Dependence of PSS Diffusion in Multilayers of Entangled PDADMA on Temperature and Salt Concentration: More than One Diffusion Constant

Abstract

Layer-by-layer (LbL) assembly is a widely used tool for engineering materials and coatings, but the dynamics of the constituent polymer chains remain poorly understood. Using neutron reflectivity, the vertical diffusion of polyanion poly(styrene sulfonate) (PSS) (Mw(PSS) = 75.6 kDa) within PSS/poly(diallyldimethylammonium) (PDADMA) (Mw(PDADMA) = 72.1 kDa) multilayers is probed, while the annealing temperature and salt concentration were varied. Only one fraction of PSS was mobile, and the other PSS molecules were assigned to a second, almost immobile fraction, with the respective diffusion constants DPSS,fast and DPSS,slow differing by about two orders of magnitude. The model with the two different mobile fractions is the simplest one that describes the time dependence of the scattering length density profiles. The relationship between the diffusion constants and the annealing temperature can be described using the Arrhenius equation. From this, the activation energies could be determined. Conditions for Fickian diffusion and for the formation of different mobile fractions are discussed.