Coupled effects of spreading solvent molecules and electrostatic repulsions on the behavior of PS-b-PAA monolayers at the air-water interface

Abstract : 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 solvent spreading 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 there still exist two regimes of compressibilities on the isotherm. A refined structural study at pH 9 combining Specular Neutron Reflectivity (SNR), Grazing-Incidence Small-Angle X-rays Scattering (GISAXS) and Atomic Force Microscopy (AFM) in liquid measurements show that: (i) PAA blocks are stretched in solution, as expected from polyelectrolyte brushes in osmotic regime; (ii) the system undergoes a spinodal decomposition during deposit at the air/water interface in presence of DMF. Upon compression, the Qxy* position of the peak associated with the spinodal structure remains almost constant but its intensity evolves strongly and passes through a maximum at intermediate pressures. This reveals two operating processes in the system: strong electrostatic repulsions between chains that prevent in-plane reorganizations and force such reorganizations to occur from the surface to the volume and progressive expulsion of the DMF molecules from the monolayer. These processes have antagonist effects on the intensity of the peak: the increase of the repulsions makes it more pronounced whereas the expulsion of solvent makes it vanish due to the loss of contrast.
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Langmuir, American Chemical Society, 2017, 33 (43), pp.12525-12534. 〈10.1021/acs.langmuir.7b02664〉
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Zineb Guennouni, Michel Goldmann, Marie-Claude Fauré, Philippe Fontaine, Patrick Perrin, et al.. Coupled effects of spreading solvent molecules and electrostatic repulsions on the behavior of PS-b-PAA monolayers at the air-water interface. Langmuir, American Chemical Society, 2017, 33 (43), pp.12525-12534. 〈10.1021/acs.langmuir.7b02664〉. 〈hal-01612065〉

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