FTIR characterization of water-polymer interactions in superacid polymers

The OD stretch of dilute HOD in H2O absorbed in a series of sulfonated syndiotactic poly(styrene) and sulfonated poly(sulfone) membranes was studied using FTIR spectroscopy to measure how the character of the sulfonate head group and the backbone polarity influenced the water-membrane interactions. Using a three-state model, the OD stretch yielded information about the populations of absorbed water participating in hydrogen bonds with polymer-tethered sulfonate groups, water in an intermediate state, or water hydrogen bonding with other water molecules. The perfluoroalkyl sulfonate moiety, which behaves as a superacid, consistently displayed the largest fraction of head-group-associated water due to its strong acidic character. Measurements of the OD stretch gave insight into the strength of the hydrogen bonds formed between water and the sulfonate groups. Water associated with the superacid displayed an OD stretch peak position that was blueshifted by 39 cm(-1) compared to the aryl-sulfonate-associated water with an OD stretching frequency that was centered at 2547 cm(-1). The polarity of the polymer backbone also affected the OD stretch peak position. As hydration increased, the OD peak stretching frequency in poly(styrene)-based membranes displayed a red shift from 2566 to 2553 cm(-1), whereas there was no OD peak maxima shift in poly(sulfone)-based membranes due to the greater amount of intermediate water in the more polar poly(sulfone) backbone system.