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

Abstract

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.

Investigation of the Morphological Changes in Nafion Membranes Induced by Swelling with Various Solvents

“…Limited results on a similar membrane-water system reported previously are in general agreement with this observation [9]. Since the mid-*/ peak represents correlation between crystalline regions within the membrane, its motion yields information about the nature of the swelling within the non-crystalline regions of the membrane separating the crystalline regions.…”

“…The primary attractive features of the layered model are its relative simplicity and its agreement with the observed linear dependency of ionic spacing with volume fraction solvent. Criticisms include lack of accountability for the observed upturn in scattering intensity at very low angles, lack of agreement between solvent induced-shift dependency of the ionic peaks and the small-angle crystalline peaks [9][10], plus some controversy surrounding the linearity of the relationship between solvent volume fraction and ionomer peak spacing [9][10].…”

“…The structure of Nafion membranes is most often investigated using small-angle scattering (SAS) methods. Numerous SAS studies by a variety of research groups [1][2][3][4][5][6][7][8][9][10] have consistently produced characteristic patterns from perfluorosulfonate ionomer membranes having three basic features: (1) a scattering maximum occurring at momentum transfer vector (q) values in excess of 0.1A-1 that has been associated with aggregation of the ionic groups, (2) a scattering maximum occurring at g~0.04A-1 that has been associated with crystalline features of the structure, and (3) an upturn in intensity in the very low q region (q < 0.01 A-1 ). Structure of the crystalline regions has also been investigated using x-ray diffraction techniques [2,11,12].…”

Ion transport in sulfonated polymers

“…In the presence of moisture, particularly, the ÀSO 3 H groups absorb atmospheric water molecules to yield the segregation of the 2$6 nm sized ionic domains in which the water molecules are surrounded by ÀSO 3 H surfaces. 39,51,54,55 This reduces the unfavorable contact between water and the hydrophobic polymer chains. If the water-rich domains create percolating networks, that is, water channels, proton conduction then occurs through the shuttling of hydrogen bonds between water molecules (Grotthuss mechanism) and/or by the diffusion of protons (vehicular mechanism) along the water channels.…”

“…The issue of phase separation should be fundamentally important in unraveling the structure-property relationship of sulfonated PEMs; however, the ill-defined, process-dependent, heterogeneous structures of many sulfonated PEMs have hampered the acquisition of quantitative information on them. For example, although scattering experiments on sulfonated PEMs have attempted to determine how the water channels are connected, 51,54,55 there is no general consensus on the exact nature of water channels located within hydrophilic phases. In order to achieve a breakthrough for enhancing the performance of PEMFCs, the development of new PEMs designed on the basis of both molecular characteristics and structural aspects is required.…”