Chapter 1 Carbon Nanotube-Reinforced Polymers a State of the art Review Karl Schulte, Florian H. Gojny, Bodo Fiedler Polymer Composites Section, Technical University Hamburg-Harburg, Hambur Jan K.w. Sandler Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, U.K. Wolfgang Bauhofer Materials in Electrical Engineering and Optics, Technical University Hamburg Harburg, Hamburg, Germany Introduction The investigation of fullerenes and especially of carbon nanotubes(CNT) has opened a totally new window for the development of polymer matrix composites with novel properties and applications. CNT, which have a number of unexpected properties, both mechanical and electrical, seem to have huge potentials as a filler i.e., as a reinforcement in nanocomposites. With the addition of only 0.05 vol% of CNT, the authors could achieve an electrical conductivity of less than 10-2S/m in an epoxy matrix. 2 With the discovery of carbon nanotubes, the research efforts initially concentrated on the better understanding of their processing conditions, modification,and properties. Initially, the application of CNT as a filler material in matrix polymers in order to improve their properties has not been in the focus of research. However, the CNT potential of quite high mechanical properties gave rise to the assumption that the fracture mechanical properties and, especially, th fracture toughness of polymers can be improved. The extremely low diameter of CNT and their high aspect ratio make them an ideal material to improve the properties of the polymer matrix, compared to glass, carbon, or aramid fibers
Chapter 1 Carbon Nanotube-Reinforced Polymers: a State of the Art Review Karl Schulte, Florian H. Gojny, Bodo Fiedler Polymer Composites Section, Technical University Hamburg-Harburg, Hamburg, Germany Jan K.W. Sandler Depanment of Materials Science and Metallurgy, University of Cambridge, Cambridge, U. K. Wolfgang Bauhofer Materials in Electrical Engineering and Optics, Technical University Hamburg-Harburg, Hamburg, Germany 1 Introduction The investigation of fullerenes and especially of carbon nanotubes (CNT) has opened a totally new window for the development of polymer matrix composites with novel properties and applications. CNT, which have a number of unexpected properties, both mechanical and electrical, seem to have huge potentials as a filler, i.e., as a reinforcement in nanocomposites. With the addition of only 0.05 vol.% of CNT, the authors could achieve an electrical conductivity of less than S/m in an epoxy matrix.'e2 With the discovery of carbon nanotubes, the research efforts initially concentrated on the better understanding of their processing conditions, modification, and properties. Initially, the application of CNT as a filler material in matrix polymers in order to improve their properties has not been in the focus of research. However, the CNT potential of quite high mechanical properties gave rise to the assumption that the fracture mechanical properties and, especially, the fracture toughness of polymers can be improved. The extremely low diameter of CNT and their high aspect ratio make them an ideal material to improve the properties of the polymer matrix, compared to glass, carbon, or aramid fibers
Polymer Composites: from Nano- to Macroscale This should lead to a reduced sensitivity to delaminations, as the CNTs, having a diameter which is a couple of hundred times smaller than that of carbon fibers can reinforce the tiny area in between the fibers of a laminated composite. They should also improve the elastic modulus of the matrix material itself. This last statement is mainly based on theoretical calculations and simulations, which predict a high fracture strength and elastic modulus for carbon nanotubes. 3-6 In this chapter, we report technologies to produce CNT-nanocomposite their resulting microstructures, and their mechanical properties Tests were performed on various epoxy-based matrices, which were reinforced with catalytically and arc- own multiple-wall carbon nanotubes(MWCNT). The arc-grown CNT were produced in a furnace, developed at the Technical University Hamburg-Harburg (TUHH), while the catalytically grown CNT were received from various sources Carbon nonotubes were used as-received, but also after the application of chromato- graphic filter methods and oxidative chemical treatment. The latter enables a cleaning from the soot and can be used as a first step in the functionalization of the CNT'surfaces. The oxidative treatment leads to the formation of carboxylic groups which support via steric hindrance the dispersion of the tubes and stabilize them in solution or in the matrix material. This is an important step for the homogenous istribution of the CNT in polymers For the homogenous dispersion of the CNT in the matrix, a number of techniques were used. The quality of the nanotube distribution in the matrix was nspected by high resolution transmission electron microscopy(tEM). 