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Investigation of Carbon Nanotube-Polymer Composites Thermal Conductivity Using Molecular Dynamics Simulations

Gharib-Zahedi, Mohammad Reza | 2015

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 47233 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Tafazzoli, Mohsen; Alaghemandi, Mohammad
  7. Abstract:
  8. In the present research, we have applied reverse non-equilibrium molecular dynamics (RNEMD) to investigate the thermal conductivity (λ) of carbon nanotube (CNT) bundles, CNT-polyamide (PA) nanocomposites, and nanoconfined PA in the vicinity of graphene layers. First, transversal heat transport in CNT bundles with and without intertube chemical bonds are examined. It is found that the transversal λ of samples with cross linkers can surprisingly increase ~9 times than that of neat CNT bundles (0.26 W m-1K-1). According to the simulation results, key parameters are overlapping vibrational density of state (DOS) profiles between the atoms forming the bridging units, cross linker in the direction of the heat flux, an ordering of the linkers as well as the chemical nature of the bridging units. The influence of a forced axial stress on the transversal λ under longitudinal stress to values ≤8% has been also studied. The observed λ reduction with increasing axial stretching can be (over)compensated by cross linkers. As a second topic of the present RNEMD simulations, heat transfer through nanometer scale interfaces consisting of either grafted or ungrafted carbon nanotube embedded in oligomeric polyamide-6,6 chains are investigated. The structural properties of the polymer in the interphase region between the nanotube and polymer have also been studied. Particular attention has been focused on the influence of the grafting density, grafted chain length and types. The λ_i value of the ungrafted composite does not exceed 0.003 W m-1K-1. For the composites with a PE grafted CNT, λ_i numbers between 0.02 and 0.07 W m-1K-1 are derived by the present RNEMD simulations.We found that the interfacial thermal conductivity is insensitive to grafted chains type, whereas it shows a strong dependence on the grafting density and grafted length. We rationalized these behaviors in terms of the modification of the structural properties such as polymer densifications at the nanotube interface, local chains conformation, and orientations. The results of this work elucidate that if chemical grafted chains are significantly less prone to collapse onto the CNT and consequently more intermingled with the free polymers, much higher aggregate thermal conductivities will be reachable. Finally, λ of nanoconfined PA in polymer-graphene nanocomposites has been calculated in a broad interval of graphene interlayer separations. The preferential alignment of the PA chains parallel to the graphene plane as well as their elongation implies that λ of the polymer in nanocomposites is larger than the neat polymer system. The structural modifications in nanoconfined polymer phases seem to support a more important phonon channel for the thermal conductivity in comparison with the neat polymer system
  9. Keywords:
  10. Carbon Nanotubes ; Graphine ; Polyamide 6,6 ; Thermal Conductivity ; Carbon Nanotubes ; Reverse Non-Equilibrium Molecular Dynamics (RNEMD) ; Polymer Nanocomposits

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