Synthesis and characterization of lyotropic liquid crystalline of N-alkyl chitosans

A series of new derivatives of chitosan-N-alkyl (methyl, ethyl, propyl and butyl) chitosans were synthesized. The degree of substitutions of one of the derivatives N-ethyl chitosan were obtained by controlled the molar ratio of the reactants. All of the products show lyotropic liquid crystalline properties regardless of the length of the carbon chains and the degree of substitution. Critical concentration ( c*) of the samples were measured by polar optical microscopy (POM). The results showed that the value c* didn't varied evidently with the increase of degree of substitution (DS from 0.16 similar to 0.82) in the case of a fixed carbon chain, but rise constantly as the length of substituent group varied from methyl to butyl. The result was explained as follow: the introduction of alkyl chain not only broke down some of intermolecular and intramolecular hydrogen bond, but also changed the molecular diameter (d) of chitosan. The former makes the rigidity of molecules weakened, so c* of all the derivatives were bigger than chitosan itself. As to the later, d didn't change with the increase of DS, thus, c* didn't vary with the increase of DS; while, d became bigger as the substituent group varied from methyl to butyl, so c* rise accordingly. This rule is in accordance with Onsager and Flory's theory of the rigid club and was farther confirmed by X-ray diffraction experiments.