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Lipopolymers are lipids with a polymer chain covalently attached to the lipid. We systematically studied the monolayer behavior of a series of lipopolymers by use of infrared reflection−absorption spectroscopy (IRRAS) and classical Langmuir film balance techniques. The lipopolymers differed in chemical composition both in the lipid chains and in the polymers. However, the lipid chains contained 18 carbon atoms in all cases. We find the lipopolymers to show a very complex phase behavior at the air−water interface, depending on such diverse parameters as polymer hydrophilicity, length of polymer, and saturation of the lipid chains. We were able to measure up to two different plateau regimes in the monolayer isotherms. One is observed for lipopolymers where the polymer part is only slightly water-soluble and is correlated with the desorption of the polymer from the water surface. This desorption transition can be termed a pancake−mushroom transition. The origin of the second plateau regime is more uncertain. Baekmark et al. (Langmuir 1995, 11, 3975) interpreted this plateau as a transition within the polymer (mushroom-to-polymer brush conformation), but in two recent publications, by Gonçalves da Silva et al. (Langmuir 1996, 12, 6547) and Baekmark et al. (Langmuir 1997, 13, 5521) this interpretation has been questioned. In the present work we show that this second transition is native to lipopolymers in the sense that it can only be observed when a lipid and polymer are present in the same molecule. On the same basis of the film balance and the IRRAS data, we are able to show that the lipid alkyl chains must be involved in the molecular processes constituting the “native” transition. To account for the origin of the “native” transition, we suggest that a process of alkyl chain condensation coupled to a strong reduction in the number of gauche isomers within the alkyl chains supply the driving force for the “native” transition.

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Lipopolymers are lipids with a polymer chain covalently attached to the lipid. At the air-water interface such lipopolymers may undergo up to two monolayer phase transitions. One is correlated with desorption of the polymer group from the surface (the so-called pancake-mushroom transition) and a second is "native" to lipopolymers in the sense that it can only be observed when lipid and polymer are present in the same molecule. In this study we present direct evidence from infrared spectroscopy showing that the "native" transition, although requiring the presence of the polymeric headgroup, is a transition solely within the lipid alkyl chains. This transition correlates to a strong reduction of the number of gauche isomers within the lipid alkyl chains. We used two poly(ethylene oxide) lipopolymers which differed only in the lipid alkyl chains. These were either fully deuterated or fully protonated. The polymer chains were protonated in both cases. Within the protonated lipopolymer, a strong ordering of the CH2 groups was observed during the "native" transition. Within the deuterated lipopolymer, the exclusive deuteration of the alkyl chains allowed us to distinguish directly between lipid and polymer. We observed that the polymer CH2 spectra remained unchanged when the lipopolymer monolayer was compressed. As it was possible to build the subtraction spectra between the two lipopolymers, we could unambiguously identify the lipid component of the protonated lipopolymer in the spectra. It was found that the strong ordering of the methylene groups occurs solely within the lipid alkyl chains.

Lipopolymers are lipids with a polymer chain covalently attached to the lipid. We studied the infrared reflection absorption behavior as a function of molecular area of Langmuir monolayers of an ether lipopolymer, DC18Gly-M35 (35 monomer units). A plateau region was observed in the monolayer isotherm. Within this region the CH2 asymmetric and symmetric stretching mode absorptions shifted toward lower absorption frequencies (6 and 4 cm-1, respectively). This indicates that the plateau is accompanied by strong local ordering in the lipopolymer, which contradicts previous suggestions that the plateau correlates with a mushroom−brush transition in the polymer.