Effect of silane structure on the properties of silanized multiwalled carbon nanotube-epoxy nanocomposites

Multiwalled carbon nanotubes (MWCNTs) were functionalized with aminosilanes via an aqueous deposition route. The size and morphology of siloxane oligomers grafted to the MWCNTs was tuned by varying the silane functionality and concentration and their effect on the properties of a filled epoxy system was investigated. The siloxane structure was found to profoundly affect the thermo-mechanical behavior of composites reinforced with the silanized MWCNTs. Well-defined siloxane brushes increased the epoxy T_g by up to 19 °C and significantly altered the network relaxation dynamics, while irregular, siloxane networks grafted to the MWCNTs had little effect. The addition of both types of silanized MWCNTs elicited improvements in the strength of the nanocomposites, but only the well-defined siloxane brushes engendered dramatic improvements in toughness. Because the silanization reaction is simple, rapid, and performed under aqueous conditions, it is also an industrially attractive functionalization route.     

The influence of matrix viscosity on MWCNT dispersion and electrical properties in different thermoplastic nanocomposites

Composites of MWCNTs having each three different levels of matrix viscosity with five different polymers (polyamide 12, polybutylene terephthalate, polycarbonate, polyetheretherketone and low density polyethylene) were melt mixed to identify the general influence of matrix viscosity on the electrical properties and the state of MWCNT dispersion. Huge differences in the electrical percolation thresholds were found using the same polymer matrix with different viscosity grades. The lowest percolation thresholds were always found in the composites based on the low viscosity matrix. The state of primary MWCNT agglomerate dispersion increased with increasing matrix viscosity due to the higher input of mixing energy. TEM investigations showed nanoagglomerated structures in the low viscosity samples which are obviously needed to achieve low resistivity values. The effect of nanotube shortening was quantified using two different viscosity grades of polycarbonate. Due to the higher mixing energy input the nanotube shortening was more pronounced in the high viscosity matrix which partially explains the higher percolation threshold.     

Influences of polymer matrix melt viscosity and molecular weight on MWCNT agglomerate dispersion

In order to study the influence of melt viscosity and molecular weight on nanotube dispersion and electrical volume resistivity, three different polycarbonates (PCs) varying in molecular weight were melt compounded with 1 wt% multiwalled carbon nanotubes (MWCNTs, Baytubes^® 150 HP) using a small-scale compounder. The experiments were performed at constant melt temperature but at varying mixing speeds, thereby applying different magnitudes of shear stress. Light transmission microscopy was used to access the state of agglomerate dispersion, and electrical resistivities of the composites were measured on pressed plates. The results indicate that with increasing matrix viscosity the agglomerate dispersion gets better when using constant mixing conditions but worse considering comparable shear stress values. To study the effect of molecular weight, in a second set of experiments melt temperatures were adjusted so that all PCs had similar viscosity and mixing was performed at constant mixing speed. As investigated on two viscosity levels, the composites based on the low molecular weight matrix showed smaller sized un-dispersed primary agglomerates as compared to composites with higher molecular weight matrices, highlighting the role of matrix infiltration into primary nanotube agglomerates as the first step of dispersion. The resistivity values of composites prepared using low viscosity matrices were lower than those of composites from high viscosity matrix.     

Relationship between dispersion metric and properties of PMMA/SWNT nanocomposites

Particle spatial dispersion is a crucial characteristic of polymer composite materials and this property is recognized as especially important in nanocomposite materials due to the general tendency of nanoparticles to aggregate under processing conditions. We introduce dispersion metrics along with a specified dispersion scale over which material homogeneity is measured and consider how the dispersion metrics correlate quantitatively with the variation of basic nanocomposite properties. We then address the general problem of quantifying nanoparticle spatial dispersion in model nanocomposites of single-walled carbon nanotubes (SWNTs) dispersed in poly(methyl methacrylate) (PMMA) at a fixed SWNT concentration of 0.5% using a ‘coagulation’ fabrication method. Two methods are utilized to measure dispersion, UV-vis spectroscopy and optical confocal microscopy. Quantitative spatial dispersion levels were obtained through image analysis to obtain a ‘relative dispersion index’ (RDI) representing the uniformity of the dispersion of SWNTs in the samples and through absorbance. We find that the storage modulus, electrical conductivity, and flammability property of the nanocomposites correlate well with the RDI. For the nanocomposites containing the same amount of SWNTs, the relationships between the quantified dispersion levels and physical properties show about four orders of magnitude variation in storage modulus, almost eight orders of magnitude variation in electric conductivity, and about 70% reduction in peak mass loss rate at the highest dispersion level used in this study. The observation of such a profound effect of SWNT dispersion indicates the need for objective dispersion metrics for correlating and understanding how the properties of nanocomposites are determined by the concentration, shape and size of the nanotubes.     

