Sum frequency generation vibrational Spectroscopy studies on molecular conformation of liquid polymers poly(ethylene glycol) and poly(propylene glycol) at different interfaces

The conformation of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) molecules at various interfaces has been investigated by sum frequency generation (SFG) vibrational spectroscopy. The interfaces studied include interfaces between pure liquid PEG (or PPG) or aqueous PEG (or PPG) solutions and air, fused silica, or solid polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). Depending on the hydrophobicity of the solid contacting media, the liquid polymers show different conformations at different interfaces, which can be correlated to molecular interactions at these interfaces. The favorable interaction between hydrophobic media and the hydrophobic segments, methylene or methyl groups, of polyethers causes an ordered conformation with these groups standing up at the interface. The unfavorable interaction between hydrophilic media and hydrophobic segments of the polyethers induces interfacial methylene or methyl groups to have a random structure or to lie down at the interfaces, indicated by the weakening or even absence of SFG signals. For comparison, interfaces between aqueous PEG or PPG solutions and air, PS, PMMA, and fused silica have also been investigated. The weak SFG signals of PEG at the PEG solution/air interface indicates that PEG molecules do not have very ordered structures at the interface, due to their strong affinities with water molecules. The absence of SFG signals of PEG at all PEG solution/solid interfaces indicates that PEG molecules do not segregate to these interfaces. PPG is more surface or interface active, and it tends to be strongly adsorbed to all interfaces. At different solid/PPG solution interfaces, adsorbed PPG molecules show conformations different from those at different solid/PPG liquid polymer interfaces. This study provides a direct in-situ observation that polyethers can have different conformations at different interfaces.