Multiwalled carbon nanotube polymer composites: Synthesis and characterization of thin films

The aim of this article was to elucidate the basic relationships between processing conditions and the mechanical and electrical properties of multiwalled carbon nanotube reinforced polymer composites. In conventional chopped fiber reinforced polymer composites, uniform distributions of fibers throughout the matrix are critical to producing materials with superior physical properties. Previous methods have dispersed carbon nanotubes by aggressive chemical modification of the nanotubes or by the use of a surfactant prior to dispersion. 1 , 2 Here, ultrasonic energy was used to uniformly disperse multiwalled nanotubes (MWNTs) in solutions and to incorporate them into composites without chemical pretreatment. Polystyrene (PS) solutions containing MWNTs were cast and spun to yield thin film MWNT composites. The rheology of PS/MWNT suspensions was modeled using the Carreau equation. MWNTs were found to align at the shear rates generated by the spin casting process. The tensile modulus and strain to failure of samples compared well to classical micromechanical models, increasing with MWNT loading. The composite films showed lower strains at the yield stress than neat PS films. The presence of MWNTs at 2.5 vol % fraction approximately doubles the tensile modulus, and transforms the film from insulating to conductive (surface resistivity, ρ, approaching 10³ Ω/□). © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2660–2669, 2002     

Preparation and characterization of alkylated carbon nanotube/polyimide nanocomposites

BACKGROUND: The development of carbon nanotube‐reinforced composites has been impeded by the difficult dispersion of the nanotubes in polymers and the weak interaction between the nanofiller and matrices. Efficient dispersion of carbon nanotubes is essential for the formation of a functional nanotube network in a composite matrix. RESULTS: Multiwalled carbon nanotubes (MWNTs) were incorporated into a polyimide matrix to produce MWNT/polyimide nanocomposites. To disperse well the MWNTs in the matrix and thus improve the interfacial adhesion between the nanotubes and the polymer, ‘branches’ were grafted onto the surface of the nanotubes by reacting octadecyl isocyanate with carboxylated MWNTs. The functionalized MWNTs were suspended in a precursor solution, and the dispersion was cast, followed by drying and imidization to obtain MWNT/polyimide nanocomposites. CONCLUSION: The functionalized MWNTs appear as a homogeneous dispersion in the polymer matrix. The thermal stability and the mechanical properties are greatly improved, which is attributed to the strong interactions between the functionalized MWNTs and the polyimide matrix. Copyright © 2009 Society of Chemical Industry     

Mechanical behavior of phenolic-based composites reinforced with multi-walled carbon nanotubes

Two types of carbon nanotubes (CNTs), the network multi-walled nanotubes (MWNTs) and the dispersed MWNTs, were used for fabricating MWNTs/phenolic composites. The MWNTs were synthesized using the floating catalyst method through the chemical vapor deposition process. The effects of the MWNT content on the mechanical properties of the composites were investigated. Modified Halpin-Tsai equation was proposed to evaluate the Young’s modulus and tensile strength of the MWNTs/phenolic composites by adopting an orientation factor and an exponential shape factor in the equation. It is found that the results obtained from the modified Halpin-Tsai equation on tensile strengths and Young’s moduli fit successfully the experimental ones. The tensile fracture surfaces of MWNTs/phenolic composites were examined using field emission scanning electron microscope to study the failure morphologies of the MWNTs/phenolic composites.     

Pseudoreinforcement effect of multiwalled carbon nanotubes in epoxy matrix composites

The carbon nanotube possesses outstanding physical properties. Theoretically, adding carbon nanotubes into a polymer matrix can remarkably improve the mechanical properties of the polymer matrix. In the present work, a series of composites was prepared by incorporating multiwalled carbon nanotubes (MWNTs) into an epoxy resin. The influences of MWNT content and curing temperature on the flexural properties of the epoxy resin were investigated. The results showed that a very low MWNT content should be used to ensure homogeneous dispersion of MWNTs in the epoxy matrix. A higher MWNT content may lead to deteriorated mechanical properties of the composites because of the aggregation of MWNTs. A decline in the flexural properties of the neat epoxy resin with increasing curing temperature was found. However, under the same curing conditions, improvement in flexural properties was observed for the composite with the low MWNT content and a mild curing temperature. The improvement was far beyond the predictions of the traditional short‐fiber composite theory. In fact, this improvement should be attributed to the retarding effect of MWNTs on the curing reaction of epoxy matrix. Therefore, the improvement in the flexural properties was only a pseudoreinforcement effect, not a nano‐reinforcement effect of the MWNTs on the epoxy resin. Perhaps, it is better for MWNTs to be used as functional fillers, such as electrical or thermal conductive fillers, than as reinforcements. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3664–3672, 2006     

