(527f) Relationship between Structural Properties and Self-Diffusion of Molecular Mixtures in Nafion By High Field Diffusion NMR in Combination with SANS and SAXS

Nafion is a commercially available perfluorosulfonic acid (PSA) polymer which exhibits properties attractive for applications, which can benefit from its use as an ion exchange membrane, a solid super acid catalyst, and a chemical sensor. It is therefore important to understand fundamentals of molecular transport within these membranes to better utilize their potential in these various applications. The Nafion structure in the presence of water consists of a hydrophobic crystalline matrix made of a polytetrafluoroethylene backbone and channels available for water diffusion. These channels are formed by the ionic side chains containing sulfonic groups. Several recent studies have focused on water transport mechanisms within these water channels. Similar studies of small organic molecules such as methanol have also been performed. However, there is still a noticeable gap in a detailed understanding of the role of the structural heterogeneity of Nafion on the intramembrane self-diffusion of water and organic molecules at the micrometer and sub-micrometer length scales. Here, we address this gap in knowledge by applying pulsed field gradient (PFG) NMR at high field up to 17.6 T and large magnetic field gradients up to 25 T/m to study self-diffusion on a broad range of microscopic length scales in Nafion 117 loaded with acetone or vanillic acid in the presence of water. Complementary studies were performed by small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS) to study structural properties of the same Nafion samples.

H-1 and C-13 PFG NMR studies revealed that at high water concentrations (i.e. > 0.5 mmol water/g Nafion), the measured self-diffusivities of all studied guest molecules in Nafion did not vary with increasing root mean square displacements (root MSDs). The maximum range of the probed root MSDs was between 0.5 and 28 micrometers. However, at lower water concentrations and in the presence of acetone, water experiences a significant stepwise decrease in the self-diffusivity with increasing root MSD at around a 5 micrometer length scale. This is in agreement with Percolation Theory that, at some sufficiently small water concentration, the hydrophilic water channels of the membrane form finite clusters and are no longer interconnected into a spanning (infinite) percolation cluster. Our PFG MMR measurements indicate that the average size of such finite clusters under our experimental conditions was around 5 micrometers. In contrast, the self-diffusivity of acetone shows no dependence on root MSD in the same samples and under the same conditions when such dependence was observed for water. To better understand why water encounters different paths available for diffusion than acetone, SANS and SAXS experiments were performed. The SANS and SAXS data suggest acetone is located in the interfacial perfluoroether regions, i.e. between the sulfonic groups in the water channels and the crystalline matrix. As acetone can diffuse through these interfacial regions, its self-diffusion coefficient does not need to be sensitive to the finite size of domains of interconnected water channels, which explains our PFG NMR data for acetone.