Spectroscopic Characterization of Sulfonate Charge Density in Ion-Containing Polymers

The charge density and hydrogen bonding with water of five different polymer membranes functionalized with various sulfonate side-chain chemistries were investigated using Fourier transform infrared (FTIR) techniques and density functional theory (DFT) calculations. The peak position of the OD stretch of dilute HOD absorbed into the sulfonated poly(sulfone) membranes was studied using FTIR to compare the charge density of the sulfonate headgroup across the different samples, which can ultimately be related to the acidity of the proton-form sulfonate moieties. The OD peak was deconvoluted to determine the percentage of headgroup-associated, intermediate, and bulk water. DFT modeling was used to calculate the charge density of each headgroup and visualize how the chemistry of the headgroup influenced the conformation of the side-chain tether. FTIR-determined OD peak positions and charge density calculations demonstrated that a perflurosulfonate containing a thioether linkage produced the most acidic sulfonate headgroup. However, the amount of headgroup-associated water calculated for this side chain was low due to the unique cis conformation of the thioether side chain. The biperfluorosulfonate side chain had very low calculated headgroup-associated water due to its bulkiness and water molecules bridging the two sulfonates. These detailed insights on local hydration of sulfonate side chains will point towards new headgroup designs for advanced membranes.