Variable-Angle SFG for the Characterization of Water Structure at Polymer Surfaces

We demonstrate that intensity-calibrated angle-scanning sum-frequency spectroscopy is well-suited to the study of water structure on polymer films. In addition to specific water–polymer interactions, many such materials are known to exhibit negative ζ-potentials and therefore can order water at an appreciable distance from the surface. The variable angle method modulates the wavevector mismatch in order to separate contributions of the bonded and diffuse layer water populations. In addition, it inherently accounts for complex thin-film local field effects that otherwise dominant the shape of the measured data. As a demonstration, we characterize the hydrogen bonding environment adjacent to poly(dimethylsiloxane) (PDMS) and polystyrene. Our measurements indicate that the majority of surface water molecules point their hydrogen atoms toward the polymer surfaces in both cases. We additionally find that water is more ordered next to polystyrene, and that water near PDMS is less perturbed by the addition of low concentration NaCl. These findings highlight the distinct molecular-level behavior of water at different hydrophobic polymer interfaces.