The Collapse of Poly(Ethylene Oxide) Monolayers

Surface pressure pi has been measured as a function of surface concentration for poly(ethylene oxide) (PEO) surface films, spread onto dieonized water at temperatures T from 8 to 28 degree C. At fixed surface coverage, the temperature coefficient of surface pressure d pi /dT was negative at submonolayer coverages and positive at higher surface coverages. This suggests, respectively, a peculiar increase and decrease of surface entropy with increasing surface coverage. A degree of ordering of water molecules with the PEO repeat unit in the surface layer is suggested as a reason for the negative value. These findings illustrate that a polymer monolayer should not in all respects be considered as two-dimensional.