Rheological study of the chitosan/glutaraldehyde chemical gel system

Chitosan dissolved in 0.1 moll^-1 acetic acid shows an apparent yield stress at very low frequencies, probably due to a structuring process yielding gel-like response. It reflects complex relaxation mechanisms once chains disentangle and relax, presumably due to incipient hydrophobic contacts reinforced by the relative stiffness of the chitosan chains, which tend to slow down reptation. When chemical cross-linkages are introduced, the weak self-associated network of chitosan is gradually replaced by a permanent covalent network as the molar ratio of aldehyde/amine groups, R, is increased. At R = 0.4 a glass-to-rubber-type transition is observed, while at R = 0.5 the form of the mechanical spectrum suggests the co-existence of a chemically cross-linked gel 'dissolved' in a second entangled network formed by chitosan chains of restricted mobility. At higher cross-linking levels (R > 1) a strong permanent gel is formed. The observed frequency dependence near the rheological gel point suggests several modes of relaxation processes.