(692d) Impact of Nanoparticle Dispersion and Surface Chemistry on the Segmental Dynamics and Transport Properties of Ionomer Nanocomposites

Incorporation of silica nanoparticles (SiNPs) into ionomer membranes has proven to be effective in reducing vanadium ion crossover, a standing obstacle with current, state-of-the-art ionomers utilized in vanadium redox flow batteries. Traditionally, these nanoparticles were understood to impact vanadium ion transport by sterically impeding the movement of ions through the ionic channels of the nanophase-segregated ionomer. However, recent small-angle neutron and X-ray scattering studies on these ionomer nanocomposites have revealed that the silica network is primarily sequestered in the fluorocarbon rich phase of the ionomer, underscoring the role played by both the SiNP dispersion state, as well as the nature of their interaction with the ionic groups in tuning ion transport in the nanocomposite membranes.

Herein, a series of solution-cast Nafion nanocomposites containing SiNPs of various surface chemistries (ranging from highly anionic to highly cationic) and at various nanoparticle loadings (ranging from 0 mass% to 10 mass% ) were synthesized, and the segmental dynamics and transport properties of these membranes was characterized. Specifically, the effect of these interactions on the segmental relaxation dynamics and water transport properties of these nanostructured membranes was captured by neutron spin echo (NSE) spectroscopy and time-resolved attenuated total reflectance Fourier transform infrared (tATR-FTIR) spectroscopy, respectively. Segmental relaxation results and viscoelastic swelling kinetics obtained from NSE and tATR-FTIR spectroscopy both show congruent trends in changes in local chain dynamics and bulk viscoelastic properties of these membranes under hydration, where maximum stiffening of polymer chains was observed at 4 mass% SiNP loading.

With the help of electron microscopy images and SANS data to characterize dispersion of silica phase within the matrix, effect of nanocomposite processing technique, viz. sol-gel method and solution casting method, is also discussed in the context of these results, where difference in evolution of silica phase with each technique appears to impact changes to both local viscosities and bulk polymer swelling dynamics differently. Finally, the relaxation dynamics of the fluorocarbon matrix were also independently characterized via broadband dielectric spectroscopy (BDS). While NSE probes dynamics at the length scale of fluorocarbon chain packing, the frequency dependent loss moduli in BDS are scanned across the bulk of dense nanocomposite membrane over a wide range of temperatures, and reflect the spatially averaged relaxations of polar segments within the ionomer nanostructure.