A novel sulfonation method that involves iridiumcatalyzed aromatic CÀH activation/borylation and subsequent SuzukiÀ Miyaura coupling with sulfonated phenyl bromides was developed for the preparation of aromatic ionomers. Superacidic fluoroalkyl sulfonic acid and less acidic aryl and alkyl sulfonic acids were efficiently incorporated into the aromatic ring of model polystyrene, and the resulting sulfonated ionomers were characterized for their properties as proton-conducting membranes. The membrane properties of ionomers containing sulfonic acid groups with different acidity strengths were compared to study the effect of acidity on the water properties, proton conductivity, and morphology. The superacidic fluoroalkyl sulfonated ionomer (sPS-S 1 ) exhibited a significantly higher proton conductivity than that of the less acidic aryl and alkyl sulfonated ionomers (sPS-S 2 and sPS-S 3 , respectively) at low relative humidity, despite a lower ion exchange capacity and lower water uptake. Hydration behaviors of the ionomers as a function of relative humidity were studied to correlate the acid strength of the sulfonates and water uptake properties. Morphology studies of the sulfonated ionomers show that sPS-S 1 has a larger hydrophilic domain than that of sPS-S 3 . Molecular dynamic simulations were performed to understand the origin of the improved proton conductivity of the superacidic ionomer at the molecular level. These simulations suggest that the enhanced proton conductivity of sPS-S 1 is due to the cumulative effect of higher acidity of the sulfonate, which leads to increased dissociation to hydronium ions and a higher degree of ionic character in the sulfonate, and better solvation of the sulfonate with water molecules.
Sulfonated poly(ether ether ketone) (S-PEEK) with 40% of degree of sulfonation was studied using full atomistic molecular dynamics simulation in order to investigate the nanophase-segregated structures, focusing on the sulfonate group and water phase at various water contents such as 10, 13, and 20 wt %. By analyzing the pair correlation function, it is found that as the water solvation of sulfonate groups proceeds more with increasing water content, the distance between sulfonate groups is increased from 4.4 Å (10 wt %) to 4.8 Å (13 wt %) to 5.4 Å (20 wt %), and the hydronium ions (H3O+) become farther apart from the sulfonate groups. The water coordination number for water and the water diffusion are enhanced with increasing water content because the internal structure of the water phase in S-PEEK approaches that of bulk water. Compared to the Nafion and Dendrion membranes, the S-PEEK membrane shows less internal structure in the water phase and smaller water diffusion, indicating that the S-PEEK has less nanophase segregation than the Nafion and Dendrion membranes.
The effects of temperature on hydrated sulfonated poly(ether ether ketone) are studied using molecular dynamics. Three different temperature conditions (298 K.15 K, 323.15 K, and 353.15 K) with two different water contents (10 wt. % and 20 wt. %) are simulated. Analyzing the pair correlation functions, it is found that there is limited temperature effect on the distribution and solvation of the sulfonate groups. The structure factor analysis shows that the temperature dependence of the nanophase-segregated morphology is not significant in the simulated temperature range. On the contrary, the structure factors S q ð Þ at $30 Å (q ¼ $0.2 Å À1 ) and $13 Å (q ¼ $0.5 Å À1 ) clearly increase with water content, indicating that the development of water channels is mostly affected by the water content. Within such water phase in the nanophase-segregated structure, the internal structure of water phase becomes more developed with decreasing temperature and increasing water content. By analyzing the mean square displacement of the water molecules, it is also found that self-diffusion of water is enhanced with the increasing temperature. From the observation that the activation energies calculated from such temperature dependency are very similar (E a ¼ 25.7 kJ=mol and E a ¼ 24.9 kJ=mol for 10 wt. % and 20 wt. %, respectively), it is inferred that the extent of the structural change in the water phase as a function of temperature is very similar between the 10 wt. % water content and the 20 wt. % water content. Compared to the bulk water (13.2 kJ=mol) and the water in Nafion (16.7-18.9 kJ=mol), it is confirmed that more nanophase-segregation enhances water transport through the membrane.
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