Two-dimensional physical networks of lipopolymers at the air/water interface

Abstract

The objective of this manuscript is to review recent film balance and interfacial rheology experiments on Langmuir monolayers of lipopolymers and lipopolymer/phospholipid mixtures at the air-water interface. In film balance experiments, we have observed that the high-film-pressure transition occurring at about 20 mN/m for mixtures containing between 40 and 100% lipopolymer, which is related to a first-order-like alkyl chain condensation, is a necessary requirement for the existence of a rheological transition. At this rheological transition, which is observed with a novel interfacial stress rheometer, dramatic increases in both the storage and loss moduli occur as the area per lipopolymer molecule is decreased. We have shown that these transitions are observed for lipopolymers prepared from both poly(ethylene glycol) and poly(ethyl oxazoline) polymer backbones. The combination of film balance and surface rheology experiments is interpreted in terms of the formation of a quasi-two-dimensional physical network in which there are two essential intermolecular interactions to impart elasticity. The first involves condensation of terminal hydrophobic alkyl chains into small aggregates at the air side of the Langmuir film. The second involves hydrogen bonding between segments of the hydrophilic polymer coil in the water subphase, mediated via water bridging.