Polymer Composites: from Nano-to macroscale 3.5 Dynamic-Mechanical Thermal Analysis The study of the thermo-mechanical behaviour was performed by dyna mechanical thermal analysis(DMTA), using a Gabo Plexor 500 N equipment Rectangular specimens, 50 mm long, 5 mm wide, and 2 mm thick, were prepared for the measurements. The tests were performed in tensile mode at a frequency of 10 Hz with a static strain of 0.6% and a dynamic strain of t0.1%. The samples were measured between-120C and 150C at a heating rate of 3C/min Results Comparison of the Multiple-Wall Carbon Nanotubes Studied Arc-grown carbon CNT form relatively stiff rods consisting of concentric multiple-wall tubes(Figure 4). Due to this production process, graphitic nanopar- ticles can be observed next to the tubes The amount of this so-called soot can exceed 30%of the total material. Therefore, a cleaning process is required before the use of CNT in the production of composites. The catalytically grown carbon nanotubes have not as much soot, but catalist particle remains are present as dark spots. Figure 5 is a transmission electron micrograph of catalytically grown CNT (produced at the University of Cambridge): CNT are extremely long, highly entangled, and with a cotton-like appearance. The tubes are highly crystalline, but compared to the arc grown ones, they have a very high defect concentration, mainly within the outer Figure 4. Transmission electron micrograph of arc-grown carbon nanotubes. A high amount of carbon nanotubes and the soot consisting of graphitic nanoparticles and carbon black are clearly seen
8 Polymer Composites: from Nano- to Macroscale 3.5 Dynamic-Mechanical Thermal Analysis The study of the thermo-mechanical behaviour was performed by dynarnicmechanical thermal analysis (DMTA), using a Gabo Eplexor 500 N equipment. Rectangular specimens, 50 mm long, 5 mm wide, and 2 mm thick, were prepared for the measurements. The tests were performed in tensile mode at a frequency of 10 Hz with a static strain of 0.6% and a dynamic strain of +0.1%. The samples were measured between -120 "C and 150 "C at a heating rate of 3 "Clmin. 4 Results 4.1 Comparison of the Multiple-Wall Carbon Nanotubes Studied Arc-grown carbon CNT form relatively stiff rods consisting of concentric multiple-wall tubes (Figure 4). Due to this production process, graphitic nanoparticles can be observed next to the tubes. The amount of this so-called soot can exceed 30% of the total material. Therefore, a cleaning process is required before the use of CNT in the production of composites. The catalytically grown carbon nanotubes have not as much soot, but catalist particle remains are present as dark spots. Figure 5 is a transmission electron micrograph of catalytically grown CNT (produced at the University of Cambridge); CNT are extremely long, highly entangled, and with a cotton-like appearance. The tubes are highly crystalline, but compared to the arcgrown ones, they have a very high defect concentration, mainly within the outer Figure 4. Transmission electron micrograph of arc-grown carbon nanotubes. A high amount of carbon nanotubes and the soot consisting of graphitic nanoparticles and carbon black are clearly seen
Chapter 1: Carbon Nanotube-Reinforced Polymers 50 Figure 5. Transmission electron micrograph of catalytically grown multiple-wall carbon nanotubes with less impurities, but with a higher defect density and strongly entangled layers, which should affect the mechanical properties. Figure 6 is a scanning electron micrograph of CVD-grown MWCNT; they grow perpendicular to the catalyst but are extremely parallel and, in addition, pronounced entanglements forming the , Figure 6. S multiple-wall carbon nanotubes The cotton pearance is due to the manufacturing CVd process, where CNt grow onto substrate plate and, in addition to the strong entanglement, parallel orientation of the tubes can be achieved. cotton-like structure are seen. Figure 7 is a transmission electron micrograph of CVD-grown MWCNT; the catalyst particles and the high defect density are clearly seen. These two findings suggest that arc-grown carbon nanotubes have a much lower defect density than the CVd tubes. As they are less entangled, they can be more easily dispersed than th ytically grown tubes
Chapter 1: Carbon Nanotube-Reinforced Polymers Figure 5. Transmission electron micrograph of catalytically grown multiple-wall carbon nanotubes with less impurities, but with a higher defect density and strongly entangled. layers, which should affect the mechanical properties. Figure 6 is a scanning electron micrograph of CVD-grown MWCNT; they grow perpendicular to the catalyst but are extremely parallel and, in addition, pronounced entanglements forming the Figure 6. Scanning electron micrograph of catalytically grown multiple-wall carbon nanotubes. The cotton-like appearance is due to the manufacturing CVD process, where CNT grow onto a catalyst-covered substrate plate and, in addition to the strong entanglement, a generally parallel orientation of the tubes can be achieved.I8 cotton-like structure are seen. Figure 7 is a transmission electron micrograph of CVD-grown MWCNT, the catalyst particles and the high defect density are clearly seen. These two findings suggest that arc-grown carbon nanotubes have a much lower defect density than the CVD tubes. As they are less entangled, they can be more easily dispersed than the catalytically grown tubes