Incorporation of 4‐aminobenzene functionalized multi‐walled carbon nanotubes in polyaniline for application in formic acid electrooxidation

Incorporation of carbon nanotubes (CNTs) in conducting polymer can lead to new composites with enhanced electrical and mechanical properties. However, the development of such composites has been hampered by the inability to disperse CNTs in polymer matrix due to the lack of chemical compatibility between polymers and CNTs. Covalent sidewall functionalization of carbon nanotube provides a feasible route to incorporate carbon nanotube in polymer. In this work, 4‐aminobenzene groups were grafted onto the surface of multi‐walled carbon nanotube (MWNT) via C? C covalent bond. Polyaniline (PANI)/MWNT composites were fabricated by electrochemical polymerization of aniline containing well‐dissolved functionalized MWNTs. The obtained composites can be used as catalyst supports for electrooxidation of formic acid. Cyclic voltammogram results show that platinum particles deposited in PANI/MWNT composite films exhibit higher electrocatalytic activity and better long‐term stability towards formic acid oxidation than that deposited in pure PANI films. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Functionalizing carbon nanotubes by grafting cyclotriphosphazene derivative to improve both mechanical strength and flame retardancy

An intumescent flame‐retardant, hex(4‐carboxylphenoxy) cyclotriphosphazene (HCPCP) was synthesized and covalently grafted on to the surface of multiwalled carbon nanotubes (MWNTs) to obtain MWNT‐HCPCP. MWNT/epoxy resin (EP) and MWNT‐HCPCP/ EP nanocomposites were prepared via thermal curing. Transmission electron microscopy results showed that a core–shell structure with MWNTs as the hard core and HCPCP as the soft shell were formed after HCPCP (10 wt%) were attached to the MWNTs. The results of flammability tests showed an increased limited oxygen index value for MWNT‐HCPCP/EP nanocomposites. The mechanical properties including tensile strength and elongation were both dramatically improved due to the better dispersion of MWNT‐HCPCP in the EP matrix. The grafting of HCPCP can improve both the dispersion of nanotubes in polymer matrix and flame retardancy of the nanocomposites. POLYM. COMPOS., 35:2187–2193, 2014. © 2014 Society of Plastics Engineers     

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     

Multiwalled carbon nanotubes grafted with polyhedral oligomeric silsesquioxane and its dispersion in poly(l-lactide) matrix

Multiwalled carbon nanotubes (MWNTs) were functionalized with polyhedral oligomeric silsesquioxane (POSS) via amide linkages. The oxidized MWNT (MWNT-COOH) was converted to the acyl chloride-functionalized MWNT (MWNT-COCl) by treating them with thionyl chloride (SOCl₂), and then MWNT-COCl was reacted with aminopropylisooctyl-POSS to prepare aminopropylisooctyl polyhedral oligomeric silsesquioxane modified MWNT (MWNT-g-POSS). Energy-filtering TEM, FTIR, and Raman spectroscopy revealed that the POSS was covalently attached to the MWNT and the weight gain due to the functionalization was determined by the thermogravimetric analyses (TGA). PLLA/MWNT and PLLA/MWNT-g-POSS composites with different MWNT loadings were first prepared by the simple solvent casting approach. A fine dispersion of MWNT-g-POSS throughout PLLA matrix was observed by SEM. Moreover, the composites were also prepared by using melt compounding under the different shearing speed of extruder via our custom-built high-shear extruder. A special feedback-type screw was used in this extruder to realize a high shearing flow field. Using this new technique, a uniform dispersion of the neat MWNTs in the PLLA matrix was found under higher screw rotation speed (≥1500 rpm) while poor dispersion was observed under low speed (<1500 rpm). The homogeneous dispersion of MWNTs throughout the PLLA/MWNT-g-POSS composites without any aggregation was observed at full range of screw rotation speed. The fractured surface of the composites showed not only a uniform dispersion of MWNTs but also a strong interfacial adhesion with the matrix, as evidenced by the presence of many broken but strongly embedded MWNTs in the matrix in the absence of debonding of MWNTs from the matrix. Incorporation of MWNTs improved the mechanical properties.     

