X-ray reflectivity of a polymer monolayer at the water/vapor interface

X-ray specular reflectivity from a monolayer of partially hydroxylated 1,2-polybutadiene (50% butyl alcohol random substitution) at the water/vapor interface was measured from below the critical wavevector for water, 0.0217 Å^-1, to Q_z ≅ 0.6 Å^-1 (where Q_z is the transferred momentum normal to the interface). The sample was prepared on a Langmuir trough, and measurements were made at five different surface densities in the high-pressure, or saturated, region of the isotherm. The measured reflectivity is interpreted to obtain a profile of the average electron density ρ(z) as a function of distance z along the surface normal. The profile has a local maximum that is approximately 10% larger than the electron density in bulk crystalline 1,2-polybutadiene. At the highest pressures the width of the liquid/vapor interface is ∼10% larger than the value calculated from thermal capillary waves using in situ measurements of the surface tension; however, for smaller pressures the measured and calculated values agree.