Metastability of Nematic Gels Made of Aqueous Chitin Nanocrystal Dispersions

Chitin nanocrystal aqueous dispersions were prepared by acid hydrolysis of crude chitin from crab shells. The resulting dispersions were studied with small deformation oscillatory experiments and polarized optical microscopy under different conditions of nanocrystal concentration, ionic strength, pH, and temperature. The chitin nanocrystal dispersions exhibited a nematic gel-like behavior with increasing solids concentration. The appearance of nematic-like structures could be explained by the Onsager theory for parallel alignment of anisotropic particles on entropic terms, while the sol-gel transition could be attributed to associative interactions between the chitin nanocrystals. With increasing ionic strength and pH, such associative interactions were enhanced, because the repulsive forces due to the electrostatic charges were reduced and, thus, stronger gels were formed. Heating of the nanocrystal dispersions led to further increases in the storage modulus (G'), which were irreversible upon cooling; the rate of G' increase (dG'/dt) was dependent on temperature.