Formation of polyelectrolyte multilayers by flexible and semiflexible chains.

Poly(sodium 4-styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) are flexible polyelectrolytes, whereas sulfated chitosan (SC) and cationic guar gum (CGG) are semiflexible polyelectrolytes. By use of a quartz crystal microbalance with dissipation (QCM-D), a zeta potential analyzer (ZPA), and atomic force microscopy (AFM), we have investigated the growth of PSS/PDDA, PSS/CGG, SC/PDDA, and SC/CGG multilayers as a function of NaCl concentration (C(NaCl)). For the same layer number, the changes of frequency (-Δf) and dissipation (ΔD) regarding PSS/PDDA multilayer increase with C(NaCl), whereas -Δf and ΔD for SC/CGG multilayer increase at C(NaCl) < 0.1 M and decrease at C(NaCl) > 0.1 M as C(NaCl) increases. In the cases of PSS/CGG and SC/PDDA multilayer, for the same layer number, -Δf and ΔD increase with C(NaCl) in the range of C(NaCl) < 0.5 M, and they decrease with the increasing C(NaCl) in the case of SC/PDDA multilayer but slightly change for the PSS/CGG multilayer at C(NaCl) > 0.5 M. QCM-D studies indicate that the growth of multilayers as a function of salt concentration is determined by the delicate balance between the weakening of electrostatic repulsion between identically charged groups and the decrease of electrostatic attraction between neighboring layers. ZPA and AFM measurements demonstrate that the extent of surface charge overcompensation and the surface morphology of the multilayers are controlled by the chain conformation.