Isothermal and nonisothermal crystallization kinetics of poly(ε‐caprolactone)/multi‐walled carbon nanotube composites

Differential scanning calorimeter (DSC) and polarized optical microscopy (POM) have been used to investigate the isothermal and nonisothermal crystallization behavior of poly(ε‐caprolactone) (PCL)/multi‐walled carbon nanotube (MWNT) composites. PCL/MWNT composites have been prepared by mixing the PCL polymer with carboxylic groups containing multi‐walled carbon nanotubes (c‐MWNTs) in tetrahydrofuran solution. Raman spectrum of c‐MWNT indicated the possible presence of carboxylic acid groups at both ends and on the sidewalls of the MWNTs. The TEM micrograph showed that the c‐MWNT is well separated and uniformly dispersed in the PCL matrix. DSC isothermal results showed that the introduction of c‐MWNT into the PCL initiates strongly heterogeneous nucleation, which induced a change of the crystal growth process. The activation energy of PCL significantly decreases by adding 0.25 wt% c‐MWNT into PCL/c‐MWNT composites and then increases as c‐MWNT content increases. The result demonstrates that the addition of c‐MWNT into PCL induces the heterogeneous nucleation at lower c‐MWNT content and then inhibits the polymer chain transportation ability during crystallization at higher c‐MWNT content. In this study, we have also studied the nonisothermal crystallization kinetics and melting behavior of PCL/c‐MWNT composites at various cooling rates. The correlation among isothermal and nonisothermal crystallization kinetics and melting behavior of PCL/c‐MWNT composites can be also discussed. POLYM. ENG. SCI., 46:1309–1317, 2006. © 2006 Society of Plastics Engineers     

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.     

Electrical conductivity and thermal properties of acrylonitrile‐butadiene‐styrene filled with multiwall carbon nanotubes

Composites of Acrylonitrile‐butadiene‐styrene (ABS) and multiwall carbon nanotubes (MWNTs) have been prepared via solution‐blending. The electrical conductivity of these composites is analyzed. The MWNT‐filled ABS shows percolation point of the electrical conductivity at low filler loadings (1–2 wt%). The micro‐structure of the composites is also analyzed by scanning electron microscopy showing that the nanotubes are dispersed quite homogeneously in the polymer‐matrix. The thermogravimetric analysis is used to study the thermal degradation of ABS/MWNTs composites in nitrogen. MWNTs tend to destabilize the ABS matrix in the 220–450°C degradation regions but improve the thermal stability in the 425–850°C regions. With further addition of MWNTs, the features of the destabilization in the 220–450°C degradation region did not change much but in the 425–850°C degradation process, the MWNTs reinforced stabilization and the quality of the char residue of amorphous carbon deposition was improved. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Multi-walled carbon nanotubes reinforced nylon 6 composites

Multiwalled carbon nanotubes (MWNT) were functionalized with amine groups using a ‘grafting to’ technique. The oxidized MWNT (MWNT-COOH) were converted to the acyl chloride functionalized MWNT (MWNT-COCl) by treating them with thionyl chloride (SOCl₂), and then MWNT-COCl was reacted with hexamethylenediamine to prepare MWNT-NH₂. The formation of MWNT-NH₂ was confirmed through the FTIR observation. MWNT-NH₂/nylon 6 composites with different MWNT loadings were prepared by the simple melt compounding approach. A fine dispersion of MWNTs throughout nylon 6 matrix was observed by SEM and TEM. The fractured surface of the composites showed not only a uniform dispersion of MWNTs but also a strong interfacial adhesion with the matrix, as evidenced by the presence of many broken but strongly embedded MWNTs in the matrix in the absence of debonding of MWNTs from the matrix. Incorporation of MWNTs improved the mechanical properties significantly. Higher thermal stability was obtained for the composites with better dispersed MWNTs.     

In situ preparation and continuous fiber spinning of poly(p-phenylene benzobisoxazole) composites with oligo-hydroxyamide-functionalized multi-walled carbon nanotubes

A graft-from approach has been performed to achieve covalent functionalization of multi-walled carbon nanotubes (MWNTs) with oligo-hydroxyamide (oHA). Pristine MWNT was first oxidized to MWNT-COOH and then functionalized to MWNT-COCl by acyl chloride. MWNT-COCl was copolymerized with oHA to produce oHA-grafted MWNTs (MWNT-oHA). The thickness of the oHA shell in MWNT-oHA is about 7.5 nm. MWNT-oHA has a remarkable solubility in polar solvents and a good thermal stability because characteristic dehydrative ring closure occurs upon heating and forms a thermally more stable benzoxazole component. MWNT-oHA has been further covalently incorporated with a rigid-rod polymer matrix, poly(p-phenylene benzobisoxazole) (PBO), through in situ polymerization. Continuous PBO–MWNT composite fibers with different MWNT compositions have been fabricated using dry-jet wet-spinning technique. The structure and morphology of PBO–MWNT composite fibers have been characterized and their mechanical, thermal, conducting properties have been investigated. The tensile modulus, tensile strength, and thermal stability of PBO–MWNT composite fibers have been improved because of a good dispersion and high alignment of MWNTs in PBO as well as enhanced interfacial interaction between these two components. Furthermore, increased conductivity has been discovered in the PBO–MWNT composite films and the inner core of the composite fibers; however, not on the outer surface. The phenomena can be interpreted using percolation model together with the heterogeneous fiber morphology and nanotube distribution over the cross-section of the fiber.     

