Towards ductile single-step polyelectrolyte complex films by means of plasticization

Polyelectrolyte complexes (PECs) show great promise as functional coatings, including as oxygen barrier coatings for food packaging, but the brittleness of dry PECs limits their application. In this work, the possibility of plasticizing polyethylenimine/poly(4-styrenesulfonic acid) (PEI/PSS) films was investigated. Three different classes of plasticizers were chosen: salt, polyols, and ionic liquids (ILs). They were successfully incorporated in the evaporation-based single-step method. Potassium bromide or sorbitol plasticized films all showed crystallization upon evaporation, while films plasticized by glycerol, polyethylene glycol, and imidazole-based ILs all showed a clear brittle-to-ductile transition in their mechanical properties. The hydrophilicity of polyols and the amphiphilicity of these ILs allowed them to form homogenous casting solutions, but also increased their water sensitivity. The ionic nature of ILs make them more efficient as plasticizers since they can replace part of the PE-PE ionic-crosslinks, resulting in a more flexible network. Overall, this work demonstrates that ductile PEC films can be formed in a single step with controlled mechanical properties through plasticization.