Polymer and Site Diffusion of PSS during PSS/PDADMA Multilayer Buildup

The spontaneous formation of polyelectrolyte multilayers or polyelectrolyte complexes depends on the (inter)diffusion of polyelectrolytes. We investigate the transport of polyelectrolytes and extrinsic sites─charged polyelectrolyte repeat units balanced by counterions─during multilayer buildup. We determine the vertical diffusion coefficient DPSS of polystyrenesulfonate (PSS) repeat units in polyelectrolyte multilayer films from poly(diallyldimethylammonium) (PDADMA) and PSS using a quartz crystal microbalance with dissipation (QCM-D) and analyzing the observed film growth. Varying the NaCl concentration cNaCl results in DPSS=(4.3±1.4)·10−20m2s·eα·cNaCl. As known from the free volume model, the prefactor is constant. For site diffusion, the exponent is also constant (α = 4.9 M–1). For polymer diffusion, however, α increases linearly with MPSS, the molecular weight of PSS. The results for site diffusion quantitatively agree with those of Fares and Schlenoff (J. Am. Chem. Soc., 2017, 139(41), 14656–14667). For MPDADMA = 117 kDa, polymer diffusion dominates when MPSS < 65 kDa; at larger MPSS, site diffusion determines multilayer buildup. To describe polymer diffusion, we use DPSS = B · MPSS–γ. B increases exponentially with cNaCl, while γ increases linearly with cNaCl. We compare these results with polymer diffusion observed by neutron reflectivity and discuss other parameters that influence site and polymer diffusion, such as the type of monovalent ion, the molecular weight of the oppositely charged polyelectrolyte, and the type of polyelectrolyte.