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Interfacial Effects in Conductivity Measurements of Block Copolymer Electrolytes.
Jonathan P Coote1, Samuel K J Adotey1, Joshua R Sangoro2
1Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
ACS Polymers Au
|August 14, 2023
Summary
Ionic conductivity in block copolymers is anisotropic. Enhanced in-plane conductivity in polystyrene-block-polymer ionic liquid (PS-block-PIL) materials is due to surface wetting layers, not through-plane depression.
Area of Science:
- Materials Science
- Electrochemistry
- Polymer Science
Background:
- Lamellar block copolymer electrolytes exhibit anisotropic ionic conductivity, with in-plane values significantly higher than through-plane.
- Previous work suggested through-plane conductivity depression due to interfacial domain layering in polystyrene-block-polymer ionic liquid (PS-block-PIL) systems.
- Understanding this anisotropy is crucial for developing efficient polymer ionic liquid electrolytes.
Purpose of the Study:
- To investigate the origin of ionic conductivity anisotropy in PS-block-PIL materials.
- To determine if anisotropy arises from through-plane depression or in-plane enhancement.
- To identify the structural factors responsible for enhanced in-plane ionic conductivity.
Main Methods:
- Impedance spectroscopy was used to measure through-plane ionic conductivity in model PIL-based systems with controlled interfacial profiles.
- Impedance spectra were acquired using a top-contact electrode configuration for various PS-block-PIL materials, a P(S-r-IL) copolymer, and a PIL homopolymer.
- Analysis focused on correlating ionic conductivity with interfacial composition, structure, and surface wetting layer formation.
Main Results:
- Through-plane conductivity measurements were insensitive to changes in interfacial composition or structure, refuting the through-plane depression hypothesis.
- Enhanced in-plane ionic conductivities were observed and correlated with the formation of a polymer ionic liquid (PIL)-rich wetting layer at the free surface.
- The enhanced in-plane conductivity aligns with geometric predictions, while through-plane conductivity remains significantly lower.
Conclusions:
- Anisotropy in PS-block-PIL ionic conductivity stems from in-plane enhancement, not through-plane depression.
- A surface-induced PIL-rich wetting layer provides a low-resistance pathway for ions, enhancing in-plane conductivity.
- Accurate structure-conductivity relationships require considering both surface and bulk contributions to impedance measurements.
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