Influence of a cyclic butylene terephthalate oligomer on the processability and thermoelectric properties of polycarbonate/MWCNT nanocomposites

The thermoelectric properties of melt-processed nanocomposites consisting of a polycarbonate (PC) thermoplastic matrix filled with commercially available carboxyl (–COOH) functionalized multi-walled carbon nanotubes (MWCNTs) were evaluated. MWCNTs carrying carboxylic acid moieties (MWCNT-COOH) were used due the p-doping that the carboxyl groups facilitate, via electron withdrawing from the electron-rich π-conjugated system. Preliminary thermogravimetric analysis (TGA) of MWCNT-COOH revealed that the melt-mixing was limited at low temperatures due to thermal decomposition of the MWCNT functional groups. Therefore, PC was mixed with 2.5 wt% MWCNT-COOH (PC/MWCNT-COOH) at 240 °C and 270 °C. In order to reduce the polymer melt viscosity, a cyclic butylene terephthalate (CBT) oligomer was utilized as an additive, improving additionally the electrical conductivity of the nanocomposites. The melt rheological characterization of neat PC and PC/CBT blends demonstrated a significant decrease of the complex viscosity by the addition of CBT (10 wt%). Optical and transmission electron microscopy (OM, TEM) depicted an improved MWCNT dispersion in the PC/CBT polymer blend. The electrical conductivity was remarkably higher for the PC/MWCNT-COOH/CBT composites compared to the PC/MWCNT-COOH ones. Namely, the PC/MWCNT-COOH/CBT processed at 270 °C exhibited the best values with electrical conductivity; σ = 0.05 S/m, Seebeck coefficient; S = 13.55 μV/K, power factor; PF = 7.60 × 10⁻⁶μW/m K⁻², and thermoelectric figure of merit; ZT = 7.94 × 10⁻⁹. The PC/MWCNT-COOH/CBT nanocomposites could be ideal candidates for large-scale thermal energy harvesting, even though the presently obtained ZT values are still too low for commercial applications.     

Effect of processing techniques on the performance of Epoxy/MWCNT nanocomposites

The aim of this study is to investigate the optimum technique to disperse Multi‐Walled Carbon Nanotubes (MWCNTs) in SC‐1 epoxy uniformly and to evaluate the effect of processing technique on the performance of SC‐1 epoxy. To achieve better dispersion, MWCNT was mixed with SC‐1 resin directly or premixed with a solvent and then mixed with SC‐1 resin after evaporating the solvent using sonication, thinky mixing and three‐roll mill methods either in isolation or combination. Flexural tests were performed to evaluate mechanical performances and results exhibit up to 27.13, 13.51, and 21.99% improvement on flexural strength, flexural modulus, and maximum strain, respectively, over neat epoxy with only 0.2 wt % loading. Dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) indicated improvement in storage modulus, T_g, inflection temperature, and residue content, respectively over neat SC‐1 epoxy. Thermal and mechanical properties at higher loading conditions were seen to either reduce or not significantly improve. This was attributed to high viscosity of nanocomposites as determined by rheological analysis which prevents good dispersion of MWNCTs into epoxy system at 0.4 wt % loading. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Cure behavior of epoxy/MWCNT nanocomposites: The effect of nanotube surface modification

The effect of carboxyl and fluorine modified multi-wall carbon nanotubes (MWCNTs) on the curing behavior of diglycidyl ether of bisphenol A (DGEBA) epoxy resin was studied using differential scanning calorimetry (DSC), rheology and infrared spectroscopy (IR). Activation energy (E_a) and rate constants (k) obtained from isothermal DSC were the same for the neat resin and fluorinated MWCNT system (47.7 and 47.5 kJ/mol, respectively) whereas samples containing carboxylated MWCNTs exhibited a higher activation energy (61.7 kJ/mol) and lower rate constant. Comparison of the activation energies, rate constants, gelation behavior and vitrification times for all of the samples suggests that the cure mechanisms of the neat resin and fluorinated sample are similar but different from the carboxylated sample. This can be explained by the difference in how the fluorinated nanotubes react with the epoxy resin compared to the carboxylated nanotubes. Although the two systems have different reaction mechanisms, both systems have similar degrees of conversion as calculated from the infrared spectroscopic data, glass transition temperature (T_g), and predictions based on DSC data. This difference in reaction mechanism may be attributed to differences in nanotube dispersion; the fluorinated MWCNT system is more uniformly dispersed in the matrix whereas the more heterogeneously dispersed carboxylated MWCNTs can hinder mobility of the reactive species and disrupt the reaction stoichiometry on the local scale.     

