Internal and interfacial structure analysis of graft‐type fluorinated polymer electrolyte membranes by small‐angle X‐ray scattering in the high‐q range

Abstract

The hierarchical structures of poly(styrenesulfonic acid)‐grafted poly(ethylene‐co‐tetrafluoroethylene) polymer electrolyte membranes (ETFE‐PEMs) with different scale ranges including lamellar spacing, interfacial thickness, and intra‐structure of conducting layers were evaluated by small‐angle X‐ray scattering in terms of background scattering (I B(q)). First, I B(q) was roughly estimated by modifying Ruland's method and then optimized to avoid overestimation using a “contribution factor,” which is defined as the contribution of I B(q) to the observed scattering intensities over the entire q range. Then, I B(q) was optimized again by using the model for the deviation from Porod's law also proposed by Ruland to select proper q‐range for interfacial thickness evaluation. The lamellar spacing, which is observed in the low‐q range, was not altered by background correction. In contrast, in the high‐q range, the interfacial thickness and the internal structures can be estimated only after correction for the background scattering data. The interfacial thickness of the final membranes (ETFE‐PEMs) is affected by both the graft‐polymerization and sulfonation processes and because the change in membrane structures is observed during propagation steps at lower ion exchange capacity (IEC) range (IECs < 2.4 mmol/g), which should affect the mechanical strength of graft‐type PEMs.

Investigation of the Lamellar Grains of Graft-type Polymer Electrolyte Membranes for Hydrogen Fuel Cell Application using Ultrasmall-angle X-ray Scattering

“…The pronounced peaks seen in the low-q region (q < 1.510 -1 nm) are observed even at high GD values (GD  59%), which indicate the strong development of the grain structures (lamellar grains) with the increase of GD. The new shoulder-like peak observed in the very low-q region (q < 1.510 -1 nm) implies the presence of the lamellar grains at a scale higher than that seen in the lamellar stacks [7][8][9]. At the localized scale, the peak corresponds probably to the average correlation distance between the two grains, which are composed of the crystalline regions and graft chains incorporated in the amorphous ETFE regions [7,18].…”

“…Based on these characterizations, the styrene was confirmed to be grafted onto the ETFE film and the grafted film was sulfonated by introduction of the −SO3H group. Figure 1 presents the Lorentz-corrected SAXS profiles of the original SAXS data [7][8][9]. This figure shows that the shoulder-like peaks to be pronounced.…”

“…In other words, the LL values, which are estimated from the peak maxima in the Lorentz-corrected SAXS profiles, should be overestimated in the previous report [7]. Accordingly, together with taking into account the results from our previous investigations [5][6][7][8][9], we will take in advantage of the one-dimentional correlation function for detailed analysis of the SAXS data for the predictions of the lamellar two-phase system. Figure 6a shows the surface LVSEM image at a magnification of 50 K for the original ETFE.…”

“…The equipment and detailed procedures for the SAXS measurements were described in our previous work [7][8][9]. Briefly, the SAXS measurement was performed using two in-house SAXS spectrometers (NIMS-SAXS-II and NIMS-SAXS-III) at the National Institute of Material Science (NIMS) and USAXS at Super Photon ring-8 GeV (SPring-8), Japan.…”

“…The obtained results showed that the ETFE-PEMs have the proton conductivities that are less dependent on the relative humidity (RH) and their mechanical strength and conductivity relate strongly to the PEM crystallinities. The hierarchical structures of the membranes were then investigated using the small-and ultrasmall-angle X-ray scattering (SAXS/USAXS) [7][8][9]. It was found that, for the ETFE-PEMs with high IECs (> 2.7 mmol/g), a higher conductivity at 30% RH and compatible tensile strength at 100% RH and 80 C, in comparison with Nafion-212, were originated respectively from the well-interconnected ion channels around the lamellar grains and the remaining lamellar crystals and grains as well.…”