Structure and Thermodynamics of Salt-Doped Polymer Blends

Abstract

A central challenge in designing novel polymeric materials is to find a broadly applicable strategy to systematically tailor microstructures in order to simultaneously optimize two or more orthogonal properties. For example, polymeric materials with high ion transport and mechanical stiffness are highly desired in water treatment membranes, ion battery electrolytes, fuel cell membranes etc. To achieve this goal, at least two components with distinct properties are usually needed, and a co-continuous microstructure, where one domain is responsible for ion transport while the other imparts mechanical strength, is favored. In this thesis, we propose that the salt-doped A/B/AB ternary system that consists of A and B homopolymer and a corresponding AB diblock copolymer is an attractive platform in accessing the bicontinuous microemulsion (BμE), where the introduction of salt improves the ionic conductivity. To help design such materials with desired properties, the influence of salt ions on the thermodynamics of mixing and phase behaviors needs to be elucidated. We start with one limit of the salt-doped ternary system that no copolymer is added. In Chapter 2, we investigated the phase behavior of LiTFSI-doped poly(ethylene-alt-propylene)/poly(ethylene oxide) (PEP/PEO) and polystyrene/poly(ethylene oxide) (PS/PEO) binary blends and observed a significant reduction of miscibility and an asymmetric phase diagram. Chapter 3 details the symmetric isopleth phase diagram of LiTFSI-doped PS/PEO/PS-b-PEO ternary blends, where a robust and wide BμE channel has been found. Chapter 4 extends the research reported in Chapter 3 to off-symmetric isopleths, where an unexpected C15 Laves phase has been observed, and isothermal phase diagrams have also been mapped out. Finally, Chapter 5 describes the influence of salts on the single-chain dimensions of PEO melts by small-angle neutron scattering and the difficulties in data analysis. Throughout the whole thesis, the main goal is to comprehensively understand salt-polymer interactions and explore the change of phase behavior compared to the salt-free system, which may help to prepare polymer electrolytes with tunable structures and properties.