Small-Angle Scattering Study of Short Pendant Chain Perfuorosulfonated Ionomer Membranes

A structural study using both small-angle X-ray and neutron scattering (SAXS and SANS) of dry and water-swollen short pendant side chain perfluorosulfonated ionomer (SPC PFSI) membranes is presented. The SAXS and SANS profiles are shown to be equivalent, confirming the phase separation between the water and the perfluorinated matrix. The study of the asymptotic behavior at large angles of the scattering curves confirms that the structure is dominated by the polymer−solvent interfacial energy. The local order model is shown to satisfactorily reproduce the scattering profiles obtained for SPC PFSI membranes depending on the equivalent weight (EW) with a low number of free parameters. However, the values of the parameters for large EWs are not in agreement with those determined from the Porod analysis due to the presence of a well-defined maximum at low q values. This maximum, attributed to a crystalline component, slightly modifies both the position and the intensity of the ionomer peak. For the low EW membrane, which generates a very large water uptake, we propose to describe the swollen membrane as a connected network of rods.