Effect of Perfluoroalkyl Endgroups on the Interactions of Tri-Block Copolymers with Monofluorinated F-DPPC Monolayers.

We studied the interaction of amphiphilic and triphilic polymers with monolayers prepared from F-DPPC (1-palmitoyl-2-(16-fluoropalmitoyl)--glycero-3-phosphocholine), a phospholipid with a single fluorine atom at the terminus of the -2 chain, an analogue of dipalmitoyl-phosphatidylcholine (DPPC). The amphiphilic block copolymers contained a hydrophobic poly(propylene oxide) block flanked by hydrophilic poly(glycerol monomethacrylate) blocks (GP). F-GP was derived from GP by capping both termini with perfluoro--nonyl segments. We first studied the adsorption of GP and F-GP to lipid monolayers of F-DPPC. F-GP was inserted into the monolayer up to a surface pressure Π of 42.4 mN m, much higher than GP (32.5 mN m). We then studied isotherms of lipid-polymer mixtures co-spread at the air-water interface. With increasing polymer content in the mixture a continuous shift of the onset of the liquid-expanded (LE) to liquid-condensed (LC) transition towards higher molecular and higher area per lipid molecule was observed. F-GP had a larger effect than GP indicating that it needed more space. At a Π-value of 32 mN m, GP was excluded from the mixed monolayer, whereas F-GP stayed in F-DPPC monolayers up to 42 mN m. F-GP is thus more stably anchored in the monolayer up to higher surface pressures. Images of mixed monolayers were acquired using different fluorescent probes and showed the presence of perfluorinated segments of F-GP at LE-LC domain boundaries.