The effect of interfacial chemistry on molecular mobility and morphology of multiwalled carbon nanotubes epoxy nanocomposite

We report on our attempts to understand the link between the nature of the CNT surface modification, dispersion in an epoxy resin and the resulting properties. Carboxylated and fluorinated nanotubes were used to synthesize nanocomposites by dispersing them separately in an epoxy resin. Dynamic mechanical analysis, using torsional deformation, was applied both parallel and perpendicular to the long axis of the multiwall nanotubes (MWNTs). Interestingly, for epoxy/MWNT (1 wt%) nanocomposites, the shear moduli in the glassy state were higher for the nanocomposites, and it's highest for the nanocomposites in which the nanotubes are parallel to the direction of applied torque. These nanocomposites also exhibited higher T_gs than the neat resin. In addition, the rubbery plateau modulus (between 150-200 °C) was higher by a factor of three for the nanocomposites. Master curves constructed using time-temperature superposition allowed us to probe low frequency dynamic moduli and further discern differences in the relaxation behavior. Samples containing fluorinated nanotubes exhibited the highest T_gs, longest relaxation times and highest activation energies relative to the carboxylated nanotube samples and the neat resin, indicative of stronger interactions. SEM and TEM studies confirmed the nanotube dispersion and alignment.     

Surfactant-Assisted Dispersion of MWCNTs in Epoxy Resin Used in CFRP Strengthening Systems

The epoxy resin used as the bonding agent in carbon fiber-reinforced polymer (CFRP) strengthening systems was modified by the infusion of multiwalled carbon nanotubes (MWCNTs). Two types of surfactants, Triton X-100 and C₁₂E₈, were used to disperse the nanotubes in the epoxy resin employing ultrasonic mixing. Dynamic mechanical analysis and tensile tests were conducted to study the effect of the surfactant-assisted dispersion of nanotubes on the thermal and mechanical properties of epoxy composites. The morphology of the epoxy composites was interpreted using scanning electron microscopy (SEM). Moreover, the effect of surfactant treatment on the structure of nanotubes was investigated by Fourier transform infrared (FT-IR). Based on the experimental results, the tensile strength and the storage modulus of the epoxy resin were increased by 32% and 26%, respectively, by the addition of MWCNTs. This was attributed to the homogeneous dispersion of nanotubes in the epoxy resin according to the SEM images. Another reason for the enhancement in the tensile properties was the reinforced nanotube/epoxy interaction as a result of the surfactant anchoring effect which was proved by FT-IR. A moderate improvement in the glass transition temperature (T _g) was recorded for the composite fabricated using Triton X-100, which was due to the restricted molecular motions in the epoxy matrix. To characterize the temperature-dependent tensile behavior of the modified epoxy composites, tensile tests were conducted at elevated temperatures. It was revealed that the MWCNT modification using surfactant substantially improves the tensile performance of the epoxy adhesive at temperatures above the T _g of the neat epoxy.     

Preparation and thermal,electrical, and morphological properties of multiwalled carbon nanotubeand epoxy composites

Multiwalled carbon nanotube (MWCNT)/epoxy composites are prepared, and the characteristics and morphological properties are studied. Scanning electron microscopy microphotographs show that MWCNTs are dispersed on the nanoscale in the epoxy resin. The glass‐transition temperature (T_g) of MWCNT/epoxy composites is dramatically increased with the addition of 0.5 wt % MWCNT. The T_g increases from 167°C for neat epoxy to 189°C for 0.5 wt % CNT/epoxy. The surface resistivity and bulk resistivity are decreased when MWCNT is added to the epoxy resins. The surface resistivity of CNT/epoxy composites decreases from 4.92 × 10¹² Ω for neat epoxy to 3.03 × 10⁹ Ω for 1 wt % MWCNT/epoxy. The bulk resistivity decreases from 8.21 × 10¹⁶ Ω cm for neat epoxy to 6.72 × 10⁸ Ω cm for 1 wt % MWCNT/epoxy. The dielectric constant increases from 3.5 for neat epoxy to 5.5 for 1 wt % MWCNT/epoxy. However, the coefficient of thermal expansion is not affected when the MWCNT content is less than 0.5 wt %. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1272–1278, 2007     

Effect of the addition of multi‐walled carbon nanotubes on the thermomechanical properties of epoxy resin

