X-ray Reflectivity of Diblock Copolymer Monolayers at the Air/Water Interface

AB diblock copolymers PS₂₆₀-b-PVP-C_nX^_24o (PS = polystyrene; PVP-C_n = alkylated poly(vinylpyridine); X^- = I-, Br^-; n = 1,4, 8, 10) have recently been found to self-assemble at the air/water interface to form circular structures referred to as surface micelles. These surface micelles are ordered into a quasi-two-dimensional array in the corresponding Langmuir-Blodgett (LB) films. Many questions arise in these systems, including the structure relationships between the air/water Langmuir films and the LB films deposited onto carbon or silicon surfaces. A detailed in situ synchrotron X-ray reflectivity study was performed to address these issues of the polymer film structure. Specular reflectance of all samples (n = 1,4,8,10) reveals that the thickness of the polyelectrolyte layer only increases by a factor of 2 as polymer surface density increases 4-fold. No significant hydration of the layer is observed. The off-specular scattering data are well fit by an exact expression for the scattering intensity from a two-dimensional-ordered hexagonal lattice with a capillary wave contribution. The micelle-micelle distances obtained are in excellent agreement with atomic force microscopy images of the LB films, confirming that no change in structure occuss in the film liftoff process. The change in micelle-micelle distance or in film thickness does not correlate in the pressure-area isotherm for n > 6. A correlation was observed only for the degree of surface ordering and the film stiffness.