Effect of carbon nanotubes functionalization on properties of their nanocomposites with polycarbonate/poly(acrylonitrile-butadiene-styrene) matrix

In this work, a comparative study evaluating the influence of different functionalization of carbon nanotubes on the properties of nanocomposites with polymeric matrix was performed. A 50/50 wt% polycarbonate (PC)/poly(acrylonitrile-butadiene-styrene) (ABS) blend was used as polymeric matrix of the nanocomposites. The comparison was made between nanocomposites reinforced with covalently functionalized multiwall carbon nanotubes (MWCNTf) and MWCNTf/nanoclay hybrid functionalization. The effect on the mechanical and morphological properties of the nanocomposites was evaluated through tensile and Izod impact tests and scanning electron microscopy and transmission electron microscopy (TEM) analyses. The thermal characterization of PC/ABS blends and nanocomposites was performed by differential scanning calorimetry (DSC). Results showed that both methods of functionalization of MWCNTs increased the stiffness and impact resistance of the nanocomposites. TEM micrographs indicated the preferred location of the reinforcements in the SAN phase of ABS. Results from DSC indicated an increase in the thermal resistance of the nanocomposites.     

Hydrogel‐MWCNT nanocomposites: Synthesis,characterization, and heating with radiofrequency fields

Hydrogel nanocomposites are attractive biomaterials for numerous applications including tissue engineering, drug delivery, cancer treatment, sensors, and actuators. Here we present a nanocomposite of multiwalled carbon nanotubes (MWCNT) and temperature responsive N‐isopropylacrylamide hydrogels. The lower critical solution temperature (LCST) of the nanocomposites was tailored for physiological applications by the addition of varying amounts of acrylamide (AAm). The addition of nanotubes contributed to interesting properties, including tailorability of temperature responsive swelling and mechanical strength of the resultant nanocomposites. The mechanical properties of the nanocomposites were studied over a range of temperatures (25–55°C) to characterize the effect of nanotube addition. A radiofrequency (RF) field of 13.56 MHz was applied to the nanocomposite discs, and the resultant heating was characterized using infrared thermography. This is the first report on the use of RF to remotely heat MWCNT‐hydrogel nanocomposites. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

The physical properties of carbon nanoparticles dispersed into NBR/LLDPE nanocomposites for corrosion protection applications

In this study, novel acrylonitrile butadiene rubber (NBR) nanocomposites with improved electrical conductivity and mechanical properties were synthesized. Carbon nanoparticles (CNP)/NBR composites and CNP‐polyethylene/NBR nanocomposites were prepared by mixing via two‐roll mill. The first type of the nanocomposite was produced to determine the percolation threshold concentration (V_c). The second type with constant CNP concentration, slightly over V_c (0.2 vol %), was synthesized to investigate the influence of polyethylene content on the mechanical, electrical and swelling behavior of nanocomposites. Only the nanocomposites with 3 vol % polyethylene loading showed electrical conductivity. However, the composites with higher polyethylene loadings showed insulating behavior due to hindrance of CNP network by polyethylene layers. Swelling measurements revealed that the change in entropy of the swelling increased with the increase in disorder level but decreased with the increase in intercalation level of CNP in the disordered intercalated nanocomposite. The increase in solvent uptake was comparable with the free volume in NBR matrix upon inclusion of nanoparticles, whereas the inhibition in solvent uptake for higher polyethylene loading was described by bridging flocculation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.     

Electrical properties of polymer nanocomposites containing rod-like nanofillers

We present an in-depth critical review of major experimental, simulation, and theoretical work in the field of conducting polymer nanocomposites containing rod-like particles such as carbon nanotubes and metal nanowires. These are a versatile class of materials that are of interest for a wide range of applications. Commercialization of various classes of conducting polymer nanocomposites is growing, yet achieving their full technological potential will hinge on the ability to engineer composites with controllable and well-defined properties, as well as aggressive exploration of new application areas. Thus, the focus of this review is to clarify key structure–property relationships, and to discuss the major gaps and greatest opportunities in the field.     

