Modeling and analysis of the relationship between molecular structure and elastic properties of nanoscale solids

Abstract

The stiffness of material has additional size-dependence when the solids are in nanoscale. It is reported that the elastic modulus of solids appeared to be larger than its conventional value when the size of solids is small. Although the strain gradient theory from continuum mechanics was developed to explain this phenomenon, the stiffening mechanism of the material is still unclear. In this thesis, we chose the molecular dynamics method to analyze the nanoscale behavior of solids and determined the origin of the elastic stiffening of polymers and nanowires from molecular simulation. A hypothesis was proposed to connect the current continuum mechanics theory with the molecular behavior of polymers. The stiffening mechanism of the solids with macromolecular molecular structure and crystal molecular structure was discussed. To validate this stiffening mechanism, simulation of elastic behavior of polymers and nanowires were carried out. Our hypothesis was supported by the simulation results. The additional rotation of molecules generates additional deformation energy, which leads to size-dependent stiffening. The conventional elastic modulus E₀ and the high order length scale parameter l₂ from our simulation results have been benchmarked with reported data. During the investigation of polymers, we further discussed the influences of molecular structure on their conventional and high order mechanical properties. The E₀ and l₂ were found to vary linearly with entanglement density or the crosslink density of chains. The results also showed that l₂ is linearly correlated with E₀. Once a reference E₀ and the correlation with l₂ are known, l₂ and the size-dependence of a solid with different molecular structure can be predicted. In the nanowire part, the significance of the directionality of the molecular bonding was discussed during the stiffening mechanism investigation. The surface lattice distortion is claimed to be account for the size-stiffening in tension. The bond rotation from strain gradient should also be considered to explain the size-dependence in bending. It is also found that the performance of piezoelectric ZnO nano-generator is able to be improved by the size-stiffening of the ZnO nanowire. In addition, we further analyzed the effect of the dislocation in the crystalline nanowire. The results show that rotations induced by the screw dislocations in nanowires can cause the size-dependent deformation. The piezoelectric performance of ZnO nano-generator can be benefit from screw dislocation, too. Conclusively, there are three factors that can cause the size-stiffening of the crystalline nanowire: surface lattice distortion, strain gradient and screw dislocation. When properly engineered, the mechanical and electric properties of nanowire can be controlled based on the requirement of industry.

Date of Award	2012
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology