Graft Polymer Physics

Abstract

Graft polymers have polymeric side chains grafted onto a common backbone and exhibit extended conformations due to steric repulsion from densely grafted side chains. The ability to modulate conformation, and thus material performance, has made graft polymers a rich area of research in the last decade. This thesis expands the fundamental understanding of the physical properties of graft polymers in order to aid in the design of novel materials and focuses in three areas: rheology, thermodynamics, and mechanical properties. First, the effect of grafting density on the linear viscoelasticity of graft polymers is investigated. We demonstrate that graft polymers experience the same relaxational modes as linear polymers and their viscoelastic behavior can be described by the same Rouse and reptation theories. The experimental data is compared to scaling models to determine the conformation of the graft polymers, and a new model is proposed to better capture the behavior of experimentally relevant graft polymers. Next, the thermodynamics of densely grafted bottlebrush block polymers is explored. Bottlebrush block polymers were prepared with homopolymer side chains added in blocks along the backbone, varying side chain and backbone length. Their order-disorder transition temperatures were measured by temperature controlled small-angle X-ray scattering. The bottlebrush block polymers display a higher segregation strength compared to linear diblock polymers at the order-disorder transition, indicative of the shielding of the segments near the backbone. The segregation strength at the order-disorder transition decreased with increasing side chain and backbone length. Finally, the mechanical properties of graft polymers with diblock side chains are studied in an attempt to produce tough and sustainable polylactide plastics. The addition of a rubber domain initially toughens the polylactide but the polymers still undergo physical aging and become brittle over time; the time to brittle failure is found to be strongly dependent on the rubber content of the graft block polymers. Pre-straining of the polymers is used to produce stronger and tougher plastics that do not embrittle upon aging.