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 containing similar to 50 mol % poly(ethylene oxide) PEO spread on water has been investigated using neutron reflectometry. The reflectivity data have been analyzed using the optical matrix method and the kinematic approximation. From the optical matrix analysis it is found that at low surface concentrations (Gamma = 0.45, 0.83 mg m(-2)), the reflectivity data are well described using a two-layer model, indicating that even at low concentrations the blocks of the copolymer are segregated. According to this model the polybutadiene (PB) block resides in a thin similar to3-5 Angstrom layer containing 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 negligible amount of PB. The amount of material in these layers determined from the reflectivity data agrees well with the spread amount. At higher surface concentrations (Gamma = 2.06 mg m(-2)), however, st more complex model is required to represent the data. Superficially, the two-layer model is modified by accommodating similar to 13% of PB in the PEG-enriched layer and by 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 aggregates of copolymer are incorporated into the region underlying the segregated monolayer structure; this is suggested by an inflection or "nonhorizontal" transition 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 supports and reinforces the observations supplied by the optical matrix fitting.