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Title:Self-assembly and additive manufacturing of bottlebrush block copolymer photonic materials
Author(s):Patel, Bijal Bankim
Director of Research:Diao, Ying
Doctoral Committee Chair(s):Diao, Ying
Doctoral Committee Member(s):Guironnet, Damien S; Leal, Cecilia; Sing, Charles E
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Additive Manufacturing
3D Printing
Block Copolymers
Directed Assembly
Bottlebrush Polymers
Abstract:The combination of block copolymer self-assembly and additive manufacturing (3D printing) provides a highly promising approach towards both direct nanopatterning for novel functional applications and efficient screening of process-structure-property relations in new materials systems. The works presented in this thesis blend a focus on innovation in additive manufacturing with fundamental inquiry into the self-assembly of bottlebrush block copolymers. In Chapter 2, I detail the design of our highly versatile hardware and software platform for melt and solution-phase benchtop AM and highlight patterning and post-deposition processing of a series of non-filament forming functional polymers. In Chapter 3, I apply this system to investigate nonequilibrium self-assembly of bottlebrush block copolymers cast from solution and demonstrate precise control of nanoscale structure and photonic properties through systematic variation of printing conditions during deposition. Remarkably, the photonic stop band of printed photonic crystals can be tuned across a range of that spans the visible spectrum from a single ink, without requiring synthetic variation or blending. Using a combination of optical property measurement, cross-sectional scanning electron microscopy, and synchrotron small-angle X-ray scattering I conclusively attribute the tuning of photonic properties to change in lamellar d-spacing over a range of greater than 70 nm. Finally, I demonstrate the mechanistic underpinning of this phenomena to be kinetic trapping of metastable chain conformations based on in situ optical microscopy and solvent-vapor annealing experiments. In Chapter 4, I present the first reported experimental investigations into the concentration-dependent solution-phase ordering of bottlebrush block copolymers in a good solvent. Through small-angle X-ray scattering, we identify the transitions from (1) uniformly mixed dilute and semi-dilute polymer chains, through the onset of compositional fluctuations (2) as the blocks begin to microphase separate, finally resulting in (3) the formation of an ordered lamellar state. Chain extension is shown to dramatically decrease with concentration in the uniformly mixed phase, followed by partial recovery with increasing concentration as microphase separation occurs. I show that it is the latter behavior that leads to the remarkable photonic properties of concentrated bottlebrush block copolymer solutions, demonstrating that the photonic stop band can be tuned across the entire range of visible wavelengths simply by varying solvent volume fraction, here under equilibrium conditions. Finally, in Chapter 5, I investigate interface-induced ordering in PS-b-PLA bottlebrush block copolymer films during thermal annealing as a straightforward route towards tuning domain orientation and photonic properties. Photonic properties were observed to saturate within minutes of annealing at 150°C, with distinct variation in transmission response as a function of film thickness. The depth of the highly aligned surface region ranged from 30 - 100 lamellar periods, greatly exceeding that commonly reported for linear block copolymer systems despite annealing for a much briefer period. The sharpness of the orientation gradient decreasing substantially with increasing film thickness, suggesting a competition between growth of aligned, heterogeneously nucleated grains at the substrate interface and orientationally isotropic, homogeneously nucleated grains throughout the bulk. Beyond the specific implications for polymer photonics, this thesis demonstrates the significant potential of bottlebrush block copolymers in rapid fabrication workflows and builds a fundamental and technical foundation for future application of directed self-assembly to enhancing ordering in bottlebrush block copolymer films.
Issue Date:2021-11-03
Rights Information:Copyright 2021 Bijal Patel
Date Available in IDEALS:2022-04-29
Date Deposited:2021-12

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