Segregation of Chemical Groups at PMMA/H₂O Interface Leads to Different Local Hydrophobicity

The local hydrophilicity of polymer surfaces is essential for many applications, such as coatings and biocompatibility, but revealing its structural origin is challenging. Here, we used poly(methyl methacrylate) (PMMA) film as a model and excavated several SFG spectral features, generated by femtosecond sum-frequency generation vibrational spectroscopy (SFG-VS), to elucidate the nature of microscopic hydrophilicity of a polymer/H₂O interface. For the first time, we successfully probed the SFG spectra of the bend-libration combination band of interfacial water, which exhibits high sensitivity to solvent-water interactions. Two local hydrophilic domains are observed at the PMMA/H₂O interface. The segregation of -OCH₃ and -CH₃ groups to the PMMA/H₂O interface results in the formation of a local hydrophobic domain, where weak solvent-water interactions, slow vibrational dynamics of OH stretching, and no ice-like interfacial water are detected. In contrast, when both C═O and -OCH₃ groups segregate to the PMMA/water interface, a local hydrophilic domain is formed, leading to strong solvent-water interactions, fast vibrational dynamics of OH stretching, and the presence of ice-like interfacial water. The water molecules around the hydrophobic domains of the PMMA surface are mainly liquid-like water rather than ice-like water. This work contributes to a molecular-level understanding of the local hydrophilicity of polymer surfaces.