Efficient Sulfonation of Syndiotactic Polystyrene Membranes Assessed by Small-Angle Neutron Scattering

The selectivity of the δ-form of syndiotactic polystyrene (sPS) toward small organic molecules enables homogeneous functionalization by sulfonation of sPS membranes via soaking in a chloroform solution of a bulky sulfonating agent such as lauroyl sulfate. Sulfonated syndiotactic polystyrene (s-sPS) is hydrophilic and shows a high proton conductivity comparable to that of Nafion, which is the benchmark in proton-exchange membrane fuel cell technology. Therefore, s-sPS might represent a good candidate for the hydrocarbon alternative to the hazardous fluorinated compounds such as Nafion in various energy conversion applications. However, for optimal performance, functionalization should be efficient across the entire volume of the membrane. Here, we report on the evaluation of the efficiency of sulfonation of sPS membranes based on a semiquantitative analysis of data measured by small-angle neutron scattering on dry and hydrated samples. By exploiting the different response of neutrons from the same compound with different degrees of deuteration, various structural details were highlighted following in-beam hydration of functionalized membranes with H₂O or D₂O. Experimental analysis of the scattering data under different contrast conditions using the one-dimensional correlation function Γ₁(x) provided evidence of homogeneous sulfonation of only the amorphous phase of the membranes, while the crystalline regions remained unaffected.