Salt Annealing and Plasticization in Chitosan:CNC Polyelectrolyte Complexes

Mixing oppositely charged polyelectrolytes can lead to the formation of Polyelectrolyte Complexes (PECs). The mechanical properties of these complexes depend on the ionic crosslinking between the polymers, which is influenced by factors such as polymer characteristics, pH, and ionic strength. By modifying the ionic crosslinks through the addition of electrolytes, it is possible to induce a plasticization process termed saloplasticity,¹ enabling the reshaping of the complexes and the adjustment of their mechanical properties.² Furthermore, the use of natural polymers to create composite PECs, such as chitosan and cellulose nanocrystals (CNC), offers an alternative to non-biodegradable materials, promoting the development of more sustainable options.

In this work, Chitosan:CNC films with different charge stoichiometries were produced by solvent casting and annealed with solutions of varying ionic strengths to manipulate their mechanical properties. Additives, such as polyethylene glycol (PEG), were also employed to enhance the plasticization of the material. The produced materials were mechanically characterized using stress-strain tests with a universal testing machine, while the elastic modulus (E') and viscous modulus (E'') were monitored using dynamic mechanical analysis (DMA). A progressive decrease in E' was observed with increasing ionic strength of the solutions used during the salt annealing process, indicating saloplastic behavior in the system composed of biodegradable polymers and nanocrystals.                                                                                                                                                                                                                                   

Acknowledgement: The authors thank FAPESP for funding this project (grant number 2021/12701-6) and for providing the G. Vedovello PhD scholarship (grant number 2023/07740-1).

References

1. Porcel, C. H. & Schlenoff, J. B. Biomacromolecules 10, 2968–2975 (2009).
2. Shamoun, R. F., Reisch, A. & Schlenoff, J. B. Adv Funct Mater 22, 1923–1931 (2012).