SAXS analysis of the thermal relaxation of anisotropic morphologies in oriented Nafion membranes

The current study uses variable temperature, small-angle X-ray scattering (SAXS) to examine the thermal relaxation behavior of oriented Nafion films as a means to evaluate the morphological stability of this ionomer at elevated temperatures. The SAXS patterns of uniaxially oriented films are characterized by strong equatorial scattering peaks which are attributed to scattering arising from the oriented ionic domains (ionomer peak ca. q = 2 nm ^-1). The intensity of the equatorial peaks - obtained from integration in the azimuthal direction (χ) - and the degree of orientation-calculated using the Hermans orientation function (f) - were monitored as a function of temperature. At lower temperatures, a constant value of f and a correlation between the β relaxation and a slight decrease in the scattering intensity of the equatorial peaks are in agreement with our earlier assignment of the β relaxation to the T _g of Nafion. At temperatures in the vicinity of the β relaxation, the static electrostatic network inhibits long-range molecular relaxation and yields a persistent anisotropic morphology. In contrast, significant changes in intensity of the equatorial peaks and values of the orientation parameter at elevated temperatures were shown to correlate well with the a relaxation observed in DMA. At temperatures in the vicinity of the a relaxation, a significant destabilization of the oriented electrostatic network occurs (i.e., through the activation of a dynamic network involving significant ion-hopping processes), thus facilitating the observed relaxation to an isotropic morphology. Therefore, morphological stability in this ionomer is principally governed by the thermal stability of the electrostatic network and not the glass transition.