Electrical and mechanical properties of carbon nanotube‐epoxy nanocomposites

In this work, electrical conductivity and thermo‐mechanical properties have been measured for carbon nanotube reinforced epoxy matrix composites. These nanocomposites consisted of two types of nanofillers, single walled carbon nanotubes (SW‐CNT) and electrical grade carbon nanotubes (XD‐CNT). The influence of the type of nanotubes and their corresponding loading weight fraction on the microstructure and the resulting electrical and mechanical properties of the nanocomposites have been investigated. The electrical conductivity of the nanocomposites showed a significantly high, about seven orders of magnitude, improvement at very low loading weight fractions of nanotubes in both types of nanocomposites. The percolation threshold in nanocomposites with SW‐CNT fillers was found to be around 0.015 wt % and that with XD‐CNT fillers around 0.0225 wt %. Transmission optical microscopy of the nanocomposites revealed some differences in the microstructure of the two types of nanocomposites which can be related to the variation in the percolation thresholds of these nanocomposites. The mechanical properties (storage modulus and loss modulus) and the glass transition temperature have not been compromised with the addition of fillers compared with significant enhancement of electrical properties. The main significance of these results is that XD‐CNTs can be used as a cost effective nanofiller for electrical applications of epoxy based nanocomposites at a fraction of SW‐CNT cost. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of annealing treatment on the structure and properties of polyurethane/multiwalled carbon nanotube nanocomposites

The thermoplastic polyurethane/multiwalled carbon nanotube (TPU/CNT) nanocomposites with high conductivity and low percolation threshold value were prepared by melting blending and annealing treatment. The effect of annealing process on the microphase structure and the properties of TPU/CNT nanocomposites was studied. It has been shown that CNT flocculation can occur in TPU/CNT nanocomposites during the annealing process. At a critical CNT content, which defined the percolation threshold, CNTs could form conductivity network. The conductive percolation threshold value of TPU/CNT nanocomposites was decreased from 10 to 4% after annealing process, and the conductivity of TPU/CNT nanocomposites with 10 vol % of CNT could reach 1.1 S/m after an annealing time of 1 h. The significant enhancement of electrical conductivity was influenced by the annealing time and the content of CNTs. The formation of CNT networks was also verified by dynamic viscoelastic characterization. The results of X‐ray diffraction and differential scanning calorimetry indicated that annealing process reinforced the microphase separation of the nanocomposites. Mechanical properties test showed that the annealing treatment was in favor of improving the mechanical properties; however, further increase in the annealing time has negative effect on the mechanical properties. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Thermoplastic vulcanizate nanocomposites based on polyethylene/reclaimed rubber: A correlation between carbon nanotube dispersion state and electrical percolation threshold

A three-step melt blending process was utilized to produce linear low-density polyethylene (LLDPE)/reclaimed rubber (RR)/carbon nanotube (CNT) nanocomposites in the presence of maleic anhydride grafted polyethylene as a compatibilizer. The effect of LLDPE/RR ratio and CNT content on the morphological, thermal, mechanical, and rheological behavior of these dynamically vulcanized LLDPE/RR nanocomposites were investigated. The morphological study showed that the RR was dispersed in the LLDPE matrix, and CNT addition led to an improved morphology as smaller RR sizes inside LLDPE were observed. The mechanical results revealed that increasing the RR content decreased the hardness, modulus of elasticity, and elongation at break while CNT improved the tensile properties and other mechanical properties. The differential scanning calorimeter analysis showed that the CNT improved the LLDPE crystallization by acting as nucleation agents. Dynamic mechanical analysis showed higher storage modulus and lower loss factor as compared to the neat blend due to mobility restrictions of the polymer chains induced by the presence of CNT. For the conditions studied, the electrical percolation threshold was found to occur at a very low CNT concentration (about 1 wt %) compared to the literature because of the specific structure produced leading to CNT residing in the LLDPE matrix and at the interface between both polymeric phases. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47795.     

