No abstract

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.

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.

Recent surface rheology and film balance experiments on monolayers of PEG lipopolymers and phospholipid/PEG lipopolymer mixtures at the air−water interface have revealed a new class of quasi two-dimensional physical networks. Two different kinds of associative interactions are necessary to form the network:  microcondensation of alkyl chains of lipopolymers to form small clusters and water molecule mediation of the interaction between adjacent PEG clusters via hydrogen bonding. In the experiments presented here, we are interested to learn whether the physical gelation is PEG specific or whether it is a more general characteristic of lipopolymers at the air−water interface. To address this topic, we have expanded our surface rheology and film balance experiments to poly(oxazoline) lipopolymers. Our experiments indicate the occurrence of a rheological transition if the poly(oxazoline) lipopolymers consist of a dioctadecylglycerol anchor. This shows that the physical gelation among lipopolymers is not a PEG-specific phenomenon. No physical gelation is found, however, if the dioctadecylglycerol anchor of the lipopolymer is replaced by a dioctadecylamine anchor. The observed importance of the hydrophobic anchor supports our previous findings that the alkyl chain condensation should be seen as one of two kinds of physical junctions necessary for the formation of the physical network.