Influence of salt on assembly and compression of PDADMAC/PSSMA polyelectrolyte multilayers

Influence of NaCl and NaBr and their concentration on assembly of polyelectrolyte multilayers and their compression behavior was studied 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. Stepwise growth of the PSSMA/PDADMAC multilayers was characterized by a quartz crystal microbalance with dissipation (QCM-D), revealing faster and slower growth of the layer thickness in higher concentration of NaCl and NaBr solutions, respectively. Moreover, QCM-D, ellipsometry and scanning force microscopy (SFM) measurements demonstrated that the multilayers were highly hydrated with a factor of 3 ∼ 9 in a wet state depending on the type and concentration of the salts used during the assembly. The type and concentration of the salts used during the assembly also influenced greatly the compression ratio of the polyelectrolyte multilayers, i.e. the compression ratio increased along with the increase of NaCl concentration, whereas it was constant when NaBr was used. The friction force of the PEI(PSSMA/PDADMAC)7 multilayers measured by SFM showed that the force peaks gradually shifted from low to high scanning frequency along with the increase of NaCl concentration, illustrating the gradual softening of the multilayers. By contrast, the peak positions in the friction force curves for all the multilayers assembled in NaBr solution kept unchanged regardless of the NaBr concentration. Contrast experiments further revealed that the compression ratios, masses in wet and wet thicknesses of the multilayers alternatingly treated in Cl− and in Br− solutions were all reversible.