Abstract
Knowledge of the transitions occurring during the formation of ion-conducting polymer films and membranes is crucial to optimize material performances. The use of non-destructive scattering techniques that offer high spatio-temporal resolution is essential to investigating such structural transitions, especially when combined with complementary techniques probing at different time and spatial scales. Here, a simultaneous multi-technique study is performed on the membrane formation mechanism and the subsequent hydration of two ion-conducting polymers, the well-known commercial Nafion and a synthesized sulfonated poly(phenylene sulfide sulfone) (sPSS). The X-ray data distinguish the multi-stage processes occurring during drying. A sol-gel-membrane transition sequence is observed for both polymers. However, while Nafion membrane evolves from a micellar solution through the formation of a phase-separated gel, forming an oriented supported membrane, sPSS membrane evolves from a solution of dispersed polyelectrolyte chains via formation of an inhomogeneous gel, showing assembly and ionic phase separation only at the end of the drying process. Impedance spectroscopy data confirm the occurrence of the sol-gel transitions, while gel-membrane transitions are detected by optical reflectance data. The simultaneous multi-technique approach presented here can connect the nanoscale to the macroscopic behavior, unraveling information essential to optimize membrane formation of different ion-conducting polymers.
Highlights
Introduction perfluorinated sulfonic side chainsThe phase separation induced by the strong repulsion between the hydrophobicIonomers and polyelectrolytes are two classes of polymeric materials extremely important for many technological applications, including batteries and fuel cells.[1,2,3,4] In fuel cells, ion-conducting backbone and the sulfonic moieties provides a hydrationdependent nanomorphology whose nature and description is still under debate despite intensive research.[12,13,14] Neutron and X-ray scattering techniques have been extensively used toDr M
Similar to what was done for Nafion, we have explored the effect of hydration on the casted membranes by exposing the sample to increasing relative humidity (RH) atmosphere after having fully dried the casted film. sulfonated poly(phenylene sulfide sulfone) (sPSS) macroscopic membrane swelling sets in when RH > 10% while no change is measured at the nanoscale by GISAXS (Figure 7a,b)
We have investigated the membrane formation process occurring during casting of two different proton conducting materials, namely the well-known perfluorosulfonated Nafion and an alternative fluorine-free aromatic sulfonated polysulfone sPSS
Summary
The structural reorganization taking place during deposition of μm-thick membranes of two polymers cast from solutions was first studied using a combined setup allowing simultaneous measurement of GISAXS/GIWAXS, EIS, and LR signals (Scheme 1). Further details about the experimental setup are explained in the Supporting Information
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