Chemical structures of liquid poly(ethylene glycol)s with different end groups at buried polymer interfaces

The interfacial conformational behaviors or structure of liquid poly(ethylene glycol)s (PEGs) with different end groups in contact with solid polymers of different hydrophobicities were investigated by sum frequency generation (SFG) vibrational spectroscopy. The solids at the interface included deuterated poly(methyl methacrylate) (d-PMMA), deuterated polystyrene (d-PS), deuterated polyethylene (d-PE), and a fluorinated polymer AF-2400; the liquids were poly(ethylene glycol) diol (PEGD), poly(ethylene glycol) methyl ether (PEGME), and poly(ethylene glycol) dimethyl ether (PEGDME). For comparison purposes, SFG has also been used to study PEG surfaces in air and in contact with fused silica. Depending on the hydrophobicity of the solid polymer, the hydroxyl O-H and methoxy OCH3 end groups and ethoxy OCH2 backbone groups of the liquid PEGs exhibited different interfacial behavior, which can be correlated to molecular interactions at these interfaces. The favorable interaction between the hydrophobic solids and the hydrophobic functional groups of the liquid PEGs causes the OCH3 end groups, and to some extent the OCH2 groups of the backbone, to become strongly ordered at the solid polymer/liquid polymer interface. We have shown that the end groups of the liquid PEGs, although comprising a small proportion of the total molecule, actually can dominate the interfacial behavior of the liquid polymer.