2 General Problems in Nanocomposite Technology The term nanocomposites means to distribute as much as possible amounts of nanoparticles in a polymer matrix. It has been shown that, when using fiber- reinforced polymers, the maximum amount of fibers in the matrix is about 70 vol. Actually, the fiber volume fraction in these materials varies between 20 and 60% Figure I summarizes the values to be achieved for various fillers used in composites In a volume element of I cm it was assumed that we have continuous fibers with a diameterof 10 um, particles(e. g, talcum) with a diameter of 1 um, and nanotubes with a diameter of 10 nm. The aspect ratios should be 20, 100, and 1000, respec tively. If we further assume a volume content of 30% for both the fibers and th particles, and only 3% for the nanotubes, we already have quite interesting results he filler element numbers are fibers, -10 0 particles, and-10 5 nano tubes, respectively. The surface area amounts to 0. 1 m for the fibers, - l m2 for he particles, and-100 m2 for the nanotubes. There is another important aspect to be found and that is the distance between the filler elements For the fibers we calculated a distance of -10 um, for the particles -l um, and for the nanotubes 100nm
4 Polymer Composites: from Nano- to Macroscale This should lead to a reduced sensitivity to delaminations, as the CNTs, having a diameter which is a couple of hundred times smaller than that of carbon fibers, can reinforce the tiny area in between the fibers of a laminated composite. They should also improve the elastic modulus of the matrix material itself. This last statement is mainly based on theoretical calculations and simulations, which predict a high fracture strength and elastic modulus for carbon nanot~bes.~-~ In this chapter, we report technologies to produce CNT-nanocomposites, their resulting microstructures, and their mechanical properties. Tests were performed on various epoxy-based matrices, which were reinforced with catalytically and arcgrown multiple-wall carbon nanotubes (MWCNT). The arc-grown CNT were produced in a furnace, developed at the Technical University Hamburg-Harburg (TUHH), while the catalytically grown CNT were received from various sources. Carbon nonotubes were used as-received, but also after the application of chromatographic filter methods and oxidative chemical treatment. The latter enables a cleaning from the soot and can be used as a first step in the functionalization of the CNT surfaces. The oxidative treatment leads to the formation of carboxylic groups, which support via steric hindrance the dispersion of the tubes and stabilize them in solution or in the matrix material. This is an important step for the homogenous distribution of the CNT in polymers. For the homogenous dispersion of the CNT in the matrix, a number of techniques were used. The quality of the nanotube distribution in the matrix was inspected by high resolution transmission electron microscopy (TEM). 2 General Problems in Nanocomposite Technology The term nanocomposites means to distribute as much as possible amounts of nanoparticles in a polymer matrix. It has been shown that, when using fiberreinforced polymers, the maximum amount of fibers in the matrix is about 70 vol.%. Actually, the fiber volume fraction in these materials varies between 20 and 60%. Figure 1 summarizes the values to be achieved for various fillers used in composites. In a volume element of 1 cm3, it was assumed that we have continuous fibers with a diameter of 10 pm, particles (e.g., talcum) with a diameter of 1 pm, and nanotubes with a diameter of 10 nm. The aspect ratios should be 20, 100, and 1000, respectively. If we further assume a volume content of 30% for both the fibers and the particles, and only 3% for the nanotubes, we already have quite interesting results: The filler element numbers are -106fibers, -10l0 particles, and -lOI5 nanotubes, respectively. The surface area amounts to -0.1 m2 for the fibers, -1 m2 for the particles, and -100 m2 for the nanotubes. There is another important aspect to be found, and that is the distance between the filler elements. For the fibers we calculated a distance of -10 pm, for the particles -1 pm, and for the nanotubes -100 nm
