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

The objective of this manuscript is to review recent film balance and inter facial rheology experiments on Langmuir monolayers of lipopolymers and lipo polymer/phospholipid mixtures at the air-water interface. In film balance e xperiments, we have observed that the high-film-pressure transition occurri ng 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 nec essary requirement for the existence of a theological transition. At this t heological transition, which is observed with a novel interfacial stress rh eometer, dramatic increases in both the storage and loss moduli occur as th e area per lipopolymer molecule is decreased. We have shown that these tran sitions are observed for lipopolymers prepared from both poly(ethylene glyc ol) and poly(ethyl oxazoline) polymer backbones. The combination of film ba lance and surface theology experiments is interpreted in terms of the forma tion of a quasi-two-dimensional physical network in which there are two ess ential 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 be tween segments of the hydrophilic polymer coil in the water subphase, media ted via water bridging.