Doctor of Philosophy (PhD)
Physics and Astronomy
Materials based on polymers and polymeric nanocomposites have properties advantageous for many applications. In many cases, the physical behavior is connected to molecular motion. Polymers are macromolecules in which both fast local motions and slower chain relaxations may contribute to the material properties. Answering fundamental questions on the molecular dynamics is challenging, because of the existence of many different processes and the strong temperature dependence of the processes. An investigation covering a broad window over many orders of magnitude in the time scale is needed. Hence, dielectric spectroscopy is an appropriate tool to enhance fundamental understanding. This dissertation pays special attention to derive clear spectra from the experiment to gain better information from the spectral shape.
In general, the dielectric permittivity is a superposition with conductivity and polarization, which challenges the delineation of spectra of pure polymer dynamics. Simultaneous data modeling on entangled polyisoprene, PI, allowed separation of the individual contributions, and more reliable information on reptation could be extracted. As an example of this improved approach, the high molecular weight transition to pure reptation, at which contour length fluctuations and constraint release cease, was found to be at least one order of magnitude higher than previously anticipated values.
The spectral shape of unentangled polymers was analyzed to test different concepts and suggestions obtained from a variety of different models. Deviating from both theoretical and empirical models, the spectra indicate processes which have not been considered before. The monomeric friction coefficient seems to depend on the position of monomer in the polymer chains. To find this influence on dielectric spectrum, the end block relaxation was introduced on unentangled polymers. The results could partially explain the discrepancy between models and experimental data and are compatible with the those from simulation and experiments on other polymers.
Analyzing polymer nanocomposites through previously built methods, it was observed that the spectral shape of chain dynamics and segmental relaxation have been both altered with the presence of nanoparticles, together with both chain and segmental dynamics being slowed down. The nanoparticle contribution seems to be a non-linear process and coupling effect needs to be considered.
Wu, Mengchun, "Dynamics of Polymer Melts and Polymer Nanocomposites Studied by Broadband Dielectric Spectroscopy" (2022). LSU Doctoral Dissertations. 5868.
Schneider, Gerald J.
Available for download on Monday, May 19, 2025