Detection of interfacial structures of poly(ethylene glycol), poly(propylene glycol) and their copolymers at liquid/solid interfaces using sum frequency generation vibrational spectroscopy

We have applied sum frequency generation (SFG) vibrational spectroscopy, a submonolayer surface sensitive analytical technique, to study interfacial structures of poly (ethylene glycol) (PEG), poly (propylene glycol) (PPG), their solutions, their copolymers, and copolymer solutions while contacting different media including air, fused silica, and various polymers. For the first time, conformations of polyether materials at various interfaces have been shown at the molecular level in situ. Depending on the hydrophobicity of the solid contacting media, the liquid polymers PEG and PPG 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, polystyrene (PS), poly(methyl methacrylate) (PMMA), and fused silica have also been investigated. In addition, we have studied conformations of pluronics at various interfaces, showing that different blocks have different interface activities. The interfaces are always dominated by the PPG segments. The studies on molecular level interfacial structures of PEG, PPG and pluronics will help us to understand and control the interfacial behavior of liquids at interfaces.