Dispersion and its relation to carbon nanotube concentration in polyimide nanocomposites

Characterization of Carbon Nanotube (CNT) dispersion in a polyimide matrix and its effect on nanocomposite mechanical properties is presented in this paper. CNT bundle aspect ratio, measured by voltage-contrast scanning electron microscopy, is determined to be the quantitative measurement of dispersion and is found to decrease with increase in nanotube concentration. The reduction of CNT bundle aspect ratio with concentration is shown to explain the less effective reinforcement observed in composites as CNT concentration is increased beyond the electrical percolation concentration. It is shown that increase in CNT concentration beyond percolation concentration does not yield proportional improvement in elastic modulus because CNT aspect ratio systematically decreases as concentration increases. A modified Cox micromechanical model that accounts for the actual nanotube bundle aspect ratio as a function of concentration, nanotube waviness and orientation is shown to predict the observed nanocomposite elastic modulus dependence upon concentration.