Impact of injection‐molding processing parameters on the electrical,mechanical, and thermal properties of thermoplastic/carbon nanotube nanocomposites

Thermoplastic nanocomposites, based on high‐density polyethylene, polyamide 6, polyamide 66, poly(butylene terephthalate), or polycarbonate and containing multiwalled carbon nanotubes (CNTs), were compounded with either neat CNTs or commercial CNT master batches and injection‐molded for the evaluation of their electrical, mechanical, and thermal properties. The nanocomposites reached a percolation threshold within CNT concentrations of 2–5 wt %; however, the mechanical properties of the host polymers were affected. For some nanocomposites, better properties were achieved with neat CNTs, whereas for others, master batches were better. Then, polycarbonate and poly(butylene terephthalate), both with a CNT concentration of 3 wt %, were injection‐molded with a screening design of experiments (DOE) to evaluate the effects of the processing parameters on the properties of the nanocomposites. Although only a 10‐run screening DOE was performed, such effects were clearly observed. The volume resistivity was significantly dependent on the working temperature and varied up to 4 orders of magnitude. Other properties were also dependent on the processing parameters, albeit in a less pronounced fashion. Transmission electron microscopy indicated that conductive samples formed a percolation network, whereas nonconductive samples did not. In conclusion, injection‐molding parameters have a significant impact on the properties of polymer/CNT nanocomposites, and these parameters should be optimized to yield the best results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Increasing the electrical conductivity of carbon nanotube/polymer composites by using weak nanotube-polymer interactions

A weak interaction between carbon nanotubes (CNTs) and polymers was found to reduce polymer-wrapping on CNT surface, decrease the contact resistance between CNTs, and increase the electrical conductivity of their composites. Thermodynamic properties such as surface energy of components, filler-polymer interactions, and wettability of carbon/polymer systems were analyzed. It was found that the graphitized CNTs filled polyoxymethylene (POM) system exhibits the weakest CNT-polymer interaction among all the investigated systems and a poor wettability. Consequently, the graphitized CNT/POM composites possess a high electrical conductivity and a low percolation threshold of 0.5 wt.% CNT loading, which is associated with the weak CNT-polymer interaction, low contact resistance between CNTs, good connectivity of CNT networks, and high crystallinity of POM in the composites. The results obtained imply that high-performance composites with optimal CNT-network structures can be designed and fabricated by fully considering the surface properties of components and CNT-polymer interactions.     

Fabrication and electrical properties of CNT/PP conductive composites with low percolation threshold by solid state alloying

The carbon nanotube/polypropylene conductive composites with a percolation threshold as low as 0.25 wt% were fabricated by solid state alloying. This solid state alloying method uses the super‐high speed mechanical shearing (at 10,000 rpm) to process the entangled catalytically grown carbon nanotubes (CNTs) and the polymer matrix in solid state. The electrical properties of the nanocomposites and the structure and distribution of CNTs were investigated. The results indicated that via the shear‐intensive process, CNTs were truncated and dispersed effectively, and their length could be controlled properly to fully exert the advantage of high aspect ratios (length‐to‐diameter ratios). At the same time, a linear structure conductive network which may considerably lower the percolation threshold was also formed by this method. Moreover, the CNTs could be further dispersed under the action of thermo energy provided by increasing the processing temperature. The super‐high speed solid state alloying method is a favorable approach for the production of low percolation threshold conductive composites of CNTs filled high viscosity resins. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Effect of surface treatment for carbon nanotubes on morphological and rheological properties of poly(ethylene oxide) nanocomposites

Poly(ethylene oxide) nanocomposites filled with functionalized multi‐walled carbon nanotubes are prepared and characterized using rheological and morphological measurements. This study investigates how the surface treatment of carbon nanotubes (CNTs) affects the CNT dispersion state. It is found that the nanocomposites have a higher effective volume fraction than the real volume fraction of the CNTs. The dispersion state of the CNTs is identified by using field emission scanning electron spectroscope and transmission electron microscope. The rheological findings indicate that there exists a percolated network structure of the CNTs in the nanocomposites, which was confirmed by electrical conductivity measurements as well as morphological observation. POLYM. ENG. SCI., 46:1350–1357, 2006. © 2006 Society of Plastics Engineers     

Influence of dispersion states of carbon nanotubes on physical properties of epoxy nanocomposites

