Reversible Dehydration-Hydration of Poly(ethylene glycol) in Langmuir Monolayers of a Diblock Copolymer Inferred from Changes in Filament Curvature

We investigated changes in the hydration state of poly(ethylene glycol) through morphological changes in Langmuir monolayers of a poly(ethylene glycol)-poly(L-lactide) (PEG-b-PLLA) diblock copolymer. When the PEG blocks were hydrated, we observed a remarkable morphology of bundles of ring-like filaments, arranged concentrically, yielding densely packed disk-like objects with a hollow centre. We attribute the uniform curvature of these filaments to a strong mismatch between the molecular volumes occupied by PLLA blocks and hydrated PEG blocks, respectively. Under the constraint that each hydrated PEG block is attached to a hydrophobic PLLA block anchored to the air-water interface, this mismatch of molecular volumes caused strong repulsion within the PEG layer, in particular when the PLLA blocks packed tightly. Induced by a transition in the ordering of the PLLA blocks, the PEG blocks lost their hydration shell and packed into a dense polymer brush, accompanied by a reduction of the pressure within the PEG layer. During this packing process, the curvature of the filaments was eliminated and the ring-like filaments fractured into small linear pieces. Upon compression, the linear pieces coalesced and formed long filaments aligned in parallel. Importantly, upon expansion of the Langmuir film, these changes in morphology were reversible, and the PEG blocks could be re-hydrated and bundles of concentrically arranged ring-like filaments were re-formed. We conclude that the change in curvature of the filaments provides a means for distinguishing between the hydrated and dehydrated states of PEG.