Kinetics of ultrasonic degradation and polymerisation degree distribution of sonochemically degraded chitosans

The process of physical degradation by means of the ultrasonic action towards chitosans with mole fraction of 2-acetamido-2-deoxy-β-d-glucopyranose units (the degree of N-acetylation, FA) in the range of 0.10≤FA≤0.28, and the weight average polymerisation degree (x̄w) in the range of 2.2≤x̄w×10−3≤3.1 has been investigated. The decrease of x̄w as well as changes in the distribution of the degree of polymerisation (P) has been determined as a function of time, FA, temperature, concentration of chitosan solution and concentration of acetic acid in the solution. The use of low-power ultrasound emitter allowed to establish that in the case of chitosan (binary heteropolysaccharide) the general rate parameter (k) increased with FA. This can be explained by the relatively stronger aggregation of macromolecules with higher FA, which results in size increase of macromolecular individuals and hence in their higher susceptibility to ultrasonic action. It was also observed that k decreased with chitosan concentration and temperature. The value of limiting degree of polimerisation (xe) was found to be influenced by structural parameters of chitosan chains (FA, aggregation). The increase of acetic acid concentration caused the increase in the k value, what indicated accelerating effect of ultrasound towards acidic hydrolysis of chitosan. The shape of the P curve of sonochemically degraded chitosans are in good correlation with the mid-point breakage concept of degradation accepted in sonochemical degradation of polymers.