THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Wednesday, April 14, 2021
9:30 AM

via

BlueJeans Video Conferencing

https://bluejeans.com/167434802

will be held the

DISSERTATION DEFENSE

for

Jiwoo Yu

"Multiscale Self-Assembly of Nanomaterials and Cellulose for Sustainability"

Committee Members:

Prof. Zhiqun Lin, Advisor, MSE

Prof. Meisha Shofner, MSE

Prof. Sung Soon Jang, MSE

Prof. Vladimir Tsukruk, MSE

Prof. Yulin Deng, ChBE

Abstract

Self-assembly is ubiquitous in nature. It is widely recognized as an external field-free means to construct intriguing structures that promise new opportunities for developing miniaturized optical, electronic, optoelectronic, and catalytic materials and devices with applications in energy and sustainability. The use of cellulose is highly advantageous for sustainable applications due to an array of intriguing attributes of cellulose, including natural abundance, morphological anisotropy, and good chemical and physical properties. In this thesis, we aim to rationally design and craft nanomaterials and cellulose composites via multiscale self-assembly and scrutinize their formation fundamentals. Specifically, hydrophilic magnetic nanoparticles (NPs) are self-assembled into 2D nanosheet, and then patchily drape on self-assembled core/shell microspheres composed of  hydrophobic magnetic nanoparticle-containing paraffin core and hydrophilic cellulose shell for potential use in water treatment. Moreover, fluorescent conjugated nanorice-shaped polymer is patterned on cellulose via flow-enabled self-assembly (FESA) for enhanced triboelectric nanogenerator.

First, a family of polymer-ligated magnetic spinel nanoparticles (NPs) with controlled size, composition, and surface chemistry are synthesized via star-like block copolymer nanoreactor strategy. They display appealing size- (i.e., 4 nm, 7 nm, and 11 nm) and composition- (i.e., CoFe2O4, NiFe2O4, and MnFe2O4) dependent magnetic properties and colloidal stability.  Magnetization~magnetic field (M~H) hysteresis curves measured at 300K reveal a clear dependency of MS, coercivity, and susceptibility on size and composition of NPs. Zero field and field cooled magnetization (ZFC-FC) curves for these magnetic spinel NPs, examined over the temperature range of 2 K-300 K, also demonstrate a good accordance with theoretical curves.

Second, magnetic NPs are also synthesized for integration with self-assembled paraffin/cellulose core/shell microspheres for potential water purification. Specifically, hydrophobic ligand-capped Fe2O3 NPs are prepared via thermolysis. Subsequently, centimetre-scale free-standing 2D magnetic nanosheets are created via rapid self-assembly of hydrophilic dithiocarbamate- and glycine-co-capped Fe2O3 NPs at oil/water interface. The chemistry and mechanism for the formation of 2D magnetic nanosheets are scrutinized. Notably, 2D magnetic nanosheets are structurally intact without the need for crosslinking. Moreover, the versatility of this robust oil/water interfacial self-assembly strategy is manifested by crafting 2D nanosheets using magnetic NPs of different sizes.

Third, multiscale self-assembly of cellulose and magnetic NPs is performed to yield hydrophobic-magnetic-NP-containing paraffin/hydrophilic-magnetic-nanosheet-patchily-covered cellulose core/shell microspheres in an environmentally benign and cost-effective manner for potential use in water treatment. The average size of microspheres can be varied by adjusting hydrophilic-liphophilic balance (HLB) of surfactant used. The hydrophilic cellulose shell renders colloidal stability of microspheres in water. As microspheres are integrated with magnetic NPs, they can be easily collected with magnets for reuse after water purification.

Finally, as another application for sustainability, triboelectric nanogenerator on self-assembled cellulosic material is studied. FESA is employed to position nanorice-shaped conjugated polymers on patterned Si substrates containing Si pillars of various shape and height. Subsequently, FESA of conjugated polymers on TEMPO-oxidized cellulose nanofibers (T-CNF) pillars is carried out for triboelectric nanogenerator application. The conjugated polymers/T-CNF pillars show an average Voc of 43.7 V/cm2, representing a 21.6 % enhancement over T-CNF pillars without FESA patterned counterpart.