Effects of different dispersion states of carbon nanotubes (CNTs) on rheological, mechanical, electrical, and thermal properties of the epoxy nanocomposites were studied. The dispersion states were altered depending upon whether a solvent was employed or not. To characterize dispersion of the CNTs, field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) were used. It was found that the nanocomposites containing poorly dispersed CNTs exhibited higher storage modulus, loss modulus, and complex viscosity than ones with well dispersed CNTs. It means that the poorly dispersed CNTs/epoxy composites have, from a rheological point of view, a more solid-like behavior. Tensile strength and elongation at break of the nanocomposites with different dispersion of CNTs were measured. Both of the well and the poorly dispersed CNTs composites showed a percolation threshold of electrical conductivity at less than 0.5 wt.% CNTs loading and the former had higher electrical and thermal conductivities than the latter. Effects of the CNTs content on the physical properties were also examined experimentally. As loading of the CNTs increased, improved results were obtained. From the morphological observation by FESEM and TEM, it was found that when the solvent was not used in the CNTs dispersion process, aggregates of pristine CNTs remained in the nanocomposites.     

Enhanced electrical properties of polycarbonate/carbon nanotube nanocomposites prepared by a supercritical carbon dioxide aided melt blending method

Polycarbonate/carbon nanotube (CNT) nanocomposites were generated using a supercritical carbon dioxide (scCO₂) aided melt blending method, yielding nanocomposites with enhanced electrical properties and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Baytubes^® C 150 P CNTs were benignly deagglomerated with scCO₂ resulting in 5 fold (5X), 10X and 15X decreases in bulk density from the as-received CNTs. This was followed by melt compounding with polycarbonate to generate the CNT nanocomposites. Electrical percolation thresholds were realized at CNT loading levels as low as 0.83 wt% for composites prepared with 15X CNT using the scCO₂ aided melt blending method. By comparison, a concentration of 1.5 wt% was required without scCO₂ processing. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO₂ processing compared to direct melt blending, but was significantly worse than that of twin screw melt compounded nanocomposites reported in the literature. A rheologically percolated network was observed near the electrical percolation of the nanocomposites. The importance of maintaining longer carbon nanotubes during nanocomposite processing rather than focusing on dispersion alone is highlighted in the current efforts.     

Effects of Carbon Fillers on Crystallization Properties and Thermal Conductivity of Poly(phenylene sulfide)

Differential scanning calorimetry and polarized optical microscopy methods were used to investigate the crystallization behavior and isothermal crystallization kinetics of poly(phenylene sulfide) (PPS)/carbon nanotube (CNT) and PPS/CNT/carbon fiber (CF) composites. In this article, the influences of CNT and CF on PPS crystallization behavior are explained. The thermal conductivity properties of composites were studied using the laser flash method. The results show that CNT increased crystallization temperature and rate and thermal conductivity greatly improved at 8 wt.% CNT content. In addition, the crystallization and thermal performance of PPS are significantly improved via synergistic effects of CNT and CF in the composites.     

A new approach to determine rheological percolation of carbon nanotubes in microstructured polymer matrices

We demonstrate the rheological percolation of carbon nanotubes (CNTs) in microstructured polymer matrices. Polymer/CNT composites are fabricated from polycarbonates with different molecular weights to diversify the microstructures, which vary with the polymer radius of gyration and entanglements. We propose a model for the dispersion of CNTs in polymer matrices, which explains the electrical and rheological properties. The percolation theory represented by a power–law relation cannot account for the rheological percolation of CNTs in this work. Therefore, we investigate the crossover points to provide a quantitative indication of the rheological percolation threshold of nanofillers in polymer matrices. For the first time, the rheological percolation threshold is determined experimentally with this definition. The effects of molecular weight and shear viscosity of the medium on the percolation of CNTs are demonstrated separately.     

Network behavior of thermosetting polyimide/multiwalled carbon nanotube composites

In previous published research, network formation has been used to understand morphology and properties in polymer nanocomposites containing carbon nanotubes (CNTs) through measurements of rheological and electrical percolation thresholds, largely in thermoplastic matrices. In this research, these tools are explored as a means to understand network transport mechanisms and changes in CNT dispersion during curing in a thermosetting matrix. Specifically, rheological and electrical measurements were performed on the uncured nanocomposites, and electrical measurements were performed on the cured nanocomposites. The resulting data were applied to a percolation model. The results showed that the uncured resin played a limited role in mediating rheological transport and that little CNT aggregation occurred during curing. The results of this initial work suggest that such a combination of techniques is applicable to understanding dispersion changes resulting from curing and provides complementary insight to that provided by electron microscopy imaging of the same phenomenon.     