Polymer Composites: from Nanoto Macroscale 100nm Figure 7. Transmission electron micrograph of catalytically grown multiple-wall carbon nanotubes with a high defect density. Catalyst particles are clearly seen 4.2 Purification s both the CVD-and arcgrown carbon nanotubes have impurities(amor phous carbon or catalyst residues), they should be cleaned. A number of purification methods have been described in the literature. In addition to chromatographic methods, the oxidative technique has been successfully used, -l6 by taking inte account the differences in the thermodynamic stability of carbon black, graphite, and the nanotubes. The wet chemical oxidation with a solution of sulfuric and nitric acids has been shown to be effective In our studies the nanotubes were refluxed in a solution of 3 parts of sulfuric and 1 part of nitric acid for 3 h at 130C. After centrifugation or membrane micro-filtration and washing with water, the nanotubes were refluxed for a couple of hours in a dilute sodium hydrogen carbonate solution to really eliminate the remaining acids. TEM clearly showed a reduction in impurities. It could be also observed that, due to this purification process, the nanotubes start to be oxidized first at their caps and structural defects, and that functional groups(mainly carboxylic, which can be used for further modification)are formed on the surfaces(Figure 8). As this process injures the tubes, the caps of the Figure 8. The oxidation process of the nanotubes leads to the formation of carboxylic and referentially on the caps or on structural defects cap opening cannot be avoide
Polymer Composites: from Nano- to Macroscale Figure 7. Transmission electron micrograph of catalytically grown multiple-wall carbon nanotubes with a high defect density. Catalyst particles are clearly seen. 4.2 Purification As both the CVD- and arc-grown carbon nanotubes have impurities (amorphous carbon or catalyst residues), they should be cleaned. A number of purification methods have been described in the literature. In addition to chromatographic methods, the oxidative technique has been successfully used,'@16 by taking into account the differences in the thermodynamic stability of carbon black, graphite, and the nanotubes. The wet chemical oxidation with a solution of sulfuric and nitric acids has been shown to be effective. In our studies, the nanotubes were refluxed in a solution of 3 parts of sulfuric and 1 part of nitric acid for 3 h at 130 "C. After centrifugation or membrane micro-filtration and washing with water, the nanotubes were refluxed for a couple of hours in a dilute sodium hydrogen carbonate solution to really eliminate the remaining acids. TEM clearly showed a reduction in impurities. It could be also observed that, due to this purification process, the nanotubes start to be oxidized first at their caps and structural defects, and that functional groups (mainly carboxylic, which can be used for further modification) are formed on the surfaces (Figure 8). As this process injures the tubes, the caps of the Figure 8. The oxidation process of the nanotubes leads to the formation of carboxylic and hydroxylic groups on the tube surfaces, preferentially on the caps or on structural defects. The cap opening cannot be avoided
Chapter 1: Carbon Nanotube-Reinforced Polymers nanotubes can be opened or totally disappear(Figure 9), which is a disadvantage of the oxidative technique. The catalysts do not only help the formation and build up of CNT; in this special case, they also contribute to the degradation of nanotube 00 9. Transmission electron micrograph of oxidized carbon nanotubes. The opening of due to the formation of functional groups is clearly seen 4.3 CNT/Epoxy Composites: Dispersion, Matrix Bonding, and Functionalization 4.3.1 Dispersion The use of nanoparticles as reinforcement elements in polymers is a common method to improve the mechanical and/or electrical properties of composites. The improvement of the fracture mechanics properties by the addition of particles can be achieved when a sufficiently good interaction between the nanoparticles and the matrix polymer takes place and when the particles are well dispersed within the matrix Our studies aimed at the improvement of the mechanical properties of poxies by the addition of multiple-wall carbon nanotubes. The quality of the dispersion and the possible interaction between the CNT and the epoxy were investigated by light microscopy. For the homogeneous distribution of carbon black in an epoxy matrix, it was sufficient to use a high-speed disperser (Ultraturrax T-25), but for the dispersion of nanotubes this approach was not optimal( Figure 10a) The nanometer-scale size of the CNt and their aspect ratio make it necessary to develop new dispersion techniques in order to brake up the intermolecular bonds leading to the formation of agglomerate In this respect, sonication has been found to be a promising alternative. suspension of the CNt in an appropriate solvent (acetone)can be sonicated with a pulse ultrasound to break up the agglomerates. This suspension can later be mixed with the epoxy and the solvent is evaporated by simple heating. The nanotubes were sonicated for 20 min at a 30% amplitude, mixed with the epoxy resin and