Electrical and Optical Properties of Conducting Poly(3-hexylthiophene)/Multi-walled Carbon Nanotube System

Polymer multi-wall carbon nanotube (MWNT) composites were prepared and characterized as part of an effort to develop polymeric materials with improved combinations of properties for potential use in solar cell applications. Multi-walled carbon nanotube (MWNT) poly(3-hexylthiophene) nanocomposites were synthesized by in situ polymerization of monomers in the presence of different amounts of MWNTs. A process is reported to efficiently disperse multi-walled carbon nanotube (MWNT) bundles in a semiconducting polymer matrix. A uniform dispersion of the nanotubes in the polymer matrix was obtained. Characterization of the nanocomposites and the effects of MWNT concentration and dispersion on the structural, optical and electrical properties were discussed. FTIR and Raman spectroscopic investigations of nanocomposites indicate that the polymer is wrapped on the nanotubes, taking up a rigid orientation through π-π stacking. The Hall voltage measurement is followed to monitor carrier concentrations and mobilities, instead of the device fabrication and hole mobility measurements.     

Enhancement of the surface and bulk mechanical properties of polystyrene through the incorporation of raw multiwalled nanotubes with the twin‐screw mixing technique

In this article, we present the effects of incorporated multiwalled nanotubes (MWNTs) on a metal surface and the bulk mechanical properties of as‐synthesized polystyrene (PS)–MWNT composites prepared with the twin‐screw mixing technique. The MWNTs used for preparing the composites were raw compounds that were not treated with any surface modifications. The morphology for the dispersion capability of the MWNTs in the PS matrix was subsequently characterized with transmission electron microscopy. Surface mechanical property studies (i.e., wear resistance and hardness) showed that the integration of MWNTs led to a distinct increase in the wear resistance and also the micro/nanohardness with up to a 5 wt % MWNT loading in the composites. Moreover, the enhancement of the wear resistance of the as‐prepared composites, in comparison with pure PS, was further identified with scanning electron microscopy observations of the surface morphology after testing. On the other hand, for bulk mechanical property studies (i.e., the tensile strength and flexural strength), the composites containing a 3 wt % concentration of MWNTs in the PS matrix exhibited the best performance with respect to the tensile strength and flexural strength. This means that this composition of MWNTs exhibited good compatibility with the PS matrix, and this can be attributed to the π–π interacting forces existing between the aromaticity of the MWNTs and PS matrix. Furthermore, at higher MWNT loadings (e.g., 5 wt %), raw MWNTs were aggregated in the polymer matrix, as observed by transmission electron microscopy. Also, this led to an obvious decrease in the tensile strength and flexural strength. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of surface modification with 3‐aminopropyltriethoxysilane on structure and mechanical property of multiwalled carbon nanotube/polycarbonate composites

Hydroxyl functionalized multiwalled carbon nanotubes (H‐MWNTs) were silanized using 3‐aminopropyltriethoxysilane (APTES) in order to improve the dispersion and interfacial interaction in composites. MWNT/polycarbonate (PC) composites filled with H‐MWNTs and silanized MWNTs (S‐MWNTs) were fabricated by melt mixing and injection molding. Fourier transform infrared spectrometry (FTIR) and energy dispersion X‐ray spectroscopy (EDS) were employed to prove the presence of APTES on the surface of S‐MWNTs. In addition, thermogravimetric analysis (TGA) was used to evaluate the relative amount of introduced APTES. The microstructure and mechanical property of both composites were investigated by scanning electron microscopy (SEM), transmission electron microscope (TEM), tensile test and dynamic mechanical analysis (DMA). The SEM and TEM images showed that S‐MWNT/PC composites had better dispersion and interfacial adhesion than H‐MWNT/PC composites. A reinforcing and toughening effect on tensile behavior of composites was obtained after silane functionalization. The storage modulus of composites increased markedly as a function of MWNTs content, especially for the composites with S‐MWNTs. In summary, the silanization can improve the dispersion of MWNTs and the interfacial adhesion between MWNTs and PC so as to enhance the mechanical properties of composites. POLYM. COMPOS., 37:1914–1923, 2016. © 2015 Society of Plastics Engineers     

