Compression-induced formation of surface micelles in monolayers of a poly(2-oxazoline) diblock copolymer at the air-water interface: A combined film balance and electron microscopy study

We investigated monolayers of the diblock copolymer poly(2-ethyl-2-oxazoline)-poly(2-nonyl-2-oxazoline) (E60N60) at the air-water interface using classical Langmuir film balance techniques and transmission electron microscopy on transferred Langmuir-Blodgett films. The isotherms exhibit three distinct regions. At low pressures the monolayer is relatively incompressible and the moderately water-soluble ethyloxazoline block is adsorbed to the interface. At intermediate pressures of approximately 10 mN/m the compressibility increases temporarily and decreases again at higher lateral pressures. Desorption of the ethyloxazoline block takes place in the intermediate high-compressibility region. This desorption process cannot be described as a phase transition. In the water-insoluble nonyloxazoline block the polymer backbone remains confined to the interface at all pressures. Electron micrographs of monolayers transferred onto mica at lateral pressures below approximately 10 mN/m showed no structures. In the high-compressibility region structures appeared that persisted at higher pressures. These structures can be described as two-dimensional micelles. The driving force for the micelle formation is the only moderate water solubility of the ethyloxazoline block. For this reason, desorption of the moderately water-soluble ethyloxazoline block and the formation of surface micelles are coupled.