Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) Copolymer 2D Single Network at the Air–Water Interface
2020
Haroun, Ferhat | El Haitami, Alae | Ober, Patrick | Backus, Ellen H. G. | Cantin, Sophie
In this work, Langmuir monolayers based on poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (PEG–PPG–PEG) triblock copolymer were in situ stabilized at the air–water interface in the presence of a cross-linking agent, benzene-1,3,5-tricarboxaldehyde (BTC), in the aqueous subphase. The reaction takes place through acid-catalyzed acetalization between the terminal hydroxyl groups of the copolymer and aldehyde functions of the BTC molecules. Mean area per repeat unit measurements as a function of the reaction time show a significant monolayer contraction associated with an increase in its compressibility modulus. In addition, Brewster angle microscopy observations indicate the appearance of higher-density two-dimensional domains, irreversibly formed at constant surface pressure. This is also confirmed on a smaller scale by atomic force microscopy (AFM). These arguments, consistent with copolymer monolayer cross-linking in acidic medium, are supported in situ at the air–water interface by sum-frequency generation (SFG) spectroscopy. Furthermore, PEG–PPG–PEG monolayer cross-linking is not evidenced in alkaline medium, in coherence with the interfacial acid-catalyzed acetalization.
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