Controlling surface properties of polyelectrolyte multilayers by assembly pH

The control of surface properties such as the morphology, roughness, stiffness, and wettability of polyelectrolyte multilayers was carried out using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(4-styrenesulfonic acid-co-maleic acid, 1:1 SS:MA) sodium salt (PSSMA 1:1) as the building blocks via adjusting the assembly pH value. We found that the surface roughness of the multilayers increased with increasing assembly pH value, and that the morphology was quite different at various assembly pH values when PSSMA was assembled as the final layer. However, the surface roughness and morphology of the multilayers (PDADMAC as the final layer) showed no apparent change at various assembly pH values. Interestingly, the stiffness of the PSSMA/PDADMAC multilayers could be tuned. Nanoindentation measurements by SFM showed that the hardness of the multilayers was very different at various pH values when PSSMA was the outermost layer. Conversely, the hardness of the multilayers remained unchanged regardless of the pH when PDADMAC was the outermost layer. The water contact angle hysteresis (the difference between the advancing and receding contact angles) of the multilayers was largely affected by both surface roughness and surface hydrophilic groups which could also be tuned by assembly pH.