Evidence of Temperature-Induced Subdiffusion of Water on the Micrometer Scale in a Nafion Membrane

Abstract

To investigate the topological features of proton transport in perfluorosulfonic membranes, we performed time-dependent (t)H NMR pulsed gradient spin-echo experiments on four samples of Nafion-115 with lambda = 0.5, 3.2, 5.8, and 12.4 water molecules per sulfonic group at different temperatures ranging from 278 to 348 K. A subdiffusive behavior of water, < z(2) > = 2D(alpha)t(alpha) with 0.75 < alpha < 1, was observed above 320 K for each of the four samples. The onset of water subdiffusion with increasing temperature, basically independent of membrane hydration, supports the hypothesis of a drop in dimensionality for the diffusion space. Diffusion-diffraction effects on the NMR echo-signal attenuation confirm that proton displacement is restricted because of an interconnected pore structure With it coherence length of similar to 1 mu m, which depends on temperature and water content

Morphological Heterogeneity by Diffusional Kurtosis NMR Spectroscopy in Perfluorosulfonic Acid/SiO2Composite Proton‐Exchange Membranes

“…The PFSA/SiO 2 membrane with 3 wt% concentration in silica shows a change in the values of E a in the low-temperature range in synchrony with the changes in the value of diffusivities measured at 50 ° C (see Figure 2 a). The above results are in agreement with that of Casieri et al [ 14 ] They observed a subdiffusive behavior for Nafi on-115 membranes above 320 K, and in this case, the mean-square displacement is given by 〈 z 2 〉 = 2 D α Δ α with α ∈ [0.75, 1]. The interpretation of the water subdiffusive process was related to the fact that the connectivity of water channels starts to decrease when the membrane temperature increases.…”

“…[ 10 ] Water channels described by this model are surrounded by partially hydrophilic side branches, forming inverted-micelle cylinders. Furthermore, water diffusion coeffi cient NMR spectroscopy measurements [12][13][14] and ionic conductivity experiments [ 15 ] showed the existence of coherent orientation of aqueous pathways in Nafi on membrane. Furthermore, the anisotropy of channel alignment in different Nafi on membranes was measured using the splitting of 2 H wideline NMR spectroscopy.…”

“…Furthermore, the recent investigation on the PFSA/SiO 2 nancomposites using TEM, SAXS, and proton transverse-relaxation NMR spectroscopy proved that the water channels are partially blocked at silica concentration larger than 3 wt%. [ 19 ] The diffusional non-Gaussianity of the Nafi on membrane was put in evidence by measuring the mean square displacement E as a function of the diffusion time Δ eff [ 14 ] A subdiffusion behavior of water was observed above 320 K and was described by 〈 z 2 〉 = 2 D α Δ α eff , with an effective power parameter 0.75 < α < 1. [ 14 ] No kurtosis parameter was used for the characterization of this propagator acting in the space coordinates.…”

“…[ 19 ] The diffusional non-Gaussianity of the Nafi on membrane was put in evidence by measuring the mean square displacement E as a function of the diffusion time Δ eff [ 14 ] A subdiffusion behavior of water was observed above 320 K and was described by 〈 z 2 〉 = 2 D α Δ α eff , with an effective power parameter 0.75 < α < 1. [ 14 ] No kurtosis parameter was used for the characterization of this propagator acting in the space coordinates. This propagator is generated by Fourier transformation of the data taken in the wave-number q space (see below).…”

“…The anomalous diffusion that is related to softer restrictions of the water diffusion due to structural and dynamic characteristics of the matrix leads to a non-Gaussian propagator. [ 14 ] The quantitative characterization of the propagator can be made using kurtosis parameters. [ 20 ] Water diffusion anisotropy and the exchange processes are not only specifi c for the morphology of PEMs, but are also present in many biological tissues, including the brain.…”