While it is beyond the scope of this chapter to give an extensive review of the field of PEMs, it is necessary that reader be aware of the types of materials that are being developed for fuel-cell applications. Regardless of the particular application (e. g., stationary, portable, or automotive power), these materials must exhibit a certain set of chemical and physical properties that are critical for optimal fuel-cell perfor­mance. Any material being used in a fuel cell must exhibit a list of properties including, but not limited to (1) high proton-conductivity (i. e., a good electrolyte), (2) negligible electrical-conductivity, (3) permeability to ions, but allow only one type of charge, (4) resistance to permeation of uncharged gases, (5) variable membrane-area and thickness, and (6) good mechanical strength. Furthermore, the membrane must be of reasonable cost and durability. Ultimately, it is the polymer chemistry and microstructure that give rise to the macroscopic performance prop­erties that are desired. A range of synthetic approaches have yielded materials that include, but are not limited to, poly(perflourosulfonic acid)s (PFSA), ion-containing polystyrene derivatives, polyarylene ethers, polysulfones, polyimides, and ion — containing block copolymers. There have been extensive studies on each of these classes of materials, but to date neutron techniques have been primarily used to study poly(perflourosulfonic acid)s, namely Nafion® [2]. Therefore, it is necessary to give a brief background on this particular material. Throughout this chapter, where specific breakthroughs have been made, studies involving other PEM materials using neutron techniques will be highlighted. On the whole, however, the overlap of neutron measurements and PEM materials has been dominated by PFSAs.

The most widely studied PFSA, Nafion®, is a product of the E. I. Dupont Chemical Company having the structure given below.

The polar perfluoroether side-chains containing the ionic sulfonate-groups have been shown to organize into aggregates, thus leading to a nanophase-separated mor­phology where the ionic domains, termed clusters [3], are distributed throughout the non-polar polytetrafluoroethylene (PTFE) matrix. In addition, the runs of tetrafl — uroethylene, of sufficient length, are capable of organizing into crystalline domains having unit-cell dimensions virtually identical to that of pure PTFE [4, 5]. The degree of crystallinity in PFSIs is generally less than ca. 10 % as a mass fraction in 1,100 equivalent-weight Nafion® (EW, the grams of dry polymer per equivalent number of SO3- groups) and has been shown to vary with EW. The complex, phase — separated morphology, consisting of crystalline, amorphous, and ionic domains, of PFSIs has been the focus of several investigations [3, 5-16]. Over the last 50 years, a wide variety of studies involving Nafion® have aimed to relate the thermal, mechanical, and fuel-cell performance properties (i. e., transport, ionic conductivity, and dielectric behaviour) to specific morphological features. Many of these studies involve neutron-scattering techniques and will be discussed in detail below.