Unique Structural Characteristics of Graft-Type Proton-Exchange Membranes Using SANS Partial Scattering Function Analysis

Abstract

The partial scattering function analysis was applied to determine the exact structure of radiation-grafted protonexchange membranes, made of poly(styrenesulfonic acid)-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE-g-PSSA). Hydrated ETFE-g-PSSA membranes were treated as a three-component system comprising the ETFE base polymer (BP), PSSA graft polymer (GP), and absorbed water. On a large length scale, polymer grains with an approximate radius of gyration (R g ) of 150 nm and a mass fractal structure with a dimension of 2.4 were observed. These grains were formed by the aggregation of phase-separated GP domains in the BP matrix. Each individual GP domain has an average R g of 9.5 nm and is composed of homogeneously distributed GP and water nanodomains that form a bicontinuous-like local structure with a mean separation distance of 2 nm. These structures were strongly supported by the first finding that PSSA GP and water interact attractively and repulsively in q-regions lower and higher than 2 nm −1 (i.e., ∼3 nm), respectively. The repulsion between GP and water at a molecular length level of <3 nm results in a lower hydration number and hence poorer conductivity at low relative humidity when compared to Nafion. The results of this study provided a mechanistic insight into membrane conductivity and structure correlations.

Microscopic Determination of the Local Hydration Number of Polymer Electrolyte Membranes Using SANS Partial Scattering Function Analysis

“…This is because the measured intensity profile contains mixed structural information on all components. This undesirable original data problem can be solved by partial scattering function (PSF) analysis, which is the quantitative decomposition of a series of intensity profiles obtained through contrast-variation small-angle neutron scattering (CV-SANS) experiments. − PSF was theoretically developed and applied to polymer nanocomposites in early studies, and recently to PEMs. , …”

“…It is worthy of showing the mathematical foundation of the decomposition of scattering intensity profiles, I ( q ), into PSFs through CV-SANS for a three-component system. Although the quantitative description can be found in S2 in the Supporting Information, we here emphasize the main equation that I ( q ) can be described using three PSF self-terms − I false( q false) = false( b 1 − b 2 false) false( b 1 − b 3 false) S 11 false( q false) + false( b 2 − b 1 false) false( b 2 − b 3 false) S 22 false( q false) + false( b 3 − b 1 false) false( b 3 − b 2 false) S 33 false( q false) where b i and S ii ( q ) are the SLD and PSF self-term of the i component ( i = 1–3), respectively. S ii ( q ) represents the structure of the i component and is determined mathematically in S2 and S3 in the Supporting Information. − …”

“…In Model-I, the PEM was composed of BP, GP, and water, and the SLD of each component can be calculated using its chemical structure and mass density. ,, b BP and b GP are constants, as listed in Table , but the SLD of water ( b W ) is a function of f D2O in the water mixture: b W = b normalD 2 O f normalD 2 O + b normalH 2 O false( 1 − f D 2 normalO false) where b D 2 O and b H 2 O are the SLD of D 2 O and H 2 O being 6.34 and −0.56 (× 10 10 cm –2 ), respectively. , Table and eq indicate that b i of all components in Model-I can be accurately determined at a given contrast (i.e., f D 2 O ). S ii of each component is therefore rigorously calculated in S3 in the Supporting Information in conjunction with Figure S3 . As a result, S BP‑BP_1 is the true profile of BP under the current hydration condition, because it is independent of the λ local value.…”

“…22−25 PSF was theoretically developed and applied to polymer nanocomposites in early studies, 25 and recently to PEMs. 22,23 In this report, we propose a new PSF analysis application to determine λ local of PEMs currently being implemented in electrochemical devices such as the benchmark material Nafion, and the well-studied radiation-grafted PEM consisting of poly(styrene sulfonic acid)-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE-g-PSSA). We demonstrate the advantages of this new method to provide new micro-to molecular level structure insights, and as well discuss the application limits for materials under high hydration conditions.…”

“…In Model-I, the PEM was composed of BP, GP, and water, and the SLD of each component can be calculated using its chemical structure and mass density. 23,30,31 b BP and b GP are constants, as listed in Table 1, but the SLD of water (b W ) is a function of f D2O in the water mixture:…”