Synchrotron SAXS and Impedance Spectroscopy Unveil Nanostructure Variations in Redox-Responsive Porous Membranes from Poly(ferrocenylsilane) Poly(ionic liquid)s

Nanostructured cellular polymeric materials with controlled cell sizes, dispersity, architectures, and functional groups provide opportunities in separation technology, smart catalysts, and controlled drug delivery and release. This paper discusses porous membranes formed in a simple electrostatic complexation process using a NH3 base treatment from redox responsive poly(ferrocenysilane) (PFS)-based poly(ionic liquid)s and poly(acrylic acid) (PAA). These porous membranes exhibit reversible switching between more open and more closed structures upon oxidation and reduction. The porous structure and redox behavior that originate from the PFS matrix are investigated by small-angle X-ray scattering (SAXS) using synchrotron radiation combined with electrochemical impedance spectroscopy. In order to gain more insight into structure variations during electrochemical treatment, the scattering signal of the porous membrane is detected directly from the films at the electrode surface in situ, using a custom-built SAXS electrochemical cell. All experiments confirm the morphology changing between more "open" and more "closed" cells with approximately 30% variation in the value of the equivalent radius (or correlation length), depending on the redox state of ferrocene in the polymer main chain. This property may be exploited in applications such as reference-electrode free impedance sensing, redox-controlled gating, or molecular separations.