Electric-Field-Assisted Layer-by-Layer Assembly of Weakly Charged Polyelectrolyte Multilayers

We describe how layer-by-layer (LbL) assembly of weakly charged. polyelectrolytes can be controlled by applying an electric field (electrophoretic LbL deposition). The rate of deposition of polyelectrolytes of cationic polyethylenimine (LPEI) and poly(acrylic acid) (PAA) can be enhanced by g increasing the magnitude of the externally applied electric field. Accordingly, a micrometer-thick film can be attained by electric-field-assisted LbL assembly with the deposition of less than 10 bilayers. However the ionization conditions or electrostatic mobility of weakly charged polyelectrolytes is highly influenced by the environmental conditions of the polyelectrolyte solution, resulting in complex film deposition behaviors in response to the variation of the electric field strengths. At an intermediate electric field strength, the adsorption of polyelectrolytes is weakened due to the electrolysis of water at the electrode surface and a subsequent reduction in the degree of ionization of the polyelectrolyte chains. However, under a strong electric field, reinforced electrophoretic deposition of polyelectrolyte chains overwhelms the effects of the electrolysis of water, leading to enhanced LbL film assembly. By exploiting this phenomenon, we were able to achieve successful film deposition even at pH values that are typically not available in the conventional LbL assembly. Additionally, we demonstrated that our method could be used to pattern the polyelectrolytes via selective deposition onto suitable electroconductive substrates for various downstream applications.