Theoretical and experimental investigations of the elastic properties of carbon nanotube-reinforced polymer thin films

Nanocomposites are a promising new class of materials for the mechanical components of microstructures such as microactuators and microresonators. This work presents a combination of theoretical and experimental investigations of the utility of carbon nanotube-reinforced composites for designing microstructures. In the theoretical part of this research, the effects of nanotube aspect ratio, dispersion, alignment, and volume fraction on the elastic modulus and longitudinal wave velocity are analyzed by recourse to the Mori-Tanaka theory. The calculated bounds on Young's modulus and wave velocity capture the trend of the experimental results reported in the literature. Polymer-matrix nanocomposites reinforced with aligned, dispersed single-walled carbon nanotubes are identified as excellent candidates for small structures with properties rivaling those of metallic- and ceramic-structures used in the current generation of microelectromechanical systems (MEMS). The experimental part of this research focuses on the manufacture and characterization of carbon nanotube-reinforced polymer thin films. A novel nanoindenter-based bending test is developed for characterizing the elastic properties of nanocomposite thin films. This technique is first numerically verified using finite element methods. Polymer thin films with known mechanical properties are then utilized to validate the technique experimentally. Next, epoxy-matrix and vinyl ester epoxy-matrix nanocomposite films (ranging from 50 to 70 μm in thickness) reinforced with low concentrations (<1% by weight) of single-walled carbon nanotubes are successfully manufactured and characterized. Finally, using carbon nanotube sheets (buckypaper), polymer-matrix nanocomposite films with high volume fractions of carbon nanotubes (30-40%) are manufactured by using two different techniques: vacuum infiltration and hot press. This relatively high content of carbon nanotubes results in a three- to four-fold increase in the elasticity of nanocomposites with respect to that of the pure polymer. A qualitative comparison between the state-of-the-art nanocomposite manufacturing technology and the predicted upper hound theoretical results highlights the enormous improvements needed in materials processing and micromachining to harness the full potential of carbon nanotube-reinforced composites for microstructure applications.