| Counterion surface segregation in ionomers is observed and the process and mechanism are investigated. Counterion segregation in films of poly(styrene-methacrylic acid) neutralized with Cs is followed using Rutherford backscattering spectrometry (RBS). Whereas as-cast films and those annealed below ∼120°C contain a uniform distribution of Cs, segregation of Cs to the air/surface interface is observed for films annealed above 130°C. At long times and high temperatures, the surface excess of Cs reaches a constant value corresponding to about two-thirds of the total Cs concentration. Using scanning force microscopy (SFM), the surface excess is found to coincide with the appearance of nanometer surface features. Scanning transmission electron microscopy (STEM) detects the presence of Cs rich vesicular aggregates 8–85 nm in diameter. Based on the RBS, SFM, and STEM studies, a model is proposed whereby cations are initially associated with free acid groups or in vesicular aggregates in the bulk. Then upon annealing, the Cs that had been associated with free acids dissociates and segregates to the surface, while the Cs in aggregates remains in the bulk.; Surface segregation is observed in all monovalent counterions investigated, K, Rb and Cs. In contrast, no surface segregation is found in divalent counterions, Ca, Sr, Ba and Zn. Furthermore, the onset temperature of surface segregation is found to be higher for Rb than for Cs. These observations are attributed to differences in the binding strength between the counterions and their acid groups.; The kinetics of this counterion surface segregation are also explored. The surface excess, z*, of Cs and the depletion layer are found to grow with the annealing time to the ½ power, as expected in a diffusion controlled surface segregation process. The dz*/dt increases with increasing temperature, and from the observed z*, diffusion coefficients for each temperature were calculated. We conclude that a fraction of the Cs counterions are able to dissociate from their acid group and diffuse as free ion through the matrix. |