Chapter 1: Carbon Nanotube-Reinforced Polyme Fiber Talcum Particle siz Number of particles Interface Figure I. Influence of particles on the surface area These values already show the problems to overcome. Nanotechnolo means that one has to deal with huge surface areas, a vast amount of nanofillers and a small distance between ther Figure 2 shows a calculation of the distance between nanotubes depending on their volume fraction. For this example, we assumed to have single-wall carbon nanotubes(SWCNT)with a diameter of ca I nm. at a volume fraction of 3 %, the distance between two tubes is just two times the diameter(ca. 3. 2 nm). If we now assume to have isotactic polystyrene(iPS) as matrix polymer with a chain diameter Spacing(nm) re 2. Correlation of the spacing between individual single-wall nanotubes(d= l um)and volu of 0.8 nm, there is only a small gap for the polymer to penetrate between the tubes The main conclusion from this result is that it is extremely pro achieve high volume contents of nanotubes in a polymer matrix. All those who
Chapter 1: Carbon Nanotube-Reinforced Polymers 5 Particle size 10 pm 1 Pm Volume content 30% 30% Number of particles - lo6 - 1O1O Interface - 0.1 m2 - I mZ Aspect ratio - 20 - 100 Figure 1. Influence of particles on the surface area. These values already show the problems to overcome. Nanotechnology means that one has to deal with huge surface areas, a vast amount of nanofillers, and a small distance between them. Figure 2 shows a calculation of the distance between nanotubes depending on their volume fraction. For this example, we assumed to have single-wall carbon nanotubes (SWCNT) with a diameter of ca. 1 nm. At a volume fraction of 3%, the distance between two tubes is just two times the diameter (ca. 3.2 nm). If we now assume to have isotactic polystyrene (iPS) as matrix polymer with a chain diameter 0.1 I' 10 100 Spacing (nm) Figure 2. Correlation of the spacing between individual single-wall nanotubes (d = 1 pm) and their volume fraction. of 0.8 nm, there is only a small gap for the polymer to penetrate between the tubes. The main conclusion from this result is that it is extremely problematic to achieve high volume contents of nanotubes in a polymer matrix. All those who
Polymer Composites: from Nano-to Macroscale report contents in the range of 10% or more might have agglomerates rather than a proper distribution E 3.1 Manufacturing of Multiple-Wall Carbon Nanotubes A number of methods have been developed to produce single -and multiple wall carbon nanotubes. The most prominent are laser ablation, arc discharge, and catalytic growth(vapor deposition(CVD)and high pressure carbon monoxide (HipCO) process In the present chapter, the MWCNt were produced at the TUHH by the arc discharge method in a Kratschmer generator. This method has previously been veloped for the production of fu today it is a common method to produce MWCNT without having a catalyst. It is based on the ignition of an electric rc between two graphite electrodes. a direct current of 20 to 30 V between the electrodes, in a helium atmosphere at a pressure of about 500 mbar, produces a plasma in which carbon is vaporized from the anode and reorganized at the cathode forming a cylindrical deposit. During the process, the distance between the electrodes and the current density have to remain constant. Inside the deposit, which has a hard shell of turbostratic graphite, MWCNT are formed. Figure 3 is a scanning electron micrograph showing the nanotube-containing core of such a deposit, produced in a joint venture of TUHH and Trinity College, Dublin. In case that 6.2 20, 0K x1. Ok 50um Figure 3. Scanning electron micrograph of the core of a deposit produced in a Kratschmer The outer shell, consisting of tropostratic graphite, has been removed. The amorphous core material contains up to 50% multiple-wall CNT
6 Polymer Composites: from Nano- to Macroscale report contents in the range of 10% or more might have agglomerates rather than a proper distribution. 3 Experimental 3.1 Manufacturing of Multiple-Wall Carbon Nanotubes A number of methods have been developed to produce single- and multiplewall carbon nanotubes. The most prominent are laser ablation, arc discharge, and catalytic growth (vapor deposition (CVD) and high pressure carbon monoxide (HipCO) process). In the present chapter, the MWCNT were produced at the TUHH by the arc discharge method in a Wtschmer generator? This method has previously been developed for the production of fullerenes, but today it is a common method to produce MWCNT without having a catalyst. It is based on the ignition of an electric arc between two graphite electrodes. A direct current of 20 to 30 V between the electrodes, in a helium atmosphere at a pressure of about 500 mbar, produces a plasma in which carbon is vaporized from the anode and reorganized at the cathode, forming a cylindrical deposit. During the process, the distance between the electrodes and the current density have to remain constant. Inside the deposit, which has a hard shell of turbostratic graphite, MWCNT are formed. Figure 3 is a scanning electron micrograph showing the nanotube-containing core of such a deposit, produced in a joint venture of TUHH and Trinity College, Dublin. In case that an Figure 3. Scanning electron micrograph of the core of a deposit produced in a Kriitschmer generator. The outer shell, consisting of tropostratic graphite, has been removed. The amorphous core material contains up to 50% multiple-wall CNT
