Adhesion and reinforcement in carbon nanotube polymer composite

Chenyu Wei
2006 Applied Physics Letters  
Temperature dependent adhesion behavior and reinforcement in carbon nanotube ͑CNT͒-polymer ͑polyethylene͒ composite is studied through molecular dynamics simulations. The interfacial shear stress through van der Waals interactions is found to increase linearly with applied tensile strains along the nanotube axis direction, until the noncovalent bonds between CNTs and molecules break successively. A lower bound value about 46 MPa is found for the shear strength at low temperatures. Direct
more » ... ures. Direct stress-strain calculations show significant reinforcements in the composite in a wide temperature range, with ϳ200% increase in the Young's modulus when adding 6.5% volume ratio of short CNTs, and comparisons with the Halpin-Tsai formula are discussed. Since their discovery in the 1990s, carbon nanotubes ͑CNTs͒ have been shown to have exceptional mechanical and unique electronic and thermal properties. 1 The large surface area and the high modulus and strength of CNTs 1 make them a good candidate as reinforcing fibers. Recent experiments have shown remarkable enhancements in elastic modulus and strength of polymer composites with an addition of small amounts of CNTs. 2-5 Polymer CNT composites have also been investigated as multifunctional materials for electric and thermal applications. 6-8 The adhesion behavior and reinforcement properties are crucial to determine the efficiency of CNTs as nanofibers and the structural stabilities in these CNT composites. While adhesion in CNT composites can be through covalent bonds with specific polymers, noncovalent van der Waals ͑VDW͒ interactions are universally present, which have been shown to play an important role for molecule structures at interface and reinforcement in CNT composite, 9 and is the focus of this study. In this letter we investigate the adhesion behaviors such as interfacial shear stress and bond breaking at large strains, reinforcements in elastic modulus, and their temperature dependence in a polymeric CNT composite, through molecular dynamics simulation method. 10 Polyethylene ͑PE͒ is chosen as a model matrix, which is described through a united atom model with bond stretching, bending, and dihedral potentials. A truncated 6-12 Lennard-Jones ͑LJ͒-type VDW interaction is included between CNTs and matrix and within matrix. The details of the force field can be found elsewhere. 11 Amber force field 12 is used for the carbon-carbon interactions on CNTs, and the use of which is justified as the embedded CNTs are expected to behave in elastic regime ͑to be discussed later͒, due to the weak VDW interactions. The Young's modulus for intrinsic CNTs with Amber force field is found within 10% difference from that with the widely used Tersoff-Brenner potential. 13 The composite system consists of 50 PE molecules with 100 repeating units and a capped 190-Å-long CNT ͑10, 0͒ representing discontinuous fibers, in a periodic unit cell ϳ26ϫ 26ϫ 200 Å 3 ͑see inset of Fig. 1 for illustration͒. The system is prepared at 600 K with individual molecules relaxed with Monte Carlo simulation beforehand, and gradually cooled down to low temperatures with a rate of 10 K / 100 ps ͑Berendsen NPT ensemble, P = 1 bar͒. The details of sample preparation can be found elsewhere. 11 Time step of 0.5 fs is used. All the data reported here are averaged over eight sample sets. To investigate the interfacial adhesion and reinforcement behavior, a tensile stress along the nanotube axis direction is applied to the composite gradually with a rate of 1 bar/ 1 ps and at each stress there is a responding strain ͑Berendsen NPT ensemble͒. In this study we focus on the properties induced by the embedded CNTs and the stress/strain rate effect is not discussed. Shown in Fig. 1 is the change in the total interfacial VDW energy, U vdw , between the CNT and the polymer as a function of strain at various temperatures. 14 In comparison, the change in the total VDW energy is also shown for a similarly prepared pure polymer bulk ͑50 PE with 100 units͒ at T = 50 K. It can be seen that the VDW interaction in the composite is much enhanced due to the presence of the nanotube. We attribute such enhancement to two following factors. ͑1͒ The large surface area and the high atomic density on the CNT: while the space ͑6.5% volume ratio͒ occupied by the CNT would only accommodate 347 matrix ͑carbon͒ atoms, there are as many as 1804 atoms on the CNT and all of them are on the surface. ͑2͒ The existence of adsorption layer around the CNT, which has a higher density compared with in bulk polymer. 9 At high temperatures the molecule density in the adsorption layer decreases due to a͒ Electronic mail: cwei@mail.arc.nasa.gov FIG. 1. The change in the total interfacial VDW energy in the PE composite as a function of applied tensile strain at various temperatures from 50 to 400 K ͑solid lines͒, in comparison with the change in the total VDW energy in bulk PE at T =50 K ͑dashed line͒. Inset: Schematic illustration of the unit cell for the composite in simulation, with tensile stress applied along the CNT axis direction. APPLIED PHYSICS LETTERS 88, 093108 ͑2006͒
doi:10.1063/1.2181188 fatcat:fa6ivzjch5ee5c3e7imfnzoy6i