The influence of multi‐walled carbon nanotubes (MWCNTs) on thermosetting epoxy is examined using dynamic mechanical analysis, thermogravimetric analysis, and differential scanning calorimetry (DSC). Specimens are prepared with loadings of 0.1 and 1 wt% MWCNTs which are dispersed in the resin using two different dispersion methods. While the storage modulus of the specimens is improved, both the glass transition temperature and the thermal stability are reduced by the addition of MWCNTs with both effects being greater for the higher MWCNT loading, for both dispersion systems. The DSC results additionally indicate that the level of residual unreacted epoxy increases progressively with the addition of the nanotubes. This finding is considered as confirmation that the MWCNTs obstruct crosslinking of the epoxy resin. POLYM. COMPOS., 38:1873–1880, 2017. © 2015 Society of Plastics Engineers     

Functionalization of multi-wall carbon nanotubes to reduce the coefficient of the friction and improve the wear resistance of multi-wall carbon nanotube/epoxy composites

Functionalization of multi-wall carbon nanotubes (MWCNTs) was achieved by grafting carboxyl groups and amino groups. Fourier transform infrared spectroscopy was used to detect the changes produced by functional groups on the surface of the MWCNTs. Three different MWCNTs were incorporated into epoxy resin and the friction and wear behavior of MWCNT/epoxy composites was investigated using a M-2000 wear testing machine at different sliding speeds under different applied loads. Scanning electron microscopy was used to observe the worn surfaces of the samples. The results indicated that the functional groups had been grafted on the surface of MWCNTs. Compared with neat epoxy, the composites with MWCNTs showed a lower friction coefficient and wear rate, and the wear rate decreased with the increase of MWCNT loading. Combining epoxy resin with MWCNTs is an efficient method to improve the wear resistance and decrease the coefficient of friction.     

Dynamic cure kinetics of epoxy/TiO2/MWCNT hybrid nanocomposites

The effect of multi-walled carbon nanotubes (MWCNTs) on cure kinetic parameters of the epoxy/amine/TiO₂ (1 wt%) resin system was studied dynamically at four heating rates using DTA. The presence of MWCNT in various amounts (0.1, 0.2, 0.4 and 0.6 wt%) neither retarded nor accelerated the cure reaction of the epoxy/amine/TiO₂ system in a considerable extent. Addition of MWCNTs increased the extent of cure of the corresponding nanocomposites, especially at higher contents up to 0.4 wt% MWCNT filled composite. However, increasing the MWCNT content to 0.6 wt% adversely affected the extent of cure due to nanoparticle agglomeration. The fracture surface morphology of the nanocomposites revealed that the cracks deviated on reaching the MWCNTs, while propagating in the polymer matrix. Fractional extent of conversion (α) was calculated using genetic algorithm. Flynn–Wall–Ozawa and Kissinger methods were used to analyze the kinetic parameters. The presence of MWCNTs did not affect the autocatalytic cure mechanism of epoxy/amine/TiO₂ resin system and also did not cause any considerable barrier effect on the curing process. Activation energy data fitted well in the cubic polynomial regression equations and the changes of E _a with respect to α proved the autocatalytic cure mechanism, being followed by all the MWCNT-containing epoxy-based hybrid nanocomposites.     

Direct functionalization with 3,5‐substituted benzoic acids of multiwalled carbon nanotube/epoxy composites

Directly functionalized multiwalled carbon nanotubes (MWCNTs) with benzene‐1,3,5‐tricarboxylic acid (BTC) and 3,5‐diaminobenzoic acid (DAB) were successfully accomplished with less structural damage as confirmed by XPS and FT‐Raman results. Their dispersibility and thermal stability were achieved after the functionalization. The functional groups on MWCNT surfaces can accelerate the curing reaction of epoxy composites remarkable inducing rather low exothermic peak temperature (T_p) and exothermic heat of reaction (ΔH). The values of activation energy (E_a) obtained from Kissinger and Ozawa methods obviously decreased with the introduction of MWCNTs, especially DAB‐MWCNTs. The dynamic mechanical properties notably enhanced with the incorporation of unmodified and functionalized MWCNTs. The crosslink density (ρ) increased and free volume fraction (f_g) decreased, resulting in dramatic increase of glass transition temperatures (T_g) and decrease of coefficient of thermal expansion. Additionally, epoxy composites exhibited low dielectric constant close to that of neat epoxy resin. From these remarkable properties, MWCNT/epoxy composites can be considered as a good candidate for high performance insulation materials. POLYM. ENG. SCI., 53:2194–2204, 2013. © 2013 Society of Plastics Engineers     

Cure kinetics and morphology of blends of epoxy resin with poly (ether ether ketone) containing pendant tertiary butyl groups

