Proton Conductive Membranes Based on Sulfonated Syndiotactic Polystyrene: evidence for hydrated channels and connected water clusters

Syndiotactic polystyrene (sPS) is able to form different kinds of co-crystalline phases with guest molecules of various size, shape and property. Several advanced materials have been already produced starting from sPS co-crystalline films (like for example fluorescent, photoreactive, chiral and paramagnetic materials) [1,2]. In particular, sulfonated sPS (sSPS) can be used as proton-conductive membrane for fuel cells, as it presents high proton conductivity (comparable with Nafion), it shows a high chemical and thermo-mechanical stability and it has a low cost [3]. The morphology of different sPS clathrates and the structural features of sSPS upon hydration can be more thoroughly understood by combining X-rays scattering, FT-IR and TEM with SANS [4,5]. By exploiting the neutron contrast variation between various hydrogenated and deuterated components of sPS and sSPS clathrates, additional and unique information about the distribution of guest molecules in the crystalline and amorphous regions and about the hydrated domains were obtained. Moreover, the SANS investigation of in-situ water absorption-desorption process in these membranes using a humidity chamber that enables to choose constant relative humidity RH over a wide range (between 5% and 95%) emphasizes that the hydration-drying processes involve both the inter-lamellar amorphous space and bulk amorphous. Moreover, the stretching of films leads to occurrence and distribution of scattering features from typical morphologies on specific directions and sectors of detection plan, which enables an unambiguous characterization of such complex polymeric systems.

A complete SANS investigation on sPS samples, starting from their crystallization with guest molecules through sulfonation process and followed by subsequent hydration, performed at SANS diffractometer KWS-2 of MLZ will be presented. The SANS results were complemented with WAXS, FTIR and TEM observations. This experimental analysis has highlighted that the morphology of these polymeric films is characterized by hydrated channels in the amorphous phase which are aligned along the stretching direction and alternate to staples of crystalline lamellae.

References:
[1]. J. Schellenberg in “Syndiotactic Polystyrene’‘, John Wiley & Sons, Inc. (2010); ISBN:978-0-470-28688-3
[2]. G. Guerra et al., J. of Pol. Sci. B, Polymer Physics, 50, 305 (2012); DOI: 10.1002/polb.23035 http://onlinelibrary.wiley.com/doi/10.1002/polb.23035/abstract
[3]. G. Fasano et al., Int. Journal of Hydrogen Energy, 36, 8038 (2013); DOI: 10.1016/j.ijhydene.2011.01.100 http://www.sciencedirect.com/science/article/pii/S0360319911001455
[4]. F. Kaneko et al., Polymer, 54, 3145 (2013); DOI:10.1016/j.polymer.2013.04.050
[5]. F. Kaneko et al., Chemistry Letters, 44, 497 (2015); http://doi.org/10.1246/cl.141179