Mechanistic analysis of nanoparticle-polymer interactions and their application in composite solid polymer electrolyte systems
Creators: name: Mitra, Anindya
Publisher: name: Graduate Studies
Year: 2023
Subjects: subject: polymer electrolytes
subject: polymer nanocomposites
subject: molecular dynamics
subject: Engineering--Chemical

description: Polymer dynamics command interest and fascination from scientists and engineers due to many unique aspects. Some of those include the fact that relevant phenomena in such systems might occur over a very wide variety of time and length scales which may result in dynamic phenomena getting masked (or conversely, getting uncovered) depending on the system. Molecular interactions play a very fundamental role in such observed phenomena. This thesis investigates molecular interactions relevant to polymeric systems using fully atomistic molecular dynamics method, with a special emphasis on composite solid polymer electrolytes. In chapter three, we study the effects of polymer-nanoparticle and nanoparticle-nanoparticle surface interactions on the dynamics and dispersion characteristics of polymer nanocomposites. We uncover the counter-intuitive observation that hydroxylated nanoparticles (with a hydrophilic surface) show greater dispersion as compared to non-hydroxylated nanoparticles (with a hydrophobic surface) in a hydrophobic polymer matrix. We attribute this to increased nanoparticle-nanoparticle repulsion caused by the surface -OH groups on the nanoparticle. In the next chapter, we tie in this observation to a composite solid polymer electrolyte system containing a polyethylene oxide (PEO) matrix and cellulose nanofillers. We investigate the effect of nanoconfinement on ionic conductivity of the polymer. Our results indicate that the confinement of the polymer with nanocellulose has a two-fold effect on the ion dynamics, with confinement increasing the average mobility of cations as well as the proportion of cations that are highly mobile. However, confinement is experienced only when distance between two cellulose nanofibrils is comparable to the radius of gyration of the PEO chain. In the final chapter, we look at the effects of nanoparticle-polymer and nanoparticle-cation interaction on the ion dynamics in a composite solid polymer electrolyte. We show that the addition of both hydroxylated and non-hydroxylated C60 fullerene nanoparticle enhance ionic mobility in comparison to the pure PEO system. However, the system with non-hydroxylated nanoparticles displays significantly higher ion mobility. This is a counter-intuitive observation; as PEO is a hydrophilic polymer, we would expect hydroxylated nanoparticles to have greater dispersion and aid in cation dynamics. Even in terms of the free energy landscape, the system containing hydroxylated nanoparticles have wider regions of stability, indicating that they should have higher cation mobility. However, we show that the polar hydroxyl groups engender stronger nanoparticle-polymer and nanoparticle-cation interactions, which has a retarding effect on the cations and thus slows down cation dynamics. Thus, we can conclude that polymer and nanoparticle dynamics in a composite solid polymer electrolyte system depend upon a complex interplay of interactions between the nanoparticles, the polymer matrix and the ionic species involved.