Structure of a Spread Film of a Polybutadiene−Poly(ethylene oxide) Linear Diblock Copolymer at the Air−Water Interface As Determined by Neutron Reflectometry

Abstract

The structure of a film of a linear diblock copolymer of polybutadiene−poly(ethylene oxide) with a molecular mass of ∼60 000 g mol-1 and containing ∼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 (Γ = 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 ∼3−5 A layer containing ∼50% (by volume) polymer and indeterminate quantities of water and PEO. The PEO is accommodated in a slightly thicker layer (∼6−11 A), and this solvated layer contains ∼30% PEO and a negligible amount of PB. The amount of material in these layers determined from the reflectivity data agrees well with the sprea...