Incorporation of silica nanoparticles (SiNPs) into perfluorosulfonated ionomers (such as Nafion) has gained popularity as a means of reducing vanadium ion crossover as compared to traditional proton exchange membranes (PEMs) for vanadium redox flow batteries. However, the impact of these nanoparticles on the structural dynamics and transport kinetics of the composite PEMs remains elusive. Therefore, elucidating such structure–property relationships can provide vital insights regarding the mechanism of crossover reduction in these composite materials. In our study, time-resolved Fourier transform infrared-attenuated total reflectance (tFTIR-ATR) spectroscopy was employed to characterize the ionomer swelling kinetics and water transport in a series of nanocomposite membranes with varying nanoparticle concentration and surface chemistry (both anionic- and cationic-functionalized surfaces). Water transport through these membranes was found to be strongly coupled to the viscoelastic response of the composite membrane. Specifically, anomalous, multi-stage viscoelastic relaxation and water diffusion was observed. We conjecture that this multi-stage behavior is caused by structural reorganization of the ionomer during hydration. In addition to tFTIR-ATR spectroscopy measurements, neutron spin echo (NSE) spectroscopy and dielectric relaxation spectroscopy (DRS) were employed to characterize the impact of SiNP surface chemistry on local chain dynamics in Nafion fluorocarbon network. While DRS provides a measure of motion of polar side-chains, NSE allows us to evaluate changes to segmental dynamics at the length scale of the ionomer backbone spacing. Thus, these combinatorial spectroscopic techniques helped to resolve the implicit interactions between SiNPs and Nafion by probing local network dynamics that span across multiple time and length scales. Interestingly, changes in bulk polymer dynamics, as measured by NSE, were found to be analogous to swelling dynamics measurements from tFTIR-ATR spectroscopy, providing unique insight into the impact of SiNPs (and interfacial interactions) on water transport in these ionomer nanocomposites.

Uncovering how the nanoparticle–polymer interface affects segmental dynamics and water transport in ionomer nanocomposites

2019 Spring National Meeting

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