Imperfect" Block Polymers: Effects of Dispersity and Morphological Defects on Block Polymer Properties"

Abstract

The chemically distinct segments of block polymers drive the formation of various microphase separated morphologies such as lamellae, cylinders packed on a hexagonal lattice and double gyroid. Previous experimental and computational studies have explored the phase behaviors of model, near-perfect block polymers with narrow molar mass distributions of the constituent blocks. However, recent reports have shown that broad block dispersity notably alters the thermodynamic phase behavior of block polymers and have explored the efficacy of using polymer dispersity to enhance performance in various applications such as nanolithography and thermoplastic elastomers. Lithium salt-doped polyether-based block polymers present an attractive system to combine desirable mechanical properties with high ionic conductivities to enable design of safe, high performance electrolytes in solid-state lithium batteries, while the effect of block dispersity in polymer electrolytes has not been studied. In this thesis, we investigate how Li salt-doped block polymer phase behavior and ion conductivities are affected by increased dispersity in the conductive poly(ethylene oxide) (PEO) domains of poly(styrene-block-ethylene oxide-block-styrene) (bSOS) polymers. We blend a series of bSOS triblock polymers with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and construct the corresponding morphology portraits as a function of Li+ loading using small-angle X-ray scattering analyses. We investigate the shift of the lamellar phase boundaries and dilation of the domain spacing caused by the increased O block dispersity. We observe that bSOS affords higher ionic conductivities than the narrow dispersity diblock control samples, as characterized by electrochemical impedance spectroscopy. We rationalize this observation based on a decreased extent of long-range ordering of the lamellar phase in the salt-doped bSOS that reduces ion diffusion pathway torturosity. In other words, the PEO domain continuity is preserved across morphological defects such as grain boundaries. We further explore the idea of continuity through grain boundaries by successfully fabricating mechanically stable nanoporous materials by etching away the matrix domain from a cylindrical phase. We utilize polystyrene/polylactide and polyisoprene/polylactide block polymers to establish the versatility of this matrix etching method, and find that highly interconnected cylinders are present in both cases. We further assess the continuity and size selectivity of the fibril network composed of cross-linked polyisoprene through a permeation experiment as a proof-of concept for future applications as ultrafiltration membranes. This thesis provides new insights into using the ‘imperfections’ in the block polymer architecture and microphase separated morphologies to realize real-world applications as Li-ion battery electrolytes and separation membranes.