Structure and morphology of model polymer electrolyte membranes based on sulfonated syndiotactic-polystyrene in the δ co-crystalline phase resolved by small-angle neutron scattering

Syndiotactic polystyrene (s-PS) is able to form different kinds of co-crystalline phases with guest molecules of different size, shape and property. Several advanced materials have been produced starting from s-PS co-crystalline films. In particular, sulfonated s-PS (s-sPS) can be used as proton-conductive membrane in some fuel cells applications, as it presents high proton conductivity (comparable with Nafion). Besides, it shows a high chemical and thermo-mechanical stability and a low cost. The morphology of different s-PS clathrates and the structural behavior of s-sPS upon hydration can be thoroughly understood by SANS. In fact, exploiting the neutron contrast variation between various hydrogenated and deuterated components of s-PS and s-sPS clathrates, additional and unique information about the distribution of guest molecules in the crystalline and amorphous regions and about the hydrated domains of the polymer were obtained. Moreover, using uni-axially deformed films the occurrence and distribution of scattering features from typical morphologies on specific directions and sectors of detection plan enable an accurate structural study of such complex polymeric systems. We report in the present paper a detailed SANS investigation of s-PS films, starting from their crystallization with guest molecules to the subsequent sulfonation and hydration. FT-IR, neutron PGAA, WAXD and cryo-TEM were used complementary to SANS to check the state of the samples after each step of the treatment process and to obtain additional structural information as support for the understanding of the SANS data. The current experimental analysis has highlighted that the morphology of these polymeric films is characterized by hydrated channels in the bulk amorphous phase alternated to stacks of crystalline lamellae, oriented along the stretching direction.