Organisation of an amphiphilic linear diblock copolymer at the air/water interface studied by neutron reflectometry

Abstract

Neutron reflectometry has been used to obtain the organization of a linear diblock copolymer at the air/water interface. To obtain the maximum information, a series of copolymers with approximately the same chemical composition but differing in isotopic labelling of the component blocks was synthesised. Each of these was spread on heavy water and a mixture of light and heavy water with zero reflective power for neutrons and the reflectivities were obtained at surface pressures of 2, 5 and 10 mN m^–1. Qualitative inspection of these reflectivity profiles suggests that a two-layer structure is formed with the poly(methyl methacrylate) block adjacent to the air. A more quantitative analysis using two-layer fits to the data generated by the optical matrix confirms this view but the parameters obtained are very sensitive to the quality of the fits. A direct analysis method has been attempted using the partial-structure-factor approach and using a Gaussian model for each layer we have been able to obtain layer dimensions, layer separations and the distribution of water in the poly(ethylene oxide)-rich lower layer which appears to be completely in the aqueous subphase.