Synchrotron X-ray Diffraction and Reflection Studies of a Polymacromonomer Monolayer at the Air−Water Interface: Transition from Aligned Single Molecules to Homogeneous Layer

Abstract

Monolayers of polymacromonomers with poly(vinyl)pyridine side chains of different lengths, polyPVP20.8 and polyPVP46.7, are studied at the air/water interface. Combination of reflection data and diffraction peaks allows the establishment of a structural model. In the expanded phase at pressures below approximately 15 mN/m, the side chains are adsorbed to the air/water interface, causing polymer backbone stretching and alignment of the single molecules. The in-plane peaks are due to the lateral electron density variation above and below the adsorption layer. The diameter of the flattened cylindrical molecules corresponds to twice the contour length of the side chains (25 and 12.5 nm, respectively) and decreases on monolayer compression by approximately 20%, due to side chain desorption. Additionally, side chain desorption causes monolayer thickening. Above the phase transition, the lateral order disappears. Obviously, the side chains of adjacent molecules mix and the monolayer is homogeneous.