Multiscale Investigation on Mechanical Behaviors of Polymeric Material Systems
Student thesis: Doctoral Thesis
Related Research Unit(s)
Polymeric material systems have received tremendous attention in both industrial and scientific communities, and can be readily found in applications across a large range of length scales, ranging from nano-scale structures, such as photoresist lithography in micro-electro-mechanical systems, to macro-scale components such as adhesive bonding in aerospace industry and civil infrastructures. Apart from these synthetic material systems, they are found in many plant natural materials. Specifically, the cell wall is the fundamental natural polymeric material system, which plays an important role in the load bearing in the plants. The durability of these material systems are mainly determined by the mechanical properties of the constituent materials and the bonded interface. In this research, a comprehensive investigation on the mechanical reliability and the interfacial integrity of polymeric material systems has been conducted computationally using the synthetic polymeric material systems and the plant cell wall as representatives, respectively. The mechanical reliability of the synthetic polymer has been studied based on the modeled cross-linked networks using molecular dynamics simulation approach. A dynamic cross-linking algorithm is developed to achieve the cross-linking process in the modeling of atomistic cross-linked network. Meanwhile, the interfacial integrity of synthetic polymeric material systems is studied based on the atomistic polymer-bonded interfaces. By reconstructing the free energy surface, the intrinsic interfacial strength is determined, and it is found that the interfacial integrity can be deteriorated significantly in the presence of water. Furthermore, the predicted mechanical properties from the molecular level are used to develop the cross-linked network at meso-scale, which enables the investigation on the effect of structural voids intrinsic to the polymeric structure. The simulated mechanical properties demonstrate the strong structural integrity of the synthetic polymeric material systems, and the environmental effects shall be considered during the long-term service life. For the natural polymeric system, i.e. the plant cell wall investigated in this research, the viscoelastic behaviors of the constituent natural polymers have been obtained using steered molecular dynamics method, which can be correlated to the viscoelasticity of bulk plants to provide a comprehensive picture of the plant viscoelastic behaviors. Notably, the lignin in the cell wall is modeled by modifying the developed cross-linking algorithm. Moreover, the mechanical behaviors of constituent natural polymers and their interactions have been investigated based on a modeled meso-scale cell wall structure, which are shown to play an important role in the mechanical properties of the plant cell wall during deformation, and thus the macroscopic performance of the bulk plants. The in-depth understanding of the mechanical behavior of plant cell wall enables one to identify the plant that is at risk of external strikes, and to provide structural supports for the plant before the irreversible decline occurs, which is important for optimizing the plant longevity and preventing the catastrophic death. The multiscale method used in this research provides a versatile tool to link the nano-level mechanical behaviors of polymeric material systems to the macro-level durability issue.