Equilibrium and nonequilibrium features in the morphology and structure of physisorbed polyelectrolyte layers

AFM is used to characterize sodium poly(styrene sulfonate) (PSS) layers physisorbed from NaCl solutions with an ionic strength ranging between 0 and 1 M NaCl. Colloidal probe tapping mode imaging shows that domains of PSS salted brushes coexist with flatly adsorbed PSS. The brush area fraction increases with rising degree of polymerization. The surface forces (as measured with colloidal probe technique) are a superposition of steric and electrostatic forces; their respective contribution is determined by the brush area fraction. Unexpectedly, the internal properties of the brush domains (brush thickness and average chain distance) are independent of the deposition salt concentration. Both properties increase with rising polymer length/degree of polymerization, whereas only the brush thickness can be controlled by the surrounding salt concentration (equilibrium feature). Furthermore, we show that the amount of brush-like physisorbed PSS chains leads to an increase in PSS surface coverage, which is also observed with other techniques after addition of salt to the deposition solution (as is extensively described in literature).