The cure kinetics and morphology of diglycidyl ether of bisphenol-A (DGEBA) epoxy resin modified with a poly (ether ether ketone) based on tertiary butyl hydroquinone (PEEK-T) cured with diamino diphenyl sulphone (DDS) were investigated using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and dynamic mechanical thermal analysis (DMTA). The results obtained from DSC were applied to autocatalytic and diffusion controlled kinetic models. The reaction mechanism broadly showed autocatalytic behaviour regardless of the presence of PEEK-T. At higher PEEK-T concentration, more diffusion controlled mechanism was observed. The rate of curing reaction decreased with increase in thermoplastic content and also with the lowering of curing temperature. The activation energies of the blends are higher than that of the neat resin. The blends showed a phase separated morphology. The dispersed phase showed a homogeneous particle size distribution. The T_g of the neat resin decreased with the decrease in cure temperature. Two T_g's corresponding to the epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum. The storage modulus of 10 and 20 phr PEEK-T blends are found to be greater than the neat resin.     

A comparative study on the properties of the different amino‐functionalized multiwall carbon nanotubes reinforced epoxy resin composites

Multiwall carbon nanotubes (MWCNTs) were amino‐functionalized by 1,2‐ethylenediamine (EDA)' triethylenetetramine (TETA), and dodecylamine (DDA), and investigated by fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and thermogravimetric analysis (TGA). The dispersion of the DDA functionalized MWCNT in DMF is better than that of the MWCNT functionalized by the EDA and the TETA. Carbon nanotubes reinforced epoxy resin composites were prepared, and the effect of the amino‐functionalization on the properties of the composites was investigated by differential scanning calorimetry (DSC), dynamical mechanical analysis (DMA), and TGA. The composites reinforced by the MWCNTs demonstrate improvement in various mechanical properties. The increase of T_g of the composites with the addition of amino‐functionalized MWCNT compared to the T_g of the composites with the addition of unfunctionalized MWCNT was due to the chemical combination and the physical entanglements between amino group from modified MWNTs and epoxy group from the epoxy resin. The interfacial bonding between the epoxy and the amino group of the EDA and the TETA‐modified MWCNT is more important than the well dispersion of DDA‐modified MWCNT in the composites for the improvement of the mechanical properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Preparation and thermal properties of bio‐based gallic acid epoxy/carbon nanotubes composites by cationic ring‐opening reaction

In order to prepare the bio‐based polymeric materials, a gallic acid epoxy resin (GA‐ER) is synthesized by using biodegradable gallic acid, and the nanocomposites of GA‐ER/glycidyl methacrylate (GMA)/multiwalled carbon nanotubes (MWCNTs) were prepared by dual hybrid cationic ring‐opening reaction. Differential scanning calorimetry (DSC) results show that the curing reaction temperature of the nanocomposites is between 150 and 225°C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results suggest that MWCNTs are homodispersing in the GA‐ER/GMA matrix when the MWCNTs content is not more than 1.0 wt%. The glass transition temperature of the nanocomposite with 0.5 wt% MWCNTs is 9.3°C higher than that of pure resin system. The initial thermal degradation temperature and degradation activation energies E_a of the nanocomposite with 1.0 wt% MWCNTs is 10°C and 68.6 kJ/mol higher than that the pure resin system, respectively. POLYM. COMPOS., 37:3093–3102, 2016. © 2015 Society of Plastics Engineers     

Thermal conductivity and dynamic mechanical property of glycidyl methacrylate‐grafted multiwalled carbon nanotube/epoxy composites

The well dispersed multiwalled carbon nanotube (MWCNT)/epoxy composites were prepared by functionalization of the MWCNT surfaces with glycidyl methacrylate (GMA). The morphology and thermal properties of the epoxy nanocomposites were investigated and compared with the surface characteristics of MWCNTs. GMA‐grafted MWCNTs improved the dispersion and interfacial adhesion in epoxy resin, and enhanced the network structure. The storage modulus of 3 phr GMA‐MWCNTs/epoxy composites at 50°C increased from 0.32 GPa to 2.87 GPa (enhanced by 799%) and the increased tanδ from 50.5°C to 61.7°C (increased by 11.2°C) comparing with neat epoxy resin, respectively. Furthermore, the thermal conductivity of 3 phr GMA‐MWCNTs/epoxy composite is increased by 183%, from 0.2042 W/mK (neat epoxy) to 0.5781 W/mK. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Characterizations on the amidized multiwalled carbon nanotubes grafted with polyaniline via in situ polymerization

