Solution self-assembly of semiflexible amphiphilic poly(D-glucose carbonate)s

Abstract

The study of the solution self-assembly of nanoscale building blocks has had significant implications on both academic and industrial communities due to its potential to achieve complex structures with bottom-up designs. With the advance of synthetic routes and assembly processes, there exist endless possibilities and combinations of unique chemistry and assembly pathways to construct desired structures but also challenges in precise control and characterization of both local and global-level molecular interactions. Inspired by natural amphiphiles (e.g., fatty acids and lipids) and their ability to self-organize into various cell constituents, synthetic amphiphilic molecules have been studied to explore the extent one can mimic complex physiological assembly processes and also successfully incorporate this knowledge to various commercial applications including organic electronics, emulsifying or filler agents as well as in biomedical applications for developing nanomedicine or gene delivery carriers. Our interest also expands beyond the self-assembly of these amphiphiles into classic bio-inspired nanostructures such as micelles and bilayers to more complex, diverse nanostructures using large molecule analogues such as amphiphilic block copolymers. Amphiphilic block copolymers provide many advantages over small molecule amphiphiles not only because of their chemical diversity and mechanical robustness but, more importantly, due to their non-ergodic nature to form kinetically trapped nanostructures inaccessible by small molecule assembly. While many studies have been dedicated to understanding different parameters that impact the assembled morphology, studies are often limited to using coil-based or vinyl-based block copolymers where the effects of many non-classical polymer chain properties are not discussed. With the recent effort to design and synthesize next generation green chemistry-based polymers, the need for a deeper understanding of unconventional chain properties resulting from both structural and chemical uniqueness focuses our attention on the exploration of different chain parameters that can impact the BCP solution assembly. Such particular chain characteristics include polyelectrolyte-like behavior, polymer chain stiffness, and the local solvophobicity of the polymer chains that impact not only local chain characteristics and global assembly properties of non-coil-based block copolymers in solution but also have implications due to their role in dictating many important biological processes observed in nature (e.g., protein folding and DNA packaging).The goal of this dissertation is to explore the effect of chain rigidity and local amphiphilicity on the polymer solution and assembly behavior using a glucose-based poly(D-glucose carbonate) (PGC). The PGC-based polymers were first developed to offer alternative, renewable-resourced materials with easy accessibility, functionality and applicability in response to recent efforts to reduce the long-term environmental impact of petrochemical plastics. The challenge, however, of using PGCs stems from its unique solution behavior and assemblies attributed to its semiflexible, hydrophobic PGC backbone that does not follow the traditional BCP behavior. To have an in-depth understanding of how the particular PGC backbone chemistry affects the local chain and global assembly behavior, both PGC-based amphiphilic block copolymers and PGC-based homopolymers were studied in solvent mixtures to observe how the change in local chain conformation due to the solvent quality change leads to a particular assembly morphology. Specifically, we examined the PGC molecules in solution to study: 1) sphere and hierarchical fibril formation of PGC-based amphiphilic BCP in different solution environment and proposed chain packing of the PGCs within the assembled nanostructures; 2) globule formation of hydrophilic PGC-based homopolymers and the effect of solvent quality on the globule-globule interaction; 3) the fine-tuning of the PGC-based amphiphilic BCP assembly using different solvent compositions and side chain chemistries. The three studies here tie into the theme of understanding the role of semiflexible and hydrophobic backbone chemistry and how a complex balance between the backbone solvation and the side chain solvation can lead to an unusual local chain and global assembly behavior. ☐ First, with a PGC-containing amphiphilic diblock copolymer system, the kinetically controlled assembly pathways were explored with tetrahydrofuran (THF)/water mixture that led to the hierarchical assembly of ribbon-like fibers with features that did not follow the traditional BCP micellar-like packing. The results here demonstrated that while the stiffness of the PGC backbone impacted the local BCP chain conformation and chain packing within the assembled nanostructure, the backbone hydrophobicity drove the unidirectional hierarchical fibril growth by the formation of soft patchy precursor particles. ☐ Second, to understand exactly how the BCP chain structures evolve with the solvent quality gradient, the hydrophilic block equivalent homopolymers were studied with a variation in solvent compositions that led to the BCP fibril assembly. Due to the hydrophobic backbone, the PGC homopolymers formed soluble globules in both water and in mixed THF/water where a complex balance between solvent swollen PGC backbone and the side chain ionization led to an interesting chain configuration within the globule and solution globule-globule interactions. ☐ Lastly, to further fine-tune the BCP assembly and tie the BCP and homopolymer studies together, the PGC BCP assembly was explored with a variation in assembly conditions. Specifically, the change in solvent composition, pH or side chain chemistries led to an interesting deviation from the initial hierarchical fibril formation where network-like morphologies, twisty fibrils or no hierarchical assembly were observed, respectively. The results suggest that these assembly parameters manipulate the pathway that the initial patchy particles link together. ☐ These findings suggest that polymers with unconventional backbone chemistries, frequently found in natural carbohydrate-based molecules, can shed light on the effects of polymer backbone rigidity and hydrophobicity on the assembly pathway and final assembled structures while also presenting opportunities to better understand how other bio-inspired green chemistry-based molecules can organize and assemble in various environmental conditions.