Small-angle scattering study of short pendant chain perfuorosulfonated ionomer membranes

A structural study using both small-angle X-ray and neutron scattering (SAX S and SANS) of dry and water-swollen short pendant side chain perfluorosulf onated ionomer (SPC PFSI) membranes is presented. The SAXS and SANS profile s are shown to be equivalent, confirming the phase separation between the w ater and the perfluorinated matrix. The study of the asymptotic behavior at large angles of the scattering curves confirms that the structure is domin ated by the polymer-solvent interfacial energy. The local order model is sh own to satisfactorily reproduce the scattering profiles obtained for SPC PF SI membranes depending on the equivalent weight (EW) with a low number of f ree parameters. However, the values of the parameters for large EWs are not in agreement with those determined from the Pored analysis due to the pres ence of a well-defined maximum at low q values. This maximum, attributed to a crystalline component, slightly modifies both the position and the inten sity of the ionomer peak. For the low EW membrane, which generates a very l arge water uptake, we propose to describe the swollen membrane as a connect ed network of rods.