Coupled Effects of Spreading Solvent Molecules and Electrostatic Repulsions on the Behavior of PS-b-PAA Monolayers at the Air–Water Interface

We describe the surface behavior of PS-b-PAA monolayers at the air/water interface using N,N-dimethyformamide (DMF) as spreading solvent. At low pH, when the PAA blocks are neutral, the surface pressure versus molecular area isotherm shows a pseudoplateau associated with the presence of remaining spreading solvent molecules in the monolayer, as we described in a former study (Guennouni et al., Langmuir, 2016). We show here that the width of the plateau decreases when increasing pH up to its complete disappearance at high pH, when PAA blocks are fully charged, although two regimes of compressibilities on the isotherm still exist. A refined structural study at pH 9 combining specular neutron reflectivity (SNR), grazing-incidence small-angle X-ray scattering (GISAXS), and atomic force microscopy (AFM) in liquid measurements shows that (i) PAA blocks are stretched in solution, as expected from polyelectrolyte brushes in the osmotic regime; (ii) the system undergoes a spinodal decomposition during deposit at t...