Browsing by Subject "Block Polymer"

Now showing 1 - 4 of 4
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    A Computational Approach to the Stability of Complex Sphere Forming Phases in Block Polymer Melts
    (2021-05) Cheong, Guo Kang
    Block polymers spontaneously self-assemble into a variety of morphologies upon cooling below their order-disorder temperature. Owing to this behavior, block polymers have various potential applications ranging from semiconductors fabrication to filtration devices. Recent discovery of the stable Frank-Kasper phases in diblock copolymer melts resulted in a shift of focus from high-symmetry morphologies to low-symmetry tetrahedrally close-packed phases. While experimentalists have been able synthesize block polymers that exhibit stable Frank-Kasper phases, they could not predictably determine the observed phases a priori. Computational tools can aid in prediction but are rooted in well-developed theories and experimental results. In this dissertation, we aim to develop theoretical understanding of the stability of Frank-Kasper phases that could aid in prediction through a computational study of block polymers guided by experimental data. To this end, we take a three-pronged approach in the dissertation. First, we examined an experimental diblock copolymer/homopolymer system which produces a variety of Frank-Kasper phases. Our computational study reproduced the salient behavior of the system and unveiled a new mechanism for the stabilization of Frank-Kasper phases. Next, we studied the disordered micelle regime, which has consequence in stabilizing metastable Frank-Kapser phases in thermal processing experiments, for conformationally asymmetric diblock copolymer melts. We uncovered a reduction in the window of stability for the disordered micelle regime with increasing conformationally asymmetric. Finally, we compared computational prediction of binary blends of high molecular weight diblock copolymer to experimental results and demonstrated their utility in accessing Frank-Kasper phases. Along with our analysis, we unveiled a potentially new mechanism that may be important in the stabilization of Frank-Kasper phases. We believe that our work in this dissertation provides additional understanding to the behavior of diblock copolymer, specifically in stabilizing Frank-Kasper phases. This work also opens up potential avenues of interest that may further our ability to tailor block polymers for specific applications.
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    Computational Study and Design of Self-Assembling Block Polymers
    (2023-01) Case, Logan
    Upon cooling below the order-disorder transition temperature, block polymers self-assemble into a wide variety of nanostructured morphologies. When paired with advances in synthetic chemistry that allow unprecedented control over the size and architecture of these block polymers, these self-assembly characteristics make block polymers excellent candidates for use in specialty materials with highly tunable properties. Potential applications of block polymers range from filtration membranes to photonic crystals. As it happens, however, the source of this exemplary potential is also one of the great barriers to its realization. The vast design spaces available for block polymers (through numbers and permutations of chemistries, and architectural features) make possible a potentially limitless variety of morphologies. At the same time, these design spaces combined with the subtlety of mechanisms driving morphology selection make finding systems which adopt those morphologies a daunting task.In this dissertation, we take a computational approach to address the challenge of designing block polymer specialty materials through two broad approaches. First, we directly address the challenges posed by these vast design spaces by developing an open-source software to automate the exploration of polymer parameter space. This software uses a particle swarm optimization algorithm to guide a search through polymer parameter space for positions where self-consistent field theory predicts a targeted morphology will be most stable compared to a set of competing phases. Second, we use computational studies of two classes of diblock blends seeking to understand the mechanisms that stabilize the low-symmetry Frank-Kasper phases in block polymers with the goal of improving the intuition that guides future efforts to design block polymer materials. In the first of these studies, we use an AB/B`C diblock ``alloy'' with miscible corona and immiscible core blocks to probe the effect of conformational asymmetry on the stability of Frank- Kasper Laves phases when the conformational asymmetry is confined to only particular particle positions. This study finds that conformational asymmetry can be either stabilizing or destabilizing for the Laves phases, depending on which particles are impacted. In the second of these studies, we attempt to identify the balance of core and corona bidispersity in AB/A`B` blends which can still enable formation of Frank-Kasper phases. Unfortunately, this latter study was complicated by a series of methodological flaws limiting its utility in the furtherance of block polymer design. Regardless, the flawed study serves as a lesson in proper study design, and the importance of carefully considering complicating factors.
