(381a) Influence of the Immobilization of an Organic Acid in Nafion on Its Microscopic Transport and Structural Properties By Advanced NMR and Small Angle Scattering Techniques

Nafion is a commercially available perfluorosulfonic acid (PSA) polymer, which is well known due to its potential and/or current applications as ion exchange membrane, solid super acid catalyst and chemical (acetone) sensor for breath analysis, just to name a few. It was recently demonstrated that an immobilization of an organic acid, such as vanillic acid (VA), in Nafion can preserve its catalytic activity in the presence of water even at high intra-polymer water concentrations corresponding up to 100% ambient relative humidity. This observation is notable given that the use of PSA polymers as catalysts has previously been restricted to organic solvent systems. There is currently no clear understanding of how immobilized organic acid molecules influence the microscopic structural and transport properties of Nafion. To close this gap, we used pulsed field gradient (PFG) NMR in combination with quasi-electric neutron scattering (QENS) to investigate diffusion of water and acetone in Nafion samples. These studies interrogated the length scale dependence of these motions in Nafion with immobilized VA, i.e. Nafion-VA membrane, as well as in the parent, VA-free Nafion membrane. Complementary structural characterization of these membranes was performed by small angle x-ray scattering (SAXS). The results of these studies indicate that in Nafion-VA membranes, the immobilized VA molecules are located in the interfacial perfluoroether regions between hydrophobic domains with sulfonic groups (viz. water channels) and the crystalline matrix of Nafion. Our diffusion data shows evidence that the presence of these acid molecules lead to a better separation of the diffusion pathways of the water and acetone. While water molecules mostly diffuse along water channels and acetone molecules mostly diffuse in the interfacial regions even in VA-free Nafion, the addition of VA leads to a stronger preference of water and acetone to their respective regions during the diffusion process. As a result, the diffusion processes of water and acetone can become more independent from each other in Nafion-VA membranes. This may explain the role of VA in preserving catalytic activity in the presence of water. The ability to control and separate better the diffusion pathways of different types of molecules in Nafion by introducing immobilized species is expected to be of growing importance in novel applications of Nafion such as chemical sensing. Such control is facilitated by understanding the relationship between microscopic structural and transport properties of Nafion membranes and can accelerate development of new applications involving molecular intramembrane transport.