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Thermal Transitions in Perfluorosulfonated Ionomer Thin-Films.

Meron Tesfaye^1,2, Douglas I Kushner², Bryan D McCloskey^1,2

¹Chemical and Biomolecular Engineering, University of California-Berkeley, Berkeley, California 94720, United States.

ACS Macro Letters

|June 2, 2022

Summary

Thin perfluorosulfonated ion-conducting polymer (PFSI) films show thickness-dependent thermal transitions. Reduced thickness increases the thermal transition temperature (T_T) in PFSI-H⁺ but not PFSI-M⁺, impacting polymer mobility and device function.

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Area of Science:

Materials Science
Polymer Chemistry
Electrochemistry

Background:

Thin perfluorosulfonated ion-conducting polymer (PFSI) ionomers are crucial in energy-conversion devices.
Their functionality is often limited by confinement-driven and surface-dependent interactions.
Understanding these interactions is key to improving device performance.

Purpose of the Study:

To investigate the effects of confinement and interface-dependent interactions on PFSI thin-films.
To explore how film thickness influences the thermal transition temperature (T_T) of PFSI.
To correlate T_T changes with polymer chain mobility and potential implications for gas transport.

Main Methods:

Fabrication of PFSI thin-films with varying thicknesses (10-400 nm).
Measurement of thermal transition temperature (T_T) using thermal analysis techniques.
Comparison of T_T for PFSI in acid (H⁺) form versus metal cation (M⁺) exchanged forms.

Main Results:

A significant increase in T_T was observed with decreasing PFSI-H⁺ film thickness (from 70 °C at 400 nm to 130 °C at 10 nm).
In contrast, T_T remained constant across different thicknesses for PFSI-M⁺.
Results indicate an interplay between surface mobility and substrate interface effects, modulated by ionomer intermolecular forces.

Conclusions:

Confinement and interface interactions significantly impact PFSI thin-film thermal transitions and chain mobility.
The nature of the counter-ion (H⁺ vs. M⁺) critically influences the thickness dependence of T_T.
These findings provide insights into optimizing PFSI-based materials for advanced energy applications.