The phase behavior and rheological properties of natural polyelectrolyte complexes with salt addition

Oppositely charged polyelectrolytes can form polyelectrolyte complexes (PECs) due to the electrostatic interactions. The structure and properties of PECs can be tuned by varying the salt concentration, as the addition of salt can facilitate the associative phase separation. In this work, PECs are prepared from two biopolymers, positively charged chitosan and negatively charged alginate. Without any salt addition, the water content of the complex phase was >95%. The addition of salt led to a decrease in the complex phase's water content with increasing shear-modulus. However, at a very high salt concentration, the shear-modulus of the complex phase decreased. In the non-linear regime, the shear modulus decreased with the increase of strain amplitude. The non-linear rheological behavior of the salt-doped PECs was further investigated using large amplitude oscillatory shear experiments. The compositions of the PECs were determined as a function of salt concentration, and the results indicate the preferential partitioning of salt into the complex phase. Small-angle X-ray scattering was used to understand the structure of the salt-doped systems. This study provides insights into the tunability of  PECs properties necessary for developing functional materials from natural polyelectrolytes.