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Title:Solution state aggregation and flow-induced alignment of conjugated polymers
Author(s):Kwok, Justin J.
Director of Research:Diao, Ying
Doctoral Committee Chair(s):Diao, Ying
Doctoral Committee Member(s):Chen, Qian; Leal, Cecilia; Schroeder, Charles
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):conjugated polymers
small angle X-ray scattering
Abstract:Solution processability is a key advantage of conjugated polymers enabling the production of organic electronics devices over large areas in a high throughout and energy efficient manner. There are two important aspects of solution processing which consists of understanding what the solution state of conjugated polymers is and how processing the solution can affect the conjugated polymer assembly and the resulting morphology of printed films. In this thesis, I focus on developing a better fundamental understanding of these two aspects, namely understanding the precise nature of pre-aggregation in donor-acceptor (D-A) conjugated polymers solutions and understanding how the fluid flows ubiquitous to solution processing can affect the conjugated polymer’s conformation and assembly to tune film morphology. In Chapter 2, I focus on unraveling a picture of the conjugated polymer solution state using small-angle X-ray scattering (SAXS). By developing a model consisting of a combination of fibrillar aggregates and dispersed polymer chains I am able to understand the scattering profiles and extract quantitative information about the conformation and pre-aggregation structure. With this model I also clarify the physical origin of the broad structure factor peaks observed in SAXS of D-A polymers. Furthermore, by demonstrating the generality of my model by fitting to a variety of different D-A polymer scattering profiles I am able to unify the seemingly different D-A polymer scattering profiles under a single interpretation. After understanding the precise nature of the conjugated polymer solution state I then move to understand how flow can affect their alignment (Chapter 3). Using microfluidics and flow-induced retardance measurements, I study the flow-induced alignment of conjugated polymers under shear and extensional flow in a systematic manner. I determine the strengths of shear and extensional strain rates required to align the conjugated polymer pre-aggregates, allowing us to estimate their relaxation time. By tuning the pre-aggregation by adjusting solvent quality I am able to investigate the interplay between pre-aggregation and flow. I find that enhanced aggregation forming longer, more rigid fibrils can drastically facilitate flow-induced alignment but that the internal structure of the pre-aggregates and the presence of interparticle interactions can diminish the effectiveness of flow. Having characterized the flow-induced alignment of conjugated polymers and synergisms with the conjugated polymer solution state I then focus on studying the effect of flow in the meniscus of a real solution printing process (Chapter 4). By carrying out a multiphysical finite element simulation of meniscus guided printing I am able to understand the evolution of the meniscus flow field and strain rate with increasing printing speed. Combining this with imaging and spectroscopy of printed films, I demonstrate that the interplay between shear rate and residence time dictates the morphological transitions observed in the printed films. The printing flow at intermediate speeds is characterized by high shear rate and low residence time leading to the planarization of the twisted D-A polymer backbone. At this transition regime flow is able to alter the conjugated polymer assembly pathway to produce highly aligned films with a four-fold increasing in charge carrier mobility. Overall, my thesis has demonstrated that the conjugated polymer pre-aggregation and the fluid flow during processing along with their interplay can be used to improve the morphology of thin films. A careful understanding of these aspects can lead to design rules during solution processing that can leverage this synergy where specifically tuned aggregation and engineered flows can enhance film morphology and electronic performance.
Issue Date:2021-12-02
Rights Information:Copyright 2021 Justin J. Kwok
Date Available in IDEALS:2022-04-29
Date Deposited:2021-12

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