Meysam Rahmat

Interaction stresses in carbon nanotube-polymer nanocomposites.

A new technique of atomic force microscopy interaction measurement is used to obtain the three-dimensional stress field in nanocomposites made of single-walled carbon nanotubes (SWNT) and poly(methyl methacrylate) (PMMA) matrix. This original approach expands the current capability of AFM from imaging and force mapping to three-dimensional stress field measurements. Latest developments in the field have been limited to three-dimensional imaging at the surface only, and one value (adhesion) force mapping. The current work provides the interaction stress results for a PMMA-SWNT nanocomposite and shows a maximum estimated load transfer of less than 7 MPa (the maximum attraction stress estimated). This value is obtained for an unfunctionalized nanocomposite and hence the interaction stress is mainly based on van der Waals interactions. This means that for this system, carbon nanotubes behave similar to an elastic-fully plastic material with a yield stress of less than 7 MPa. This phenomenon illustrates why carbon nanotubes may not show their strong mechanical properties (yield strength of above 10 GPa) in polymeric nanocomposites.

Interaction energy and polymer density profile in nanocomposites: a coarse grain simulation based on interaction stress

A coarse grain approach was selected to simulate nanocomposites made of single-walled carbon nanotubes (SWNTs) and poly(methyl methacrylate) (PMMA). Unlike common coarse grain simulations, which employ Lennard-Jones models as the force field between the beads, the current approach used experiment-based data, and hence is considered a semi-empirical approach. Interaction stress data obtained from atomic force microscopy experiments under water were multiplied by ten to represent the interaction stresses in vacuum and then fed into the simulation. A new planar approach was introduced to simplify the three-dimensional unit cell into a two-dimensional plane. Furthermore, preliminary one-dimensional simulations were carried out to acquire a trade-off between simulation time and accuracy of the results. It was shown that the final results were independent of the initial conditions and converging parameters (the parameters that define the convergence rate). Two sets of planar simulations were performed to model pure PMMA systems and PMMA–SWNT nanocomposites. Polymer chain configuration and density profiles for these systems were obtained and compared. The surface effect on the polymer configuration and density profile was captured and demonstrated to be identical for the two systems. The polymer simulations showed a core section with a constant density, where the surface effect from the free surface did not influence the behaviour of the polymer chains. The effect of nanotube on polymer morphology was observed by layered structures of polymer chains around the nanotube, with preferable bands of peaks and valleys in the radial density profile. Finally, a new method was presented to calculate the interfacial binding energy for nanocomposites. The value of 0.44 kcal mol−1 A−2 obtained for the PMMA–SWNT nanocomposite was shown to be in good agreement with the previously reported results obtained from atomistic simulations.

Molecular Dynamics Simulation of Single-Walled Carbon Nanotube – PMMA Interaction

Mechanical performance of nanocomposites is strongly dependent on the interaction properties between the matrix and the reinforcement. Therefore, the aim of this work is to investigate the carbon nanotube – polymer interaction in nanocomposites. With the ever-increasing power of computers, and enormous advantage of parallel computing techniques, molecular dynamics is the favourite technique to simulate various atomic and molecular systems for this application. In order to simulate nanocomposites using molecular dynamics techniques, a stepwise approach was followed. First, a single-walled carbon nanotube was modelled as the reinforcing material. The validity of the model was examined by applying simple tension boundary conditions and comparing the results with the literature. Next, PMMA chains, with different geometries and molecular weights, were modelled employing the chemical potentials extracted from the literature. The last step included the modelling of the nanotubes surrounded by the matrix material and the investigation of the energy minimization for the system. Based on the results, the non-covalent interaction energy between a single-walled carbon nanotube and the PMMA matrix was obtained.