Chapter 1: Carbon Nanotube-Reinforced Polymers 11 nanotubes can be opened or totally disappear (Figure 9), which is a disadvantage of the oxidative technique. The catalysts do not only help the formation and build up of CNT; in this special case, they also contribute to the degradation of nanotubes. Figure 9. Transmission electron micrograph of oxidized carbon nanotubes. The opening of the caps due to the formation of functional groups is clearly seen. 4.3 CNTIEpoxy Composites: Dispersion, Matrix Bonding, and Functionalization 4.3.1 Dispersion The use of nanoparticles as reinforcement elements in polymers is a common method to improve the mechanical and/or electrical properties of composites. The improvement of the fracture mechanics properties by the addition of particles can be achieved when a sufficiently good interaction between the nanoparticles and the matrix polymer takes place and when the particles are well dispersed within the matrix. Our studies aimed at the improvement of the mechanical properties of epoxies by the addition of multiple-wall carbon nanotubes. The quality of the dispersion and the possible interaction between the CNT and the epoxy were investigated by light microscopy. For the homogeneous distribution of carbon black in an epoxy matrix, it was sufficient to use a high-speed disperser (Ultraturrax T-25), but for the dispersion of nanotubes this approach was not optimal (Figure 10a). The nanometer-scale size of the CNT and their aspect ratio make it necessary to develop new dispersion techniques in order to brake up the intermolecular bonds, leading to the formation of agglomerates. In this respect, sonication has been found to be a promising alternative. A suspension of the CNT in an appropriate solvent (acetone) can be sonicated with a pulse ultrasound to break up the agglomerates. This suspension can later be mixed with the epoxy and the solvent is evaporated by simple heating. The nanotubes were sonicated for 20 min at a 30% amplitude, mixed with the epoxy resin and
Polymer Composites: from Nano-to macroscale (a) ■圈■ Pure epoxy square side 10 mm Figure 10: Light microscopy of a series of CNT/epoxy nanocor es with different amounts of nanotubes:(a)CVD-grown CNT, 0,0.1,0.2, and 0.4 wt % (b )arc-grown carbon nanotubes, 0.4, 0.8, and 1.5 wt % The arc-grown carbon nanotubes tend to show less agglomerations than the CVD-grown CNT. sonicated again for 10 min, During sonication, the samples have to be cooled to avoid curing of the composite. Finally, the composites were cured for 5 h at 80C and for 3 h at 130C in vacuum for the post curing This approach allowed to substantially reduce the size of the agglomerates( Figure 10b) A further reduction of the agglomerate size can be achieved by a combination of sonication and oxidative cleaning. The functional groups which develop on the surfaces on the CNT lead to steric hindrance and electrostatic interactions with the solvent hence to a better distribution of the nanotubes in the epoxy matrix(Figure 11). Figure 11. Transmission electron micrograph of a CNT/epoxy composite Oxidation followed by sonication leads to an improved dispersion. 8
12 Polymer Composites: from Nano- to Macroscale (a\ 1 scale unit = 10 mm \-/ 4 ..., +I (P"; A -4 i II i P a II L-.. - -. . - Pure epoxy -.m 0.1 wt.% a 0.2 ~1.~70 a 0.4 WI.% IB[I Figure 10: Light microscopy of a series of CNTIepoxy nanocomposites with different amounts of nanotubes: (a) CVD-grown CNT, 0,0.1,0.2, and 0.4 wt.%; (b) arc-grown carbon nanotubes, 0.4,0.8, and 1.5 wt.%. The arc-grown carbon nanotubes tend to show less agglomerations than the CVD-grown CNT. sonicated again for 10 min. During sonication, the samples have to be cooled to avoid curing of the composite. Finally, the composites were cured for 5 h at 80 "C and for 3 h at 130 "C in vacuum for the post curing. This approach allowed to substantially reduce the size of the agglomerates (Figure lob). A further reduction of the agglomerate size can be achieved by a combination of sonication and oxidative cleaning. The functional groups which develop on the surfaces on the CNT lead to steric hindrance and electrostatic interactions with the solvent, hence to a better distribution of the nanotubes in the epoxy matrix (Figure 11). Figure 11. Transmission electron micrograph of a CNTlepoxy composite. Oxidation followed by sonication leads to an improved dispersion.18