A Molecular Dynamics Study of Material Behavior Controlled by Interface

In this work, the behaviour of nano-structured materials that is controlled by the interface is studied using Molecular Dynamics (MD). Four different types of nano-structured materials were investigated: (1) the sintering behaviour of nanoparticle; (2) the evolution of bamboo-like nanowires; (3) the mechanical property of the interlamellar phase of semicrystalline polymers; and (4) the mechanical property of the interlamellar phase of biodegradable polymers. In the MD simulation of nanoparticle sintering, it is observed that the particles can reorient themselves to match their crystalline orientations at the beginning of the sintering and thereby form different types of necks between different particles. This leads to different mechanisms of matter redistribution at the different necks. It has also been observed that the particles switch the mechanism of matter transportation halfway through the sintering process. None of these can be handled by the continuum model. However, assuming the right scenario, the continuum theory does agree with the MD simulation for particles consisting of just a few thousand atoms. In the multi-scale MD simulation of the evolution of bamboo-like nanowires, the microstructure evolution behaviour of the bamboo nanowire is observed very different to the conventional bamboo structure polycrystals. When the materials reduce to the nano-size, different evolution behaviour occurs: the low angle tilt grain boundary (GB) tends to be eliminated by forming a bending crystal form and dislocation slip might occur when raise the temperature; the large tilt GB is found stable at low temperature but the GB diffusion is very sensitive to the temperature; An interesting microstructure evolution behaviour of the nanowire with the small radius starting with the large angle GB is observed. A new hcp grain is nucleated from the triple point of the bamboo structure. In the multi-scale MD study of the mechanical property of the interlamellar phase of semicrystalline biodegradable polymers, it is found that the mechanical stiffness of interlamellar phase below Tg is mainly governed by the LJ interaction along the polymer backbone. Therefore, good polymer chain entanglement enhances the LJ interaction and increases the mechanical strength. Although the amorphous interlamellar phase is not the idea elastomer when temperature is above the glass transition temperature, it also shows the elastomer behaviour above Tg when we examine the number of long chains inside the amorphous interlamellar phase. The results of this study further support Pan's entropy spring theory by showing the Young's modulus drop lags behind the biodegradation process at temperatures above the glass transition temperature. For the amorphous interlamellar phase below the glass transition temperature, the Young's modulus drops quickly as the chain scissions quickly reduce the polymer chain entanglement.