E.T. Thostenson et al. Composites Science and Technology 61(2001)1899-1912 Because the interaction at the nanotube/ matrix inter face is critical to understanding the mechanical behavior of nanotube-based composites, a number of researchers have investigated the efficiency of interfacial stress transfer. Wagner et al. [67 examined stress-induced fragmentation of multi-walled carbon nanotubes in polymer films. Their nanotube-containing film had a thickness of approximately 200 um. The observed frag mentation phenomenon was attributed to either pro- cess-induced stress resulting from curing of the polymer or tensile stress generated by polymer deformation and transmitted to the nanotube. from estimated values of nanotube axial normal stress and elastic modulus 500nm Wagner and co-workers concluded that the nanotube, polymer interfacial shear stress is on the order of 500 Fig 13. Fracture mechanisms in nanotube-based composites[] MPa and higher. This value, if reliable, is an order of magnitude higher than the stress-transfer ability of cur- rent advanced composites and, therefore, such inter- faces are more able than either the matrix or the bridging by the nanotubes. They also used short-fiber nanotubes themselves to sustain shear. In further work composite theory to demonstrate that 10% by weight of Lourie and Wagner [68-70 investigated tensile and carbon fibers(about 5% by volume) in the research of compressive fracture in nanotube-based composites Tibbetts and McHugh [63] would be required to achieve Stress transfer has also been investigated by raman he same increase in elastic modulus with 1%(by spectroscopy. Cooper and co-workers [71] prepared composite, but to take full advantage of the exceptional Raman band(2610 cm -)to a lower 28<99 weight) of carbon nanotubes composite specimens by applying an epoxy-resin/nano Fig. 13 shows significant pull-out of the nanotubes tube mixture to the surface of an epoxy beam. After the rom the matrix. Clearly there is reinforcement as evi- specimens were cured, stress transfer between the poly denced by improvements in stiffness and strength of the mer and the nanotubes was detected by a shift in the g avenu ber. The stifness, strength, and resilience of carbon nanotubes, shift in the G Raman band corresponds to strain in the strong interfacial bonding is critical. Jia et al. [64 graphite structure, and the shift indicates that there is showed that the nanotubes can be initiated by a free- stress transfer, and hence reinforcement, by the nano- radical initiator, AiBN (2, 2'-azobisisobutyronitrile), to tubes. It was also concluded that the effective modulus open their bonds In their study of carbon-nanotube/ of single-walled nanotubes dispersed in a composite poly(methyl methacrylate)(PMMA) composites, the could be over 1 TPa and that of multi-walled nanotubes possibility exists to form a C-c bond between the was about 0.3 TPa In their investigation of single-wal nanotube and the matrix. Gong et al. [65] investigated led nanotube/epoxy composites, Ajayan et al. [72]sug- surfactant-assisted processing of nanotube composites gest that their nearly constant value of the Raman peak with a nonionic surfactant. Improved dispersion and in tension is related to tube sliding within the nanotube interfacial bonding of the nanotubes in an epoxy matrix bundles and, hence, poor interfacial load transfer resulted in a 30% increase in elastic modulus with between the nanotubes. Similar results were obtained by addition of I wt% nanotubes Schadler et al. [73. Multi-walled nanotube/epoxy com Lordi and Yao [66] looked at the molecular mechan- posites were tested in both tension and compression ics of binding in nanotube-based composites. In their The compressive modulus was found to be higher than work, they used force-field-based molecular-mechanics the tensile modulus of the composites, and the raman calculations to determine the binding energies and slid peak was found to shift only in compression, indicating ing frictional stresses between pristine carbon nanotubes poor interfacial load transfer in tension and different polymeric matrix materials. The binding Even with improved dispersion and adhesion, micro energies and frictional forces were found to play only a mechanical characterization of these composites is diffi minor role in determing the strength of the interface. cult because the distribution of the nanotubes is The key factor in forming a strong bond at the interface random. Thus, attempts have been made to align nano- is having a helical conformation of the polymer around tubes in order better to elucidate the reinforcement the nanotube. They suggested that the strength of the mechanisms. Jin et al. [74] showed that aligned nano- interface may result from molecular-level entanglement tube composites could be obtained by mechanical of the two phases and forced long-range ordering of the stretching of the composite. X-ray diffraction was used to determine the orientation and degree of alignmentbridging by the