Control of Hierarchical Order and Development of Stimuli-Responsive Self-Assembled Materials Across Multiple Length Scales

Chin, Stacey Megan

doi:https://doi.org/10.21985/n2-tckw-hz41

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Control of Hierarchical Order and Development of Stimuli-Responsive Self-Assembled Materials Across Multiple Length Scales

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One of the grand challenges in materials chemistry and nanochemistry is the development of functional materials through ordered, hierarchical structures using synthetic building blocks. Nature has done this through evolution of molecular components such as nucleic acids, saccharides, lipids, amino acids, and inorganic crystals. The precise spatial positioning of these components gives rise to highly complex functions, such as the movement of muscles or the growth of tissues. In the context of these biological systems, this work developed novel stimuli-responsive materials based on interactions and hierarchical ordering of low molecular weight molecules known as peptide amphiphiles (PAs). The first system of supramolecular-covalent hybrid polymers was created by synthetically modifying a peptide amphiphile capable of self-assembling into supramolecular filaments with an initiator for atom-transfer radical polymerization. This enabled the growth of thermo-responsive oligo(ethylene glycol) methacrylate (OEGMA) polymers covalently attached to the nanofiber surface. Arrangement of these nanofibers in an aligned tubular scaffold resulted in macroscopic anisotropic actuation of the hybrid material, with a stronger response perpendicular to the direction of alignment. This behavior emulates the anisotropic actuation observed in biological muscle tissue, which is driven chemically. Computational and experimental results indicated that the anisotropy was due to its hierarchical structuring, in part from the tethered nature of the thermo-responsive molecular structures to the supramolecular filaments as well as due to the structural reinforcement of the microstructure. From this work, explorations of how to further control the macroscopic domains of alignment were carried out using a direct ink-writing approach to apply shear. 3D printing was used to order the supramolecular assemblies into various patterns of alignment within the hybrid structure. With printed patterns of alignment, structures were able to display curling instead of shrinking behavior, attributed to differences in drag force and therefore alignment between layers of printed material. In order to create more uniformly patterned alignment, cellulose nanocrystals were then incorporated into the PA printing solution, which significantly improved the rheological properties of the ink such that large free-standing structures could be printed without the addition of cross-linking calcium ions. In order to create similar printed structures but without the need for rheological additives, an all-PA ink system was developed containing β-cyclodextrin and adamantane moieties for supramolecular host-guest cross-linking. These all-PA materials were also printable and maintain their structure without the addition of ionic cross-linkers or other rheological additives, which could be important for biological applications of these materials. Finally, the formation of hierarchical peptide amphiphile structures was investigated on a microscopic level by using annealing processes in confined environments. The reversible formation of micron-sized, highly ordered liquid crystalline superstructures was observed upon heating, consisting of thousands of bundled PA nanofibers packed in a hexagonal lattice. It is suggested that the inter-fiber spacing decreases significantly at elevated temperatures due to increased ion condensation and the entropic release of water bound to the PA surface. This effect is analogous to the lower critical solution temperature behavior observed in thermo-responsive polymers such as OEGMA, but has not been previously observed within supramolecular assemblies. Taken together, these findings demonstrate the versatility of PA molecules towards controlling the formation of hierarchically ordered materials and their applications in the development of stimuli-responsive functional materials.

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