Reorientation of carbon nanotubes in polymer matrix composites using compressive loading
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Purified single-walled nanotubes (SWNTs) were dispersed in an epoxy polymer and subjected to uniaxial compressive loading. The orientation and stress in the nanotubes were monitored in situ using polarized Raman microscopy. At strains less than 2%, the nanotubes reorient normal to the direction of compression, thereby minimizing the local strain energy. Above 2% strain, the Raman peak shift reaches a plateau. A new analytical model, which approximates the SWNT reorientation by varying the aspect ratio of a representative spheroid, predicted the rotation behavior of nanotubes under load. The results of this model suggest that the observed plateau of the Raman peak shift is caused by both polymer yielding and interfacial debonding at the ends of nanotubes.
I. INTRODUCTION
Carbon nanotubes have an enormous potential as a reinforcing phase in polymer composites. The high fracture strain and elastic moduli of nanotubes would result in great improvements in the mechanical properties of polymer composites over current carbon fiber-reinforced composites.1 In addition, the unique electrical, thermal, and optical properties of carbon nanotubes provide multiple functionalities in polymer composites, permitting automatic alteration of their properties depending on the environmental conditions.2 However, many issues remain to be resolved to achieve the predicted mechanical property improvements for nanotubes-polymer composites. One key issue is whether standard micromechanical models can be used to describe the behavior of nanosized particles. Many researchers have discovered that nanotubes behave differently than larger diameter reinforcement particles when subjected to a mechanical load. Because of their small size, nanotubes can dynamically rotate in a composite to minimize the elastic strain energy.3–7 Frogley, et al.7 showed that the reorientation of nanotubes embedded in an elastomer along the direction of applied tensile strain can explain the observed increase in elastic modulus and that this is a potential way of aligning nanotubes.6 Wood et al. found that nanotubes would reorient in a polymer composite at strains above the yield point of the polymer.5 They measured the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0139 1026
http://journals.cambridge.org
reorientation of the nanotubes using Raman microscopy and cited a plateauing of the Raman peak shift with strain as evidence of the lack of stress transfer to the nanotubes. In addition, numerous researchers have measured a plateau in the Raman peak shift with strain and explained this as either a debonding of the nanotube/polymer interface or a yielding of the polymer matrix.7–10 We present a new micromechanical model that accounts for the reorientation of nanotubes under mechanical load by making the orientation of the tubes a function of the applied load. We use this model to interpret the stress-dependent Raman peak shifts obtained from the strained nanotubes. More importantly, this new model represents a new cont
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