P188, a triblock copolymer of the form poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) is known as an effective membrane sealant. Using a dipalmitoylphosphatidylcholine monolayer at the air-water interface to mimic the outer leaflet of the cell membrane, we have examined the interaction between P188 and a phospholipid film. Atomic force microscopy (AFM) was used to investigate the morphological changes in the lipid monolayer induced by P188 insertion upon transferring the lipid/polymer film from the air-water interface onto a solid substrate. Our AFM results confirm that lipid packing regulates poloxamer insertion such that P188 only changes the morphology of a lipid monolayer when the lipid packing density is below that of the insertion threshold. By combining phase imaging and different driving forces in tapping mode AFM, our results further help reveal the distribution of poloxamers in the lipid monolayer with nanometer resolution.