Hydrophobic-Interaction-Induced Alignment of Polymers at the Solid/Liquid Interface Studied by Infrared−Visible Sum Frequency Generation

Abstract

The effects of surface hydrophobicity and solvent on the orientation and ordering of amphiphilic neutral polymers adsorbed at the solid/liquid interface have been studied by infrared−visible sum frequency generation (SFG) vibrational spectroscopy. SFG spectra have shown that poly(propylene glycol) (PPG) and poly(ethylene glycol) (PEG) adsorb with their hydrophobic moieties preferentially oriented toward hydrophobic polystyrene surfaces. However, the hydrophobic moieties in PPG and PEG do not show anisotropic orientation when these polymers adsorb at the hydrophilic silica/water interface. Water is a critical factor for mediating the orientation and ordering of hydrophobic moieties in polymers adsorbed at hydrophobic interfaces. When methanol is substituted in place of water as the solvent, neither PPG nor PEG displays structural ordering when adsorbed on polystyrene substrates. This work demonstrates that hydrophobic surfaces and water as the solvent are required to induce the anisotropic orientation of adsorbed amphiphilic neutral polymers at the solid/liquid interface.