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.     

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.     

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.     

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.     

Space durable polymer/carbon nanotube films for electrostatic charge mitigation

Low color, flexible, space environmentally durable polymeric materials possessing sufficient surface resistivity (10⁶-10¹⁰ Ω/square) for electrostatic charge (ESC) mitigation are of interest for potential applications on Gossamer spacecraft as thin film membranes on antennas, large lightweight space optics, and second surface mirrors. One method of incorporating intrinsic ESC mitigation while maintaining low color, flexibility, and optical clarity is through the utilization of single-walled carbon nanotubes (SWNTs). However, SWNTs are difficult to uniformly disperse in the polymer matrix. The approach reported herein employed amide acid polymers endcapped with alkoxysilane groups that could condense with oxygen containing functionalities that were present on the ends of SWNTs as a result of the oxidative purification treatment. These SWNTs were combined with the endcapped amide acid polymers in solution and subsequently cast as unoriented thin films. Two examples possessed electrical conductivity (measured as surface resistance and surface resistivity) sufficient for ESC mitigation at loading levels of ≤0.08 wt% SWNT as well as good retention of thermo-optical properties. The percolation threshold was determined to lie between 0.03 and 0.04 wt% SWNT loading. Electrical conductivity of the film remained unaffected even after harsh mechanical manipulation.     

Mechanical properties and electrical conductivity of carbon-nanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene

Composites of polyamide-6 and carbon nanotubes (NT) have been prepared on a corotating twinscrew extruder. It is shown by transmission electron microscopy (TEM) that the nanotubes are dispersed homogeneously in the polyamide-matrix. The electrical conductivity of these composites was analyzed and compared to carbon black filled polyamide-6. It is found that the NT-filled polyamide-6 shows an onset of the electrical conductivity at low filler loadings (4-6 wt%). In agreement with rheological measurements this onset in the conductivity is attributed to a percolation of nanotubes in the insulating matrix polymer. Tensile tests of the NT-composites show a significant increase of 27% in the Young's modulus, however the elongation at break of these materials dramatically decreases due to an embrittlement of the polyamide-6. Blends of these composites and Acrylonitrile/butadiene/styrene (ABS) have been prepared by extrusion. It is shown by TEM measurements that the nanotubes are selectively located in the polyamide-6. These selectively filled polyamide-6/ABS-blends show a highly irregular, cocontinuous morphology. Due to the confinement of the conductive filler to one blend component these materials show an onset in the electrical conductivity at very low filler loadings (2-3 wt%). These findings are explained by a double percolation effect. The NT-filled blends show superior mechanical properties in the tensile tests and in IZOD notched impact tests.     

Magnetic field alignment and electrical properties of solution cast PET-carbon nanotube composite films

Single-wall carbon nanotubes (SWNTs) were dispersed in a polyethylene terephthalate (PET) matrix by solution blending and then cast onto a glass substrate to create flexible films. Various SWNT loading concentrations were implemented (0.5, 1.0, and 3.0 wt%), and the processing method was repeated to produce films in the presence of magnetic fields (3.0 and 9.4 T). Alignment of the SWNTs in the PET matrix was characterized by Raman spectroscopy. Impedance spectroscopy was utilized to study the electrical behavior of the nanocomposites. It was concluded that SWNT concentration and dispersion are the key variables for improving electrical conductivity, while alignment plays a secondary role. Interestingly, it appears that a magnetic field may prove to be a novel method for improving the dispersion of unmodified SWNTs by disrupting van der Waals interactions.     

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.     

Electrical and rheological percolation in poly(vinylidene fluoride)/multi‐walled carbon nanotube nanocomposites

Nanocomposites of poly(vinylidene fluoride) (PVDF) and multi‐walled carbon nanotubes (MWCNTs) were prepared through melt blending in a batch mixer (torque rheometer equipped with a mixing chamber). The morphology, rheological behavior and electrical conductivity were investigated through transmission electron microscopy, dynamic oscillatory rheometry and the two‐probe method. The nanocomposite with 0.5 wt% MWCNT content presented a uniform dispersion through the PVDF matrix, whereas that with 1 wt% started to present a percolated network. For the nanocomposites with 2 and 5 wt% MWCNTs the formation of this nanotube network was clearly evident. The electrical percolation threshold at room temperature found for this system was about 1.2 wt% MWCNTs. The rheological percolation threshold fitted from viscosity was about 1 wt%, while the threshold fitted from storage modulus was 0.9 wt%. Thus fewer nanotubes are needed to approach the rheological percolation threshold than the electrical percolation threshold. Copyright © 2010 Society of Chemical Industry     