Controlling the miscibility of polyethylene/layered silicate nanocomposites by altering the polymer/surface interactions

Polyethylene/layered silicate nanocomposites are synthesized utilizing three types of polymeric surfactants/compatibilizers in order to influence the miscibility of polyethylene with the nanoparticle surface. The additives are designed so that they can play the role of a polymeric surfactant modifying the hydrophilic clay or of a compatibilizer with the organoclay. Model additives, especially synthesized for this study, included: polyethylene chains, which possess either a single functional end-group or multiple functional groups along the chain, as well as functional diblock copolymers. Maleic anhydrite grafted polyethylene with a low degree of functionalization was used as well. The structure of the resulting micro- or nanocomposites was investigated by X-ray diffraction and transmission electron microscopy. Immiscible hybrids as well as intercalated and/or exfoliated nanocomposites are obtained in a controlled way, depending on the kind of additive and its concentration in the mixture. The most important factor controlling the structure and the properties is the ratio of additive to nanoparticles. The rheological properties of the hybrids correlate well with the final micro- or nanostructure.     

Visualization of nanomechanical mapping on polymer nanocomposites by AFM force measurement

Nanocomposite based on an elastomer, natural rubber (NR), and pristine multi-walled carbon nanotubes (MWCNT) was prepared using a two-roll mill mixer. The high shearing stress induced homogeneous dispersion of 5 phr. MWCNTs in NR matrix. A procedure based on combination of Johnson-Kendall-Robert (JKR) contact mechanics and “two-point method” together with AFM force measurements, was successfully used to visualize nanomechanical mapping on the resulting nanocomposites. Topography, elastic modulus, and adhesive energy distribution maps were obtained at the same point and at the same time in a single scan. Such maps were successfully used to identify and characterize CNTs and NR regions in nanocomposites. The intermediate modulus region formed around CNTs was investigated on the quantitative evaluation in real space and demonstrated the existence of interaction between CNTs and NR matrix.     

The effect of clay dispersion on the properties of LLDPE/LLDPE‐g‐MAH/montmorillonite nanocomposites

In this work, three coupling agents presenting different grafting contents and molecular weights were used to prepare linear low density polyethylene (LLDPE)/linear low density polyethylene grafted with maleic anhydride (LLDPE‐g‐MAH)/montmorillonite nanocomposites with various morphologies. The clay dispersion was analyzed at the micrometric level by scanning electron microscopy and at the nanometric level by X‐ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that coupling agents having intermediate molecular weights led to the highest exfoliation extents, whereas the coupling agent presenting the highest molecular weight led to a poor delamination of the clay platelets. The properties of the nanocomposites produced and of their LLDPE/LLDPE‐g‐MAH reference blends were analyzed. It was shown that the best improvements in mechanical and barrier properties are not necessarily achieved for the nanocomposites, exhibiting the highest exfoliation extents. The length of the tactoids also plays a crucial role on the macroscopic properties. In addition, a high level of delamination could result in a loss of reinforcement effect, due to the inherent flexibility of the individual clay platelets. Finally, the strength of the clay/polymer interface, which was evaluated through surface tension measurements, seems to play a significant role on the properties of the nanocomposites. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers.     

Preparation and thermal properties of multiwalled carbon nanotube/polybenzoxazine nanocomposites

In this study, we prepared nanocomposites comprising multiwalled carbon nanotubes (MWCNTs) and polybenzoxazine (PBZ). The MWCNTs were purified through microwave digestion to remove most of the amorphous carbon and metal impurities. After purification, MWCNTs were treated with H₂SO₄/HNO₃ (3 : 1) to introduce hydroxyl and carboxyl groups onto their surfaces. Raman spectroscopy revealed the percentage of nanotube content improved after prolonged microwave treatment, as evidenced by the decrease in the ratio of the D (1328 cm^?1) and G (1583 cm^?1) bands. For the untreated MWCNTs, the I_D/I_G ratio was 0.56. After microwave treatment for 40 min, the value decreased to 0.29, indicating that the percentage of nanotube content improved. Dynamic mechanical analyses (DMAs) revealed that the storage moduli and the T_gs of the MWCNTs/PBZ nanocomposites were higher than that of the pristine PBZ. This is due to the nanometer‐scale MWCNTs restricting the motion of the macromolecular chains in the nanocomposites. Transmission electron microscopy (TEM) image revealed that the MWCNTs were well dispersed within the PBZ matrix on the nanoscale when the MWCNT content was less than 2.0 phr. The coefficient of thermal expansion (CTE) of the nanocomposites decreased on increasing the MWCNTs content. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Graphene/polyethylene nanocomposites: Effect of polyethylene functionalization and blending methods