Rheological and conductive percolation laws for syndiotactic polystyrene composites filled with carbon nanocapsules and carbon nanotubes

Semicrystalline syndiotactic polystyrene (sPS) composites with carbon nanocapsule (CNC) and carbon nanotube (CNT) fillers were prepared and good filler dispersion confirmed by electron microscopy. Their rheological and electrical properties were investigated to reveal the effect of filler aspect ratio. Amorphous atactic polystyrene (aPS) was used to prepare composites with a CNT filler to elucidate the effect of matrix tacticity. Percolation scaling laws are applied and the threshold concentration and exponent are determined. Above a threshold, the magnitudes of storage modulus (G′) and conductivity are related to the level of percolation network as well as the intrinsic properties of the matrix and filler. Master curves are obtained provided that an appropriate percolation function is selected. Different scaling laws are validated for the G′ and conductivity results.Composites with CNTs show a much lower threshold than those with CNCs. A lower threshold is derived from the G′ results compared to that obtained from the conductivity data regardless of the filler aspect ratio and matrix tacticity. Owing to the pronounced nucleating effects of CNT, crystalline sPS composites exhibit a four times larger conductivity threshold compared to their amorphous aPS counterparts, although their rheological thresholds are similar.     

Structure and properties of polypropylene‐wrapped carbon nanotubes composite

By means of in situ graft method, polypropylene (PP)‐wrapped carbon nanotubes (CNTs) composite were prepared. Infrared spectroscopy (IR) results showed that there was covalent linkage between PP and CNTs via maleic anhydride (MAH) grafting. Owing to the uniform dispersion of CNTs and covalent adhesion between PP and CNTs, the tensile strength of PP‐wrapped CNTs composite was higher than that for neat PP by 110%, and a 74% increase as compared to the CNTs/PP (with the same CNTs content) composite. The further test showed a strong mechanical behavior with up to 113% increase in Young's modulus of the neat PP. Based on the uniform dispersion of CNTs, the electrical conductivity of PP‐wrapped CNTs composite increased sharply by up to seven orders of magnitude with 4 wt % CNT fillers. As a result, the volume resistivity was decreased with increase in the CNT content that could be governed in a percolation‐like power law with a relatively low percolation threshold. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Influence of modified carbon nanotube on physical properties and crystallization behavior of poly(ethylene terephthalate) nanocomposite

Poly(ethylene terephthalate) (PET) nanocomposites reinforced with a very small quantity of modified carbon nanotube (CNT) were prepared by melt compounding using a twin‐screw extruder. The introduction of carboxylic acid groups on the surfaces of the nanotube leads to the enhanced interactions between the nanotube and the polymer matrix through hydrogen bonding formation. The thermal stability, mechanical, and rheological properties of the PET nanocomposites are strongly dependent on the interfacial interactions between the PET and the modified CNT as well as the dispersion of the modified CNT in the PET. The introduction of the nanotube can significantly influence the non‐isothermal crystallization behavior of the PET nanocomposites. This study demonstrates that a very small quantity of the modified CNT can substantially improve the thermal stability and mechanical properties of the PET nanocomposites, depending on the dispersion of the modified CNT and the interfacial interactions between the polymer matrix and the modified CNT. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

Isotactic polypropylene–vapor grown carbon nanofibers composites: Electrical properties

Nanocomposites have been obtained by dispersing various amounts of vapor grown carbon nanofibers within isotactic polypropylene. Thermal investigations done by differential scanning calorimetry and dynamic mechanical analysis revealed the effect of the vapor grown carbon nanofibers on the melting, crystallization, α, and β relaxations. Direct current electrical features of these nanocomposites have been investigated and related to the thermal features of these nanocomposites. The effect of the loading with carbon nanofibers on the electrical properties of these nanocomposites is discussed within the percolation theory. The percolation threshold was estimated at about 5.5% wt carbon nanofibers. The temperature dependence of the direct current conductivity is analyzed in detail and it is concluded that the electronic hopping is the dominant transport mechanism. A transition from one‐dimensional hopping towards a three‐dimensional hopping was noticed as the concentration of carbon nanofibers was increased from 10% wt to 20% wt carbon nanofiber. The possibility of a differential negative resistivity is suggested. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45297.     

