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CNT-Polstyrene Composites

(1) Computational Methods
      The reactive empirical bond-order (REBO) for C-H and C-C interactions (parameterized by Brenner et. al.) for C-F and F-F (Graves et. al.) and F-H interactions (parameterized by Sinnott et. al.) is used for short range covalent interactions. The long range van der Waal interactions are determined by coupling a 12-6 Lennard-Jones potential to the REBO potential.
      The system temperature is maintained at 300 K using Langevin thermostats. After ion deposition, the system is equilibrated at 400 K and then cooled back down to 300K. Periodic boundary conditions are applied to the direction of the nanotube axis, and the time step is 0.2 fs.

(2) Normal Incidence
a) System Setup
  • Polystyrene is initially crystalline.
  • Vary the depth of the nanotube in the composite relative to the surface (1-3 layers of polymer covering the nanotube)
  • Two composite geometries are considered: (I) CNT/PS-//, (II) CNT/PS-⊥
  • Deposit C3F5+ ions in a beam of 50 ions onto the composite surface
  • Incident energy = 50 or 80 eV/ion
  • Surface temperature maintained at 300 K
    		•
    b) Results
          The blue spheres are carbon atoms in the substrate, the green spheres are carbon atoms in the ions, and the gold spheres are the incident fluorine atoms.

    i) Processes that Occur during Ion Beam Deposition on CNT/PS-// Composites (50 eV/ion)
      Substrate etching
    (# of C/ion)    		 Trapped C
    from ion (%)    		 # of
    intact C3F5    		 Functionalization
    of CNT    		 Penetration
    depth
    (nm)    		 Major trapped
    ion species
    Chemical adsorption
    of ion species    		 Cross-links
    between CNT and PS
    PS only 0 63.3 13 N/A N/A 2.1 C3F5
    CNT/PS-//-1 0.3 49.3 7 Yes Yes 1.8
    CNT/PS-//-2 0.1 42.0 11 No No 1.9
    CNT/PS-//-3 0.2 40.1 12 No No 2.2

    ii) Processes that Occur during Ion Beam Deposition on CNT/PS-⊥ Composites (50 eV/ion)
      Substrate etching
    (# of C/ion)    		 Trapped C
    from ion (%)    		 # of
    intact C3F5    		 Functionalization
    of CNT    		 Penetration
    depth
    (nm)    		 Major trapped
    ion species
    Chemical adsorption
    of ion species    		 Cross-links
    between CNT and PS
    CNT/PS-⊥-1 0.2 61.7 16 Yes No 3.1 C3F5
    CNT/PS-⊥-2 0.2 74.7 15 Yes No 3.3
    CNT/PS-⊥-3 0.3 57.3 17 No No 3.2

    iii) Processes that Occur during Ion Beam Deposition on CNT/PS-// Composites (80 eV/ion)
      Substrate etching
    (# of C/ion)    		 Trapped C
    from ion (%)    		 # of
    intact C3F5    		 Functionalization
    of CNT    		 Penetration
    depth
    (nm)    		 Major trapped
    ion species
    Chemical adsorption
    of ion species    		 Cross-links
    between CNT and PS
    PS only 2.0 40.7 3 N/A N/A 2.8 CF2
    CNT/PS-//-1 1.2 37.3 4 Yes Yes 2.5
    CNT/PS-//-2 1.2 38.7 1 Yes Yes 2.9
    CNT/PS-//-3 1.3 54.7 5 Yes No 3.3

    iv) Effect of Incident Energy: Chemical bonding information of the trapped ion species (deposition on CNT/PS-//-1)

    v) Comparison between PS and PS/CNT Composite
    • At 50 eV/ion, the fraction of trapped C atoms from the incident ions:
      • PS: 63.3%
      • CNT/PS-//: 40-50%
    • At 80 eV/ion, substrate etching (# of C/ion):
      • PS: 2.0
      • CNT/PS-//: 1.2-1.3

    c) Conclusions
    • Ion-beam deposition at low energies cross-link nanotubes to the polymer backbones at the composite surface.
    • Current efforts are focusing on exploring the orientation and incident energy requirements for cross-linking between nanotubes and polymers and investigating higher energy deposition events.

    (3) Incident Angle Effects
    a) System Setup
          The dimensions of the composite are 8.0 nm × 5.7 nm × 3.0 nm. The matrix is crystalline polystyrene and the filler is a (10,10) single walled carbon nanotube. The carbon nanotube is placed under two layers of polystyrene chains.
          The incident ion beam is 20 C3F5+ ions at 30°, and 5 C3F5+ ions at 45°; the incident energies are 50 eV and 80 eV.

    b) Results
          Results from the incident energy of 50 eV at 30°, 45° beam angle after equilibration are as follows:
    Incident Angle Average Penetration (Å) % of Ions Implanted Bonds
    30° 15.4 55 % NO
    45° 13.9 45 % NO

          Results from the incident energy of 80 eV at 30°, 45° beam angle after equilibration are as follows:
    Incident Angle Average Penetration (Å) % of Ions Implanted Bonds
    30° 16.2 50 % 1
    45°   0.0   0 % NO

    The ions with the incident angle of 45° scattered and "bounced off" the structure when their energy was 80 eV.

          This study shows that carbon nanotubes can be functionalized with polyatomic ion beam deposition and can induce covalent crosslinks between the otherwise unfunctionalized carbon nanotube and polystyrene chains. The chemical functionalization of the carbon nanotube increases with higher energies. As the incident angle increases from the zero degree surface normal, the amount of scattering increases.

    We provide open source codes for MD simulations: C-H REBO MD code, C-F-H REBO MD code, and C-O-H REBO MD code.


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    Last Update: Wednesday, May 27, 2005


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