Doctor of Philosophy (PhD)
Physics and Astronomy
This dissertation concentrates on the morphology and the dynamics of bottlebrush polymers and their relationship with the material properties. In general, the macroscopic behavior is determined by microscopic properties, like structure and dynamics. Therefore, the understanding of these properties is essential. Bottlebrushes belong to the family of branched polymers with a high grafting density of side chains, covalently bonded to the backbone. Due to steric repulsions, elongated or star-like shapes occur instead of random coil conformations, known from linear polymers. Besides these morphological changes, bottlebrushes show a reduced entanglement density compared to linear counterparts. Consequently, the dynamics change and super soft and hyper elastic materials result. To investigate the morphology and the dynamics, this dissertation is split in two parts. In the first part, the structure of bottlebrush polymers was studied by small-angle neutron scattering. A transition from elongated to spherical shapes occurs, by changing the size ratio of backbone to side chains. In the spherical region, bottlebrushes can be considered as star polymers with negligible core size and thus, can be seen as shell-only particles. In this context, structural changes depending on the molecular weight of the linear host matrices and the bottlebrush volume fraction were investigated. This showed a decreasing radius of the bottlebrushes, induced by the increasing molecular weight of the host matrices. Additionally, clustering starts at the highest volume fraction. The second part concentrates on the dynamics of bottlebrush polymers, investigated by different techniques. Dielectric spectroscopy showed a reduced segmental relaxation time for bottlebrush polymers compared to their single linear side chains. Field cycling measurements revealed an entanglement free bottlebrush even if the molecular weight is high. Quasi-elastic vi neutron scattering tracks the methyl group dynamics and segmental relaxation. While the methyl group dynamics are independent of the side chain length and the momentum transfer, the segmental relaxation times depend on both, i.e., the molecular weight of the side chains and the momentum transfer. This strong 𝑄-dependence suggests non-Gaussian behavior. A new approach toward a master curve of the mean square displacement was developed, considering the separate contributions from methyl groups and the segments.
Bichler, Karin Julia, "Morphology and Dynamics of Bottlebrush Polymers" (2020). LSU Doctoral Dissertations. 5244.
Available for download on Sunday, April 25, 2027