 Hz to 10⁶ Hz after stopping the extruder (recovery after shearing) and during cooling (non-isothermal crystallization). For a sample with a MWNT content of 2 wt% the AC conductivity shows a tremendous increase with time after shearing was stopped. This conductivity recovery is explained by the reorganization of the conducting network-like filler structure, which was partially destroyed by the shear. The reformation kinetics of filler clusters is assumed to be due to a cooperative aggregation. For conductive fillers in a thermoplastic matrix the kinetics of cooperative aggregation is coupled to the electrical percolation. The reorganization of the percolation network can be related to reformation of (i) the local contact regions between the nanotubes (separated by polymer chains) and (ii) to the reorientation of nanotubes oriented in the shear flow. The conductivity recovery is less pronounced for samples with MWNT concentrations well above the percolation threshold. During cooling of the melt to temperatures below crystallization a significant decrease in the conductivity and permittivity was detected. This is consistently expressed in the conductivity and permittivity spectra and can be explained by reduction of the amorphous phase (high ion mobility) on expense of the crystalline phase and/or by crystalline regions in the contact region between tubes.     

Dispersion of pristine single-walled carbon nanotubes using pyrene-capped polystyrene and its application for preparation of polystyrene matrix composites

A pyrene-capped polystyrene (PyPS) is synthesized by an anionic polymerization method and acts as dispersant for dispersion of pristine single-walled carbon nanotubes (SWCNTs). Through a well-known π-stacking interaction confirmed qualitatively by proton nuclear magnetic resonance and fluoroscopic analyses, PyPS is strongly but noncovalently adsorbed onto the nanotube surface, affording highly uniform and stable SWCNT dispersion in chloroform with the nanotube content as high as 250 ± 30 mg L⁻¹. Since no direct chemical reaction takes place on the nanotubes, their intrinsic electronic structure is maintained, thus ensuring them as functional fillers for application in conductive polymer composites. The so-obtained dispersion is subsequently used to prepare polystyrene matrix composites. A solution-based process adopted here preserves the good nanotube dispersing state in dispersion into the composites. Hence, the resultant composites show good optical transmittance and a low electrical percolation threshold of 0.095 wt.% SWCNTs. In comparison, the composites with absence of PyPS prepared by the same process have a relatively high percolation threshold of 0.28 wt.% SWCNTs.     

Influence of twin-screw extrusion conditions on the dispersion of multi-walled carbon nanotubes in a poly(lactic acid) matrix

Twin-screw extrusion using a co-rotating Berstorff ZE25 extruder was applied to disperse multi-walled carbon nanotubes (MWNT) in poly(lactic acid) (PLA). The masterbatch dilution technique was used whereas four different masterbatches were produced under variation of MWNT content, screw profile, temperature profile, and rotation speed which then were diluted to composites with 0.75 wt% MWNT under varied process conditions. The state of dispersion was investigated by light microscopy from which a dispersion index was quantified. Transmission electron microscopy was performed to observe the MWNT dispersion and network formation in the sub-micron scale.The state of MWNT dispersion within the diluted composites was predominated by the state of filler dispersion in the masterbatches. High rotation speed (500 rpm) that still ensures a certain residence time of the melt combined with a screw profile containing mainly mixing elements were found to be highly convenient to disperse and distribute the MWNT in the PLA matrix as well during masterbatch production as the dilution step. The temperature profile showed less influence, however, an increasing profile resulted in slightly better nanotube dispersions. By means of these processing conditions a percolation set was performed indicating an electrical percolation threshold below 0.5 wt% MWNT content as measured on compression molded samples.     

Low percolation threshold in single-walled carbon nanotube/high density polyethylene composites prepared by melt processing technique

A new method was developed to disperse carbon nanotubes (CNTs) in a matrix polymer and then to prepare composites by melt processing technique. Due to high surface energy and strong adsorptive states of nano-materials, single-walled carbon nanotubes (SWNTs) were adsorbed onto the surface of polymer powders by spraying SWNT aqueous suspected solution onto fine high density polyethylene (HDPE) powders. The dried SWNTs/powders were blended in a twin-screw mixture, and the resulting composites exhibited a uniformly dispersion of SWNTs in the matrix polymer. The electrical conductivity and the rheological behavior of these composites were investigated. At low frequencies, complex viscosities become almost independent of the frequency as nanotubes loading being more than 1.5 wt%, suggesting an onset of solid-like behavior and hence a rheological percolation threshold at the loading level. However, the electrical percolation threshold is ∼4 wt% of nanotube loading. This difference in the percolation thresholds is understood in terms of the smaller nanotube-nanotube distance required for electrical conductivity as compared to that required to impede polymer mobility. The measurements of mechanical properties indicate that this processing method can obviously improve the tensile strength and the modulus of the composites.     