Excellent tensile ductility in highly oriented injection-molded bars of polypropylene/carbon nanotubes composites

Multi-wall carbon nanotubes (MWNTs) grafted with alkyl chain were used for reinforcement of polypropylene (PP). For achieving excellent tensile properties, the as-prepared PP/MWNTs composites were subjected to a unique injection molding, as so-called dynamic packing injection molding, to induce a highly oriented structure with both PP chains and MWNTs aligned along the shear flow direction. Not only Young's modulus and tensile strength were enhanced, as expected for oriented materials, but also more importantly composites containing only 0.1-0.3 wt% MWNTs were much ductile compared with the polymer matrix. The addition of PP-g-MMA made a drop in the elongation at break to only 15%; however, it could be improved to 80-100% after incorporation of small amount of MWNTs. This improvement in ductility could be ascribed to: (1) the increased mobility of both the PP chains and MWNTs, as they are oriented along tensile deformation direction and (2) the bridging effect of the oriented MWNTs on the crack development during tensile failure.     

多壁碳纳米管／环氧树脂复合材料性能研究

采用物理机械方法与化学方法相结合的手段,制备了多壁碳纳米管（MWNTS）／环氧树脂（Epoxy）复合材料。通过力学拉伸试验测试了MWNTs/Epoxy复合材料拉伸强度和拉伸模量与MWNTS添加量的关系,利用扫描电镜（SEM）分析了MWNTS／Epoxy复合材料的拉伸断面,并用表面电阻测试仪对所制备的碳纳米管复合材料进行了电学性能测试。结果表明：经过化学酸化的方法处理后的MWNTS在复合材料中的分散得到了改善,力学性能也得到了明显的提高,但酸处理后的复合材料的电学性能明显低于未处理的复合材料。     

Effects of nanotube modification on the dielectric behaviors and mechanical properties of multiwall carbon nanotubes/epoxy composites

Multiwall carbon nanotubes (MWNTs) were modified by three methods, namely, oxidizing the tubes and opening both ends, filling the tubes with Ag, and grafting the tubes with hexamethylene diamine. Modified MWNTs/epoxy composites were prepared by melt‐mixing epoxy resin with the tubes. Transmission electron microscope images showed that the modified MWNTs can be dispersed in the epoxy matrix homogeneously. The dielectric behaviors and mechanical properties of the composites were investigated. The dielectric and mechanical properties of the modified MWNTs/epoxy composites were considerably improved compared with those of the epoxy matrix. The tensile strengths of the Ag‐filled, opened, and grafted MWNTs composites at the same filler content of 1.1 wt% were higher by ～30.5%, 35.6%, and 27.4%, respectively, than that of neat epoxy. The Izod notched impact strength of the grafted MWNTs/epoxy composite with filler content of 1.1 wt% was approximately four times higher than that of neat epoxy. A dielectric constant of ～150 of the composite with 1.1 wt% Ag‐filled nanotubes was observed in the low‐frequency range, which was ～40 times higher than that of the epoxy matrix. The proper modification of nanotubes provides a way to improve the properties of the polymer‐based composites. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Synthesis and characterization of multi-walled carbon nanotubes reinforced polyamide 6 via in situ polymerization

Polyamide 6 (PA6)/carbon nanotubes (PA6/CNTs) composites have been prepared by in situ polymerization of ε-caprolactam in the presence of pristine and carboxylated multi-walled carbon nanotubes (MWNT and MWNTCOOH). Viscosity measurements show that adding 0.5 wt% of carbon nanotubes (CNTs) does not affect the molecular weight of PA6. Compared with pure PA6, the yield strength of PA6/CNTs composites loaded with 0.5 wt% CNTs is almost unchanged, and the tensile strength is increased slightly. Dynamic mechanical analysis (DMA) demonstrates that both the storage modulus (E′) and glass transition temperature (T_g) of the PA6/CNTs composites increase, particularly for PA6/MWNTCOOH, indicating there is some chemical bonding between PA6 and MWNTCOOH. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultra small-angle X-ray scattering (USAXS) show that MWNT and MWNTCOOH are well dispersed in PA6 matrix. Comparison of the USAXS data with a stiff-rod model and wormlike rod model reveals that the CNTs are quite flexible, regardless the degree of chemical modification. Due to the flexibility of CNTs, mechanical properties of the PA6/CNTs composites are marginally enhanced.