(Near-) Ambient Pressure XPS for Studying Surface Reconstruction of Hydrated Polymers*

Polymer restructuring in response to environmental conditions has made surface characterization of polymers designed for use in hydrated applications difficult. Analysis techniques that provide nanoscale surface composition (e.g. X-ray Photoelectron Spectroscopy (XPS)) have been largely limited to use in vacuum, where the surface presentation no longer matches that of hydrated polymers. Consequently, characterization of the hydrated surface has been relegated to contact angle goniometry and vibrational spectroscopy techniques such as Raman and Sum Frequency Generation (SFG). We present the first synchrotron (Near-) Ambient Pressure XPS (AP-XPS) studies performed on polymer surfaces in contact with water vapor. Water adsorption on polydimethylsiloxane-based polymers was found to increase substantially for those containing even minimal amounts of amphiphilic side chains. The presence of hydrogen-bonding side chains also correlated with greater increases in water adsorption and decreases in silicon-character oxygen (associated with the polymer backbone) at 6 Torr relative to non-hydrogen-bonding side chains.