Solution studies of the chitin-lithium chloride-N,N-di-methylacetamide system

Light scattering and viscosity measurements on dilute or moderately concentrated solutions of chitin in N,N-dimethylacetamide (DMAc) containing 5% or 8% of LiCl are reported. Chitins with d.p. lower than that of commercial samples were obtained by treatment of these solutions with ultrasound. The curve c/Hovs. c exhibits a maximum at c values inversely related to the molecular weight of the polymer and is consistent with a model in which, at relatively low concentration, the polysaccharide chains intertwine whereas at high concentration they become hyper-entangled, eventually resulting in an inhomogeneous network. Light scattering measurements on the dilute solutions containing 5% of LiCl allowed an evaluation of the molecular weight, second virial coefficient, and radius of gyration. The values of the Mark-Houwink parameters a and K and the relationship between radius of gyration and molecular weight were obtained. An evaluation of the linear expansion factor, αs, using the interpenetration function, gives values close to unity. This fact, together with the observed high values of the second virial coefficient, implies that the chitin molecules are rather stiff in this solvent system. The persistence length was evaluated by means of the Yamakawa-Fujii and Benoit-Doty theories for worm-like chains. Derivatives of chitin prepared in DMAc-5% LiCl in a homogeneous phase under mild conditions had high degrees of substitution without main-chain degradation.