SANS Study of the Effects of Water Vapor Sorption on the Nanoscale Structure of Perfluorinated Sulfonic Acid (NAFION) Membranes

Abstract

Several poly(perfluorosulfonic acid) membranes (NAFION, EW ) 1100) with the same sulfonic acid content were systematically investigated with SANS under in-situ water vapor sorption and/or with bulk water to quantify the effects of relative humidity (RH), membrane processing (melt-extruded and solution-casting), prehistory (pretreated at 80°C and as-received), and thickness on the nanoscale structure at room temperature. The sorption isotherm (water uptake vs RH) of the membranes showed a strong correlation between the interionic domain distance (L ion ) and RH. The melt-extruded membranes showed evidence of partial alignment of better organized ionic domains than those solution-cast. Pretreating the membranes resulted in a larger L ion and a broader scattering over the entire range of RH. The ionic peak of the melt-extruded membranes (as-received and pretreated) became more symmetric and narrower with sorption time. Diffusion coefficients of water vapor, based on structural evolution and Fick's second law, are in the range of 1 × 10 -7 -3 × 10 -7 cm 2 /s for both extruded (pretreated and as-received) membranes. A thickness-dependent crystalline feature around Q ≈ 0.03 Å -1 was also observed.

Mechanics-based model for non-affine swelling in perfluorosulfonic acid (PFSA) membranes

“…The non-affine swelling behavior is assumed to be independent of temperature. (iii) Non-affine deformation is here associated with the structural reorganization at the nanoscale [11,[18][19][20][21]34,35]. Alternatively, this type of deformation could also be attributed to the lamellar dilution of rod-like particles [33] or to randomness in the nanostructure at different scales.…”

“…It is well established that the swelling of PFSA membranes is non-affine. For example, the microscopic swelling strain (defined as the inter-cluster distance) is much larger than the macroscopic swelling strain (which is related to the water content of the membrane) [11,[18][19][20][21]29,[33][34][35]. In this study, since we assume that the volume is nearly filled by close-packed RVEs of the same size, the inter-cluster distance is the outer diameter of the RVEs, 2b.…”

“…These water-filled hydrophilic domains are separated from the hydrophobic polymer matrix resulting in a two-phase nanostructure as observed through a vast number of scattering experiments [9,[20][21][22][23][24][25][34][35][36][37]. Upon water uptake, 3 water molecules initially ionize the SO 3 groups and remain bound to them.…”

“…Upon water uptake, 3 water molecules initially ionize the SO 3 groups and remain bound to them. Additional water molecules are then free to move through the ionomer, causing growth of the clusters and consequently the macroscopic swelling of the membrane [8,9,11,24,34,[38][39][40][41].…”

“…Consequently, the magnitude of the strain of the individual water-filled nano-domains is higher than that of the overall macroscopic swelling strain of the material. This relationship is referred to as non-affine deformation and has been shown to occur for PFSA membranes by a number of studies in literature [11,[18][19][20][21][22]29,[33][34][35]. Despite the growing body of literature on the sorption behavior [7,8,10,24,34,35,38,40,[44][45][46][47][48][49][50] and nano-structural modeling and characterization [11,13,[17][18][19][20][21][22]28,29,31,[33][34][35][36]39,49,51,52] of PFSA ionomers, the relationship between the water uptake and nanostructure, temperature, mechanical properties and membrane pretreatment is not well established.…”