Ms. Sujin Lee

Thesis Title: " Cholesteric phase of Chitin Nanocrystals Under Confinement

Monday, January 10, 2022 at 11 am

Bluejeans link: https://bluejeans.com/448110544/1137

Committee Members:

Prof. Mohan Srinivasarao (Advisor), School of Materials Science and Engineering, School of Chemistry and Biochemistry

Prof. Elsa Reichmanis (Advisor), Department of Chemical & Biomolecular Engineering, Lehigh University

Prof. Nicholas Hud, School of Chemistry and Biochemistry

Prof. David Collard, School of Chemistry and Biochemistry

Prof. Will Gutekunst, School of Chemistry and Biochemistry

Summary

Chitin nanocrystals (ChNCs) naturally self-assemble into a “cholesteric” liquid crystalline phase (ChLC) above a critical concentration in water. The helical structure of the ChLC selectively reflects the incident white light through Bragg reflection, and the average wavelength l of the reflected light depends on the pitch of helix, p. However, due to the random alignment of the cholesteric structure, it is challenging to study the self-assembly process and utilize the optical properties of ChNCs. Confinement of the cholesteric phase is an effective strategy to produce well-aligned structures. In this study, we document the alignment of the cholesteric phase of ChNCs in confined space. When ChNCs are confined to spherical droplets, they exhibit various textures ranging from twisted bipolar structure to concentric layers depending on the size of the droplets. The interplay of elastic constants coupled with surface anchoring dictates the configuration of the droplets. When droplets were made into microgels by adding precursors to the droplet followed by photopolymerization, the microgels displayed anisotropic swelling behavior in water, which is governed by the twisted structure, in response to temperature. Using an external magnetic field, we demonstrated the various alignments possible for a cholesteric fluid composed of ChNCs in the presence of magnetite particles. In the absence of a magnetic field, the ChNCs exhibit strong planar anchoring in droplets. Upon application of the field, the helical axis of the twisted structure in the droplets aligned parallel to the field and displayed a nested cup configuration. Finally, since the ChNCs are polydisperse, we fractioned them according to their length and studied the concentration and length dependence of the cholesteric pitch. It was found that the helical twisting power (HTP), which is a proportionality constant of the ChNCs content in water (W) and inverse pitch (1/p0), given by 1/p0 = HTP·W, increased with the rod length. The HTP of the ChNCs was nearly 5 times lower than that of cellulose nanocrystals having same length.