Planar Phase Separation of Weak Polyelectrolyte Brushes in Poor Solvent

Poly(acrylic acid) (PAA) brushes were synthesized on silicon wafer substrates using a silane linker and Z-group RAFT method, which enables the controlled variation of brush characteristics. Atomic force microscopy (AFM) was used to characterize the brush morphology in the dry state, which ranged from isolated globules to micelles to a continuous film featuring dimples. Surface morphology was primarily determined by total grafted polymer concentration with a critical polymer concentration between 4.5 and 6.5 × 10⁻²³mol/nm², where the polymer rich regions transition from micellar islands to a continuous film with dimples. However, even at the highest grafting densities and brush strand lengths used in this study, a flat homogenous film was not formed. Morphological consequences of the solvent and thermal history were shown to persist into poorly solvated state after being dried. X-ray photoelectron spectroscopy (XPS) was used to quantify the degree of remaining salt after drying. It was found that a higher relative amount of salt was retained after drying in the lower grafting density samples.