Flow Behavior of Chitosan Particles

Abstract

54-18:544.774 Chitin and chitosan with degree of deacetylation (DD) 57, 85, 89, and 90% were obtained as 250-μm microparticles. Their behavior with shear flow in aqueous and acidic solutions was studied. It was found that chitosan microparticles with various DD converted into gels as a result of swelling in water. Chitosan hydrogel particles with DD >85% were destroyed in shear flow without destroying the chains. The macromolecules were ordered in the direction of the flow.Powdered chitosan (CS) is an exceedingly practical product for preparing solutions with broad applications [1]. The powder particles are usually produced by milling CS produced by deacetylation of chitin (CT). This type of chemical transformation destroys the crystal structure of the starting CT. CS with a high degree of deacetylation (DD, >80%) is practically amorphous and dissolves in weakly acidic solutions, in particular HOAc (2%) [2]. Such CS samples swell and form hydrogels in H 2 O.A solvent is not always weakly acidic but often turns out to be closer to neutral or simply H 2 O. In such instances, CS inevitably swells and forms hydrogel particles. Studies of the behavior of CS in H 2 O are important because the hydrogel particles are exceedingly sensitive to the effects of external fields, especially deformations due to flow. The flow of such a solution is characteristically non-Newtonian and was studied in the present work taking into account swelling of CS in a shear flow.CT and CS with DD 57, 85, 89, and 90% were selected for the experiments. Microparticles (250 μm) were prepared by milling samples in a ball mill. The change in the size of CT (d CT ) and CS (d CS ) microparticles after swelling was determined using an optical microscope. The swelling of samples as a function of CS DD was judged from the d CS /d CT ratio. The behavior with shear flow of the selected samples in H 2 O and acidic solutions was studied on a Rheotest-2 instrument [3].The tests showed that increasing DD resulted in increased swelling of CS samples in H 2 O. The swelling was greater for DD ≥ 90% (Fig. 1). This indicated that there was a certain characteristic DD value above which the ability of H 2 O to penetrate into the bulk of the CS samples increased sharply. The CS appeared as hydrogel microparticles that were susceptible to deformations, especially with flow.A comparison of swelled microparticles in shear flow showed that CS had typical non-Newtonian behavior that was more apparent for samples with high DD under large shear stresses (σ) (Fig. 2). The appearance of such behavior, i.e., breaks in the curves for γ < 400 s -1 , was due to deformations of the microparticles. Then, the samples acted as Newtonian fluids for γ > 400 s -1 . Hydrogels with DD ≥ 90% became paste-like. Surely, this resulted from coalescence of compressed deformed hydrogel particles in the flow.In acidic solution, CS not only swelled but also dissolved. These processes were kinetic in nature, depended on the DD, and occurred evenly throughout the whole solution with vigorous ...