Since its recent successful isolation, graphene has attracted an enormous amount of scientific interest due to its exceptional physical properties. Graphene incorporation can improve electrical and mechanical properties of polymers including polyethylene (PE). However, the hydrophobic nature and low polarity of PE have made effective dispersion of nano-fillers difficult without compatibilization. Graphene was derived from graphite oxide (GO) via rapid thermal exfoliation and reduction. This thermally reduced graphene oxide (TRG) was blended via melt and solvent blending with linear low density PE (LLDPE) and its functionalized analogs (amine, nitrile and isocyanate) produced using a ring-opening metathesis polymerization (ROMP) strategy. TRG was well exfoliated in functionalized LLDPE while phase separated morphology was observed in the un-modified LLDPE. Transmission electron micrographs showed that solvent based blending more effectively dispersed these exfoliated carbon sheets than did melt compounding. Tensile modulus was higher for composites with functionalized polyethylenes when solvent blending was used. However, at less than 3 wt.% of TRG, electrical conductivity of the un-modified LLDPE was higher than that of the functionalized ones. This may be due to phase segregation between graphene and PE, and electrical percolation within the continuous filler-rich phase.     

Compatibilised LDPE/LLDPE/nanoclay nanocomposites: I. Structural,mechanical, and thermal properties

Nanocomposites of LDPE/LLDPE/nanoclay have been prepared using a lab‐scale co‐rotating twin screw extruder. Using XRD, tensile testing, AFM, TGA, effects of some material properties and one processing parameter on mechanical and thermal properties of the prepared nanocomposites were evaluated. Tensile properties indicated that all the prepared nanocomposites exhibited a significant improvement in elastic modulus and toughness compared to pristine LDPE/LLDPE blends of the same composition. Thermal stability of nanocomposites in the air and nitrogen atmosphere was improved. XRD patterns and AFM micrographs showed semi‐exfoliated and intercalated microstructures for the prepared nanocomposites with different orders of mixing.     

Charge transfer properties and photoelectrocatalytic activity of TiO2/MWCNT hybrid

The vertically aligned multiwalled carbon nanotube (MWCNT) arrays on tantalum foils were successfully coated with TiO₂ nanoparticles by a hydrothermal process. The prepared TiO₂/MWCNT hybrid was characterized by scanning electron microscopy and transmission electron microscopy. The charge transfer properties and photocatalytic degradation of rhodamine B with and without bias potential under UV irradiation were investigated. The MWCNTs promoted the separation of photoinduced carriers in the TiO₂, thus enhanced photocatalytic activity. Applying bias potential on the photoanode further enhanced its catalytic activity. The efficient charge transportation and high photoelectrocatalytic activity towards degradation of rhodamine B made this hybrid material promising for photocatalyst and for the development of photoelectrical devices.     

Electrical conductivity and self-temperature-control heating properties of carbon nanotubes filled polyethylene films

Electrical properties of polyethylene and carbon nanotube composite films were investigated, when the composite films were set in heating box or under electric field at constant voltage. The composite films were prepared by gelation/crystallization from dilute solution. The mixture of ultra-high molecular weight polyethylene (UHMWPE) and branched low molecular weight polyethylene (LMWPE) was used as matrix, and multi-walled carbon nanotubes (MWNTs) were used as fillers. The filler content was chosen to be 10 wt% (ca. 5.25 vol%) which is a relatively higher loading than the percolation threshold to ensure to act as heating element in plane heater of composite film. The focus was concentrated on the temperature dependences of electric conductivity by external heating and by exothermic effect concerning self-temperature-control heating properties which were measured for the three kinds of UHMWPE-LMWPE composites with the same content of MWNTs in the composites. When a certain voltage was applied to the composite, the surface temperature of film reaches the equilibrium value within less than 100 s. The maximum surface temperature as the equilibrium state of the resultant composite film can be easily controlled by adjusting the composite ratio represented as UHMWPE/LMWPE. The high efficiency of heating and wide adjustability of stable temperature suggested its good application in high efficient plane heater.