nanotubes. They also used short-fiber composite theory to demonstrate that 10% by weight of carbon fibers (about 5% by volume) in the research of Tibbetts and McHugh [63] would be required to achieve the same increase in elastic modulus with 1% (by weight) of carbon nanotubes. Fig. 13 shows significant pull-out of the nanotubes from the matrix. Clearly there is reinforcement as evi￾denced by improvements in stiffness and strength of the composite, but to take full advantage of the exceptional stiffness, strength, and resilience of carbon nanotubes, strong interfacial bonding is critical. Jia et al. [64] showed that the nanotubes can be initiated by a free￾radical initiator, AIBN (2,20 -azobisisobutyronitrile), to open their p bonds. In their study of carbon-nanotube/ poly(methyl methacrylate) (PMMA) composites, the possibility exists to form a C–C bond between the nanotube and the matrix. Gong et al. [65] investigated surfactant-assisted processing of nanotube composites with a nonionic surfactant. Improved dispersion and interfacial bonding of the nanotubes in an epoxy matrix resulted in a 30% increase in elastic modulus with addition of 1 wt.% nanotubes. Lordi and Yao [66] looked at the molecular mechan￾ics of binding in nanotube-based composites. In their work, they used force-field-based molecular-mechanics calculations to determine the binding energies and slid￾ing frictional stresses between pristine carbon nanotubes and different polymeric matrix materials. The binding energies and frictional forces were found to play only a minor role in determing the strength of the interface. The key factor in forming a strong bond at the interface is having a helical conformation of the polymer around the nanotube. They suggested that the strength of the interface may result from molecular-level entanglement of the two phases and forced long-range ordering of the polymer. Because the interaction at the nanotube/matrix inter￾face is critical to understanding the mechanical behavior of nanotube-based composites, a number of researchers have investigated the efficiency of interfacial stress transfer. Wagner et al. [67] examined stress-induced fragmentation of multi-walled carbon nanotubes in polymer films. Their nanotube-containing film had a thickness of approximately 200 mm. The observed frag￾mentation phenomenon was attributed to either pro￾cess-induced stress resulting from curing of the polymer or tensile stress generated by polymer deformation and transmitted to the nanotube. From estimated values of nanotube axial normal stress and elastic modulus, Wagner and co-workers concluded that the nanotube/ polymer interfacial shear stress is on the order of 500 MPa and higher. This value, if reliable, is an order of magnitude higher than the stress-transfer ability of cur￾rent advanced composites and, therefore, such inter￾faces are more able than either the matrix or the nanotubes themselves to sustain shear. In further work, Lourie and Wagner [68–70] investigated tensile and compressive fracture in nanotube-based composites. Stress transfer has also been investigated by Raman spectroscopy. Cooper and co-workers [71] prepared composite specimens by applying an epoxy-resin/nano￾tube mixture to the surface of an epoxy beam. After the specimens were cured, stress transfer between the poly￾mer and the nanotubes was detected by a shift in the G0 Raman band (2610 cm 1 ) to a lower wavenumber. The shift in the G0 Raman band corresponds to strain in the graphite structure, and the shift indicates that there is stress transfer, and hence reinforcement, by the nano￾tubes. It was also concluded that the effective modulus of single-walled nanotubes dispersed in a composite could be over 1 TPa and that of multi-walled nanotubes was about 0.3 TPa. In their investigation of single-wal￾led nanotube/epoxy composites, Ajayan et al. [72] sug￾gest that their nearly constant value of the Raman peak in tension is related to tube sliding within the nanotube bundles and, hence, poor interfacial load transfer between the nanotubes. Similar results were obtained by Schadler et al. [73]. Multi-walled nanotube/epoxy com￾posites were tested in both tension and compression. The compressive modulus was found to be higher than the tensile modulus of the composites, and the Raman peak was found to shift only in compression, indicating poor interfacial load transfer in tension. Even with improved dispersion and adhesion, micro￾mechanical characterization of these composites is diffi- cult because the distribution of the nanotubes is random. Thus, attempts have been made to align nano￾tubes in order better to elucidate the reinforcement mechanisms. Jin et al. [74] showed that aligned nano￾tube composites could be obtained by mechanical stretching of the composite. X-ray diffraction was used to determine the orientation and degree of alignment. Fig. 13. Fracture mechanisms in nanotube-based composites [62]. 1908 E.T. Thostenson et al. / Composites Science and Technology 61 (2001) 1899–1912