Carbon nanotube - polymer interface

We illustrate CNT-polymer interface morphology using a CNT/PS and a CNT/ epoxy system. Rod specimen of CNT/PS composite with 1 mm diameter was fabricated using an extrusion process, with a CNT content of about 1 wt.%. Tensile failure surfaces of the CNT/PS composite rod were examined under a field emission seanning electronic microscope (FESEM) and transmission electron microscope (TEM). CNT/epoxy (EPON SU-8 photo resist) thin film with 0.1 wt.% CNT and 5.8/xm in thickness was fabricated by spin-coating mixture of CNT and epoxy on to a silicon wafer. The fracture surface of CNT/ epoxy specimens was examined under FESEM and TEM after shaft-loaded test (inset of Fig. 13.7). 

A fracture surfaee of the MWNT/epoxy film is shown in Fig. 13.7, where pullouts near the matrix eraek are seen. Close examination indicate that these pullouts were MWNTs eovered by epoxy. The pullout morphology indicates 

7 Fracture surface of CNT/epoxy thin film, oriented in radial direction. CNT bundles are seen coated with epoxy. Schematic of the shaft- loaded test setup is shown in the inset. 

8 TEM of CNT in epoxy (a) a longitudinal section of CNT, no physical boundary is seen between CNT and the matrix, (b) CNT pulled out from the matrix. Note that a thin layer (about 3 nm) of polymeric material adheres to the surface of CNT. 

Due to the difficulties in devising experiments to study the CNT-polymer interface, molecular modeling may serve to elueidate the importance of various factors constituting the interfacial characteristics of CNT reinforced polymer composites. To extend our understanding on CNT-polymer interactions, the interfacial adhesion characteristics between CNTs and a group of polymers (Table 13.2) are studied through molecular mechanics simulations. In this study, we are only concerned with non-bond interactions. 

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Wong M, Paramsothy M, Xu XJ, Ren Y, Li S, Liao K (2003). Physical interactions at carbon nanotube-polymer interface. Polymer 44 7757-7764. 

Frankland, S. J. V., Caglar,A., Brenner, D. W. and Griebel, M. Molecular Simulation of the Influence of Chemical Cross-Links on the Shear Strength of Carbon Nanotube-Polymer Interfaces. The Journal of Physical Chemistry B 2002 106 3046-8. 

Frankland, S.J.V. Caglar, A. Brenner, D.W. Griebel, M. Molecular simulation of the influence of chemical cross-links on the shear strength of carbon nanotube-polymer interfaces. J. Phys. Chem. B 2002, 106, 3046-3048. 

For CNTs not well bonded to polymers, Jiang et al. [137] established a cohesive law for carbon nanotube/polymer interfaces. The cohesive law and its properties (e.g., cohesive strength, cohesive energy) are obtained directly from the Lennard-Jones potential from the van der Waals interactions. Such a cohesive law is incorporated in the micromechanics model to study the mechanical behavior of carbon nanotube-reinforced composite materials. CNTs indeed improve the mechanical behavior of composite at the small strain. However, such improvement disappears at relatively large strain beeause the eompletely debonded nanotubes behave like voids in the matrix and may even weaken the composite. The increase of interface adhesion between CNTs and polymer matrix may significantly improve the composite behavior at the large strain [138]. 

Figure 10.1 reprinted fr om Advanced Materials, Vol. 18,2006, Authors Barber B A, Cohen S R, Eitan A, Schadler L S and Wagner H D, Title Fracture transitions at a carbon nanotube/polymer interface, pp. 83-87, Copyright (2007), with permission from Wiley-VCH. 

C. Velasco-Santos, A.L. Martinez-Hernandez, and V.M. Castano, Carbon nanotube-polymer nano composites The role of interfaces. Composite Interfaces, 11 (8-9), 567-586, 2005. 

Several studies on the characterization and fabrication of carbon nanotube-polymer nanocomposites have highlighted the important roles of the parameters discussed in Chapter 2 (such as, orientation, dispersion, and interfacial adhesion) in determining the properties of the composites. Jia et al. [75] used an in situ process for the fabrication of a PM M A/ M WNT composite. An initiator was used to open up the Jt bonds of the MWNTs in order to increase the linkage with the PMMA. The formation of C—C bonds results in a strong interface between the nanotubes and the PMMA. 

Li and Chou [73, 74] have reported a multiscale modeling of the compressive behavior of carbon nanotube/polymer composites. The nanotube is modeled at the atomistic scale, and the matrix deformation is analyzed by the continuum finite element method. The nanotube and polymer matrix are assumed to be bonded by van der Waals interactions at the interface. The stress distributions at the nanotube/polymer interface under isostrain and isostress loading conditions have been examined. They have used beam elements for SWCNT using molecular structural mechanics, truss rod for vdW links and cubic elements for matrix. The rule of mixture was used as for comparison in this research. The buckling forces of nanotube/ polymer composites for different nanotube lengths and diameters are computed. The results indicate that continuous nanotubes can most effectively enhance the composite buckling resistance. 

Foster J, Singamaneni S, Kattumenu R, Bliznyuk V (2005). Dispersion and phase separation of carbon nanotubes in ultrathin polymer films. J. Colloid and Interface Science 287 167-172. 

Nanocarbon hybrids have recently been introduced as a new class of multifunctional composite materials [18]. In these hybrids, the nanocarbon is coated by a polymer or by the inorganic material in the form of a thin amorphous, polycrystalline or single-crystalline film. The close proximity and similar size domain/volume fraction of the two phases within a nanocarbon hybrid introduce the interface as a powerful new parameter. Interfacial processes such as charge and energy transfer create synergistic effects that improve the properties of the individual components and even create new properties [19]. We recently developed a simple dry wrapping method to fabricate a special class of nanocarbon hybrid, W03 /carbon nanotube (CNT) coaxial cable structure (Fig. 17.2), in which W03 layers act as an electrochromic component while aligned... 

Nanowires increase efficiency because they directly deliver the electrons from the interface of the nanowire with the polymer to their electrode, while the electrode holes travel in the opposite direction to the tip of the wire and pass through the very thin polymer layer before reaching their electrode. Earlier carbon nanotube and nanowire designs were not directly connected to their electrodes and therefore did not provide the electrons with a direct path. 

Eitan, A., Jiang, K. Y., Dukes, D., Andrews, R., and Schadler, L. S. 2003. Surface modification of mul-tiwalled carbon nanotubes Toward the tailoring of the interface in polymer composites. Chemistry of Materials 15 3198-201. 

Moreover, the carbon nanotube density is low. Thus, there is a considerable interest in using nanotubes to fabricate composite materials from the point of view of mechanical reinforcement. However, the above models assume a perfect adhesion of the nanotubes to the matrix. In practice, the interface can fail. Of course, this lowers the stress transfer and the reinforcement by the nanotubes. This is why controlling the surface chemistry of the nanotubes and their interactions with a polymer matrix are also critical challenges. 

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