Materials engineering depends on a thorough understanding of the structure-property relationships in order to rationally design better materials. In polymer science, information about the structure of the material generally comes from scattering techniques, such as wide angle X-ray scattering or neutron scattering. These scattering experiments produce information about the material's structure in reciprocal space, so there can at times be ambiguity in the results when transitioning to real space. Transmission electron microscopy (TEM) is widely used in the materials science community to produce direct, atomic-scale images of hard materials. However, special techniques are needed to image polymeric materials with TEM due to the inherent radiation sensitivity in these soft materials. A series of techniques termed cryo-EM have been developed by the structural biology community to produce atomic-resolution images of proteins. Cryo-EM has only sparingly been applied to synthetic polymers, yet it is a promising tool to advance polymer science.
This dissertation focuses on the application of cryo-EM techniques to study the atomic structure of synthetic polymers. The atomic-scale images of the polymers are combined with other material characterization data, molecular dynamics simulations, and TEM image simulations in order to understand the governing interactions that control self-assembly at the atomic level. The direct, atomic-scale imaging of synthetic polymer materials serves to advance material engineering by uncovering the structure-property relationships starting at the atomic scale.
Chapters 2, 3, and 4 provide detailed studies in using polypeptoids, which are synthetic polymers with a monomer unit similar to peptides, to uncover the effect of single atom substitutions, ionic interactions, and fixed charges on the self-assembly of these materials. Polypeptoids are a great model system to study self-assembly because their unique submonomer synthesis method grants the ability to precisely vary single atoms within the polymer structure. Chapter 2 starts with using this ability to study the effect of halogen substitutions on the self-assembly and crystal motifs of polypeptoid nanosheets. Chapter 3 investigates the effect of fixed charges and condensed counterions in polypeptoid nanofibers. Chapter 4 details an investigation into the effect of fixed charges and charge density on polypeptoid self-assembly.
Chapter 5 applies the same cryo-EM techniques to a conventional diblock copolymer in poly(ethylene oxide)-b-polystyrene (SEO). New synthetic routes were established to create single crystals of SEO with a lithium salt whereby the lithium salt was fully incorporated in the crystal. The single crystals of SEO were imaged with atomic-scale resolution to study how the arrangement of polymer chains changes in response to different amounts of lithium salt. Image simulation based on known crystal structures was used to bring greater understanding to the atomic-scale images.