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     

Preparation and properties of carbon nanotube/binary‐polymer composites with a double‐segregated structure

A carbon nanotube (CNT)/poly(methyl methacrylate) (PMMA)/ultrahigh molecular weight polyethylene (UHMWPE) composite containing a double‐segregated structure was formalized by means of a facile mechanical mixing technology. In the composite, the CNTs were decorated on the surfaces of PMMA granules, and the CNTs decorated granules formed the continuous segregated conducting layers at the interfaces between UHMWPE particles. Morphology observations confirmed the formation of a specific double‐segregated CNT conductive network, resulting in an ultralow percolation threshold of ～0.2 wt %. The double‐segregated composite containing only 0.8 wt % CNT loading exhibited a high electrical conductivity of ～0.2 S m^?1 and efficient electromagnetic shielding effectiveness of ～19.6 dB, respectively. The thermal conductivity, temperature‐resistivity behaviors, and mechanical properties of the double‐segregated composites were also studied. This work provided a novel conductive network structure to attain a high‐performance conducting polymer composite at low filler loadings. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39789.     

Processing and properties of carbon nanotube/poly(methyl methacrylate) composite films

A systematic study of the reinforcement of single‐walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes, and vapor‐grown carbon nanofibers (VGCNFs) in poly(methyl methacrylate) (PMMA) is reported. SWNT/PMMA composite films with various SWNT concentrations (from 0.5 to 50 wt % with respect to the weight of PMMA) were processed from nitromethane. Two types of SWNTs were used: SWNT‐A, which contained 35 wt % metal catalyst, and SWNT‐B, which contained about 2.4 wt % metal catalyst. Properties of different nanotubes containing composites were compared with 15 wt % carbon nanotubes (CNTs). Property enhancement included electrical conductivity, mechanical properties, and solvent resistance. The thermal degradation of PMMA in the presence of CNTs in air and nitrogen environments was studied. No variation in the thermal degradation behavior of PMMA/CNT was observed in nitrogen. The peak degradation temperature increased for the composites in air at low CNT loadings. Dynamic and thermomechanical properties were also studied. At a 35 wt % SWNT loading, a composite film exhibited good mechanical and electrical properties, good chemical resistance, and a very low coefficient of thermal expansion. Property improvements were rationalized in terms of the nanotube surface area. Composite films were also characterized with Raman spectroscopy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Electrical and thermal conductivity and tensile and flexural properties of carbon nanotube/polycarbonate resins

Adding conductive carbon fillers to insulating thermoplastic polymers increases the resulting composite's electrical conductivity. Carbon nanotubes (CNTs) are very effective at increasing composite electrical conductivity at low loading levels without compromising composite tensile and flexural properties. In this study, varying amounts (2–8 wt %) of CNTs were added to polycarbonate (PC) by melt compounding, and the resulting composites were tested for electrical conductivity (1/electrical resistivity), thermal conductivity, and tensile and flexural properties. The percolation threshold was less than 1.4 vol % CNT, likely because of CNTs high aspect ratio (1000). The addition of CNT to PC increased the composite electrical and thermal conductivity and tensile and flexural modulus. The 6 wt % (4.2 vol %) CNT in PC resin had a good combination of properties for electrical conductivity applications. The electrical resistivity and thermal conductivity were 18 Ω‐cm and 0.28 W/m · K, respectively. The tensile modulus, ultimate tensile strength (UTS), and strain at UTS were 2.7 GPa, 56 MPa, and 2.8%, respectively. The flexural modulus, ultimate flexural strength, and strain at ultimate flexural strength were 3.6 GPa, 125 MPa, and 5.5%, respectively. Ductile tensile behavior is noted in pure PC and in samples containing up to 6 wt % CNT. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010