Dextran-based polyelectrolyte multilayers: Effect of charge density on film build-up and morphology

We have studied the growth process of thin polyelectrolyte (PE) films fabricated by the layer-by-layer assembly (LbL) and composed of Dextran sulfate with high (DexS H) and low (DexS L) sulfation rate and poly(allylamine hydrochloride) (PAH). Film growths were monitored by combining Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), Surface Plasmon Resonance (SPR) and Atomic Force Microscopy (AFM). Even though, the two films growth up to 10 bilayers, QCM-D showed that polyelectrolyte pairs do not display similar behaviours. (PAH/DexS H) systems lead to linear growth, i.e. amounts deposited increase both for PAH and DexS H, while the PAH/DexS L pair generated zig-zag shaped asymmetric growth. Film water contents were determined by QCM-D solvent exchange and SPR experiments. DexS L contains less water than DexS H and in agreement with the QCMD dissipation values that suggest the formation of more rigid films in the case of DexS L than DexS H. Surface morphology investigated by AFM display distinct surface patterns since DexS H form thin films with fibril-like morphology covering all the surface while heterogeneous films with puddle-like aggregates were imaged in the case of DexS L. Difference of charge compensation and charge neutralisation between both systems likely lead to dissimilar growth mechanisms that are tentatively proposed in this paper.

Automated summary

The growth process of thin polyelectrolyte films composed of Dextran sulfate with high and low sulfation rate and poly(allylamine hydrochloride) was studied using various monitoring techniques. Different growth behaviors were observed between the two systems, leading to differences in film water content and surface morphology. Charge compensation and neutralization variations likely contribute to the dissimilar growth mechanisms observed.