Investigation of Creep Behavior in Epoxy-Based Polymer Systems Using Molecular Dynamics Simulations
Student thesis: Doctoral Thesis
Related Research Unit(s)
Epoxy-based polymers have been widely utilized for various functions in different application areas ranging from nanoscale structures, such as electronic devices and microelectronic, to macroscale components in aerospace and building constructions. Specially, epoxy-based materials are versatile with advantages on mechanical properties, which are applied commonly as structural components and bonded materials. The structural stability and creep strength under constant loadings are the major concerns for these polymer materials during long-term service. To enhance their creep resistance, the incorporation of carbon nanotubes (CNTs) into epoxy matrix has become a useful strategy. The substantial improvement in creep resistance requires systematic design and manipulation of molecular structures, which largely relies on the systematic study of related parametric effects, the comprehensive understanding of creep response as well as the prediction of long-term performance in CNT reinforced epoxy nanocomposites. In addition, mechanical durability in epoxy bonded material systems is generally related to the interfacial properties between epoxy and substrate. In this research, a comprehensive investigation on the creep behavior in epoxy-based material systems, including composites and layer structure, has been conducted through molecular dynamics simulations. The effects of CNT length, CNT weight fraction and CNT dispersion state on the creep response of nanocomposites has been studied with creep response and system dynamics from atomistic scale. The results suggest that the CNT-epoxy nanocomposites with superior creep resistance can be achieved in the design by optimizing the CNT-associated parameters. For the bilayer structure, the interfacial creep behavior in the epoxy-silica system has been investigated using steered molecular dynamics method. The results demonstrate the threshold stress for the onset of interfacial creep of bilayer material system under different loading conditions. The relationship between creep displacement and the applied constant force is quantified by an analytical model. The microstructural changes during creep process are captured to demonstrate creep deformation process, and unravel the mechanism of creep behavior in different material systems. The effect of elevated temperature on the interfacial creep behavior is also analyzed to reveal the interfacial properties and the role of CNTs. The fundamental knowledge of creep mechanism in epoxy-based material systems can provide a bottom-up approach to understand the creep deformation from the nanoscale, and lay the foundation for the predictions of creep response and further study of environmental effects on creep behavior for long-term services. The results revealing the strengthening mechanism in CNT-epoxy nanocomposites using computational approach also provide suggestions to the optimal design rule for CNT-polymer nanocomposites and related bonded structures with enhanced creep resistance and other expected mechanical properties in engineering applications.