Self-Patterning Polyelectrolyte Multilayer Films: Influence of Deposition Steps and Drying in a Vacuum.

Typically, laterally patterned films are fabricated by lithographic techniques, external fields, or di-block copolymer self-assembly. We investigate the self-patterning of polyelectrolyte multilayers, poly(diallyldimethylammonium) (PDADMA)/poly(styrenesulfonate) (PSS)short. The low PSS molecular weight (Mw(PSSshort) = 10.7 kDa) is necessary because PSSshort is somewhat mobile within a PDADMA/PSSshort film, as demonstrated by the exponential growth regime at the beginning of the PDADMA/PSSshort multilayer build-up. No self-patterning was observed when the PDADMA/PSS film consisted of only immobile polyelectrolytes. Atomic force microscopy images show that self-patterning begins when the film consists of seven deposited PDADMA/PSSshort bilayers. When more bilayers are added, the surface ribbing evolved into bands, and circular domains were finally observed. The mean distance between the surface structures increased monotonously with the film thickness, from 70 to 250 nm. Scanning electron microscopy images showed that exposure to vacuum resulted in thinning of the film and an increase in the mean distance between domains. The effect is weaker for PSSshort-terminated films than for PDADMA-terminated films. The mechanism leading to domain formation during film build-up and the effect of post-preparation treatment are discussed.