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    Disordered Block Polymers for Highly Selective Water Filtration Membranes
    (2020-06) Hampu, Nicholas
    Reliable access to clean water is a major and growing societal challenge. Selective membrane technologies are expected to play a critical role in sustaining the water economy due to their energy efficient filtration of wastewater. High performing water filtration membranes require both high water permeability and high size-selectivity to ensure that purified water is produced in a cost-effective manner. Existing ultrafiltration membranes contain continuous and interconnected pores that allow for the rapid transport of water, satisfying the requirement of high water permeability. However, they typically exhibit broad pore size distributions that limit their size-selectivity and prohibit their application in highly precise separations. Block polymers represent a potentially powerful alternative class of materials for improved size-selectivity due to their self-assembly into well-defined domains of uniform size at the nanoscale. Removing one of the blocks generates the uniform pores required for precise separations. By coating a thin block polymer selective layer onto a commercially available ultrafiltration membrane, it may be possible to simultaneously obtain both high water permeability and high size-selectivity in a single membrane. However, commercialization of block polymer membranes has been impeded by technological challenges associated with producing continuous pores in an industrially scalable fabrication process from the typically observed ordered block polymer morphologies. Rather than targeting these ordered morphologies, this thesis aims to utilize the disordered state of block polymers to produce higher performing and potentially more scalable membranes. By kinetically trapping disordered state composition fluctuations, a disordered and co-continuous morphology can be obtained and subsequently converted into uniform and continuous pores without the need for challenging processing techniques. Chapter 1 introduces key concepts in block polymer self-assembly, including the order-disorder transition and composition fluctuations. Chapter 2 provides a summary of the technological requirements of an ideal water filtration membrane and discusses various strategies to integrate block polymers into these systems. Chapter 3 contains an overview of the various synthetic, processing, and characterization techniques employed throughout the thesis. Chapter 4 describes proof-of-concept results demonstrating that thermal cross-linking can be used to kinetically trap disordered state composition fluctuations. Chapter 5 details a strategy that introduces temporal control by using thermally stable photocuring strategies to arrest the disordered state. Chapter 6 describes a fundamental investigation into the temperature dependent morphological evolution of block polymers in the disordered state. Chapter 7 examines the use of large amplitude oscillatory shear to precisely control the domain structure of disordered block polymers. Chapter 8 integrates all these findings into the development of a novel co-casting technique to fabricate composite membranes with both high water permeability and high size-selectivity in a potentially scalable manner.
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    Polymer-Based Ion Gels as a Versatile Platform of Solid Electrolytes
    (2018-07) Tang, Boxin
    Ion gels are a versatile class of functional materials. Combining the excellent electrical properties such as high ionic conductivity and capacitance of the ionic liquid (IL) and the mechanical integrity of the polymer, the composite materials have led to a variety of applications such as electrolyte-gated transistors (EGTs), electroluminescent, and electrochromic soft materials. This thesis is built up from previous research on the electrical and mechanical properties of the ABA triblock polymer-based ion gels and continues to improve properties of the materials for electrochemical device applications. In the first part of the thesis work, the objective is to improve the existing ABA triblock polymer systems with poly(ethylene oxide) (PEO) or poly(methyl methacrylate) (PMMA) as the IL-solvating midblock by combining the merit of the low Tg from PEO and hydrophobicity from PMMA into one system. As a result, poly(styrene-b-ethyl acrylate-b-styrene) (SEAS) triblock polymer was developed. The ion gels made with SEAS demonstrate similarly high ionic conductivity as the PEO-based ion gels, which are significantly improved from those of the PMMA-based ion gels. By shortening the midblock size of the triblock polymer, a synergistic improvement of both the ionic conductivity and the modulus can be achieved. Additionally, the EGTs made by SEAS-based ion gels demonstrate superior stability under humidity compared with EGTs made by SOS-based ion gels. In the following two projects of the thesis work, the polymer platform changes from petroleum-based polymers with hydrocarbon backbones to renewable aliphatic polyesters with the potential aim of EGTs in biocompatible applications. To achieve the ion gels, both physical and chemical crosslinked-systems have been explored. The physically crosslinked ABA aliphatic polyester triblock ion gels demonstrate good mechanical integrity and can be successfully printed under similar conditions as the previous systems, and demonstrate improved ionic conductivity from the PMMA-based ion gels. In addition, the resulting ion gels also demonstrate efficient hydrolytic degradation under basic condition. In a different approach, chemically crosslinked poly(lactide) (PLA)-based ion gels can be synthesized from a facile one-pot method. Owing to a smaller volume fraction in ion-insulating domain, the ion gel demonstrates an excellent ionic conductivity at low polymer concentration. Meanwhile, the ion gel also possesses a high toughness owing to the chemical crosslinks. The thin chemically crosslinked PLA-ion gels can be laminated onto EGTs via a cut-and-stick method. On the other hand, the bulk ion gel demonstrates a good electromechanical response with high electromechanical sensitivity with the applied strain and a low hysteresis between stretching and unstretching.