Browsing by Subject "Polymer Physics"

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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    Structure and Dynamics of Particle Forming Diblock Copolymer Melts and their Blends
    (2022-11) Mueller, Andreas
    Complex micellar packings which mimic transition-metal alloy crystal structures known as Frank Kasper phases have been serendipitously identified in a range of soft matter since the early 1990s. The set of known soft Frank Kasper (FK) phases presently includes A15, σ, C14, C15, and one instance of Z alongside closely related dodecagonal quasicrystals (DDQCs). These structures boast low symmetry unit cells containing ≥ 7 particles of ≥ 2 distinct shapes and sizes– a notable deviation from the two particles of a single type populating the canonical body centered cubic (BCC) lattice. The discovery of Frank Kasper phases in ostensibly simple diblock copolymer melts cemented the universality of this behavior across soft matter and triggered widespread reevaluation of the phase behavior of particle-forming diblock copolymers aimed at establishing far-reaching geometric principals underlying the formation of these low symmetry phases. This work addresses these concerns from two directions. First core-homopolymer/diblock (A′/AB) blends where diblock particle cores are swollen with core-block homopolymer were demonstrated to form thermodynamically stable Frank Kasper phases, even in diblock systems that do not form them in the bulk. These ideas were subsequently expanded towards low-molecular weight A′/AB blends, where A′ molecular weight was tuned to dictate AB diblock chain packing in the blend, and thus the ensuing impact on particle packing lattice symmetry. These works established simple A′/AB blending as a general strategy for forming Frank Kasper phases. Notably, these experiments were originally designed in analogy to a surfactant system, underscoring the universality of the geometry of complex phase formation across different types of system. The second thrust of this work focused on the nature of the metastable DDQC, which often forms in advance of equilibrium Frank Kasper phases– hence many Frank Kasper phases are known as quasicrystalline approximants. Initially, this involved establishing the conditions for DDQC formation in a crystalline amorphous poly(ethylene oxide)-block-poly(2-ethylhexyl acrylate) OA diblock copolymer with a minority poly(ethylene oxide) fraction. The OA diblock was demonstrated to undergo breakout crystallization at sufficiently low temperatures, erasing the melt particle-packing microstructure. Melting the semicrystalline state below the order-disorder transition of the diblock enable direct access to a supercooled glass-like packing of particles, which offered a platform from which a DDQC could nucleate. Quenching to the same temperature from the thermally disordered state (above the order-disorder transition) instead BCC to nucleate, which directly transitioned to σ, underscoring the requirement of disorder for the formation of the DDQC. Last, X-ray photocorrelation spectroscopy (XPCS) experiments were performed on a binary blend of a pair of poly(styrene)-block-poly(1,4-butadiene) diblock copolymers which forms DDQC at short anneal times, before ultimately transitioning to σ. These XPCS measurements revealed a wealth of dynamic information wherein σ apparently displays faster grain dynamics compared to DDQC at the same temperature in the same system, attributed to the differing grain structures of each phase.
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    Structure and mechanical properties of elastomeric block copolymers.
    (2010-12) Alfonzo, Carlos Guillermo
    This research presents the synthesis (by anionic polymerization and catalytic hydrogenation) and characterization of two types of block copolymers: CMC and XPX. In CMC, C is glassy poly(cyclohexylethylene) and M, the matrix, can be semicrystalline poly(ethylene) E, rubbery poly(ethylene-alt-propylene) P, or rubbery poly(ethylethylene) EE, or a combination to yield: CPC, CEEC, CEC, CPEEC and CEPC, with fC ≈ 0.18 – 0.30. XPX materials have X = CEC, fC ≈ fE, and fP ≈ 0.40 – 0.60. Block copolymer phase behavior and morphology were examined through a combination of DSC, rheology, SAXS, WAXS and TEM. CMC materials are meltordered due to block thermodynamic incompatibility with TODT > Tg (C) ≈ 147 °C and show lamellar or C cylinder morphologies. The design of XPX yields melt disordered materials up to high Mn with microphase segregation induced by E crystallization. Two high Mn XPX polymers are melt ordered above Tm(E) and show two correlation lengths in SAXS assigned to the C – E and X – P length scales. TEM images indicate that all XPX materials, irrespective of melt segregation, are characterized by composite glassy and crystalline hard domains dispersed in rubbery P at room temperature. Tensile and recovery testing at room temperature show that CMC and XPX materials, with the exception of plastic CEC, behave as thermoplastic elastomers with tunable properties. Interestingly, melt disordered XPX materials have competitive mechanical properties comparable to the strongest CMC polymers, but with advantageous processing. For melt ordered CMC, Tprocess > TODT, which is dependent on Mn, while for melt disordered XPX, Tprocess > Tm(E) ≈ 100 °C independent of Mn. The deformation of melt disordered XPX materials, probed by recovery studies and WAXS, suggests that deformation is first taken by P, then E and finally C, which causes ultimate failure, as agreed in the literature for conventional SBS and SIS thermoplastic elastomers. This implies that strain recovery in XPX materials can be comparable to that of CPC if materials contain low hard block content or are stretched to strains below the onset of E deformation. Finally, a collection of data of mechanical properties, namely modulus E, strain at break εb, tensile strength σTS and tension set εs, obtained from CMC, XPX and previously reported materials were examined. Most notably, E and εs were found to be strongly correlated with the volume fractions of C and E, as properties increase with (fC + fE)δ, where δ = 1 – 2.4. Ultimate properties such as σTS and εs are unaffected by changes in composition as failure is dictated by that of the hard domains and values are similar above a minimum amount of hard block. In addition, E, σTS, and εb are inversely correlated to rubber entanglement molecular weight Me, which implies that modulus and ultimate properties are affected by the ability of the rubber network to redistribute stress by entanglement slippage. However, εs is unresponsive to Me variations, which indicates that irrecoverable deformation in these materials results from deformation of the hard domains.