Poly (ethylene glycol) -block-poly (propylene glycol) -block-poly (ethylene glycol) Copolymer 2D Single Network at the Air-Water Interface

Haroun, F.; El Haitami, A.; Ober, P.; Backus, E.H.G.; Cantin, S.

Langmuir 36(31): 9142-9152


ISSN/ISBN: 0743-7463
PMID: 32686418
DOI: 10.1021/acs.langmuir.0c01398
Accession: 072331332

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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.