Polymer compatibility effects on morphologies from mixed brushes probed by high pressure GISANS (HP-GISANS), specular and off-specular reflectivity

By means of Grazing Incidence Small Angle Neutron Scattering under hydrostatic pressure ( H P - GISANS) and neutron reflectometry at the solid-liquid interface, we compare effects of pressure and temperature on two model homopolymer mixed brush systems in thin (<100 nm) film geometry: a weakly incompatible (poly(methyl methacrylate)/poly(2-(dimethylamino)ethyl methacrylate)) (PMMA/PDMAEMA) and strongly incompatible ((poly((2,2,3,3,4,4,5,5-octafluoro)pentyl methacrylate))/poly(2-(dimethylamino)ethyl methacrylate)) (POFPMA/PDMAEMA) one. A dual responsive, temperature and pressure-sensitive, polymer (PDMAEMA) in both systems allows to explore the role of mixing incompatibilities between tethered macromolecules with solvent and also pressure on the morphological changes in the PDMAEMA-containing nanostructures. Our findings indicate distinct pressure-dependent response in both lateral (HP-GISANS) and transversal (specular NR) nanostructure depending on the chemical composition. Specular ToF-NR results suggest that temperature effects are found to be counteracted by pressure. We conclude that mixing non-idealities and residual unoccupied volume from hydrophobic domains within the investigated mixed brushes, similar to what is anticipated for homopolymer brushes, are polymer-compatibility dependent. Moreover, pressure enhances access to such nanocavities as evidenced by enhanced D 2 O retention within the brush in proximal distance to the substrate by the strongly incompatible POFPMA/PDMAEMA mixed brush. GISANS analysis indicates the presence of uncorrelated cylindrical hydrophobic nanostructures within a PDMAEMA-based swollen matrix. Moreover, off-specular scattering (OSS) analysis demonstrates that the spacing between hydrophobic domains and the associated surface coverage by hydrophilic regions exhibits changes with pressure only for the strongly incompatible POFPMA/PDMAEMA mixed brush. Our H P - GISANS and off-specular scattering results suggest that the length scale of phase separation on the sample surface along the beam direction can strongly depend on the compositional variation in the mixed brushes.