The aggregative behavior of hydrophobically modified chitosans with high substitution degree in aqueous solution

Nowadays chitosan, CHIT, is becoming one of the most interesting raw materials because of the actual and potential applications in several technological fields due to its chemical versatility, natural abundance and ecological compatibility. By applying a very versatile and quantitative amination reaction, linear aliphatic chains of length variable from five to 12 carbon atoms have been grafted on the chitosan backbone at 10% in mole of the glycosidic units. Newtonian viscosity and fluorescence probe emission have been used to analyze the intra- and inter-aggregation properties of these modified chitosans, Cn-CHITs, and to compare them with those of chitosan itself. Viscosity in the dilute concentration range showed that for CHIT and C5-CHIT the behavior is quite similar highlighting that the insertion of this short aliphatic chain does not modify the rigidity and/or the interaction with the solvent. For C6, C8, and C10-CHIT, the increasing length of the pendant promotes a progressively more efficient intra-aggregation of the polymer as shown by the reduction of its hydrodynamic radius. Converging evidences are obtained by the analysis of the fluorescence spectra of pyrene, used as probe, in the aqueous solutions of the grafted polymers, which show how different is the environment in which pyrene is located, if short or long chained polymers are analyzed. In fact, the spectra in CHIT, C5-CHIT, and C6-CHIT are quite similar among them and indicate a scant hydrophobic quality of the aggregates for these short grafted polymers even in concentrated solution. In contrast, the presence of C8-CHIT, C10-CHIT, and C12-CHIT concentrated solutions gives rise to a progressive and evident modification of the pyrene spectra, which resemble those of this fluorescent probe in a hydrocarbon solvent, thus indicating the formation of well-defined hydrophobic pools. However, in the limit of very concentrated solutions, the specificity of the grafted chains of different length is lost.