Rheometric study of the gelation of chitosan in a hydroalcoholic medium.

The formation of chitosan physical hydrogels without any external cross-linking agent was studied. The gelation took place in an acetic acid-water-propanediol solution. The time to reach the gel point was determined by rheometry and gelations from different initial conditions could be compared. The influence of different parameters on gelation such as the polymer concentration, the degree of acetylation (DA) of chitosan and the composition of the initial solvent were investigated. The fractal morphology of the sample was not affected by the composition of the system. The number of junctions per unit volume at the gel point varied only with the initial number of chain entanglements per unit volume. Then, below an initial concentration of 1.5% (w/w), physical chain entanglements were insufficient and more junctions had to be formed to induce gelation. Over this value, only the kinetics allowing to replace entanglements by stable physical junctions played a key parameter. This kinetics was influenced by several parameters such as DA, temperature or the initial proportion water/alcohol. The acetyl groups played an important role in the formation of hydrophobic interactions, mainly responsible for gelation. The study of the influence of the gelation media revealed two critical points at 40% and 70% of water in the initial solvent, probably due to conformational changes and then to different modes of gelation. These physical hydrogels being used for cartilage regeneration, their final rheological properties were studied as a function of their degree of acetylation, the polymer concentration and the solvent composition in the initial solvent. Our results allowed us to define an optimal gelation condition for our application, corresponding to: DA=40%, a proportion water/alcohol of 50/50 and a polymer concentration of 1.5%.