Surface structure of Nafion in vapor and liquid.

The microstructure of Nafion varies in response to changes in hydration. Thus, it undergoes a transition from tightly packed bundles of inverted micelles with aqueous cores and fused hydrophobic shells ("macaroni bundles") at low hydrations to normal type ("spaghetti") micelles at high hydrations. It was postulated recently that a similar "macaroni-spaghetti" transition, i.e., breakup of surface-aligned macaroni to randomly oriented spaghetti, takes place at the polymer surface when the external medium is changed from vapor to liquid water, which can explain some puzzling features of Nafion and similar microphase-separated ionomers. The resulting (nonequilibrium) structures may remain confined to a few nanometers thick surface region. Here, this picture is corroborated using grazing-incidence small-angle X-ray scattering (GISAXS), contact angle, and atomic force microscopy (AFM). The enhanced alignment of bundles adjacent to the surface in vapor, similar to the effect of biaxial stretching, is elucidated by GISAXS of spin-cast Nafion films. It is inferred from the characteristic change in relative intensities and position of the ionomer peak in the X-Y (in-plane) and Z (out-of-plane) directions with varying X-ray penetration depths into the film. However, contact angle measurements show that the relatively smooth and very hydrophobic surface of Nafion in vapor transforms to a hydrophilic surface, when vapor as the external medium is replaced with liquid water. In addition, AFM indicates that the surface roughness significantly increases in liquid. The results demonstrate that the surface region of Nafion and similar microphase-separated materials may be indeed subject to drastic structural variations, even though the extremely slow relaxation of the solid matrix may preclude propagation of such changes into the bulk. These effects may have a profound effect on the macroscopic characteristics of Nafion membranes, such as hydration and conductivity, as well as their functioning as ion-selective barriers in electrochemical and other applications.