Polyelectrolyte Spin-Assembly: Effect of Ionic Strength and Spinning Rate on the Growth of Multilayered Thin Films

Polyelectrolyte spin assembly (PSA) of multilayers is a sequential process featuring adsorption of oppositely charged polyelectrolytes from dilute solutions undergoing spin-coating flow. We investigated the dependence of multilayer film build-up of poly(sodium-4-styrene sulfonate) and poly(allylamine hydrochloride) on solution ionic strength and spin rate. We observed that at fixed spin rate the PSA growth rate shows a non-monotonic dependence on salt concentration. It first increases and then decreases with increasing the solution ionic strength. This observation is a manifestation of two competing mechanisms driven by electrostatic interactions and shear flow, which control film build up. This non-monotonic behavior is explained in the framework of a Flory-like theory of multilayer formation from polyelectrolyte solution under shear flow. We performed molecular dynamics simulations to further elucidate the factors controlling the film structure.