A comparison of complexation induced brittleness in PEI/PSS and PEI/NaPSS single-step coatings

In the field of coatings, there is a major trend towards the reduction of volatile organic compounds by developing water-based alternatives for traditional solvent-based paints. Complexes of oppositely charged polyelectrolytes have recently emerged as a promising candidate for such coatings. Recently, we developed an ammonia-evaporation induced complexation approach to form polyelectrolyte complex films of two weak polyelectrolytes in a single step. Here, we extend this method by applying it to a combination of PEI/PSS, a weak polycation and a strong polyanion. Homogenous solutions at different ratios were successfully prepared. After casting and drying, dense and optically transparent layers were formed. As a comparison, PEI/NaPSS was also studied. Contrary to PEI/PSS, the pH in PEI/NaPSS films remains high during drying and NH₃ evaporation, so PEI does not acquire a positive charge, thereby preventing complexation. We show that this leads to large differences in the properties of the films, with the PEI/NaPSS films being more sensitive to water, more ductile, and more prone to phase separation. We also found that the mechanical properties of the films depend strongly on the charge ratio: films made at a PEI/PSS ratio between 2:1 and 1:1 formed the strongest complexes, resulting in films that swell relatively little in water, but that are also brittle and prone to cracking during drying. In this work, we report a systematic study into this promising system and show its potential and tunability.