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Polypeptoid Chain Conformation and Its Role in Block Copolymer Self-Assembly

  • Author(s): Yu, Beihang
  • Advisor(s): Segalman, Rachel A
  • et al.
Abstract

Polymer chain conformation underlies polymer physical properties and impacts many of polymer functionalities. Understanding chain conformation is critical for predicting and controlling the structures and properties in polymeric systems. In block copolymers, chain conformation of the consisting blocks closely impacts the thermodynamics of microphase separation and the resultant structures, which are key to block copolymers as functional materials. However, the understanding of chain conformation effects beyond coil–coil block copolymers is yet nascent, partially due to the challenge to precisely control chain conformation without introducing other complicating factors.This dissertation utilizes sequence-defined polypeptoids to install precise chain conformation control into traditional polymer systems, to examine the role of chain conformation in block copolymer self-assembly. First, the polypeptoid chain conformation is examined in terms of local stiffness, overall chain size, and response to solvent quality by comparing chemically identical helical and coil polypeptoids in dilute solution. The detailed understanding from molecular length scale reveals that the helical secondary structure, driven by steric hindrance from side chains, makes the polypeptoid chains locally stiffer but overall more compact than the coil analogues. Further, we show these helical chains are relatively insensitive to solvent conditions due to their sterically defined nature. Then, through the design of model polypeptoid-containing block copolymer systems, we are able to study the effects of the helical chain conformation, which has distinct space-filling characteristics from chemically analogous coil chains, on the melt self-assembly of block copolymers. In the lamellae-forming system, the helical chain conformation is shown to decrease the order–disorder transition temperature through a combination of decreasing the enthalpic interaction between dissimilar blocks and experiencing amplified chain stretching. Further, polypeptoids are used as a conformation tuning handle and are shown to modulate network phase formation and stability as a conformationally tunable interfacial segment, which demonstrates the importance of chain conformation at the vicinity of the interface in determining the morphology of block copolymers. The findings in this dissertation highlight the importance of chain conformation on the self-assembly thermodynamics of block copolymers, and polypeptoids as highly-controlled, precise polymers to aid the fundamental understanding of polymeric materials.

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