Pentadecane functionalized graphite oxide sheets as a tool for the preparation of electrical conductive polyethylene/graphite oxide composites

Covalent functionalization of pentadecane-decorated thermally reduced graphite oxide (GO) sheets has been studied as a tool for the preparation of polyethylene/GO composites exhibiting rheological and electrical percolation thresholds. It was accomplished through pentadecane based radical addition onto unsaturated bonds located on the GO sheets' surface using dicumyl peroxide as hydrogen abstractor. This chemical functionalization influences the affinity of the formed pentadecane grafted GO sheets for various solvents. Then, the compounding of the composites pentadecane grafted GO/PE was performed at a processing temperature of 140 °C with 25, 20, 15, 10, 8 and 5 wt% loadings. Rheological and electrical percolation thresholds were found between 10 and 15 wt% for polyethylene/pentadecane functionalized graphene oxide composites while the composite graphite/PE at the same loading percentage did not reach any percolation threshold.     

A large increase in the thermal conductivity of carbon nanotube/polymer composites produced by percolation phenomena

We have studied the thermal conductivity of carbon nanotube/polymer composites as a function of the CNT volume fraction using a steady-state measurement technique. The results show a large increase in the thermal conductivity at a small loading of carbon nanotube volume fraction (ca. 1.4 vol.%). The remarkably high increase in the thermal conductivity is described well by thermal transport through networks of carbon nanotubes in the polymer matrix, following a critical power law indicating percolating behavior, which shows the thermal percolation in the vicinity of the electrical percolation threshold concentration.     

Influence of injection molding on the electrical properties of polyamide 12 filled with multi-walled carbon nanotubes

Microinjection-molded and compression-molded polyamide (PA12) matrix composites filled with 0.67, 1.33, 2 and 4 wt% multi-walled carbon nanotubes (MWNTs) were prepared from twin-screw extruded pellets. The compression molded samples have an electrical percolation threshold close to 1.2 wt%. Coupled rheological and electrical measurements show that their electrical properties start decreasing as soon as shear begins and are partially restored during flow, suggesting successively breakage and reconstruction of a percolating network. On the other hand, the electrical properties of the microinjection molded composites are close to the matrix ones, showing that cooling is too fast for the MWNTs to form a network. There is some electrical anisotropy in these composites, as evidenced by a greater conductivity measured in the flow direction. However polarized Raman spectroscopy analysis does not reveal a significant orientation of the MWNTs.     

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.     

Conductivity and mechanical properties of well-dispersed single-wall carbon nanotube/polystyrene composite

The morphologies, electrical and mechanical properties and structure of polystyrene (PS) composites with varying concentrations of single-wall carbon nanotubes (SWNT) are analyzed. Using Raman spectroscopy and electron microscopy, we demonstrate that initial thermal annealing of SWNT significantly improves their dispersion in PS. In dielectric measurements, the annealed SWNT/PS composites show higher electrical conductivity and a lower percolation threshold (less than 0.3 wt%) than the raw SWNT/PS composites, which provides further evidence of good dispersion of the annealed SWNT in PS. Raman spectra of composites under tension show good transfer of an applied stress from the polymer matrix to SWNT. However, mechanical moduli of the annealed SWNT/PS composites are only increased slightly. The reason for this discrepancy remains unclear.     

Electrical,rheological and electromagnetic interference shielding properties of thermoplastic polyurethane/carbon nanotube composites

Electrically conducting rubbery composites based on thermoplastic polyurethane (TPU) and carbon nanotubes (CNTs) were prepared through melt blending using a torque rheometer equipped with a mixing chamber. The electrical conductivity, morphology, rheological properties and electromagnetic interference shielding effectiveness (EMI SE) of the TPU/CNT composites were evaluated and also compared with those of carbon black (CB)‐filled TPU composites prepared under the same processing conditions. For both polymer systems, the insulator–conductor transition was very sharp and the electrical percolation threshold at room temperature was at CNT and CB contents of about 1.0 and 1.7 wt%, respectively. The EMI SE over the X‐band frequency range (8–12 GHz) for TPU/CNT and TPU/CB composites was investigated as a function of filler content. EMI SE and electrical conductivity increased with increasing amount of conductive filler, due to the formation of conductive pathways in the TPU matrix. TPU/CNT composites displayed higher electrical conductivity and EMI SE than TPU/CB composites with similar conductive filler content. EMI SE values found for TPU/CNT and TPU/CB composites containing 10 and 15 wt% conductive fillers, respectively, were in the range ?22 to ?20 dB, indicating that these composites are promising candidates for shielding applications. © 2013 Society of Chemical Industry     

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.     

