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- All Subjects: Morphology
- Creators: Rajagopalan, Jagannathan
Description
The exceptional mechanical properties of polymers with heterogeneous structure, such as the high toughness of polyethylene and the excellent blast-protection capability of polyurea, are strongly related to their morphology and nanoscale structure. Different polymer microstructures, such as semicrystalline morphology and segregated nanophases, lead to coordinated molecular motions during deformation in order to preserve compatibility between the different material phases. To study molecular relaxation in polyethylene, a coarse-grained model of polyethylene was calibrated to match the local structural variable distributions sampled from supercooled atomistic melts. The coarse-grained model accurately reproduces structural properties, e.g., the local structure of both the amorphous and crystalline phases, and thermal properties, e.g., glass transition and melt temperatures, and dynamic properties: including the vastly different relaxation time scales of the amorphous and crystalline phases. A hybrid Monte Carlo routine was developed to generate realistic semicrystalline configurations of polyethylene. The generated systems accurately predict the activation energy of the alpha relaxation process within the crystalline phase. Furthermore, the models show that connectivity to long chain segments in the amorphous phase increases the energy barrier for chain slip within crystalline phase. This prediction can guide the development of tougher semicrystalline polymers by providing a fundamental understanding of how nanoscale morphology contributes to chain mobility. In a different study, the macroscopic shock response of polyurea, a phase segregated copolymer, was analyzed using density functional theory (DFT) molecular dynamics (MD) simulations and classical MD simulations. The two models predict the shock response consistently up to shock pressures of 15 GPa, beyond which the DFT-based simulations predict a softer response. From the DFT simulations, an analysis of bond scission was performed as a first step in developing a more fundamental understanding of how shock induced material transformations effect the shock response and pressure dependent strength of polyurea subjected to extreme shocks.
ContributorsLi, Yiyang (Author) / Oswald, Jay (Thesis advisor) / Rajan, Subramaniam D. (Committee member) / Solanki, Kiran (Committee member) / Chamberlin, Ralph (Committee member) / Rajagopalan, Jagannathan (Committee member) / Arizona State University (Publisher)
Created2017
Description
Physical vapor deposition (PVD) of phase-separating multicomponent alloy films generates a rich variety of unique self-organized nanoscale morphologies. However, an understanding of how the different material and process parameters influence the formation of these nanostructures is limited. My dissertation aims to bridge this gap by developing phase-field models that can predict an entire spectrum of nanostructures as a function of processing conditions and composition in multicomponent alloys under a set of material-specific constraints. Firstly, I developed a numerical model to simulate nanoscale phase separation in codeposited immiscible binary alloy films. An investigation on the influence of deposition rates, phase-fraction, and temperature, on the evolution of self-assembled nanostructures yielded many characteristic patterns, including well-known morphologies such as the lateral and vertical concentration modulations, as well as some previously undocumented variants. I also simulated phase-separation in ternary alloyed PVD films, and studied the influence of deposition rate and composition on the evolution of self-assembled nanostructures, and recorded many novel nanoscale morphologies. I then sought to understand the role of material properties such as elastic misfit due to lattice mismatch between phases, grain boundaries formed in polycrystalline films, and the interplay of interphase and surface boundaries at the film surface. To this end, I developed phase-field models of binary PVD film deposition that incorporated these constraints and studied their role in altering the temporal and spatial characteristics of the evolving morphologies. I also investigated the formation of surface hillocks and the role of surface and interfacial energies in their evolution. By studying the change in total free energy across the different deposition models, I established that, in addition to influencing the temporal and spatial characteristics of nanoscale structures in the films, this quantity is also responsible for driving morphological transitions as the rate of deposition is increased. Insights gained from this computational study will demonstrate the viability of these models in predicting experimentally observed morphologies and form a basis for understanding the various factors involved in driving phase-separation and morphological transitions. In addition, morphology maps will serve as templates for developing new pathways for morphology control in the manufacturing of PVD alloy films.
ContributorsRaghavan, Rahul (Author) / Ankit, Kumar (Thesis advisor) / Rajagopalan, Jagannathan (Committee member) / Mushongera, Leslie T (Committee member) / Arizona State University (Publisher)
Created2021