Title

Rational Design and Self-Assembly Studies of Mixed-Graft Block Copolymers

Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Engineering (DEng)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Zhong, Mingjiang

Abstract

The self-assembly of block copolymers (BCPs) represents a promising route to the bottom-up fabrication of nanostructured materials, which have potential applications in areas including microelectronics, separations, photonics, and energy storage. The versatility of BCPs in various applications draws on their highly tunable bulk properties, and their high performance relies on the nanostructures having precisely defined size, morphology, orientation, and molecular functionality. Control over BCP nanostructure and bulk properties can be exercised through judicious control of various aspects of molecular structures, such as molecular weight, chemical composition, stereochemistry, and chain architecture or topology. This thesis focuses on control of polymer properties and self-assembly through the design of mixed-graft block copolymers (mGBCPs), which consist of two or more types of polymeric side chains grafted from a linear backbone in a random, alternating, or pseudo-alternating sequence. mGBCPs can phase separate with the backbone serving as the interface of the blocks, and the side chains dominate their self-assembly behavior, determining the self-assembled nanodomain sizes and morphologies. In this thesis, the current literature on the synthesis and self-assembly of mGBCPs is first reviewed, paying special attention to the synthetic routes used, which affect the side chain sequence and grafting density of the side chains in the resulting mGBCPs. Subsequently, we investigate the self-assembly of Janus mGBCPs, in which the A and B side chains are arranged in a pseudo-alternating sequence along the backbone. Ordered nanostructures with an ultra-small domain size down to 2.8 nm (half-pitch) can be obtained using the mGBCP architecture since the covalently linked backbone stabilizes the mGBCP phase separation. Furthermore, the self-assembled nanostructures and thermomechanical properties of the mGBCPs can be independently encoded into two different features of the mGBCP architecture, the side chain length and backbone length, respectively. We then synthesize mGBCPs in which A and B side chains are randomly distributed along the backbone. These mGBCPs have similar self-assembly behaviors to the Janus mGBCPs and can achieve diverse self-assembled morphologies. As this synthetic method is more scalable than that of the Janus mGBCPs, we also investigate the processing of the mGBCPs via electrospinning and demonstrate improved processability compared to a linear diblock copolymer analogue. The last part of the thesis focuses on ternary mGBCPs, which contain three types of side chains. The bulk self-assembly of the ternary mGBCPs is investigated, revealing a distorted hexagonal phase. Studies of the thin-film self-assembly of the ternary mGBCPs support our hypothesis that the orientation of the self-assembled nanodomains can be controlled by designing a ternary mGBCP in which one block preferentially wets the air or substrate interface, forcing the remaining blocks to orient perpendicularly to the substrate. Perpendicularly oriented nanodomains are often needed for applications involving nanopatterning and nanotemplating, such as nanolithography, membrane fabrication, and nanoparticle patterning.

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