Chapter 1: Carbon Nanotube-Reinforced Polymers ultra-fine metal powder(Ni, Fe, Co, Cr)is added to the anode, single-wall carbon nanotubes can be found on the walls of the reactor. The currently most successfully commercialized route is based on the CVD technique. Catalyst particles, such as Fe, Ni, or other metal catalysts, are introduced onto a substrate which is placed in a furnace. The growth of filaments(carbon nanotubes)is most sensitive to the reaction conditions, such as the pressure of the vaporized carbon source(benzene, toluene, etc. ) the purity and flow rate of the carrier gas, the residence time for thermal decomposition, and the temperature of In addition to the arc-grown CNT produced at the TUHH, catalytically grown CNT, received from various sources, were also used 3.2 Treatment of Carbon Nanotubes The dispersion of the nanotubes was performed either via direct stirring of the CNts into the epoxy using an Ultraturrax T-25 disperser, or via a sonication method. The sonicator, a Bandelin Sonoplus HD2200, generates a pulsed ultrasound whereby the nanotubes were dispersed in the epoxy resin, apu/ A purification of the CNTs has been performed by oxidation using a mixture of sulfuric and nitric acid at 100C for 3 h The separation of the oxidized nanotubes from the reaction solution was performed via centrifugation and membrane microfiltration The oxidized carbon nanotubes were then functionalized with different types of amines, such as triethylenetetramine, ethylenediamine and polyether amines. The functionalization has been performed by refluxing a suspension of oxidized CNTs with the amine for 12 h to ensure a quantitative conversion. The separation of the nanotubes from the solution was again performed centrifugation and membrane microfiltration 3 Matrix Polymers The epoxy resins Araldite LY556 with the hardener HY932 and Ruetapox 64 with a polyetheramine hardener (Jeffamineo T-403)were used. The JeffamineT-403 can be used as hardener and/or as flexibilizer for epoxies and leads to thermosets with improved ductility 3.4 Electron Microscopy Transmission electron microscopy (TEM) was performed with an EM400 type microscope from Philips. Scanning electron microscopy(SEM)was performed with a Gemini GSMIC-848 apparatus
Chapter I: Carbon Nanotube-Reinforced Polymers 7 ultra-fine metal powder (Ni, Fe, Co, Cr) is added to the anode, single-wall carbon nanotubes can be found on the walls of the reactor. The currently most successfully commercialized route is based on the CVD technique. Catalyst particles, such as Fe, Ni, or other metal catalysts, are introduced onto a substrate which is placed in a furnace. The growth of filaments (carbon nanotubes) is most sensitive to the reaction conditions, such as the pressure of the vaporized carbon source (benzene, toluene, etc.), the purity and flow rate of the carrier gas, the residence time for thermal decomposition, and the temperature of the furna~e.~ In addition to the arc-grown CNT produced at the TUHH, catalytically grown CNT, received from various sources, were also used. 3.2 Treatment of Carbon Nanotubes The dispersion of the nanotubes was performed either via direct stirring of the CNTs into the epoxy using an Ultraturrax T-25 disperser, or via a sonication method. The sonicator, a Bandelin Sonoplus HD2200, generates a pulsed ultrasound whereby the nanotubes were dispersed in the epoxy resin? A purification of the CNTs has been performed by oxidation using a mixture of sulfuric and nitric acid at 100 "C for 3 h. The separation of the oxidized nanotubes from the reaction solution was performed via centrifugation and membrane microfiltration. The oxidized carbon nanotubes were then functionalized with different types of amines, such as triethylenetetramine, ethylenediamine and polyetheramines. The functionalization has been performed by refluxing a suspension of oxidized CNTs with the amine for 12 h to ensure a quantitative conversion. The separation of the nanotubes from the solution was again performed via centrifugation and membrane microfiltration. 3.3 Matrix Polymers The epoxy resins Araldite LY556 with the hardener HY932 and Ruetapox LV 0164 with a polyetheramine hardener (JeffamineB T-403) were used. The JeffamineB T-403 can be used as hardener andlor as flexibilizer for epoxies and leads to thermosets with improved ductility. 3.4 Electron Microscopy Transmission electron microscopy (TEM) was performed with an EM400 type microscope from Philips. Scanning electron microscopy (SEM) was performed with a Gemini GSMIC-848 apparatus