Computational Studies of Vitrimers, Semicrystalline Polymers and Metals: Deformation, Actuation and Fabrication

Abstract

Molecular dynamics (MD) provides valuable insights into the structure, dynamics, and properties of materials, especially when experimental data is scarce or difficult to obtain. In this thesis, we use MD simulations to study three classes of materials: vitrimers, semicrystalline polymers, and metals. These materials exhibit diverse and complex behaviors under thermal and mechanical loads.

Vitrimers are a promising alternative material system that has been recently designed to address the non-recyclability of thermoset plastics. In vitrimers, dynamic cross-links allow for malleability under thermal stimuli. Despite their several advantages, the lack of understanding of their mechanical response has limited the widespread use of these materials for aerospace applications. In this thesis, we develop a capability to simulate dynamic cross-linking reactions of vitrimers under general thermomechanical loading conditions using a topological reaction scheme within MD. We demonstrate the application of the model towards damage healing of a vitrimer system at the molecular level. The work will allow us to: (1) develop innovative computational models of the complex interplay between chemistry and mechanics in vitrimers, (2) link the evolution of thermomechanical properties to underlying molecular mechanisms, and (3) understand the molecular-scale mechanisms of the creep behavior of vitrimers which is hard to characterize by experiments.

Another application studied is the simulation of semicrystalline polymer--based twisted and coiled polymer actuators (TCPA). These actuators can achieve large actuation strokes by exploiting the material anisotropy due to the amorphous and crystalline phases of Nylon. An all-atom molecular dynamics (MD) simulation is used to investigate the thermal behavior of the two phases and to determine their coefficients of thermal expansion (CTE) and glass-transition temperature, separately. Based on this data, we develop a finite element model for TCPA and validate the results against experimental data. The model is used to explore the effects of actuator parameters such as chirality, twist angle, and material anisotropy on the actuation performance. The CTE anisotropy is found to be the dominant factor as compared to the elasticity tensor for the large actuation. This study provides a comprehensive understanding of the physics of TCPA and provides directions for their optimization and performance.

The final application studied is Additive manufacturing (AM) of metal parts. Additive manufacturing involves a gradual modification in the size and shape of solids due to the addition of new layers of the material on top of the existing ones that result in large thermal gradients under non--equilibrium phenomena. Atomistic simulations are used to model the additive manufacturing process at the nanoscale to establish process-property correlations. To this end, we study the evolution of defect structure as a function of process parameters: cooling time, melt thickness, substrate temperature, and soft vs. hard inclusions. We find that the defect content can be significantly reduced by raising the temperature of the powder bed to a critical temperature. A critical advantage of this approach is that simulations can be used to perform alloy design, as demonstrated by simulating the effect of the addition of a hard and soft inclusion on the defect structure.