A multiwalled carbon nanotubes (MWCNTs) were carboxylated after refluxing with sulfuric and nitric acids. These attached carboxylic acid groups were further condensated with o‐phenylene diamine into amide catalyzed by dicyclohexyl carbodiimide (DCC). The obtained amidized MWCNTs were in situ‐polymerized with aniline monomers to graft a conducting polyaniline (PANI) onto MWCNT (ES‐g‐MWCNTs) through the polymerization occurring in the ortho‐ and meta‐positions. The reduced conductivity of the MWCNT after carboxylation can be recovered after grafting with PANI, which owns a strong λ_max at the near infrared region due to the extended conjugation from MWCNTs to PANI. Transmission electronic microscopic pictures show a gradual broadening of the MWCNT diameter after carboxylation, amidization, and polymerization. The weight loss from the thermogravimetric thermograms due to the carboxylations of MWCNTs, amidized MWCNTs, and the PANI grafted MWCNTs into CO₂ can be used to estimate the degree of carboxylation, amidization, and grafting of PANI. The degree of carboxylation of MWCNT calculated from ESCA spectrum is around 23% close to that estimated from TGA thermogram. The doping level of redoped PANI‐grafted MWCNT is found to be 27.78% much less than the maximum 50% of neat PANI. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Multi-walled carbon nanotubes modified by sulfated TiO2 - A promising support for Pt catalyst in a direct ethanol fuel cell

We report the synthesis of multi-walled carbon nanotubes coated with sulfated TiO₂ (S-TiO₂/MWCNTs) as a promising support for Pt catalyst in a direct ethanol fuel cell. Highly dispersed Pt nanoparticles were supported on the S-TiO₂/MWCNT composites by NaBH₄ reduction procedure (Pt-S-TiO₂/MWCNTs). The presence and nature of the catalyst were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy. The size of the sulfated TiO₂ product was about 8 nm, and that of the Pt nanoparticle on the S-TiO₂/MWCNT composites was about 5 nm. The Pt-S-TiO₂/MWCNTs were used to study the electrochemical ethanol oxidation reaction using cyclic voltammetry, chronoamperometry and impedance spectroscopy. The results show that Pt-S-TiO₂/MWCNT catalysts show higher catalytic activity for ethanol oxidation compared with Pt supported on non-sulfated TiO₂/MWCNT composites and commercial Pt/C catalysts.     

Thermal and mechanical stabilities of composite films from thiadiazol bearing poly(amide-thioester-imide) and multiwall carbon nanotubes by solution compounding

An amino acid containing poly(amide-thioester-imide) (PATEI) possessing a conjugated thiadiazol ring was shown to be effective for dispersing multiwall carbon nanotubes (MWCNTs) in N,N′-dimethylacetamide. Through casting of these dispersions, MWCNT/PATEI composite films were successfully fabricated on substrates and showed no signs of macroscopic aggregation. To increase the compatibility between PATEI matrix and MWCNTs, carboxyl-functionalized MWCNTs (f-MWCNTs) were used in this study. The f-MWCNTs were dispersed homogeneously in the PATEI matrix while the structure of the polymer and the MWCNTs structure were stable in the preparation process as revealed by transmission electron microscopy. Tensile tests and thermal analysis were carried out on free-standing composite films for different MWCNT loading levels. Results showed that overall mechanical and thermal properties of the composites were greatly improved as compared with the neat PATEI film. Fourier transform infrared spectroscopy, powder X-ray diffraction, and field emission electron microscopy were also used to evaluate the MWCNT/PATEI composite system.     

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

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

Effect of functionalized carbon nanotubes on the thermal conductivity of epoxy composites

Direct functionalized carbon nanotubes (CNTs) were utilized to form the heat flow network for epoxy composites through covalent integration. A method of preparing a fully heat flow network between benzenetricarboxylic acid grafted multi-walled carbon nanotubes (BTC-MWCNTs) and epoxy matrix is described. A Friedel-Crafts modification was used to functionalize MWCNTs effectively and without damaging the MWCNT surface. Raman spectra, X-ray photoelectron spectra and thermogravimetric analysis reveal the characteristics of functionalized MWCNTs. The scanning electron microscope images of the fracture surfaces of the epoxy matrix showed BTC-MWCNTs exhibited higher solubility and compatibility than pristine-MWCNTs. The MWCNTs/epoxy composites were prepared by mixing BTC-MWCNTs and epoxy resin in tetrahydrofuran, followed by a cross-linking reaction with a curing agent. The BTC was grafted onto the MWCNTs, creating a rigid covalent bond between MWCNTs and epoxy resin and forming an effective network for heat flow. The effect of functionalized MWCNTs on the formation of the heat flow network and thermal conductivity was also investigated. The thermal conductivity of composites exhibits a significant improvement from 0.13 to 0.96 W/m K (an increase of 684%) with the addition of a small quantity (1-5 vol%) of BTC-MWCNTs.     

