Detection of Hydrophobic End Groups on Polymer Surfaces by Sum-Frequency Generation Vibrational Spectroscopy

We report the successful application of SFG to detect segregation of end groups on polymer surfaces. Two groups of polymer samples are studied: one is polyurethane with different surface-modified end groups, the other is poly(ethylene glycol) (PEG) with different end groups. For each group of polymers, both hydrophobic and hydrophilic end groups are chosen. With the surface sensitivity of SFG, we have found that hydrophobic end groups [e.g., methoxy on PEG or poly(dimethylsiloxane) (PDMS) on polyurethane] tend to segregate to the polymer surface in air. However, the hydrophilic end groups (e.g., hydroxyl group on PEG or PEG on polyurethane) remain in the bulk so that the surfaces that are exposed to air are covered by the polymer backbones. Although contact-angle measurements and XPS results can demonstrate that polymer surfaces indeed have been modified by different end groups, only SFG can show the surface structure at the molecular level.