Structural evolution of water swollen perfluorosulfonated ionomers from dry membrane to solution

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.…”

“…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].…”

“…The polymer matrix, in turn, exerts a counter force due to its elastic deformation, thereby limiting the swelling [7,11,18,26,27,[40][41][42]. Thus, the swollen state represents an equilibrium between two opposing forces: (i) the osmotic swelling pressure driving the water volume to increase, and (ii) the elastic deformation of the polymer matrix which exerts a counter pressure on the water-filled hydrophilic domains driving water volume to decrease [11,18,20,30,40,41,43]. As a result of this interplay between water uptake and matrix deformation during swelling, a continuous structural reorganization and coalescence of the water-filled domains occur at the nanoscale.…”

“…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.…”

Surface orientation of hydrophilic groups in sulfonated poly(ether ether ketone) membranes

“…It was suggested that SPEEK exhibited similar microstructure conformation in a polar solvent DMAc [38]. A packing sphere 28 model of the sulfuric groups in post-sulfonated SPEEK, similar to Nafion ® [43], either isolated or in small spherical clusters of two or three interacting sulfonic groups, was further proposed quite recently by Wu et al [39]. The flexible perfluoroalkylether (PFAE) side chain in Nafion ® provides the possibility for forming lamella clusters, leading to rod-like [14] or plate-like [39,44] hydrophilic domains.…”