The effects of triethylenetetramine grafting of multi‐walled carbon nanotubes on its dispersion,filler‐matrix interfacial interaction and the thermal properties of epoxy nanocomposites

This study describes the influence of triethylenetetramine (TETA) grafting of multi‐walled carbon nanotubes (MWCNTs) on the dispersion state, interfacial interaction, and thermal properties of epoxy nanocomposites. MWCNTs were first treated by a 3:1 (v/v) mixture of concentrated H₂SO₄/HNO₃, and then TETA grafting was performed. Chemically grafted MWCNT/bisphenol‐A glycidol ether epoxy resin/2‐ethyl‐4‐methylimidazole nanocomposites were prepared. TETA grafting could establish the connection of MWCNTs to the epoxy matrix and transform the smooth and nonreactive MWCNT surface into a hybrid material that possesses the characteristics of both MWCNTs and TETA, which facilitates homogeneous dispersion of MWCNTs and improves nanotube‐epoxy interfacial interaction. Therefore, the impact property, glass transition temperature, thermal stability, and thermal conductivity of epoxy nanocomposites are enhanced. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Preparation and properties of multiwalled carbon nanotube/epoxy‐amine composites

Different amounts of multiwalled carbon tubes (MWCNTs) were incorporated into an epoxy resin based on diglycidyl ether of bisphenol A and both epoxy precursor and composite were cured with 4,4′‐diamino diphenyl sulfone. Transmission and scanning electron microscopy demonstrated that the carbon nanotubes are dispersed well in the epoxy matrix. Differential scanning calorimetry measurements confirmed the decrease in overall cure by the addition of MWCNTs. A decrease in volume shrinkage of the epoxy matrix caused by the addition of MWCNTs was observed by pressure–volume–temperature measurements. Thermomechanical and dynamic mechanical analysis were performed for the MWCNT/epoxy composites, showing that the T_g was slightly affected, whereas the dimensional stability and stiffness are improved by the addition of MWCNTs. Electrical conductivity measurements of the composite samples showed that an insulator to conductor transition takes place between 0.019 and 0.037 wt % MWCNTs. The addition of MWCNTs induces an increase in both impact strength (18%) and fracture toughness (38%) of the epoxy matrix with very low filler content. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

A study on rheological behavior of MWCNTs/epoxy composites

Rheological behaviors of multiwalled carbon nanotubes (MWCNTs)/epoxy composites with various MWCNT contents were investigated by using a time sweep and frequency sweep experiment with oscillatory rheometry. The functional groups on the acid-treated MWCNTs were investigated by fourier transfer-infrared spectroscopy (FT-IR). The composites containing acid-treated MWCNTs exhibited faster gel time than pure epoxy resins. The storage (G′) and loss (G″) moduli of the composites showed solid-like behavior owing to interaction between the MWCNTs and the epoxy resins. The 1.0 wt% MWCNT composites had the highest crosslinking activation energy (E_c) due to good dispersion and strong interfacial bonding. These results imply that three-dimensional crosslinking might take place among the hydroxyl group in epoxy resins and the carbonyl or hydroxyl group in acid-treated MWCNTs.     

Effects of amino‐functionalized carbon nanotubes on the properties of amine‐terminated butadiene–acrylonitrile rubber‐toughened epoxy resins

Amino‐functionalized multiwalled carbon nanotubes (MWCNT‐NH₂s) as nanofillers were incorporated into diglycidyl ether of bisphenol A (DGEBA) toughened with amine‐terminated butadiene–acrylonitrile (ATBN). The curing kinetics, glass‐transition temperature (T_g), thermal stability, mechanical properties, and morphology of DGEBA/ATBN/MWCNT‐NH₂ nanocomposites were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis, a universal test machine, and scanning electron microscopy. DSC dynamic kinetic studies showed that the addition of MWCNT‐NH₂s accelerated the curing reaction of the ATBN‐toughened epoxy resin. DSC results revealed that the T_g of the rubber‐toughened epoxy nanocomposites decreased nearly 10°C with 2 wt % MWCNT‐NH₂s. The thermogravimetric results show that the addition of MWCNT‐NH₂s enhanced the thermal stability of the ATBN‐toughened epoxy resin. The tensile strength, flexural strength, and flexural modulus of the DGEBA/ATBN/MWCNT‐NH₂ nanocomposites increased increasing MWCNT‐NH₂ contents, whereas the addition of the MWCNT‐NH₂s slightly decreased the elongation at break of the rubber‐toughened epoxy. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40472.