Detecting molecular-level chemical structure and group orientation of amphiphilic PEO-PPO-PEO copolymers at solution/air and solid/solution interfaces by SFG vibrational spectroscopy

The molecular chemical structure and group orientation of a series of pluronic copolymers at both solution/air and solid/solution interfaces have been investigated using sum frequency generation (SFG) vibrational spectroscopy. Both solution/air and solid/solution interfaces are dominated by hydrophobic PPO blocks. The methyl groups on PPO blocks orient at the interface differently, depending on the solution concentration or the solid contacting media. For the saturated solution/air interface, the methyl groups on PPO blocks tend to orient preferentially along the interface normal and show a relatively ordered structure. However, at very low concentrations, pronounced changes in methyl group orientation have been observed, which explains the low-concentration break on the surface tension data vs log concentration curve. The solid/solution interface is also dominated by PPO blocks, and the orientation of the methyl groups on PPO blocks varies with different solid contacting media. Different conformations of copolymers at different solid/solution interfaces can be related to interfacial molecular interactions. The favorable interactions between hydrophobic surfaces and hydrophobic methyl groups on PPO blocks cause more ordered structures, with these groups preferentially oriented along the interface normal. The interactions between less hydrophobic surfaces and hydrophobic methyl groups are not very favorable, causing a less ordered methyl group arrangement at the interface. We have demonstrated that SFG is an appropriate technique to obtain a molecular-level picture of pluronics at different interfaces.