Date of Award
Doctor of Philosophy (PhD)
Materials Science and Engineering
Controlled degradation of polymers finds various applications in fields ranging from the design of functional soft materials to recycling of polymers. In several of these applications, the characteristic length scale at which relevant processes occur ranges from nanometers to microns, typically referred to as the mesoscale. Although analytical models and continuum approaches inform our current understanding, analysis of degradation at the mesoscale is exceptionally limited. For modeling degradation at the mesoscale, we use the Dissipative Particle Dynamics (DPD) technique and the LAMMPS simulation software. Within the DPD framework, we model controlled degradation or the breaking of covalent bonds within a polymer as a stochastic process that reproduces first order degradation reaction kinetics. A known limitation of the DPD approach is polymer chains crossing through each other. Previous researchers had developed a modified segmental repulsive potential (mSRP) framework which prevents such crossing of polymers by introducing extra repulsion between the bonds of polymer chains. We modified the existing model in LAMMPS to enable switching off the extra repulsion when a bond is broken. We implemented this feature within the LAMMPS framework, and it is now available for the general scientific community as a part of the online open-source project. Later, we extended this feature to introduce the extra repulsion when a bond is formed to simulate the hydrosilylation reaction used in the synthesis of polymer derived ceramics. As a model polymer network for studying degradation, we use the tetra-arm polyethylene glycol (tetra-PEG) based hydrogel films. Tetra-PEG networks have a uniform network structure and hence superior mechanical properties. We tracked the degradation iii of these networks by measuring the evolution of the weight average molecular weight and dispersity during degradation. By tracking the fraction of degradable bonds broken, we identified the “reverse gel point”, the point where the polymer network dissolves into the surrounding solvent. Additionally, we tracked the erosion or mass loss from the degrading network by accounting for polymer fragments which dissociate and diffuse away from the network. We identified that the mass loss from the network depends on the initial thickness of the hydrogel films. As a second system, we modeled the controlled degradation of nanogels that are either suspended in a single solvent or adsorbed onto a liquid-liquid interface. Controlled degradation of nanogels at an interface provides a dynamic approach to control interface topography at the nanoscale. We tracked the degradation of these particles by analyzing the evolution of their shape and size along with the molecular weights and dispersity in the system. In bulk, the particles swell almost homogenously while at the interface, the particles spread and cover the interface as degradation occurs. We found that the reverse gel point for these particles varies with the total initial number of precursors. The evolution of particle shape and size is significantly affected by the surrounding solvent and the surface tension between the two liquid phases. The final part of this dissertation focuses on developing an initial framework to extend the above approach to model degradation of polyolefin melts under a local temperature gradient. The long term goal of this project is to study thermal degradation of polyolefins caused by introducing microwave absorbing nanosheets and subjecting the polymer to microwave irradiation.
Palkar, Vaibhav, "Mesoscale Modeling of Controlled Degradation in Polymer Networks and Melts" (2023). All Dissertations. 3357.
Author ORCID Identifier