Carbon nanotube dispersion and characteristics: Thermomechanical properties and conductivity of polyimide nanocomposites
Abstract
The properties of polymer nanocomposites are significantly affected by Carbon Nanotube (CNT) dispersion. Even though dispersion is recognized by the scientific community as one of the main issues in nanocomposite synthesis, a method to characterize CNT dispersion has not yet been universally accepted. In the present work, a thorough and systematic approach to determine dispersion and characteristics of CNTs within a polyimide (PI) matrix and their effect on nanocomposite thermo-mechanical and electrical properties is presented. CNT dispersion was characterized by voltage contrast scanning electron microscopy, applying high accelerating voltage to render images of the nanotube network embedded in the polyimide matrix and measuring CNT bundle diameter, contour length and end-to-end distance. Nanotube dispersion and characteristics were studied in nanocomposites synthesized with eight different sources of CNTs. CNT bundle aspect ratio—a quantitative measurement of dispersion—is found to decrease with increase in nanotube concentration for Single-Walled CNTs (SWNTs) synthesized via high pressure carbon monoxide process (HiPCO). The observed reduction of nanotube aspect ratio with concentration is modeled using the concept of percolation onset from percolation theory. Multi-Walled CNTs (MWNTs) tend to form highly entangled CNT agglomerates instead of bundles and show worse distribution of CNTs within the PI matrix than HiPCO SWNTs. The distribution of polymer and CNTs is seen to worsen as CNT concentration increases, regardless of the source of CNTs used for composite synthesis. The quality of nanotube dispersion is shown to explain the lack of expected increase in mechanical reinforcement in PI-CNT composites with concentration. Nanocomposites synthesized with HiPCO SWNTs show better dispersion characteristics and greater reinforcement than MWNT composites. A modified Cox micromechanical model that accounts for the actual change in nanotube bundle aspect ratio with concentration, nanotube waviness and orientation is able to predict the observed nanocomposite elastic modulus. CNTs characteristics are also a fundamental aspect of nanocomposite conductivity. The experimentally determined percolation threshold for CNTs-minimum concentration at which a conductive network is formed—is fairly predicted from CNT aspect ratio using percolation theory concepts, whereas the percolation exponent was found to be correlated with CNT length and waviness. The analysis methodology and results presented in this work help explain property variability in polymer nanocomposites and establish a solid base for study and characterization of nanocomposites, which is essential for maximization of the potential of CNTs for polymer matrix improvement.
Degree
Ph.D.
Advisors
Pipes, Purdue University.
Subject Area
Polymer chemistry|Chemical engineering|Nanoscience
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