Structure and Ion Transport Property of Sulfonated Block Copolymer Thin Films

Abstract

Ion-conducting polymer electrolytes such as polymer-salt complexes, hydrophilic vinyl polymers bearing acid groups or quaternary salts have been widely investigated for their uses in a range of electrochemical devices

Li-ion batteries, fuel cells, electro-active actuators and chemical sensors. For such devices, ion transport properties of polymer electrolytes play an important role in determining performance, thus stimulated intensive research efforts on developing a new polymer electrolyte. In this research, I fabricate polymer electrolytes for humidity sensors and Li-ion batteries having improved properties.First, fast responsive dual-mode humidity sensors are developed with a series of self-assembled poly(styrenesulfonate-methylbutylene) (PSS-b-PMB) block copolymers where the PSS chains provide hygroscopic nature. In dry state, the PSS-b-PMB films exhibit hexagonal cylindrical morphology possessing hydrophobic PMB cylinders dispersed in a hydrophilic PSS matrix. Under the levels of humidity, the PSS-b-PMB films self-display discernible reflective color changes, which cover almost entire visible light spectrum from violet (RH = 20%) to red (RH = 95%). This is due to the fact that the PSS matrix absorbs water molecules yielding anisotropic swelling of the film along the film thickness direction. In addition, the sensors exhibit a few orders of magnitude changes in impedance with exposure to humid air owing to the strong polyelectrolyte characteristics of PSS chains. Remarkably, the time to complete the changes in color and/or impedance is only 5s regardless of RH gradients, as rationalized by well-connected hydrophilic PSS matrices in HEX structure, offering short water diffusion pathways in nanostructured block copolymer thin films.Second, bicontinuous polymer electrolyte membranes for Li-ion batteries are fabricated using three-dimensionally (3D) ordered polyurethane (PU) inverse opal templates upon filling Li-salt-doped poly(ethylene oxide) (PEO) electrolyte into the pores. The free-standing inverse opal templates composed of UV-curable polymer provide high chemical and mechanical stability, while the co-continuous Li-salt-doped PEO phases enable fast Li-ion transport along the nanoscale ionic pathways. A 2-fold enhanced Li-ion conductivity is demonstrated from the membranes fabricated from the inverse opal templates, compared to ill-defined PEO/PU blend polymer electrolytes. This is rationalized by the structural advantages of bicontinuous morphology having continuous ion-conducting domains, which create less tortuous Li-ion transport pathways.