Browsing by Subject "polymer physics"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Mechanisms of Chain Exchange in Block Copolymer Micelles
    (2015-11) Lu, Jie
    Mechanisms of equilibration in block copolymer micelles were investigated in detail using time resolved small angle neutron scattering (TR-SANS). The model polymers used in this study were polystyrene-b-polyethylenepropylene (PS-PEP) diblock copolymers and corresponding triblock copolymers (PS-PEP-PS, PEP-PS-PEP). When dissolved in squalane, the polymers self assembled into spherical micelles with the PEP blocks forming the solvated coronas, and undiluted PS blocks as the micelle cores. Normal and selectively deuterated equivalent polymers with controlled molecular weight, narrow molecular weight distribution and composition were synthesized by anionic polymerization of styrene and isoprene followed by the selective saturation of the polyisoprene blocks. The structure of polymer micelles were characterized using dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). A contrast matching strategy was employed for the TR-SANS experiments, where separately prepared deuterated and protonated micelles were mixed at equal volume fractions in a solvent containing 42 vol% h-squalane and 58 vol% d-squalane. Chain exchange reduces the mean contrast of the micelle cores in the solvent mixture, thus reducing the SANS scattering intensity, providing a method to characterize the dynamics of the process as a function of time. In this thesis, several aspects of chain exchange mechanisms were investigated. The hypothesis of hypersensitivity of chain exchange rate to the core block length, and the single chain exchange mechanism, were first tested and confirmed in the PS-PEP model micelle system. The chain exchange mechanisms in PEP-PS-PEP and PS-PEP-PS micelles were then investigated, and a remarkable effect of molecular architecture on the chain exchange rate is documented. In addition, this study explores the facilitating role of the corona chains in molecular exchange. It was found that adding PEP homopolymers of size comparable to the PEP blocks into dilute PS-PEP micelle solutions can significantly retard the chain exchange rate. Decreasing the corona block fraction in the PS-PEP polymers also reduced the chain exchange rate, and the concentration dependence of the chain exchange relaxation time constant. Finally, we extended our scope to chain exchange between micelles away from equilibration, i.e., micelle hybridization of two populations of PS-PEP micelles of different sizes. The results of this work suggested quantitatively different mechanisms when the micelle systems are away from equilibration, and a concentration effect was found, even when the micelles are still dilute.
  • Thumbnail Image
    Item
    Molecular Simulations of Phase Behavior for Polymer Blends and Block Polymers
    (2018-05) Chen, Qile
    The wide variety of phase behavior associated with polymer mixtures and block polymers enables unprecedented opportunities in developing novel polymeric materials with desired properties. However, the molecular design space of multi-component polymer systems is now so vast that guidance from theory and modeling is essential. The greatest challenge of predictive materials design is the lack of accurate and precise simulation methods in computing the phase diagram of polymer systems, due primarily to difficulties in (i) transferring polymer molecules between condensed phases and (ii) the sensitivity of phase diagram with respect to the interaction parameters used in the simulations. The overarching goal of this thesis is to address the above two problems. In this thesis, advanced sampling techniques of Monte Carlo simulations and accurate molecular models were developed to allow for the accurate and precise calculation of the mixing thermodynamics for binary mixtures. Furthermore, a case study of predictive materials design is presented, where molecular dynamics simulations were employed to explore the phase diagram of block oligomers with various chain lengths, volume fractions, and chain architectures, and thus, to guide the experimental synthesis for molecules with desired microphase morphologies. The work in this thesis lays a solid foundation for predictive materials discoveries using molecular simulations.
  • Thumbnail Image
    Item
    Unraveling Structure-Property Relationships in Polymer Blends for Intelligent Materials Design
    (2016) Irwin, Matthew
    Block polymers provide an accessible route to structured, composite materials by combining two or more components with disparate mechanical, chemical, and electrical properties into a single bulk material with nanoscale domains. However, the characteristic lengthscale of these systems is limited, and the choice of components is restricted to those that are able to undergo microstructural ordering at accessible temperatures. This thesis details routes to overcoming these limitations through the addition of a lithium salt, a blend of homopolymers, or both. Chapter 2 describes a study wherein complex sphere phases such as the Frank-Kasper sigma phase can be observed in otherwise disordered asymmetric block polymers through the addition of a lithium salt. Chapter 3 discusses the development and characterization of a ternary polymer blend of an AB diblock copolymer and A and B homopolymers doped with a lithium salt. Detailed characterization showed that doping blends that are otherwise disordered with lithium salt induced microstructural ordering and largely recovers the phase behavior of traditional ternary polymer blends. A systematic study of the ionic conductivity of the blends at a fixed salt concentration demonstrates that, at a given composition, disordered, yet highly structured blends consistently exhibit better conductivity than polycrystalline morphologies with long range order. Chapter 4 extends the methodology of Chapter 3 and details a systematic study of the effects of cross-linker concentration on the performance of polymer electrolyte membranes produced via polymerization-induced microphase separation that exhibit a highly structured, globally disordered microstructure. Finally, Chapter 5 details efforts to develop a water filtration membrane using a polyethylene template derived from a polymeric bicontinuous microemulsion. Throughout all of this work, the goal is to better understand structure-property relationships at the molecular level in order to ultimately inform design criteria for materials where simultaneous control over morphology and mechanical, chemical, or electrical properties is important.