Browsing by Subject "polymer blends"

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    Controlling Cocontinuous Polymer Blends with Nanofillers Jammed at the Interface
    (2017-08) Bai, Lian
    Cocontinuous polymer blends are composed of two or more immiscible or partially miscible polymers coexisted within the same volume in multiple interpenetrated networks. They can be created by either melt compounding of immiscible polymers or phase separation of partially miscible polymer pairs via spinodal decomposition. The polymer blends with cocontinuous structure have significantly improved mechanical properties and they have applications in conductive plastics, porous membranes for filtration and tissue scaffolds for drug delivery devices. As the cocontinuous morphology is in a non-equilibrium state, the thermodynamic instability causes the morphology to coarsen during post-mixing processing, which is a major drawback for applications. In order to control and optimize the phase morphology, nanofillers have been localized and jammed at the interface as an effective method to suppress the coarsening and stabilize the cocontinuous structure during annealing. However, the mechanisms involved in the morphology stabilization by interfacial nanofillers are not yet fully understood. This thesis seeks to systemically study the structure-processing-properties relationships of nanofiller stabilized cocontinuous polymer blends by providing insight to these three questions: (1) how do thermodynamic factors determine nanofiller localization and their morphology stabilization ability? (2) How do kinetic factors affect nanofiller migration during melt compounding and coarsening suppression during annealing? (3) How is the morphology dynamics of nanofiller stabilized polymer blends connected to their rheology response during annealing? Concerning the thermodynamic factors, this thesis approaches the problem via incorporating nanofillers with different surface properties into cocontinuous polymer blends. The different hydrophobicities of silica nanoparticles and the different polarities of graphene nanoplates determine the different localization of these nanofillers in the polymer blends. Nanofiller localization in one polymer phase or at the interface is explained by the system’s tendency to minimize its free energy. Wetting coefficients, which are derived from the Young’s equation and calculated based on the surface energies of nanofillers and two polymer components, have been applied to predict the nanofillers localization in the polymer blends. Concerning kinetic factors, different processing parameters during melt compounding were systemically investigated to study their effect on the migration and localization of nanofillers and their corresponding morphology stabilization ability during annealing. The proper sequence of addition of components is crucially significant to achieve the interfacial localization: nanofillers were generally premixed with the thermodynamically less favorable phase, and then melt compounded with the thermodynamically more favorable phase to enable nanofillers to migrate from the premixed phase to the interface. The effect of different melt compounding time is also systemically studied in the cocontinuous polymer blends stabilized by graphene nanoplates, and we found blends with short melt compounding time have more nanofillers jammed at the interface and more effective coarsening suppression ability during annealing. In order to correlate the morphology dynamics with rheology, we combined rheology time sweeps with morphology information from confocal microscopy, scanning electron microscopy and transmission electron microscopy. We found that morphology coarsening results in shrinkage of interfacial area and jamming of interfacial nanofillers. The nanofillers jammed at the interface contributed to stabilization of the cocontinuous morphology and formation of a 3D nanofiller network. The nanofiller network gave rise to the increase of storage modulus during annealing and the typical gel-like behavior in rheology frequency sweeps.
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    Structure and Thermodynamics of Neutral and Charged Block Copolymer-Based Materials
    (2022-08) Zhang, Bo
    Next-generation materials are often required to exhibit two (or more) orthogonal properties simultaneously. One example is polymer electrolytes, as both facile ion transport and mechanical robustness are desired. However, these orthogonal properties are hard to achieve in single-component systems, because ion transport usually requires high chain mobility while high chain rigidity or low chain mobility is desired for mechanical stability. One way to overcome this challenge is to develop co-continuous nanostructured materials, such that one domain provides ion transport while the other imparts mechanical robustness. A promising predictable and tunable co-continuous structure is the bicontinuous microemulsion from ternary blends of an AB diblock copolymer and the corresponding A and B homopolymers. However, the structure and thermodynamics of such ternary mixtures are not fully elucidated, even in the limit of neutral ternary blends. Moreover, little is known about ion-containing ternary blends. Therefore, the focus of this thesis work is to understand the fundamental phase behavior of these systems and to ultimately provide insight into the rational design of functional materials. In Chapter 2, we investigate the phase behavior of neutral ternary blends comprising a linear diblock copolymer and the corresponding homopolymers. The impacts of block copolymer compositional asymmetry on ordered, disordered, and macrophase-separated regions of the ternary phase prism are discussed. In Chapter 3, we expand the research to ternary mixtures involving a bottlebrush diblock copolymer and the corresponding linear homopolymers. The overall phase behavior closely resembles that of linear ternary mixtures, except for an unconventional spatial distribution of the homopolymers. Chapters 4 and 5 focus on the self-assembly of charged diblock copolymers, serving as the starting point for the investigation of charged ternary blend phase behavior. Chapter 4 details the phase behavior of a series of symmetric charged diblock copolymers, where the effective interaction parameter was found to increase linearly with the increase in charge fraction. Chapter 5 extends the work to a different model system with a relatively nonpolar charged block. A tilted, “chimney”-like order-disorder transition boundary was observed. However, the composition windows of the ordered phases remain nearly unchanged. Overall, the findings from this thesis research provide valuable insight into the structure and thermodynamics of neutral and charged polymer mixtures, and will inform the rational design of nanostructured polymer electrolytes with tunable structure and properties.