Ordering of Poly(ethylene oxide)/Poly(propylene oxide) Triblock Copolymers in Condensed Films Stephen M. O’Connor, † Stevin H. Gehrke, † and Gregory S. Retzinger* ,‡ Departments of Chemical Engineering and of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, Ohio 45267 Received August 25, 1998. In Final Form: February 4, 1999 Triblock copolymers of the form PEORPPOPEOR [where PEO is poly(ethylene oxide) and PPO is poly- (propylene oxide)] form stable insoluble monolayers at both the air-water and solid-water interfaces. At the air-water interface, the nominal equilibrium spreading area of copolymers that have the same PPO content, or “core”, increases with increasing PEO content by an amount equivalent to the area of the hydrated ethylene ether subunits. With compression, a PEO-dependent transition from an expanded to a more condensed monolayer occurs at ∼10 dyn‚cm -1 , presumably because of condensation of PEO segments onto the core and out of the plane of the surface. Consistent with these results and interpretation, the thickness of adsorbed films of copolymers from series that have fixed cores increases approximately linearly with ethylene oxide content. At collapse of copolymer films, the nominal area per moleculesregardless of the number of ether functions in the PEO segmentsis that of the hydrated PPO core. On the basis of these data, we propose models for the structure and ordering of triblock copolymers in condensed films, and we relate these models to the gradation in protein resistance observed for such films. Introduction Triblock copolymers of the form PEO R PPO  PEO R [avail- able commercially as Pluronic polyols; where PEO is poly- (ethylene oxide) and PPO is poly(propylene oxide)] are water-soluble, nonionic surfactants that have many industrial and medical applications. The general utility of these copolymers derives, in part, from the ease with which the surface properties of the rudimentary polyether can be changed. When the lengths of the hydrophobic PPO and hydrophilic PEO “blocks” are simply varied, a wide range of chemically similar but physicochemically distinct species of amphiphiles can be made. These distinct species, in turn, can have different applications. 1 Because surfaces coated with PEO resist the adsorption of proteins, 2 the more hydrophilic versions of the triblock polyethers have been used as coatings to passivate hydrophobic polymeric surfaces destined to contact living tissues and tissue fluids, e.g., blood, in vivo, or ex vivo. 3-15 Existing theories hold that the relatively hydrophobic PPO block of a copolymer anchors the molecule to the intended surface 16-22 while the hydrated hydrophilic PEO chains extend out into the solution, forming a “brush layer” 23-25 that inhibits protein adsorption by steric repulsion. 4,5,10,11,15,26-30 Packing of the surfactants in the brush layer is claimed to be dictated by net forces of attraction operating both between the PPO block and the underlying hydrophobic surface and between adjacent PPO blocks. 16 Others question the existence of such a brush layer, believing the energy required to create and maintain such a layer to be too great to derive by simple adsorption. 31 We are interested in general principles governing the adsorption of proteins to surfaces and consequent protein- * To whom correspondence should be addressed. 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