Small angle X-ray scattering investigation of multiarm star sulfonated polystyrene based ionomer membranes

• The SAXS characterization of SPS and SPS-b-PFPMA based star ionomers are presented.
• The distance between ionic clusters decreases with IEC and increases with the number of arms.
• Hydrated membranes show similar trends with IEC, but with ∼1–2 nm larger distances.
• Ionic clusters exist in SPS core of phase separated blocks in SPS-b-PFPMA ionomers.

A series of multiarm star sulfonated polystyrene (SPS) and SPS-block-poly(2,2,3,3,3-pentafluoropropyl methacrylate) block copolymer (SPS-b-PFPMA) based ionomer membranes have been prepared and characterized by Small Angle X-ray Scattering (SAXS). SPS membranes showed well-defined ionomer peaks corresponding to an average distance of ∼7 nm between the ionic clusters. The effects of different ion exchange capacities (IEC) and different number of arms on the ionomer peak were investigated. Larger IEC values increased the number of ionic clusters and decreased the average distance between the ionic clusters. At constant IEC, the average distance between the clusters increased with the number of arms of the star polymers. Upon hydration, the ionic clusters got swollen and the distance between the ionic clusters increased by ∼1–2 nm. Power law decay of intensity at high-q region with a slope of −4 indicated well-separation of hydrophilic SPS clusters in the matrix of hydrophobic PS domains with sharp interfaces. In the case of SPS-b-PFPMA membranes, in addition to ionomer peak, a second more intense peak at lower q corresponding to a spacing of ∼25 nm was observed. This larger length scale structuring was attributed to the phase separation of SPS block at the core and PFPMA block at corona. Both the distance between the ionic clusters and the length of larger scale structuring decreased with increasing IEC indicating attractive interactions between ionic clusters. The results are important in understanding the effect of IEC, number of arms and the composition of the block copolymer arms on the morphology of multiarm star sulfonated polystyrene membranes which have potential for higher proton conductivity polyelectrolyte membranes.

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