Electrical conductivity and rheological behavior of multiphase polymer composites containing conducting carbon black

Multiphase polymer composites of carbon black (CB), polypropylene (PP) and low density polyethylene (LDPE) were prepared by melt‐mixing method to reduce the amount of CB in the conductive composites. SEM images showed that CB preferably located in LDPE phase and formed electrically conductive path. The measurement of conductive properties showed that the ternary materials possessed lower percolation than binary composites of CB/PP or CB/LDPE, the former was ～6 wt% and the latter was 9–10 wt%. Positive temperature coefficient (PTC) effects of the binary and ternary composites were investigated, indicating that the latter exhibited a relatively high PTC intensity. A rheological percolation estimated by a power law function is 2.66 wt% of CB loading, suggesting an onset of solid‐like behavior at low frequencies. This difference between the electrical and rheological percolation thresholds may be understood in terms of the smaller CB–CB distance required for electrical conductivity as compared with that required to impede polymer mobility. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Rheology and properties of melt-processed poly(ether ether ketone)/multi-wall carbon nanotube composites

Poly(ether ether ketone) (PEEK)/multi-wall carbon nanotube (MWNT) composites containing up to 17 wt% filler were prepared using a twin screw extruder. Transmission electron microscopy (TEM) images reveal that the MWNTs were homogeneously dispersed in the PEEK matrix. Linear viscoelastic measurements show that both complex viscosity and moduli increase with increasing MWNT concentration. The storage modulus, G^′ exhibits a dramatic seven order increase in magnitude around 1 wt%, leading to a solid-like low-frequency behaviour at higher loadings; the effect can be attributed to network formation at a rheological percolation threshold. Rheotens measurements show that the melt strength also increases significantly on addition of nanotubes, however, the drawability decreases. An analytical Wagner model was used to calculate the apparent elongational viscosity over a wide range of elongational rates, and to reveal significant increases on addition of MWNTs, with a similar threshold behaviour. The electrical response is also dominated by percolation effects, increasing by nearly 10 orders of magnitude from 10⁻¹¹ to 10⁻¹ S/cm, on the addition of only 2 wt% MWNTs. In contrast, the thermal conductivity and tensile elastic modulus of the composites increased linearly with nanotube content, rising by 130% and 50%, at 17 wt% MWNTs, respectively.     

填充型导电高分子复合材料的逾渗理论进展

总结了粒子填充型导电体系逾渗网络的形成及逾渗理论概况，以热力学和动力学的观点对导电粒子种类、形状、浓度、分布和分散、基体树脂的种类、粒子与基体树脂的相互作用、加工条件等影响逾渗的重要因素作了系统的分析，阐明了体系的导电性、微观形貌、流变性能和物理性能的相互关系。对低逾渗值填充聚合物尤其是炭黑复合材料的成型与加工提供了理论指导。     

Dispersion of carbon nanotubes through amphiphilic block copolymers: Rheological and dielectrical characterizations of poly(ethylene oxide) composites

In this article, we report on some properties of polymer nanocomposites prepared from dispersions of multiwall carbon nanotubes (CNT) in aqueous solution prepared using amphiphilic block copolymers. These nanocomposites are made of polyethylene oxide as matrix and CNT wrapped with copolymers as fillers. We investigated the rheological and electrical behavior of such composites with the objectives of underlined the effect of wrapping. Two rheological and only one electrical percolation thresholds have been observed and related to polymer–CNT and CNT–CNT networks. The low values of these percolation thresholds agree with a homogeneous dispersion of CNT in the matrix. We also demonstrated that specific wrapping may induce an increase of electrical conductivity without affecting too much the viscosity of the melt. POLYM. COMPOS., 2012. © 2011 Society of Plastics Engineers