Dynamic mechanical and morphological properties of polycarbonate/multi-walled carbon nanotube composites

Dynamic mechanical and morphological properties of the polycarbonate (PC)/multi-walled carbon nanotube (MWNT) composites were studied by dynamic mechanical thermal analysis (DMTA) and X-ray diffractometry, respectively. For the without annealed PC/MWNT composites containing the higher content of the MWNT (≥7.0 wt%), double tan δ peaks were observed, which could be explained by the phase separation morphology model. For the annealed PC/MWNT composites, a broad single tan δ peak was observed. From the X-ray diffraction of the annealed PC/MWNT composites, it was observed that more regular structure of the PC was obtained, which was consistent with the result of the thermal analysis of the annealed PC/MWNT composites. From the dynamic mechanical properties, thermal analysis, and X-ray diffraction of the annealed PC/MWNT composites, it is suggested that PC/MWNT composites show a broad single tan δ peak and partially crystalline structure of the PC in the PC/MWNT composites by annealing.     

Ultra-low electrical percolation threshold in carbon-nanotube-epoxy composites

Epoxy composites based on aligned CVD-grown multi-wall carbon nanotubes with weight fractions ranging from as low as 0.001 up to 1 wt% were produced. The resulting electrical properties were analysed by AC impedance spectroscopy. The composite conductivity σ follows a percolation scaling law of the form σ∝(p−p_c)^t with the critical mean concentration p_c to form a conductive network of approximately 0.0025 wt% and an exponent, t, of 1.2. The results are compared to previous studies investigating the percolation behaviour of entangled carbon nanotubes and spherical carbon black particles in the same matrix processed under similar conditions. The experimental percolation threshold for the aligned nanotubes used in this study represents the lowest threshold observed for carbon-nanotube-based polymer composites yet reported.     

Enhancement of the mechanical properties of poly(styrene-co-acrylonitrile) with poly(methyl methacrylate)-grafted multiwalled carbon nanotubes

Poly(methyl methacrylate) (PMMA) was grafted onto multiwalled carbon nanotubes (MWNTs). Composites of PMMA-grafted MWNTs and poly(styrene-co-acrylonitrile) (SAN) were prepared by solution casting from tetrahydrofuran. Since PMMA is miscible with SAN, the two polymers mix intimately to facilitate the dispersion of PMMA-grafted MWNTs in the SAN matrix. The intimate mixing is evidenced by the transparency of the composites. The incorporation of PMMA-grafted MWNTs to SAN (effective MWNT content=0.5-2 wt%) leads to increases in storage modulus at 40 °C, Young's modulus, tensile strength, ultimate strain, and toughness by 90, 51, 99, 184 and 614%, respectively. Such simultaneous increases in stiffness, strength, ductility and toughness of a polymer by rigid fillers are rarely observed.     

Filler reaggregation and network formation time scale in extruded high‐density polyethylene/multiwalled carbon nanotube composites

A high‐density polyethylene (HDPE) masterbatch containing 20.2 wt% multiwalled carbon nanotubes (MWNTs) was melt diluted with neat HDPE using two different methods: a twin screw microcompounder and a single‐screw extruder. The electrical properties of these composites were assessed using bulk electrical conductivity measurements, their mechanical properties were evaluated using tensile tests and dynamic mechanical analysis (DMA), and percent crystallinity was determined by wide angle x‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). A percolation threshold (p_c) of 4.5 wt% MWNTs was found in compression‐molded samples. Extruded samples were prepared with nanotube concentrations below and above the compression‐molded percolation threshold (2 and 7 wt% MWNTs) and passed through the extruder twice before entering a low‐shear melt annealing zone. Different melt annealing times were used and their effects on the electrical and mechanical properties of the resulting quench‐cooled composites were evaluated. Results showed that extruded composites were nonconductive, indicating that a conductive nanotube network did not form on the time scale of these experiments. Annealing time also did not affect significantly the mechanical properties of the resulting solid composites. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers