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Impact of Nanoparticles on the Segmental and Swelling Dynamics of Ionomer Nanocomposite Membranes

Balwani, Apoorv, Faraone, Antonio, Davis, Eric M.
Macromolecules 2019 v.52 no.5 pp. 2120-2130
batteries, energy, hydrophilicity, hydrophobicity, models, nanocomposites, nanoparticles, neutrons, perfluorocarbons, polymers, spectroscopy, storage technology, vanadium, viscoelasticity
While ionomer nanocomposites show promise for advancing the viability of energy storage technologies, such as the vanadium redox flow battery, the improvements in ion selectivity are appreciable only insofar as demonstrating feasibility of these hybrid materials and are largely insufficient for wide-scale implementation of such technologies. This is in part due to our lack of understanding of how the introduction of nanoparticles (NPs) alters the morphology and dynamics of the resultant ionomer nanocomposite. Herein, we employ time-resolved attenuated total reflectance–Fourier transform infrared (tATR-FTIR) spectroscopy, as well as neutron spin echo (NSE) spectroscopy, to capture the viscoelastic creep/swelling kinetics and segmental dynamics, respectively, of these ionomer nanocomposite membranes under hydration. The polymer swelling data from tATR-FTIR spectroscopy experiments was fit to a three-element model to quantify the relaxation time constant for each nanocomposite at various NP loadings. In addition, NSE spectroscopy experiments on hydrated ionomer nanocomposites showed, in general, a stiffening of the segmental dynamics with increasing NP loading as well as thermal history. In addition, data from these experiments, in conjunction with previous morphological and thermomechanical studies on Nafion–SiNP nanocomposites, suggest that the NPs reside at the interface of the hydrophobic and hydrophilic phases of the ionomer, thereby altering the vanadium ion transport via slowdown of fluorocarbon chain dynamics. Results from this study have profound implications for controlling ion selectivity in these ionomer nanocomposite membranes.