Water dynamics in ionomer membranes by field-cycling NMR relaxometry

The dynamic behavior of water within two types of ionomer membranes, Nafion and sulfonated polyimides, has been investigated by field-cycling nuclear magnetic relaxation. This technique, applied to materials prepared at different hydration levels, allows to probe the proton motion on a time scale of the microsecond. The NMR longitudinal relaxation rate R1 measured over three decades of Larmor angular frequencies ω is particularly sensitive to the host-water interactions and thus well suited to study fluid dynamics in restricted geometries. In the polyimide membranes, we have observed a strong dispersion of R1(ω) following closely a 1/ω law in a low-frequency range (correlation times from 0.1 to 10 μs). This is indicative of a strong interaction of water with “interfacial” hydrophilic groups of the polymeric matrix (wetting situation). On the contrary, in the Nafion, we observed weak variations of R1(ω) at low frequency. This is typical of a nonwetting behavior. At early hydration stages, the proton-proton inter-dipolar contribution to R1(ω) evolves logarithmically, suggesting a confined bidimensional diffusion of protons in the microsecond time range. Such an evolution is lost at higher swelling where a plateau related to 3D diffusion is observed.