Liu, Lei (2009-05). Structure-Property Relationships in Carbon Nanotube-Polymer Systems: Influence of Noncovalent Stabilization Techniques. Doctoral Dissertation. Thesis uri icon

abstract

  • A variety of experiments were carried out to study the dispersion and microstructure of carbon nanotubes in aqueous suspensions and polymer composites with the goal to improve the electrical conductivity of the composites containing nanotubes. Epoxy composites containing covalently and noncovalently functionalized nanotubes were compared in terms of electrical and mechanical behavior. Covalent functionalization of nanotubes is based on chemical attachments of polyethylenimine (PEI) whereas noncovalent functionalization takes place through physical mixing of nanotubes and PEI. The electrical conductivity is reduced in composites containing covalently functionalized nanotubes due to damage of the tube?s conjugated surface that reduces intrinsic conductivity. Conversely, the mechanical properties are always better for epoxy composites containing covalently functionalized nanotubes. Clay particles were used as a rigid dispersing aid for nanotubes in aqueous suspensions and epoxy composites. When both nanotubes and clay were introduced into water by sonication, the suspension is stable for weeks, whereas the nanotubes precipitate almost instantly for the suspension without clay. In epoxy composites, nanotubes form separated clusters of aggregation, whereas a continuous threedimensional nanotube network is achieved when clay is introduced. Electrical conductivity of the epoxy composite is shown to significantly improve with a small addition of clay and the percolation threshold is simultaneously decreased (from 0.05 wt% nanotubes, when there is no clay, to 0.01 wt% when 2 wt% clay is introduced). The addition of clay can also improve the mechanical properties of the composites, especially at higher clay concentration. Weak polyelectrolytes (i.e., pH-responsive polymers) were also studied for their interaction with nanotubes and the electrical properties of the dried composite films. When dispersed by sonication, Nanotubes show pH-dependent dispersion and stability in poly(acrylic acid) water solution, as evidenced by changes in suspension viscosity and cryo-TEM images. The nanotube suspensions were then dried under ambient conditions and the composite films exhibit tailorable nanotube dispersion as a function of pH. The percolation threshold and maximum electrical conductivity are reduced when the pH is changed from low to high. Some other pH-responsive polymers were also studied, but their pH-dependent viscosity and conductivity were not as large or reversible as poly(acrylic acid).
  • A variety of experiments were carried out to study the dispersion and
    microstructure of carbon nanotubes in aqueous suspensions and polymer composites
    with the goal to improve the electrical conductivity of the composites containing
    nanotubes. Epoxy composites containing covalently and noncovalently functionalized
    nanotubes were compared in terms of electrical and mechanical behavior. Covalent
    functionalization of nanotubes is based on chemical attachments of polyethylenimine
    (PEI) whereas noncovalent functionalization takes place through physical mixing of
    nanotubes and PEI. The electrical conductivity is reduced in composites containing
    covalently functionalized nanotubes due to damage of the tube?s conjugated surface that
    reduces intrinsic conductivity. Conversely, the mechanical properties are always better
    for epoxy composites containing covalently functionalized nanotubes.
    Clay particles were used as a rigid dispersing aid for nanotubes in aqueous
    suspensions and epoxy composites. When both nanotubes and clay were introduced into
    water by sonication, the suspension is stable for weeks, whereas the nanotubes precipitate almost instantly for the suspension without clay. In epoxy composites,
    nanotubes form separated clusters of aggregation, whereas a continuous threedimensional
    nanotube network is achieved when clay is introduced. Electrical
    conductivity of the epoxy composite is shown to significantly improve with a small
    addition of clay and the percolation threshold is simultaneously decreased (from 0.05
    wt% nanotubes, when there is no clay, to 0.01 wt% when 2 wt% clay is introduced). The
    addition of clay can also improve the mechanical properties of the composites, especially
    at higher clay concentration.
    Weak polyelectrolytes (i.e., pH-responsive polymers) were also studied for their
    interaction with nanotubes and the electrical properties of the dried composite films.
    When dispersed by sonication, Nanotubes show pH-dependent dispersion and stability in
    poly(acrylic acid) water solution, as evidenced by changes in suspension viscosity and
    cryo-TEM images. The nanotube suspensions were then dried under ambient conditions
    and the composite films exhibit tailorable nanotube dispersion as a function of pH. The
    percolation threshold and maximum electrical conductivity are reduced when the pH is
    changed from low to high. Some other pH-responsive polymers were also studied, but
    their pH-dependent viscosity and conductivity were not as large or reversible as
    poly(acrylic acid).

publication date

  • May 2009