Structure of a spread film of a polybutadiene-poly(ethylene oxide) linear diblock copolymer at the air-water interface as determined by neutron reflectometry

The structure of a film of a linear diblock copolymer of polybutadiene-poly (ethylene oxide) with a molecular mass of similar to 60 000 g mol(-1) and c ontaining similar to 50 mol % poly(ethylene oxide) PEO spread on water has been investigated using neutron reflectometry. The reflectivity data have b een analyzed using the optical matrix method and the kinematic approximatio n. From the optical matrix analysis it is found that at low surface concent rations (Gamma = 0.45, 0.83 mg m(-2)), the reflectivity data are well descr ibed using a two-layer model, indicating that even at low concentrations th e blocks of the copolymer are segregated. According to this model the polyb utadiene (PB) block resides in a thin similar to3-5 Angstrom layer containi ng similar to 50% (by volume) polymer and indeterminate quantities of water and PEG. The PEG is accommodated in a slightly thicker layer (similar to6- 11 Angstrom), and this solvated layer contains similar to 30% PEG and a neg ligible amount of PB. The amount of material in these layers determined fro m the reflectivity data agrees well with the spread amount. At higher surfa ce concentrations (Gamma = 2.06 mg m(-2)), however, st more complex model i s required to represent the data. Superficially, the two-layer model is mod ified by accommodating similar to 13% of PB in the PEG-enriched layer and b y adding a third similar to 100 Angstrom layer containing a mixture of PB ( 10%) and PEO (15%). From this model it can be inferred that at this surface concentration any PEO brushlike structure is accompanied by a higher-level structure containing "submerged" PB. It is feasible that surface aggregate s of copolymer are incorporated into the region underlying the segregated m onolayer structure; this is suggested by an inflection or "nonhorizontal" t ransition in the surface-pressure isotherm. This behavior is not dissimilar to that found for adsorbed films of polystyrene-b-poly(ethylene oxide) at the air-water interface. Analysis using the kinematic approximation support s and reinforces the observations supplied by the optical matrix fitting.